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Bioinspired, Functional Nanoscale Materials

Permanent Link: http://ufdc.ufl.edu/UFE0042115/00001

Material Information

Title: Bioinspired, Functional Nanoscale Materials
Physical Description: 1 online resource (136 p.)
Language: english
Creator: Jun, In-Kook
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: binary, electroosmotic, nanocomposite, nanostructure, self, sers
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Functional nanomaterials in nature exhibit many unique functions and optical and mechanical properties. Examples of this include the dry adhesion of a gecko s foot, the reduced drag on a shark s skin, the high strength and toughness of nacre, and the superhydrophobic self-cleaning of a lotus leaf. This dissertation is devoted to creating unique and enhanced properties by mimicking such functional materials. We have developed a novel self-pumping membrane, which does not require an applied voltage. The self-pumping membrane harvests chemical energy from a surrounding fluid and uses it for accelerated mass transport across the membrane. A device such as this has promising applications in implantable or remotely operating autonomous devices and membrane-based purification systems. Reproducible and highly active surface enhanced Raman scattering (SERS) substrates were developed using a bottom-up self-assembly technology. With their high sensitivity and good reproducibility, the developed nanostructures (gold nanoparticle and nanohole arrays) as SERS substrates are very promising for applications such as ultra-sensitive detectors for chemicals and reproducible sensors for chemical and biological molecules. Binary colloidal crystals were created using a simple, fast, and scalable spin-coating technology. Although further investigation of the procedure is needed to improve the ordering of particles in the individual layers, the developed assembly technology has a promising outlook in applications such as optical integrated circuits and high-speed optical computing. Inorganic-organic nanocomposites were realized by assembling synthesized gibbsite nanoplatelets using the electrophoretic deposition and infiltration of a monomer followed by polymerization. Via surface modifications of gibbsite nanoplatelets, nanocomposites were further reinforced with covalent linkages between the inorganic platelets and organic matrix.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by In-Kook Jun.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Hess, Henry.
Local: Co-adviser: Jiang, Peng.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-02-28

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042115:00001

Permanent Link: http://ufdc.ufl.edu/UFE0042115/00001

Material Information

Title: Bioinspired, Functional Nanoscale Materials
Physical Description: 1 online resource (136 p.)
Language: english
Creator: Jun, In-Kook
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: binary, electroosmotic, nanocomposite, nanostructure, self, sers
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Functional nanomaterials in nature exhibit many unique functions and optical and mechanical properties. Examples of this include the dry adhesion of a gecko s foot, the reduced drag on a shark s skin, the high strength and toughness of nacre, and the superhydrophobic self-cleaning of a lotus leaf. This dissertation is devoted to creating unique and enhanced properties by mimicking such functional materials. We have developed a novel self-pumping membrane, which does not require an applied voltage. The self-pumping membrane harvests chemical energy from a surrounding fluid and uses it for accelerated mass transport across the membrane. A device such as this has promising applications in implantable or remotely operating autonomous devices and membrane-based purification systems. Reproducible and highly active surface enhanced Raman scattering (SERS) substrates were developed using a bottom-up self-assembly technology. With their high sensitivity and good reproducibility, the developed nanostructures (gold nanoparticle and nanohole arrays) as SERS substrates are very promising for applications such as ultra-sensitive detectors for chemicals and reproducible sensors for chemical and biological molecules. Binary colloidal crystals were created using a simple, fast, and scalable spin-coating technology. Although further investigation of the procedure is needed to improve the ordering of particles in the individual layers, the developed assembly technology has a promising outlook in applications such as optical integrated circuits and high-speed optical computing. Inorganic-organic nanocomposites were realized by assembling synthesized gibbsite nanoplatelets using the electrophoretic deposition and infiltration of a monomer followed by polymerization. Via surface modifications of gibbsite nanoplatelets, nanocomposites were further reinforced with covalent linkages between the inorganic platelets and organic matrix.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by In-Kook Jun.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Hess, Henry.
Local: Co-adviser: Jiang, Peng.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-02-28

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042115:00001


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BIOINSPIRED, FUNCTIONAL NANOSCALE MATERIALS


By

IN-KOOK JUN
















A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2010

































2010 In-Kook Jun


































To my family and friends,
for their unwavering support









ACKNOWLEDGMENTS

First and foremost, I thank my mentors and guides, Drs. Henry Hess and Peng

Jiang for their brilliance, humility, and patience. Directly and indirectly, they have taught

me far more things than what can be articulated in this note of acknowledgement. I

would like to thank my committee members Drs. David Norton, Wolfgang Sigmund,

Valentin Craciun, and Yiider Tseng for their time, guidance and constructive comments.

I thank all the Hess group members: Thorsten Fischer, Rob, Parag, Isaac, Ashu,

and Yoli, and former members, Shruti and Krishna. I also thank all the Jiang group

members: Stanley, Nick, William, Erik, Tzung-hua, and Hongta, and visiting professors

Dr. Xuefeng Liu from Jiangnan University and Dr. Satoshi Watanabe from Kyoto

University for their constructive feedback and assistance in research. I was very

fortunate to be a member of both groups.

Finally, and most importantly, I express my thanks and gratitude to my parents

and friends in Korea, who have always supported me in spirit along this journey. My

parents and friends gave me the courage to pursue my dreams, and encouraged me to

be a successful researcher with an aggressive attitude in life and work. I am always

going to be grateful for having them by my side. Right now, I am so missing my Mom in

the Heaven.









TABLE OF CONTENTS


page

A C KN O W LEDG M ENTS ....... .. .............. ............................................. ............... 4

LIST OF TABLES .......... ..... .. ...................... ............. ...... ............... 8

LIS T O F F IG U R E S .................................................................. 9

LIST OF ABBREVIATIONS ............ ............... .................................... 12

ABSTRACT .............. ............................... ......... 14

CHAPTER

1 INTRODUCTION ................ .......... .......... ......... 16

Bioinspired, Engineered Functional Materials.......... ....... ..... ............... 16
S cope of T hesis ................... .... .......... ...................... ................. 17
Self-pumping Membranes for Synthetic Transport Systems.......................... 17
Nanoscale SERS Substrates for Effective Detecting Systems...................... 17
Binary Colloidal Crystals for Optical Systems ........................................ .. 18
Organic-inorganic Nanocomposites for Mechanical Systems........................ 18

2 A BIOMIMETIC, SELF-PUMPING MEMBRANE........................ ............. 21

Background............................... .......... ........... 21
Electroosm otic Flow .......................... .......................................... ...... 22
Applications of Electroosmotic Micropumps ........... ..... ...... .............. 24
Challenges for Electroosm otic Pum ps................................... .................... 25
Requirements of Self-pumping Membranes.......................................... 25
Electrochemical Systems for the Self-pumping Membrane............................... 26
Compartment-less Fuel Cell System for the Self-pumping Membrane ................... 27
R results and D discussion ................................................ ............. 28
C conclusions ............... ...... ......... ......... .. ......... ............... 31
Materials and Methods...... ...................... ...... ......... 32
Membrane Preparation........................... ......... 32
C ham ber D design ......... ............ ......... ............ .............. .............. 32
Flow M easurem ents ............................ .......... ............... .............. 33

3 GOLD NANOPARTICLE-NANOHOLE ARRAYS AS SERS SUBSTRATES........... 41

B a c k g ro u n d .............................. ................ ......... .. .............. ............ ............... 4 1
Raman Scattering & Surface Enhanced Raman Scattering (SERS) ................ 41
Mechanisms for SERS Enhancement ........................ .... ........ .. 42
Various Techniques for SERS Substrates........ .... .................................. 44









S E R S A p p lica tio n s ............................................................. ................ 4 5
Challenges for SERS Substrates ........................................... .................. ........ 46
Raman Scattering Intensity Measurement / SERS EFF Calculation ................ 47
Simulation of Effects of Size and Spacing on Electromagnetic Field................ 49
Particle Self-assembly Approach ................. ............... ............... 50
Particle Aggregates for SERS ............... ................. .................... 50
Self-assembled Particles as a SERS Substrate.............. .......... .................. 51
STV/PEG-GNP Arrays... ..................... ....................... 54
Periodic Nanostructure Approach ............................................ ......... ... ............... 55
Periodic Nanostructures as SERS Substrates........................................... 55
Periodic Nanostructures from Non-close-packed Particle Arrays..................... 57
Nanohole Arrays on a Glass Substrate .... ............................ .. ... ............ 59
STV/PEG-GNP Arrays on Nanohole Arrays ................... ................ 60
Concept of GNP-nanohole Arrays .......................... ............... 60
Fabrication of GNP-nanohole Arrays..................................... 60
Optical and SERS Properties of STV/PEG-GNP Arrays on Nanohole Arrays.. 61
C o n c lu s io n s ............. ......... .. .. ......... .. .. ......... .................................. 6 2
Materials and Methods........................................ .......... 63
M materials .......................................... ........... 63
Instrum entation ...... ................................................... ............... 63
Gold Nanohole Arrays on a Glass Slide ........ .... ..................... ............... 63
STV/PEG-GNP Arrays on Gold Nanohole Arrays ............................. ........ 64
Absorbance of Substrates ................................ ............... 64
Raman Spectra Measurements...... ..................... ................. 64
Calculation of Enhancem ent Factors....................................... .................... 65

4 BINARY COLLOIDAL CRYSTALS ............................... ............... 77

Results and Discussion.......................................... ............... 79
Conclusions .............. ....................... ............. ........................... 82
Materials and Methods................................ ............... 82
Materials and Substrates.......................... ..... ........ ............... 82
Instrumentation......................... ......... 83
Non-close-packed Colloidal M onolayer ........................ .............. ............... 83
Ternary Layer Hexagonally Non-close-packed Colloidal Structure .................. 84

5 BIOINSPIRED, ORGANIC-INORGANIC NANOCOMPOSITES ........................... 90

Assembly of Colloidal Nanoplatelets........................ ...................... 91
Synthesized Gibbsite Nanoplatelets.............................................. ................. 91
Assembly of Gibbsite Nanoplatelets by Electrophoretic Deposition ................ 92
Mechanical Properties of Nanocomposites .............. ...... .................. 92
Assembly of Surface-roughened Nanoplatelets ................. ............. ............... 93
Silica-coated Gibbsite Nanoplatelets......... ......... ........ ... ............ 93
Assembly of Silica-coated Gibbsite Nanoplatelets ..................................... 93
Mechanical Properties of Nanocomposites .............. ...... .................. 94
C conclusions ............... .. ......... .... ........ ................... ......... 94









Materials and Methods................................ ............... 95
M aterials................................................. ............... 95
Instrumentation......................... ...........95
Synthesis of Gibbsite Nanoplatelets....................... ....... .. ............. 96
Surface Modification of Gibbsite Nanoplatelets with TPM .............................. 96
Coating of Gibbsite Nanoplatelets with Silica ............ ................................ 97
Electrophoretic Deposition........................................ ................. 97
M mechanical Test ...................... .......................................... 98

6 CONCLUSIONS AND OUTLOOK .......................................... ............... 102

Self-pum ping M em brane................................................................................. 102
Reproducible and Highly SERS-active Substrates ............................. 102
Potential Applications as a Hybrid Biosensor .................... ..... ............. 102
B inary C olloidal C rystals .................................................................. 103

APPENDIX

A ELECTROOSMOTIC FLOW ............................................................................. 105

Electroosmotic Flow in One-wall Channel................... ..................... 105
Electroosmotic Flow in Cylindrical Tube .......... ............. ..... ...... ........... 105

B S E LF-P U M P IN G FLO W ............. ......... .. ....................................... ............... 110

Flow Rate as a Function of Tracer Velocity ......................... .............. 110
Conductivity of W working Fluid ................ .. ................. ...... ......... 110
Flow Rate as a Function of Current ...... ............ ............... ............... 111
Flow Rate at Zero Opposing Pressure.............................. .............. 113

C FLOW RATE MEASUREMENT............. ......... .......................... 115

Length-converting M ethod ...................... .. ............. ................... ............... 115
M ass-converting M ethod..................................................... ........................... 115
Current-m monitoring M ethod .............................. ................. ......................... 115
Particle Image Velocimetry Method .......... ... ..... ...... .............. ............... 115
Concentration-monitoring Method............................... .................... 116

LIST OF REFERENCES ................. ........................... ............... 120

B IO G RA PH ICA L SKETC H ........................ ....................................... ............... 136









LIST OF TABLES


Table page

3-1 Calculated SERS enhancement factor on the fabricated substrates from
measured data of Raman intensities at 999.2 cm-1 and 1023 cm- ........ ........ 66









LIST OF FIGURES


Figure page

1-1 Functional nanoscale materials in nature: a) moth-eye, b) gecko's foot, c)
lotus leaf. ................................................... ................... ................ 20

2-1 Electroosmotic flow in a channel.. ................... .. ...... .. ............. 34

2-2 Various types of electroosmotic pumps: a) packed-column EP, b) porous
monolith column EP, c) open channel EP, and d) porous membrane-based
E P ............. .................................. .......................... 34

2-3 The self-pumping membrane: a) gold and platinum electrodes deposited on
the opposing surfaces of a track-etched polycarbonate membrane. b) surface
and c) cross-section of the track-etched membrane ................ ..... ........... 35

2-4 The dimensions and photograph of the experimental set up. ............................. 36

2-5 The experimental set up. The membrane is mounted submerged in solution
to create a compartment connected to the larger reservoir by the membrane
and a narrow channel................................... ..................... .... 36

2-6 The electroosmotic pumping membrane. Flow rate and electric current as a
function of external voltage in an aqueous solution. The inset shows a
rescaled plot of the voltage range from -1 V to + 1 V. .......... ................. 37

2-7 Flow rate dependent on current in electroosmotic pumping. In the
electroosmotic flow, the measured flow rate increases linearly with increasing
m measured current. ....................... .. ................ ................................. 37

2-8 The self-pumping membrane. Flow rate and electric current as a function of
time in a 0.01% hydrogen peroxide solution as the gold and platinum
electrode are externally connected and disconnected .................. .............. 38

2-9 Scanning electron microscopy images of the membrane after operation show
tracer particles accumulated on the membrane surface and within the pores,
possibly leading to a reduction in pumping efficiency. ......... .......................... .. 38

2-10 Flow rate dependent on current in the self-pumping. In the self-pumping flow,
the measured flow rate increases linearly with increasing measured current..... 39

2-11 Calculation of tracer velocity at low flow rates. At a low velocity of tracers,
individual tracer positions can be accurately determined and tracers can be
follow ed from fram e to fram e ................ ............................. ...... ......... 39









2-12 Calculation of tracer velocity at high flow rates. At a high velocity of tracers,
tracers moving in the center of the channel appear as streaks while tracers
adsorbed to the channel surface appear as dots.......... .......... ............ 40

3-1 Various characteristic energies: a) Rayleigh scattering, b) Stokes Raman
scattering, and c) anti-Stokes Raman scattering. ........... ... ...... ............ 67

3-2 Schematic diagrams of a) a surface plasmon polariton (or propagating
plasmon) on a flat surface and b) a localized surface plasmon on a
nanostructured surface ................................................ ............... 67

3-3 Various SERS substrates: a) Ag film on nanospheres, periodic
nanostructures by b) nanosphere lithography and c) electron-beam
lithography, and d) colloidal aggregates ..................................... ................. 68

3-4 SERS applications: a) in vivo glucose sensing equipment consisted of SERS
spectroscopy, implanted substrate, beam directing optics, and collection lens
and b) identification of cancer genes by Raman labels...................... ............... 69

3-5 Representative Raman spectrum of benzenethiol adsorbed on a SERS
substrate ................ ...... ... ................................. ......... 69

3-6 The SERS enhancement calculated based on finite element methods in the
case of nanoparticle arrays.. .............. ................................ ..... .......... 70

3-7 Schematic diagram depicting self-assembly of gold nanoparticles using a
f lo w c e ll ..................................................................................... 7 1

3-8 SEM images of 30 nm gold nanoparticle arrays with a gap between particles
on glass substrates ................................................ .............. 72

3-9 Schematic diagram depicting the fabrication procedures for making GNP-
nanoho le arrays................................................ ........................ ......... ... 73

3-10 SEM images of nanohole arrays: a) 330 nm nanohole arrays and b) 400 nm
nanohole arrays .... ........ ......... .............. ................ ..... .......... 74

3-11 SEM images of nanohole-GNP arrays: a) 330 nm nanohole arrays covered
by 30 nm gold particles and b) 400 nm nanohole arrays covered by 30 nm
gold particles. ............. ....... .............. ............. 74

3-12 Absorption spectra on a) gold nanoparticle (GNP) arrays and nanohole
arrays and b) nanohole arrays covered by gold nanoparticles ........................ 75

3-13 Raman spectra of benzenethiol absorbed on a) flat god surface, b) gold
nanoparticle (GNP) arrays, c) 330 nm nanohole arrays covered by gold
nanoparticles, and d) 400 nm nanohole arrays........................... ............. .. 76









4-1 Photonic crystals: a) photonic crystals in 1-D, 2-D, and 3-D and b) a photonic
band gap compared to an electronic band gap...................................... ..... 85

4-2 Schematic illustration of the procedure for fabricating binary hexagonal
arrays of silica spheres by using monolayer nonclose-packed colloidal
crystals as substrates ................................................................ ............ 86

4-3 Monolayer of nonclose-packed silica particles (300 nm) fabricated by the
spin-coating technique.............................................. .................... .... ..... 86

4-4 Binary hexagonal arrays of silica spheres: a, b) 300 nm particles array on
300 nm particles array (300/300), and c, d) 400 nm particles array on 300 nm
particles array (400/300).......................................... ........................... .... ... 87

4-5 Pressure effects on the ordering of particles in the second layer: a, b) 0.2
MPa for 2 min, and c, d) 0.33 MPa for 2 min in 300 nm particles arrays on
300 nm particles arrays (300/300) ............ .............................. ............ ... 88

4-6 Cross-sections of hexagonal arrays of silica spheres: a, b) binary layer of 400
nm particles array on 345 nm particles array (400/345) and c) ternary layer...... 89

5-1 Hard biological tissues and their microstructures: a) tooth, b) vertebral bone,
c) shell, d) Enamel made of long needle-like crystals with soft protein matrix,
e) dentin and bone made of plate-like crystals. ......... ................................... 99

5-2 A model of biocomposites: a) a schematic diagram of staggered mineral
crystals embedded in protein matrix and b) a simplified model showing the
load-transfer mechanism in the mineral-protein composites.............................. 99

5-3 Nanocomposite of colloidal nanoplatelets: a) TEM image of gibbsite
nanoplatelets, b) SEM image of a free standing gibbsite-ETPTA
nanocomposite, and c) tensile strain-stress curves ........................... ........... 100

5-4 Nanocomposite of surface-roughened nanoplatelets: a) TEM image of acid-
leached silica-coated gibbsite nanoplatelets, b) SEM image of silica-coated
gibbsite-PEI-ETPTA nanocomposite on an ITO electrode.............................. 101

C-1 Various methods for measuring the flow rate: a) length-converting method, b)
mass-converting method, c) current-monitoring method, and d) particle
im age velocim etry m ethod.................................... ............................... ....... 118

C-2 The double membrane configuration having compartment 1 and
compartment 2 in the concentration-monitoring method............................... 118

C-3 Plot of function, f, with time. The diffusion constant, D, and the pumping rate,
k, by the electroosmotic pump can be estimated from the slope. ................... 119











ADP

AFM

APTCS

ATP

CTAB

DI

DMFC

DNA

EBL

ETPTA

FEG-SEM

FEM

FIB

FITC

GNP

HIV

ICP

IEP

ITO

LB

LBL

LSPR

MFON

NIR


LIST OF ABBREVIATIONS

Adenosine diphosphate

Atomic force microscopy

Acryloxypropyl trichorosilane

Adenosine triphosphate

Cetrimonium bromide

Deionized

Direct methanol fuel cell

Deoxyribonucleic acid

Electronic beam lithography

Ethoxylated trymethylolpropane triacrylate

Field emission gun scanning electron microscopy

Finite element method

Focused ion beam

Fluorescein isothiocyanate

Gold nanoparticle

Human immunodeficiency virus

Inductively coupled plasma

Isoelectric point

Indium tin oxide

Langmuir-Blodgett

Layer-by-layer

Localized surface plasmon resonance

Metal film on nanosphere

Near-infrared









NSL

ORC

PBG

PC

PDMS

PEG

PEI

PEMFC

PML

PMMA

PNIPAM

PVP

RIE

SEM

SERS

SERS EF

STV

TEM

TEOS

TPM

UV


Nanosphere lithography

Oxidation-reduction cycle

Photonic band gap

Photonic crystal

Poly(dimethylsiloxane)

Poly(ethylene glycol)

Poly(ethylenimine)

Proton exchange membrane fuel cell

Perfect matched layer

Poly(methyl methacrylate)

Poly(N-isopropylacrylamide)

Polyvinylpyrrolidone

Reactive ion etching

Scanning electron microscopy

Surface enhanced Raman scattering

Surface enhanced Raman scattering enhancement factor

Streptavidin

Tranmission electron microscopy

Tetraethyl orthosilicate

Trimethoxysilyl propyl methacrylate

Ultra-violet










Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

BIOINSPIRED, FUNCTIONAL NANOSCALE MATERIALS

By

In-kook Jun

August 2010

Chair: Henry Hess
Cochair: Peng Jiang
Major: Materials Science and Engineering

Functional nanomaterials in nature exhibit many unique functions and optical and

mechanical properties. Examples of this include the dry adhesion of a gecko's foot, the

reduced drag on a shark's skin, the high strength and toughness of nacre, and the

superhydrophobic self-cleaning of a lotus leaf. This dissertation is devoted to creating

unique and enhanced properties by mimicking such functional materials.

We have developed a novel self-pumping membrane, which does not require an

applied voltage. The self-pumping membrane harvests chemical energy from a

surrounding fluid and uses it for accelerated mass transport across the membrane. A

device such as this has promising applications in implantable or remotely operating

autonomous devices and membrane-based purification systems.

Reproducible and highly active surface enhanced Raman scattering (SERS)

substrates were developed using a bottom-up self-assembly technology. With their high

sensitivity and good reproducibility, the developed nanostructures (gold nanoparticle

and nanohole arrays) as SERS substrates are very promising for applications such as









ultra-sensitive detectors for chemicals and reproducible sensors for chemical and

biological molecules.

Binary colloidal crystals were created using a simple, fast, and scalable spin-

coating technology. Although further investigation of the procedure is needed to improve

the ordering of particles in the individual layers, the developed assembly technology has

a promising outlook in applications such as optical integrated circuits and high-speed

optical computing.

Inorganic-organic nanocomposites were realized by assembling synthesized

gibbsite nanoplatelets using the electrophoretic deposition and infiltration of a monomer

followed by polymerization. Via surface modifications of gibbsite nanoplatelets,

nanocomposites were further reinforced with covalent linkages between the inorganic

platelets and organic matrix.









CHAPTER 1
INTRODUCTION

In nature, functional nanoscale materials exhibit unique optical, mechanical, and

functional properties, usually obtained from hierarchical structures.[1] For example, a

moth's eye has many hexagonally shaped facet lenses on which highly ordered arrays

of nanoscale nipples are located.[2] The hierarchical nanoscale nipple arrays generate

the enhanced light-sensitivity of light-craving moths. The gecko has an exceptional

ability to climb rapidly up smooth vertical surfaces due to the hierarchical toe structure

consisting of a myriad of nanoscale spatulae.[3] The nanoscale hair-like structures on

micro-scale mound-like structures protruding from its leaf are responsible for the

superhydrophobicity in a Lotus-leaf.[4]

Bioinspired, Engineered Functional Materials

The driving force to investigate and mimic nature's structures comes from the

belief that nature's structures are most efficient. Researchers have tried to mimic

various natural structures, such as the dry adhesion of a gecko's foot, the reduced drag

on a shark's skin, the iridescent color of a morpho-butterfly, the high strength and

toughness of nacre, and the superhydrophobic self-cleaning of a lotus leaf.[3-6]

Biomolecular motors such as kinesin and myosin are capable of transporting

analytes in a biosensor.[7, 8] They gain mechanical energy from hydrolyzing ATP to

ADP and inorganic phosphate at high efficiency. Biomolecular motors could potentially

replace a pump and a power supply in a smart dust biosensor; they may be integrated

with the housing and communication unit. Biofuel cells, which generate electricity using

glucose as fuel from the fluid, have been developed.[9] They gain electrical energy from

oxidizing fuel and reducing oxygen. Highly active catalysts for the electrochemical









reactions and electron transfer systems have been investigated to improve the

performance. Hydrogen peroxide, hydrazine, ethanol, and methanol have been

explored as alternative fuels for the electrochemical reactions in fuel cell systems to

enhance their stability, durability, and power density.

Artificial antireflection coatings are used in displays, optical components, and solar

cells; however, current antireflection technologies such as quarter-wavelength multilayer

films and nanoporous coatings are expensive and suboptimal. By mimicking the moth-

eyes, non-close-packed nipples in the sub-300 nm size range, effective antireflection

coatings have been recently developed with a bottom-up self-assembly technique.[10-

12]

Scope of Thesis

Self-pumping Membranes for Synthetic Transport Systems

Hybrid biosensors face the fundamental challenge of limited stability, due to their

biological components and the difficulty of obtaining macroscopic observable signals.

Conventional transporting systems such as electroosmotic pumps are robust and

stable; however, they still require a highly miniaturized pump and power supply. It has

been shown that some electrochemical reactions can induce mass transport by

converting the chemical energy gained from the surrounding fluid into the mechanical

energy with no need of an externally applied voltage. In this work, we design such a

synthetic transport system and investigate the effectiveness and efficiency in mass

transport.

Nanoscale SERS Substrates for Effective Detecting Systems

One of the challenges in biosensors is achieving high sensitivity. Surface

enhanced Raman scattering (SERS) can be utilized to amplify the signal. It is well









known that Raman signals dramatically increase when metallic nanostructures with

wavelength scale topography are used as SERS substrate. The maximum SERS

enhancement has been reported up to 1014 in nanoparticle aggregates.[13] Moreover,

Raman spectroscopy is a label-free technique, so a tagging procedure is not necessary.

In this work, we fabricate several nanostructures with a bottom-up self-assembly

technique and investigate their capabilities as SERS substrates.

Binary Colloidal Crystals for Optical Systems

The development of integrated optical circuits with photonic crystals has been

greatly impeded by its reliance on expensive and complex nanofabrication techniques

such as electron-beam lithography (EBL) and focused ion-beam (FIB). Bottom-up

colloidal self-assembly and subsequent templating nanofabrication can provide a much

simpler, faster, and inexpensive alternative. However, current colloidal self-assemblies

are limited to a low volume, laboratory-scale production. In this work, we investigate a

bottom-up self-assembly approach via a spin-coating technology to create non-close-

packed binary colloidal crystals.

Organic-inorganic Nanocomposites for Mechanical Systems

The nacreous layer of mollusk shells has an intricate brick-and-mortar

nanostructure which makes the shells exceptionally tough and stiff. Various bottom-up

self-assembly techniques such as layer-by-layer (LBL), ice-templated crystallization,

spin-coating, gravitational sedimentation, and centrifugation, have been explored to

mimic the nacre structure. In this work, we assemble gibbsite nanoplatelets, having high

aspect ratio (diameter-thickness ratio), via a simple, inexpensive, and scalable

elelctrophoretic deposition. We generate organic-inorganic nanocomposites by









infiltrating monomer between platelet layers and polymerizing monomer, and investigate

their mechanical properties.























IP"w 5 urr
PP'I_

;e k

... L'


Figure 1-1. Functional nanoscale materials in nature: a) moth-eye, b) gecko's foot, c)
lotus leaf. Adapted from [2, 4, 14-16].









CHAPTER 2
A BIOMIMETIC, SELF-PUMPING MEMBRANE

Biological membranes accelerate materials exchange across the membrane by

active, ATP-dependent transport through specialized channel proteins. Similarly, the

integration of "pumping" driven by chemical energy harvested from the fluid into a

synthetic membrane is highly desirable from an engineering point of view, since it would

obviate the need for external devices, such as pumps or centrifuges, to drive flow

across the membrane. Here, a novel pumping-membrane with no need of applied

voltage is described. Instead of the externally applied voltage, electrodes deposited on

the opposing surface of a membrane generate a transmembrane potential from the

electrochemical reactions (the hydrolysis of hydrogen peroxide in aqueous solution).

Short-circuiting the electrodes permits an ionic current to flow between the electrodes,

which in turn, creates a flow of about 100 nL cm-2 min-1. Future applications of such self-

pumping membranes may include implantable or remotely operating autonomous

devices and membrane-based purification systems.

Background

Microfludic devices have advantages for mixing, separating, and sensing reagents

due to their laminar flows and small diffusion distances compared to macro-scale

devices.[17] Microfluidic devices require small volumes of samples (from 100 nL to 10

pL), are disposable due to low production costs, and are amenable to high throughput

due to processing assays in parallel. Micropumps as fluidic transport system are

essential in operating microfluidic devices.









Various Types of Micropumps

Micropumps can be divided into two categories according to their operating

mechanisms: (1) displacement pumps, which exert forces on the working fluid through

moving surfaces and (2) dynamic pumps, which add energy to the working fluid by

increasing momentum or pressure directly.[18] The displacement pumps usually rely on

moving parts such as check valves, oscillating membranes, or turbines for the constant

delivery of a fluid. Diaphragm pumps, which make up the majority of reported

displacement micropumps, are actuated by piezoelectric, thermopneumatic,

electrostatic, and electromagnetic mechanisms.[19-22] On the other hand, dynamic

pumps generate the kinetic energy of the fluid by electroosmosis, electrowetting,

thermocapillary, and electrochemical, electrohydrodynamic, and magnetohydrodynamic

mechanisms without moving parts.[23-28] Dynamic pumps are advantageous in

micrometer-scale devices where the high surface-to-volume ratio is favorable.

Electroosmotic Flow

Electroosmotic micropumps are of significant promise for a wide variety of

applications.[17, 29, 30] Briefly, surface charges within a channel attract counter ions

which experience a force directed along the channel axis when an electric field is

applied across the channel (Figure 2-1). The viscous drag between the counter ions and

fluid in turn exerts a force on the fluid that is localized at the channel wall, inducing a

plug-like flow profile.[31]

In electrophoresis, the solids (the particles) are moving in the applied electrical

field. Moving particles can drag fluid (water) molecules, generating an electrophoretic

fluid flow through channels. On the other hand, in electroosmosis, the solid (the channel

wall) is stationary, while the fluid is moving in the applied electrical field. When the solid









contacts the liquid, the ions having charge opposite to that of the solid are attracted to

the solid forming an electric double layer.[30, 32, 33] Electroosmotic flow is induced by

an applied electric field to the electric double layer.

Various Types of Electroosmotic Micropumps

Based on the pumping elements, there are various types of electroosmotic

micropumps such as packed column, porous monolith pumps, open-channel pumps,

and membrane-based pumps.[29, 34-44] Packed-column electroosmotic pumps can

produce high pressures (Figure 2-2a).[29, 34, 35] The interstitial spaces between the

packed particles are parallel passages for fluid flows. The pumping power closely

depends on the size of the packed particles. In the pumps with large particles, high flow

rates can be achieved, while pumping pressures decrease. Physical geometries of the

channel such as tortuosity, porosity, and effective pore size and properties of the

working fluid are controlled to optimize the performance of electroosmotic pumps.

A porous monolith column is prepared by polymerizing the column of a monomer,

a crosslinker, a free radical initiator, and a porogenic solvent (Figure 2-2b).[36, 37] The

materials can be an organic polymer or an inorganic silica monolith. The advantage of a

porous monolith column is the elimination of frits or filters which are used in packed-

column electroosmotic pumps. The pore size and porosity as well as chemistry and

crosslinking density can be easily controlled to achieve the desired flow rate and

pressure.

Channel electroosmotic pumps use narrow capillaries or microchannels as

pumping elements (Figure 2-2c).[38-40] By consisting of hundreds of parallel and small

microchannels, both high pumping rates and high pressures can be achieved. Channel









electroosmotic pumps can be easily integrated on a microfluidic platform by standard

microfabrication techniques and are robust and reliable due to the simplicity of design.

The pump rate is controlled by the electric field and the size and number of channels.

Porous membrane pumps have many channels through the thin membrane

(Figure 2-2d).[41-44] Since the channels are short, a high electric field is achieved with

a low applied voltage. Due to the large number of microchannels, it is possible to

generate high pressures at high flow rates. Supporting frames are often used to secure

the robustness of the membrane and to apply voltage as electrodes. Various

membranes such as glass, alumina, and polymer are reported to show excellent

pumping performances. Alumina membranes with highly aligned nanochannels are

reported to show high flow velocities at low voltage.

Applications of Electroosmotic Micropumps

Electroosmotic pumps can transport liquid samples with significant flow rates and

pressures. Without moving parts, electroosmotic pumps generate plug-like and laminar

fluid flows, which can be controlled by applied voltages. The electroosmotic pumps are

favorable in microfluidic devices where the surface-to-volume ratios are high.[45]

Due to the unique characteristics of electroosmotic micropumps, many potential

applications have been suggested. Electroosmotic micropumps have great potential in

liquid drug delivery and biological sample assays.[30] Electroosmotic micropumps have

been integrated into proton exchange membrane fuel cells (PEMFCs) to remove water

from cathodes.[46] Electroosmotic pumps have also been used as fuel delivery systems

in direct methanol fuel cells (DMFCs).[47] Compact micropumps having high heat

dissipation rates are essential in microelectronics as cooling systems. Electroosmotic

micropumps made of glass frits have been reported as effective cooling systems.[48]









Challenges for Electroosmotic Pumps

Although electroosmotic pumps have great potentials as transport systems, there

are still challenges to overcome for real applications. Nonpolar liquids such as oil are

difficult to transport by the electroosmotic pump and the pumping function degrades

over time due to the interaction with working fluids. The bubbles generated on the

electrodes can interfere with the pumping function, especially in closed-loop fluidic

devices. The efficiency of the pump should also be improved.

Requirements of Self-pumping Membranes

To construct a self-pumping membrane which generates a fluid flow by harvesting

chemical energy from a fluid and converting the chemical energy into the kinetic energy,

there are several requirements to be satisfied.

First, the electrochemical reactions on the electrodes deposited on the opposite

surfaces of a membrane should generate a transmembrane potential. From this point of

view, battery systems such as primary cells (e.g. Daniell cell) and secondary cells (e.g.

lead-acid cell) and fuel cell systems can be candidates.

Second, the electrochemical reactions should generate new ions or protons as

products. For example, the protons generated on the anode will drag the fluid flow in the

electric field. On the other hand, the transition from Fe3+ to Fe2+ doesn't generate new

ions or protons for dragging the fluid flow.

Third, the electrodes should not be changed by the reduction-oxidation reactions.

In battery systems, the anode themselves dissolve into the solution, while metal ions

reduce on the cathode. In this view point, fuel cell systems are preferable since

electrodes in fuel cell systems can continue working by being fed with fuels without a

change of electrode.









Fourth, the fluid flow should not interfere with the electrochemical reactions. The

reactants for the electrochemical reaction (oxidation) on the anode and their products

should not interfere with the electrochemical reaction (reduction) on the cathode and

vice versa. Thus, "compartment-less" electrochemical systems are most promising

candidates for the self-pumping membrane.

Electrochemical Systems for the Self-pumping Membrane

Electrochemical systems utilized in the research on chemotaxis or catalytic

nanomotors can offer promising candidate systems for the self-pumping membrane

satisfying the above requirements. Metal rods or surfaces (Pt, Au/Ni, catalyst, Pt/Au,

Ag) in the hydrogen peroxide solution show autonomous movement, rotational motion,

and transportation of cargos by ejecting small oxygen bubbles or generating an

interfacial tension force or a difference in diffusion coefficient due to the catalytic

decomposition of hydrogen peroxide.[49-53] However, it was proved later that the

autonomous movements of Pt/Au nanorods were due to the self-electrophoresis.

Protons generated by the electrochemical reaction of hydrogen peroxide flow in the

catalytically induced electric field inducing autonomous movements of nanorods.[54, 55]

In further research, Pt/Au or Au/Ag rods in hydrogen peroxide solution have shown an

autonomous movement and the speed and direction of movement can be controlled.[56,

57] A convective fluid flow was shown on the Ag/Au patterned surface in hydrogen

peroxide solution.[58]

Other electrochemical systems have also been investigated for autonomous

movement. Glucose oxidase/bilirubin oxidase fibers were propelled in the interface

between electrolyte and air due to the electrochemical reactions of glucose and









oxygen.[59] A Pd/Au patterned surface in hydrazine solution showed electroosmotic

flow in a catalytically induced electric field.[60]

Compartment-less Fuel Cell System for the Self-pumping Membrane

The electrochemical systems developed in the investigations of chemotaxis and

catalytic nanomotors have been applied to compartment-less fuel cell systems. In

compartment-less fuel cells, a differential in the ability of the two electrodes to catalyze

the anodic and cathodic reaction enables the creation of an electric potential and

removes the need for an ion-exchange membrane. A compartment-less hydrogen

peroxide fuel cell with Au and Ag as catalytic electrodes was developed generating the

maximum current density of 2.9 mA cm-2. [61] In this fuel system, the bubble generation

on the anode interfered with further electrochemical reactions. While Ni electrodes

generated less bubbles, they were also less-active for the electrochemical reaction of

hydrogen peroxide.

Compartment-less glucose/oxygen fuel cells have been developed.[9, 62-65] A

biofuel cell having an anode functionalized with surface-reconstituted glucose oxidase

and a cathode modified with cytochrome c and cytochrome oxidase generated a

maximum power of 4 pW.[62] The current density generated in biofuel cells having an

anode immobilized with glucose oxidase and a cathode immobilized with a bilirubin

oxidase reached up to 10 mA cm-1.[9] The maximum power density from the biofuel cell

having an anode functionalized with glucose dehydrogenase complex and a Pt cathode

was 930 nW cm-2.[64]

Compartment-less fuel cells using methanol or ethanol as fuels were also

reported.[66, 67] Fuel cells with a nickel hydroxide anode and a silver oxide cathode

using a fuel mixture of methanol and hydrogen peroxide achieved the maximum power









density of 28.73 pW cm-2.[66] The maximum power density from fuel cells having an

anode of alcoholdehydrongenase associated with carbon nanotubes and a cathode of

bilirubin-Pt nanoparticle composite was recorded at 200 pW cm-2.[67] Although there

are membrane-less fuel cells, these fuel cells separate fuels by laminar flow instead of

ion-exchange membranes, so they don't satisfy the requirements of an electrochemical

system for self-pumping membrane.[68, 69]

While glucose is the fuel of choice for most studies, and significant advances have

been achieved in the design of enzyme-based biofuel cells,[70] in this work, hydrogen

peroxide is adopted as the fuel to simplify the electrode design and to utilize a

combination of platinum and gold electrodes known as effective catalysts for the

electrochemical decomposition of hydrogen peroxide to drive electroosmotic flows.[56]

Results and Discussion

To construct the self-pumping membrane, platinum and gold films of 25 nm

thickness (and a 5 nm titanium adhesion layer) are sputter-deposited on the opposing

surfaces of track-etched polycarbonate membranes with a pore diameter of 0.96 pm, a

thickness of 18 pm and a porosity of 12% (Figure 2-3, 2-4). The platinum electrode and

the gold electrode are electrically connected by an external switch.

In the proposed self-pumping membrane, on the platinum electrode, hydrogen

peroxide decomposes into oxygen, protons, and electrons. The generated electrons

flow along the electrical connection, while the generated protons flow through the pores

of the membrane under the catalytically induced electric field. These electrons and

protons are consumed on the gold electrode by combining with hydrogen peroxide to

produce water.









The membrane is then integrated into a fluid chamber designed to facilitate the

measurement of nL s-1 pumping speeds at near-zero backing pressure (Figure 2-5).

Flow rates are measured by microparticle image velocimetry,[71] using fluorescent

microspheres (1 pm diameter) as tracers. To amplify the flow velocity, the flow is

monitored in a narrow channel of 51 pm height, 320 pm width, and 6 mm length.

The proper functioning of the experimental setup was validated by providing an

external voltage to the membrane submersed in a solution of water and tracer

microspheres. The dependence of electric current and particle velocity as a function of

external voltage (Figure 2-6) followed the behavior expected for the hydrolysis of water,

which has a decomposition potential difference of 1.23 V.[72] Positive numbers in flow

rate and current correspond to flow/current from the gold electrode to the platinum

electrode, while negative numbers signify the opposite direction. The flow rate is

calculated from the measured velocities of tracers based on the relationship between

maximum velocity and the flow rate in a rectangular channel.[73] The flow rate

increases linearly with increasing voltage across the membrane up to about 1.2 V. At an

applied voltage of 1V the flow rate is -0.94 nL s-1 and the current is -1.9 A. For applied

voltages above 1.4 V, flow rate and current increase linearly with increasing voltage with

slopes of -43 nL s- V1 and -120 pA V-1, respectively. This implies a conductivity of the

working fluid (water and tracer particles) of 2.0 pS cm-1, which is close to the

conductivity of water.

Using the Helmholtz-Smoluchowski equation to calculate the electroosmotic flow

through the membrane while assuming the zeta potential of polycarbonate[71] to be -27

mV and considering the pressure-induced reverse flow caused by the resistance of the









small outlet channel, the flow rate as a function of current can be calculated. While the

calculation shows the observed linear dependence of flow rate on current with a slope

of 3 nL pA-1 s1, it also shows that the high resistance of the small detection channel

relative to the membrane resistance reduces the net flow through the membrane about

30-fold relative to the expected electroosmotic flow at zero pressure. In the

electroosmotic pump, the observed pumping efficiency varies from 0.4 to 0.5 nL pA- s-

(Figure 2-7), which is lower than the calculated 3 nL pA- s1.

Pumping in the absence of an external voltage is activated by the addition of

hydrogen peroxide to the aqueous solution at a concentration of 0.01 wt%. At this low

concentration of hydrogen peroxide, the formation of gas bubbles at the electrodes is

avoided. The platinum and gold electrodes are connected to a switch and an

amperemeter. Fluid flow is dependent on the state of the switch: when the switch is

closed, flow across the membrane commences from platinum to gold (Figure 2-8); when

the switch is open, the flow rate is near zero, initially with a small flow resulting from

small initial pressure differences. In less than 30 s, the flow rate reaches 0.9 nL s- and

the current reaches 0.26 pA. When the switch is opened, the flow rate rapidly ceases. In

subsequent switching cycles, the "closed switch" flow rates decreased by 20% after 270

min. This reduction is likely to be the result of a falling hydrogen peroxide concentration

due to its consumption at the electrodes or the result of clogging of the pores with tracer

particles (Figure 2-9).

The observed flow direction (from platinum to gold) is consistent with the proposed

pumping mechanism. The observed pumping efficiency of 3 nL pA- s1 matches the

above calculated electroosmotic pumping efficiency (Figure 2-10). This agreement









supports the hypothesis that the pumping results from electroosmosis and not from the

formation of gas bubbles at the electrodes. The observed current density on the order of

3 mA m-2 at a hydrogen peroxide concentration of 0.01 wt% is in good agreement with

Paxton et al.'s observation of 1 mA m-2 at a concentration of 0.006 wt%.[56] Using our

model of the system, we can estimate a flow rate at zero opposing pressure of 25 nL s-1

and a stall pressure of 1 Pa in the self-pumping membrane; both parameters are typical

for microfluidic pumps.[72]

Future improvements to such self-pumping membranes can focus on the

performance of the fuel cell component, the pump component, or their interaction. The

fuel cell can be improved by the use of better electrodes and alternative fuels, in

particular glucose. Compartment-less biofuel cells have reached current densities on

the order of 10 A m-2,[9, 73] which means that an improvement of four orders of

magnitude is possible. The electroosmotic pump can be optimized by increasing the

zeta potential, and tuning the hydrodynamic resistance of the pump and outlet, e.g. by

adjusting the pore diameter. Miao et al. described an electroosmotic pump which

creates a more than ten-fold higher flow rate for a given amount of current and

membrane area.[43] Finally, the current-voltage characteristics of the fuel cell power

source and the electroosmotic pump should be matched to maximize power conversion

efficiency. In the present design, the voltage drop across the electroosmotic pump is on

the order of 2 mV. Thus, the pump uses only a small fraction of the electromotive force

provided by the fuel cell.

Conclusions

Despite its performance limitations, the self-pumping membrane described here is

a first step towards replicating the ability of biological membranes to harvest chemical









energy from the surrounding fluid and use it for accelerated mass transport across the

membrane. Micro and macroscopic devices for drug delivery, sensing and purification,

as well as oil recovery and removal may benefit from this technology.

Materials and Methods

Membrane Preparation

A titanium adhesion layer (5 nm) and platinum and gold films (25 nm) were

deposited on the opposing surfaces of a polycarbonate membrane (pore diameter of

0.96 pm porosity of 12% and thickness of 18 pm determined from analysis of SEM

images, Isopore membrane filter, Millipore, Ireland) with a multi-target sputtering system

(Kurt J. Lesker CMS-18, Clairton, PA). The membrane was glued to a polycarbonate

frame (instant Krazy glue, Elmer's Product Inc., Columbus, OH). The platinum and gold

electrodes were connected to metal wires using silver paste (Leitsilber 200 Silver Paint,

Ted Pella Inc., Redding, CA). The resistance between the electrodes after

manufacturing was measured to be 0.2 MO.

Chamber Design

A narrow channel (51 pm high, 320 pm wide, 6 mm long) between two open

chambers was patterned with Polydimethylsiloxane (PDMS, Sylgard 184 silicone

elastomer kit, Dow Corning Corporation, MI) using a mold. The prepared chamber-

channel layer and the membrane layer were assembled on glass to form the

experimental set-up as shown in Figure 2-4.

For electroosmotic pumping, the working fluid is DI water (18 kO cm, Millipore Inc.,

Billerica, MA) with yellow-green fluorescent microspheres (1 pm diameter, 0.002% solid

loading, FluoSphere amine-modified microsphere, Molecular Probes, Eugene, OR). An

external voltage is applied between the electrodes with a DC regulated power supply









(EXTECH instruments, Waltham, MA). For the self-pumping membrane, 0.01 wt% H202

solution (Aldrich, St. Louis, MO) is added. The platinum and gold electrodes are

electrically connected by clamping the metal wires connecting platinum and gold

electrodes together.

Flow Measurements

The narrow channel between the two chambers was imaged with an Eclipse

TE200U epi-fluorescence microscope (Nikon, Melville, NY) equipped with an X-cite 120

lamp (EXFO, Ontario, Canada), a 10X objective, a cooled CCD camera (Andor iXon,

Andor Technology, Windsor, CT), and a FITC filter cube (no. 48001, Chroma

Technology Corp, Rockingham, VT). The focal plane was set to the center of the

channel where moving tracers were clearly visible. At low flow rates, images were

acquired at 100 ms intervals with 20 ms exposure times (Figure 2-11). Flow rates were

calculated by measuring the particle displacement after 1 s. At high flow rates, images

were acquired at 1 s intervals with 100 ms exposure times (Figure 2-12). Flow rates

were calculated by measuring the length of the streak generated by the moving particle

and dividing by the exposure time. The current was measured with an amperemeter

(Digital Multimeter 34410A, Agilent Technologies, Santa Clara, CA).














Low
Premure


Deprotonaled / i iiini
Silariol groups Flow


High
D Presfure











Pressure
Driver flow


Figure 2-1. Electroosmotic flow in a channel. Adapted from [74].


+ electrodes All depth=20pm


d)


S M ianlhic pumping
Pup rm.,.chold

An-dc C Clak, Ac"ic hwsing
NI nih





In. Chimbe Ot 0M ChmbLer
Flo. In- -R-*Fowou


Geb electrloes anchor paltem


Figure 2-2. Various types of electroosmotic pumps: a) packed-column EP, b) porous
monolith column EP, c) open channel EP, and d) porous membrane-based
EP. Adapted from [32, 36, 39, 43].


InIle -


GlaSs Wall


IIOulel


Output EOF / -v ,r





- I *3UU. l

7 5-530pm ;
Inpu 3(4-800-um










a Au electrode Pt electrode
H2,O, 2H' 2e 2H.O H,O, -0 O, 2H- + 2e


10 Pm


Figure 2-3. The self-pumping membrane: a) gold and platinum electrodes deposited on
the opposing surfaces of a track-etched polycarbonate membrane. b) surface
and c) cross-section of the track-etched membrane imaged by scanning
electron microscopy enable measurements of pore diameter, porosity and
thickness of the membrane.











SI 2
t7 3+2 20
15

I 05 5 l t 8
,I 0 20I


IIt


t1.0: .3. i 45 22











Figure 2-4. The dimensions and photograph of the experimental set up.
~ ~ ~ ~ ~ ~ ~ ~~-i '[..i&. "!f:...:. .:". 5
~ ~ ~ ~ ~ ~ i .:4i8: """ .i..." .l :

;:::: ii ,;: : .: ". :. :":;: :
+= i~i .=L i ...." .. ..=. ='.:i'=.i 'r~iii =r a .=...===... ,

Fi"" r ":". The i m en"""n "n '"g ap ":" ;ihie e" ......... '


25 nm Pt







25 nm Au


H202 02 + 2H* + 2e-

H* H+
e-









H202 + 2H- + 2e- 2H20


Figure 2-5. The experimental set up. The membrane is mounted submerged in solution
to create a compartment connected to the larger reservoir by the membrane
and a narrow channel. Flow through the membrane forces fluid through the
narrow channel where the flow velocity can be measured by particle-tracking
microscopy.


















20- C


-20-


-150


-100


-50-


0 0


0 -50


-100


Voltage (V)


Figure 2-6. The electroosmotic pumping membrane. Flow rate and electric current as a
function of external voltage in an aqueous solution. The inset shows a
rescaled plot of the voltage range from -1 V to + 1 V. Error bars show the
standard deviation of tracer particle velocities.


30 -
3 electroosmotic flow L
20-
S10- 0


-10- El
-20
-30- a
-80 -60 -40 -20 0 20 40 60 80
Current (iA)


Figure 2-7. Flow rate dependent on current in electroosmotic pumping. In the
electroosmotic flow, the measured flow rate increases linearly with increasing
measured current.


ow rate ^ 1.0 0 Pt Au 2
current .1 0 9 P t Au

S0 .0 ........................ .. .. .. .. .. ..........
Pt-Au 0
-0.5 A -1 P
1.0 Au Pt 00 0
L.L -2


-1.0 -0.5 0.0 0.5
\/nltang (\/\


............... ... ..... .. ..O .. .. ... .. ..................
~Qocc~,cacjac~j 0


Au Pt


f fl
0 C


4















' 0.8
-J

(U,
S0.4


LL
0.0


-0.4


0.3
<
0.21


0.1
o

0.0


-0.1


0 50 100 150 200 250
Time (min)


Figure 2-8. The self-pumping membrane. Flow rate and electric current as a function of
time in a 0.01% hydrogen peroxide solution as the gold and platinum
electrode are externally connected and disconnected. Error bars show the
standard deviation of tracer particle velocities.


Figure 2-9. Scanning electron microscopy images of the membrane after operation
show tracer particles accumulated on the membrane surface and within the
pores, possibly leading to a reduction in pumping efficiency: a) Au surface of
membrane and b) cross-section of membrane.











1.0 -


0.8

U,
-J 0.6
S
a,
S0.4

LL
0.2


0.0
0.00


0.05 0.10 0.15 0.20 0.25
Current (iA)


0.30


Figure 2-10. Flow rate dependent on current in the self-pumping. In the self-pumping
flow, the measured flow rate increases linearly with increasing measured
current.


Figure 2-11. Calculation of tracer velocity at low flow rates. At a low velocity of tracers,
individual tracer positions can be accurately determined and tracers can be
followed from frame to frame (time between frames: 100 ms; exposure time:
20 ms). Each velocity data point is the average of the velocity of 10 tracer
particles, which is determined by dividing the distance advanced after 10
frames by 1 s.


0 self-pumping flow 8
0* VJ





















Figure 2-12. Calculation of tracer velocity at high flow rates. At a high velocity of
tracers, tracers moving in the center of the channel appear as streaks while
tracers adsorbed to the channel surface appear as dots. The tracer velocity is
calculated by dividing the streak length by the exposure time (100 ms). Each
velocity data point is the average of 10 tracer particle velocities.









CHAPTER 3
GOLD NANOPARTICLE-NANOHOLE ARRAYS AS SERS SUBSTRATES

Raman spectroscopy is a noninvasive technology that enables label-free detection

of molecules. However, the Raman signal is very weak due to the small inelastic Raman

scattering cross section. Surface enhanced Raman scattering (SERS) can greatly

increase the Raman signal by electromagnetic enhancement and chemical

enhancement. The SERS enhancement factor was reported to be 1014 with silver or

gold nanoparticle aggregates as SERS substrates.[13] In this range of SERS

enhancement, even single molecules can be detected.

The SERS substrates should have high enhancements of Raman signal and show

reproducible enhancements for sensing or detecting applications. Particle aggregates or

fractals show high SERS enhancements, but the reproducibility is poor due to their

irregular structures. On the other hand, periodic nanostructures fabricated by

lithographical techniques exhibit better reproducibility, but their SERS enhancements

reported in literature are lower than that of particle aggregates.

Background

Raman Scattering & Surface Enhanced Raman Scattering (SERS)

Raman spectroscopy is a critical technique for structural analysis of molecules

which relies on inelastic scattering of visible light. Raman scattering is attributed to the

excitation and relaxation of vibrational modes of a molecule (Figure 3-1). Because

different functional groups have different characteristic vibrational energies, the

molecular structures of every molecule can be probed by the inelastic Raman

scattering. However, since Raman scattering cross sections are typically 14 orders of

magnitude smaller than those of fluorescence, the Raman signal is several orders of









magnitude weaker than fluorescence emission. Thus, the applicability of Raman

scattering is restricted to structural analysis.[75]

However, a dramatically enhanced Raman signal has been obtained with a

technique called surface enhanced Raman scattering (SERS). When the scatterer is

placed on or near roughened noble-metal substrates, the magnitude of the Raman

scattering signal can be greatly enhanced. This SERS enhancement of the signal

transforms Raman spectroscopy from a structural analytical tool to a structural probe

with single-molecule sensitivity.[76]

Mechanisms for SERS Enhancement

The mechanism of SERS enhancement remains an active research topic. There

are two mechanisms which contribute to the SERS effect: an electromagnetic

enhancement and a chemical enhancement. In the chemical enhancement, new

electronic states are created from chemisorption between the metal and adsorbate

molecules. They serve as resonant intermediate states in Raman scattering. Charge

transfer excitations can occur at about half the energy of the intrinsic intramolecular

excitations of the adsorbate. The existence of charge-transfer state increases the

probability of a Raman transition by providing a pathway for resonant excitation. This

mechanism is site-specific as well as and analyte-dependent and contributes an

enhancement factor of about 100.[77]

The electromagnetic enhancement arises from focusing an electromagnetic field

via plasmon resonance of the metallic substrate on the metal surface. Surface plasmon

polaritons propagate along the metallic surface and are trapped on the surface because

of the resonant interaction between the surface charge oscillation and the

electromagnetic field of the light (Figure 3-2a).[78] When light interacts with









nanostructures much smaller than the incident wavelength, surface plasmon polaritons

are localized (Figure 3-2b). When the localized surface plasmon resonance (LSPR) of

nanostructures on a sliver or gold substrate is excited by visible light, strong

electromagnetic fields are generated due to selective absorption and scattering of the

resonant electromagnetic radiation. When the analyte molecule is subjected to these

intensified electromagnetic fields, the intensity of the inelastic Raman scattering

increases. Electromagnetic enhancement contributes an average enhancement factor

of over 10,000.[75]

The resonant frequency of the conduction electrons in a metallic nanostructure

depends on the size, shape, and material of the structure. In the case of a spherical

nanoparticle of radius a that is irradiated by z-polarized light of wavelength A (where a is

much smaller than A), by solving Maxwell's equations using a quasi-static

approximation, the resulting solution for the electromagnetic field outside the particle is

given by


Eou (x,y,z)= Eoz- ( out a3Eo x x+ y y+z (3-1)
(-i ++2so._ r 3r

where Ei, is the dielectric constant of the metal nanoparticle, and Eout is the dielectric

constant of the external environment. When the dielectric constant of the metal is

roughly equal to -2,out, the electromagnetic field is enhanced relative to the incident

field. In the case of silver and gold, this condition is met in the visible region of the

spectrum.[79]

The enhancement factor for SERS is calculated as









IE^\\IE, -<)_ [iw IN^}2
EFs(o 4 -, [iE VA ] (3-2)
E Na(RSV)/ Nvol]

The Raman enhancement effect is a result of enhancing both the incident

excitation, Eout(w), and the resulting Stokes' shifted Raman, Eout(w- o),

electromagnetic fields. The overall enhancement scales roughly as E4. A small increase

in the local field produces large enhancements in the Raman scattering. The

enhancement factor from experimental measurements is given by the right-hand side of

equation. It is the SERS enhanced Raman intensity, ISERS(wv), normalized by the

number of molecules bound to the enhancing metallic substrate, Nsurf, divided by the

normal Raman intensity, INRS(WO), normalized by the number of molecules in the

excitation volume, Nvol.

Various Techniques for SERS Substrates

Various SERS substrates have been developed: electrodes roughened by an

oxidation-reduction cycle (ORC), island films, colloidal nanoparticles, and surface-

confined nanostructures. ORC-roughened electrodes provide reproducible, in situ SERS

substrates with moderate (~ 106) enhancement factors.[75]

Metal island film substrates are easy to fabricate and the LSPR wavelength can be

tuned by varying the film's thickness and confluence. However, the enhancement

factors achieved with these films are generally smaller (~ 104 105) than those

observed with other SERS substrates. Surface-confined nanostructures can be

produced by several fabrication schemes, including electron-beam lithography, colloid

immobilization, and soft lithography (Figure 3-3). With substrates fabricated via electron









beam lithography, enhancement factors as large as 108 have been achieved by

controlling the inter-particle spacing.[80]

Colloidal nanoparticle substrates are well suited to solution-phase SERS studies.

The Kneipp research group probed small (100 150 nm) silver colloid aggregates dosed

with crystal violet molecules. The large (1014) single-molecule enhancement was

attributed to large electromagnetic fields generated by fractal-pattern clusters of silver

colloid nanoparticles.[13] In colloidal aggregates substrates, there are a small number of

hot spots, which occur at the junction between nanoparticles. Theoretical modeling

shows the strong electromagnetic field between nanoparticles separated by < 1 nm is

due to the surface junction excitation and the efficient interaction of the molecular wave

function with the wave function of the excited metal surface.[81]

SERS Applications

SERS holds great potential as an ultra-sensitive and selective tool for the

identification of biological or chemical agents. The narrow and well-resolved bands

allow simultaneous detection of multiple analytes. As water has a very weak Raman

signal, investigation of biological samples can also be carried out. SERS offers a

method for multicomponent or multiplexed detection of low-concentration analytes,

either by directly revealing the target analyte or by indirectly detecting the fingerprint of

a molecular label.

SERS has been applied to the signal transduction mechanism in a prototype for an

implantable glucose sensor (Figure 3-4). Glucose was detected and quantified in the

physiological range with an accuracy approaching the requirements as a biomedical

device. SERS has also been applied in the detection of trace levels of chemical warfare

agents. Silver nanowires have been used as a substrate to detect 2,4-dinitrotoluene, the









most common chemical indicator of buried landmines and explosives, at a sensitivity of

0.7 pg.[82, 83]

SERS-active molecules have been implemented as labels on the analyte of

interest. Raman labels have been used to identify cancer genes, thus avoiding the

introduction of undesirable radioactive DNA labels.[84] ssDNA coupled with a gold

nanoparticle and a SERS label has been used to detect DNA hybridization events. Also,

multiplexed detection has been used to distinguish hepatitis A, hepatitis B, HIV, Ebola,

smallpox, and anthrax with a detection limit of 20 fM.[85]

Challenges for SERS Substrates

The Raman spectroscopy using SERS provides much more information about

molecular structure and the local environment in condensed phases than electronic

spectroscopy techniques like fluorescence. Minor changes in the orientation of an

adsorbate can be detected as slight variations yields measurable shifts in the locations

of Raman spectral peaks. The abrupt decay of the electromagnetic fields ensures that

only adsorbate molecules on or near noble-metal substrate are probed. This technique

is well suited for analyses performed on molecules in aqueous environments, because

water has an extremely weak Raman signal intensity.[75]

However, the inherent limitation of the technique is that the substrates should be

made of silver, gold, or copper. Other materials are not usable unless they are applied

as thin coatings on SERS-active materials. SERS has limited applicability when the

molecule of interest is not adsorbed directly onto the substrate.

The primary bottleneck has been the reproducible preparation of well-defined,

reliable, and stable substrates with a high SERS-activity.[86] Colloidal substrates tend

to aggregate and thus the molecular surface coverage changes with time. Variation in









the fabrication processes used to produce SERS-active substrates leads to inconsistent

optical properties and discrepant enhancement factors. In the currently used

nanofabrication techniques, the SERS enhancement factors can fluctuate by up to an

order of magnitude for substrates fabricated with seemingly identical methodology.

Regularly arranged monodispersed colloidal gold and silver particles on

functionalized metal or glass substrates or well-ordered nanostructured surfaces

produce SERS with good reproducibility and stability.[87] As the SERS intensity

depends on the excitation of the localized surface plasmon resonance (LSPR), it is

important to control all of the factors influencing the LSPR, namely the size, shape,

particle interspacing, and the dielectric environment to maximize signal and ensure

reproducibility.

Reproducibility can be improved by creating a long-range pattern with sub-

micrometer periodicity on the substrate. Nanosphere lithography (NSL) has excellent

control over nanometer scale features by utilizing self-assembled polymer nanospheres

as vapor deposition masks [88]. Electron-beam lithography (EBL) on a scanning

electron microscope has also been used to produce regular elongated particle arrays for

SERS with optimization control based on center-to-center spacing.[89]

However, these nanofabrication techniques have relatively lower enhancements

than colloidal aggregates. These techniques are also limited by higher cost of

fabrication, the availability of equipment, the substantial expertise required and the low

surface area of the metal structure.

Raman Scattering Intensity Measurement / SERS EFF Calculation

To evaluate the performance of prepared metal nanoparticles array as a SERS

substrate, benzenethiol is used as a model compound. In addition to the excellent









affinity to gold surfaces, benzenethiol molecules have a large Raman cross-section,

presumably as a result of chemical enhancement alongside electromagnetic

enhancement.[82] Each peak in the Raman spectra corresponds to the vibrational

specific mode: the peak at 1572 cm-1 to C-C stretching, the peak at 1072 cm- to C-S

stretching, the peak at 1021 cm- to in-plane ring deformation, and the peak at 997 cm-

to S-H bending, as shown in Figure 3-5.[90]

Gold nanoparticles arrays are placed in a 5 mM solution of benzenethiol in 200-

proof-ethanol for 45 min and then rinsed in 25 ml of 200-proof-ethanol for several

minutes. The samples are allowed to air-dry for 20 min, after which the Raman spectra

are measured.[91] A flat gold film is sputter-deposited on a glass slide using the same

deposition condition used for the control sample for Raman spectra. Raman spectra are

measured with a Renishaw inVia confocal Raman microscope using a 785 nm diode

laser at 15 mW with an integration time of 10 s and a 40 pm2 spot size.

Raman scattering intensity is also measured in an aqueous environment.[13] The

excitation source is an argon-ion laser pumped Ti:sapphire laser operating at 830 nm

with a power of about 200 mW at the sample. Dispersion is achieved using a Chromex

spectrograph with a deep depletion CCD detector. A water immersion microscope

objective (363, NA 0.9) is brought into direct contact with a 30 ml droplet of sample

solution for both excitation and collection of the scattered light. Scattering volume is

estimated with a cylinder of diameter derived from the applied power and the intensity

and length derived from depth-of-field consideration.

The SERS enhancement factors are calculated from data collected using confocal

Raman spectrometers in which the surface enhancement, G, is defined as follows:









G = CNAo-hIurf
G= (3-3)
RIbulk

The intensity of the Raman peak obtained at the SERS surface, Isrf, is compared to that

obtained for a solution, Ibulk, of concentration co. NA is Avogadro's number, o is the

surface area occupied by the adsorbate, R is the roughness factor of the surface, and h

is a parameter defined by the confocal volume of the spectrometer.[92]

Simulation of Effects of Size and Spacing on Electromagnetic Field

If SERS substrates with various parameters are assessed and optimized by

modeling their electromagnetic characteristics, time and effort can be saved in

laboratory preparation and experimental testing.

The field enhancement is strongly dependent on physical parameters such as the

surface morphology, the dielectric constants used to perform the modeling, and the

excitation conditions. Finite element electromagnetic modeling was applied to predict

the Raman enhancement with a variety of SERS substrates with differently sized,

spaced, and shaped morphologies with nanometer dimensions.[93]

The electromagnetic waves must satisfy Maxwell's equations within the modeling

domain, and adhere to the boundary equations at the interface between the media and

the scatterer. Finite element methods (FEM) using Comsol Multiphysics software have

been employed to provide numerical solutions for each substrate. For a radiation

condition, the perfect matched layers (PML) boundaries method and a low-reflection

boundary condition are applied. The output of the modeling process is a two-

dimensional map of the electric field intensity, which can be used to calculate the

Raman enhancement G(r, w). When the polarization of the scattered light is the same









as that of the incident light, the expected electromagnetic enhancement of the Raman

signal may be expressed to a first approximation as

2 2 4
E,(cL) E"(O) E(r, co)
G(r, ) = ( ) 2E (co) 2 ( ) (3-4)
Free (L) Efree () En (0)

where E(r, w) is the total predicted electric field at position r, and Einc(w) is the electric

field associated with the incoming electromagnetic radiation.

Particle Self-assembly Approach

Particle Aggregates for SERS

Kneipp et al reported that the SERS enhancement of 1014 was achieved with

colloidal silver solution, where silver nanoparticles slightly aggregated.[13] The

enhancement was calculated from comparison between the concentration of analyte in

the colloidal silver solution and the concentration in control solution (without colloidal

silver particles) having the same level of Raman signal. The general method to calculate

the enhancement factor (G) will be discussed in the next section. The possibility of

single molecule detection was demonstrated by the change in the distribution of Raman

signal when the average number of analyte is one.[13] Single hemoglobin molecules in

a junction between silver nanoparticles could be detected and the maximum SERS

enhancement in the hot spot was estimated to be 1010.[94]

The SERS enhancement has an advantage in the applications in an aqueous

solution due to very weak Raman signal of water. The SERS imaging in a living cell was

successful by using colloidal gold particles.[95] The biological molecules including

adenine, L-cysteine, L-lysine, and L-histidine, were successfully detected by using gold

nanoparticle aggregates as SERS substrates, where the enhancement was 107-109 in

bulk solution.[96]









The hot spot of high enhancement, which is generally located in a junction

between particles, is sensitive to the spacing between particles as well as the

wavelength and polarization of the excitation laser, as shown in Figure 3-6.[76, 93, 97]

The SERS enhancement up to 1014 in a junction between Ag-coated particles was

reported by calculation based on the extended Mie theory (the classical electromagnetic

theory of spherical particles).[94]

According to the mechanism for the electromagnetic enhancement, the localized

surface plasmon resonance (LSPR) is excited to generate a high SERS enhancement

when electromagnetic radiation is incident upon substrates with the same

wavelength.[75] To generate a high SERS enhancement, the wavelength of

electromagnetic radiation was tuned to the excitation of LSPR with different laser

sources. Alternatively, the excitation of LSPR, which is indicated as a peak in the

extinction spectrum, was tuned to the wavelength of electromagnetic radiation with

using various size and shape of particle and controlling the interparticle spacing. The

plasmon absorption was tuned by using different sizes of gold nanoparticles in aqueous

solution.[98] By assembling silver nanoparticles with DNA bases (adenine, guanine,

cytosine, and thymine) or surface-modifying gold nanoparticles with ATP, the

interparticle spacings between particles in particles aggregates were controlled so that

the plasmon absorption could be tuned.[99, 100]

Self-assembled Particles as a SERS Substrate

For a SERS substrate, a three-dimensional multilayered film with assembled gold

nanoparticles was prepared by using the Langmuir-Blodgett (LB) method.[90] The

Raman signals from self-assembled particles as a SERS substrate were ~ 107 higher









than those from the control (a bare substrate). The enhancements depended on the

Raman peak as well as the particle size and the film thickness.

Periodically ordered arrays of nanoparticles were devised to produce reproducible

SERS enhancement. To predict or estimate the SERS enhancement on the ordered

array of nanoparticles and understand the effects of parameters such as the particle

size, shape, and interparticle spacing, several models were designed to solve the

electromagnetic field satisfying the Maxwell's equations. Finite element methods (FEM),

T-matrix method, RLC circuit analogy were employed to solve the electromagnetic field

on nanoparticle arrays.[93, 101, 102]

It is worthwhile noting that the maximum value of enhancement (Gmax) obtained

from the hot spot should be distinguished from the average value of enhancement (Gave)

calculated over the entire surface of the substrate. The maximum enhancement factor

was used to emphasize the capability of single-molecule detection, for example, in

Kneipp's paper,[13] while the average enhancement factor was used to demonstrate the

effectiveness of the nanostructure as a SERS substrate, for example, in Jung's

paper.[90] Since the portions of hot spot in the entire surface of substrate are much less

than 1 %, the average enhancement (Gave) is several orders lower than the maximum

enhancement factor (Gmax).[103]

The enhancement factors are maximized at the wavelengths of the incident light

which are close to the excitation of the localized surface plasmon resonance (LSPR).

The maximum enhancement factor (Gmax) on periodic arrays of silver nanoparticles

calculated based on the Finite element methods was increased up to 108 with

decreasing separation between particles.[93] Based on the T-matrix theory, the









maximum calculated enhancement factor (Gmax) on one-dimensional arrays of silver

nanoparticles was calculated to be 109.[101] The average enhancement factor (Gave) on

two-dimensional square arrays of gold nanospheres calculated based on the RLC circuit

model is about 108, where Gave depends on the ratio of interparticle spacing and particle

size.[102]

Controlling the interparticle spacing and particle size is important to achieve a high

enhancement factor (Gave) in terms of generating large area of hot spots in a junction

between particles. Periodically ordered arrays of nanoparticles functionalized with

surfactant molecules were assembled to control the interparticle spacing between

particles. Gold nanoparticles of 50 nm diameter functionalized with

cetytrimethylammonium bromide (CTAB) were self-assembled to generate hexagonally

close-packed monolayer with the interparticle spacing of 8 nm.[104] The average

enhancement factors (Gave) were calculated to be up to 108 at the near-infrared (785

nm) excitation by comparing ratios of the SERS peak intensities to the corresponding

unenhanced signals from neat analyte films. Close-packed arrays of silver nanoparticles

of 20 nm capped with surfactant molecules of oleic acid and oleylamine were

assembled on the poly (N-isopropylacrylamide) (PNIPAM) film, where the interparticle

spacing could be controlled by altering the temperature since PNIPAM is a temperature-

sensitive polymer.[105] Controlling the interparticle spacing from less than 4 nm to 24

nm, brought the plasmon resonance peak closer to the laser excitation wavelength,

generating larger Raman signals of an analyte. Two-dimensional hexagonal close-

packed arrays of gold nanoparticles functionalized with resorcinarene tetrathiol had

interparticle spacing less than 1 nm.[106, 107] The average enhancement factors (Gave)









were determined using peak integration ratios of the SERS peak intensities to the

corresponding unenhanced signals from neat analyte films. The SERS enhancement

factors (Gave) of 107 with NIR excitation were shown in large particle arrays.

Most investigation has been carried out based on the direct relationship between

extinction/absorption and SERS enhancement. On the other hand, Lu et al claim that

the connection between extinction/absorption and SERS enhancement is indirect and

qualitative at best since the spatial distribution of collective resonances should be

considered.[108] Bulk-like resonances (proportional to the volume) have a large

contribution to absorption, while surface-like resonances (proportional to the surface)

have a large contribution to SERS enhancement. For example, a high SERS

enhancement was observed at the wavelength of excitation laser where there is no

resonance in the absorption/extinction.

STV/PEG-GNP Arrays

In this work, gold nanoparticle (30 nm) functionalized with streptavidin and

polyethylene glycol (PEG) chain are assembled into hexagonally close-packed arrays

using a flow cell, as shown in Figure 3-7. The size of streptavidin immobilized on the

nanoparticles is about 4 nm.[109] Double-sided tapes are assembled as spacers on a

clean glass slide and a thin glass slip is covered to make a flow cell.

Streptavidin/PEGylated gold nanoparticles (STV/PEG-GNP) solution monodispersed in

distilled water is flown through the flow cell. Solutions are pipeted from one side of the

flow cell and sucked out from the other side by capillary action. After 5 minutes, rinsing

is done by flowing enough volume of DI water to remove excessive gold particles and

the remaining salts in the solution. Solvent is then evaporated at the room temperature

generating STV/PEG-GNP arrays on a glass slide.









The self-assembled STV/PEG-GNP arrays are shown in Figure 3-8. They are non-

close-packed arrays due to spacers such as streptavidin and PEG. The interparticle

spacing corresponds to two-fold of the streptavidin size (8 nm). Most part of the surface

are covered with a monolayer of particle arrays, though some areas are not covered

with particles.

In the absorption spectrum, there is a peak at the excitation wavelength of 544

nm indicating the excitation of localized surface plasmon resonance (LSPR), as shown

in Figure 3-12a. The SERS spectrum at the excitation wavelength of 785 nm is shown in

Figure 3-13a. Benzenethiol is used as an analyte as it has a good affinity to gold and

forms a monolayer on gold surface. The SERS enhancement factor (Gave) is about

5x105 which is calculated from the SERS peak intensity at the Raman shift of 999.2 cm

1 (S-H bending + in-plane ring deformation mode).[110] Although the wavelength of the

excitation laser (785 nm) is not close to the excitation of LSPR (544 nm), there is a high

SERS enhancement of 5x105.

Periodic Nanostructure Approach

Periodic Nanostructures as SERS Substrates

To generate reproducible SERS substrates, periodic nanostructures have been

fabricated by various micro/nanofabrication techniques. Silver nanoparticle arrays were

produced by electro-beam lithography.[ 11] The important factors in SERS

enhancements such as particle size, shape, and interparticle spacing could be

controlled. Triangular silver nanoparticle arrays were fabricated by nanosphere

lithography (NSL).[112] The excitation wavelength of localized surface plasmon

resonance (LSPR) was controlled with various dimensions of the nanoparticles and the









wavelength of the excitation laser was tuned with tunable laser systems. According to

the systematic investigation, the SERS enhancement was maximized when the

excitation wavelength of LSPR was located between the wavelength of the excitation

laser and the wavelength of the Raman scattered photon by the analyte molecules. The

SERS enhancement factor (Gave) was calculated up to be 108.

Hexagonally ordered arrays of nanovoids as SERS substrates were fabricated by

self-assembly of sacrificial nanospheres and electrochemical deposition of gold.[113-

115] The fabricated nanostructures had either gold flat surfaces or corrugated surfaces

depending on the thickness of gold film. Surface plasmon polaritons propagate along

the flat gold surface and scatter at the rims of shallow dishes forming delocalized Bragg

modes. On the highly corrugated surface with nanovoids, surface plasmon polaritons

were localized in the nanovoids forming localized Mie modes. The Bragg plasmon mode

depends on the incident angle and sample orientation, while the Mie plasmon mode

depends on the geometry of nanostructure.

Maximum SERS enhancements occurred when the excitation laser is incident at

the wavelength of the excitation of LSPR on the substrate and the Raman scattered

photon is also coincide with the excitation of LSPR.[92, 115] The measured SERS

enhancement factor (Gave) was 710 8 on a highly corrugated substrate with nanovoids

of 350 nm diameter, where the wavelengths of both excitation and Raman scattered

photons were in the absorbance peak (corresponding to the excitation of LSPR). It was

claimed that it was not possible to achieve completely uniform SERS enhancements

over whole Raman scattering modes, because the wavelengths of Raman scattered

were all different and could not be matched to the excitation of LSPR all together.









The excitation of localized surface plasmon resonance (LSPR) could be tuned to

near infrared (NIR) wavelength, by controlling the nanovoid size and the film

thickness.[116] The use of NIR laser sources can be favored since the photochemical

reactions can be activated and the fluorescence from adsorbed molecules can interfere

with the Raman signals. The SERS enhancement factor of 3x106 was obtained at the

NIR of 1064 nm.

Three-dimensional nanostructures were fabricated for reproducible and highly

SERS-active substrates.[117, 118] Inverse opal films fabricated by self-assembling a

binary mixture of sacrificial latex microbeads and gold nanoparticles showed stable and

reproducible SERS enhancements. The alumina membranes decorated with gold

nanoparticles as SERS substrates had advantages of efficient light interaction on the

wall of cylindrical pores with minimal absorption and scattering. The SERS

enhancement factor (Gave) was calculated to be 106.

Periodic Nanostructures from Non-close-packed Particle Arrays

Various non-close-packed arrays of nanopillars, nanodots, nanoholes, nanovials,

and nanovoids, could be achieved based on the hexagonally non-close-packed

nanoparticle arrays fabricated by a colloidal self-assembly.[l 19-123] Concentrated silica

nanoparticles (20 vol%) dispersed in ethoxylated trimethyllolpropane triacrylate

(ETPTA) monomer were spin-coated on a silicon wafer. Then, ETPTA monomer is

rapidly photopolymerized to immobilized silica particles on the substrate. After removing

the polymer matrix by an oxygen plasma etching, colloidal monolayer of hexagonally

non-close-packed silica particles is fabricated. The interparticle spacing between

particles is about 1.4 times of the particle diameter, which is explained by keeping a









minimal volume fraction of silica particle in ETPTA matrix due to the centrifugal force

during spin-coating.[1 19]

Periodic nanopyramid arrays are fabricated using the non-close-packed

nanoparticle arrays as deposition masks during Cr deposition.[91] The silica particles

are removed by dissolving in hydrofluoric acid aqueous solution, generating Cr

nanohole arrays on silicon wafer. Inverted pyramid arrays of silicon are produced by

wet-etching in KOH solution, where Cr nanohole arrays play the role of etching masks.

After removing Cr layer with a Cr etchant and depositing gold with thickness of 500 nm,

the deposited gold layer is transferred onto a glass substrate using a polyurethane

adhesive. The SERS enhancement on the fabricated nanopyramid arrays is 7x105

Instead of depositing gold and transferring onto a glass substrate, polymer replication

can be used to generate nanopyramids of ETPTA and, by deposition of a gold layer,

gold coated nanopyramid arrays are fabricated.[124] The SERS enhancement is

improved to 108 due to the unbroken tips of nanopyramid. The strong concentration of

electromagnetic field near sharp tips of nanopyramids is contributed to the high SERS

enhancement. The charged analytes can be concentrated to the surface of

nanopyramid arrays under externally applied electric field, strengthening the Raman

signals.[125]

The metal film over nanosphere (MFON) as a SERS substrate is fabricated by

depositing gold on the hexagonally non-close-packed array of silica nanoparticles

without a polymer etching process.[126] Contrary to conventional MFON substrates, the

fabricated MFON consists of gold islands of 10 nm and gaps of less than 10 nm. Gold

islands form only on the polymer wetting layer during the deposition of gold layer. The









SERS enhancement is up to 108 due to the delocalized Bragg plasmon modes along

the periodic nipple structure and of the localized Mie plasmon modes in gold islands.

Disordered arrays of gold half-shells as SERS substrates are fabricated by

depositing gold on non-close-packed arrays of silica nanoparticles, transferring to a

glass substrate, and removing the silica particles in HF solution.[127] The strong

concentration of electromagnetic field near the sharp edge of half-shell and hot spots

between half-shells contribute to the measured high SERS enhancement (Gave) of 1010.

On the other hand, a maximum SERS enhancement factor (Gmax) of larger than 1010 in

the hot spot near sharp tips of gold nanocrescent moons was reported.[128]

Nanohole Arrays on a Glass Substrate

In this work, nanohole arrays are fabricated based on the colloidal self-assembly

for hexagonally non-close-packed arrays and the nanosphere lithography, as shown in

Figure 3-9. To generate nanoparticle arrays on glass slides instead of silicon wafers,

PMMA is spin-coated as a sacrificial layer. The concentrated silica nanoparticles

dispersed in ETPTA monomer are spin-coated on a PMMA coated glass slide

generating hexagonally non-close-packed arrays of nanoparticles. After

photopolymerization of ETPTA to immobilize particles on the substrate, ETPTA and

PMMA are etched by using nanoparticles arrays as an etching mask. Silica

nanoparticles are removed by ultrasonication in ethanol and the remaining ETPTA and

PMMA are removed in acetone, generating nanohole arrays on a glass substrate.

The fabricated gold nanohole arrays with different sizes (330 and 400 nm) are

shown in Figure 3-10. They are hexagonally non-close packed arrays of nanoholes. In

the absorption spectrum of 330 nm nanoparticles arrays, there is a peak at the

excitation wavelength of 722 nm, indicating the excitation of localized surface plasmon









resonance, as shown in Figure 3-12a. In the case of 400 m nanoparticles arrays, a peak

is present at the excitation wavelength of 816 nm. These excitations of LSPR are close

to the used excitation wavelength of 785 nm.

STV/PEG-GNP Arrays on Nanohole Arrays

Concept of GNP-nanohole Arrays

In the absorption spectra of STV/PEG-GNP arrays, 330 nm gold nanohole arrays

fabricated in our study, and 400 nm nanohole arrays, the excitation of localized surface

plasmon resonance are located at the wavelengths of 544 nm, 722 nm, and 816 nm,

respectively. To maximize the SERS enhancement, the excitation of LSPR should be

between the wavelengths of the excitation and the Raman scattered photons, according

to literatures. Thus, there are two ways to maximize the SERS enhancement such as

tuning the excitation of LSPR to the wavelengths of excitation and the Raman scattered

photons by controlling the geometry of the substrates and tuning the laser excitation

wavelength by using tunable laser systems.

The laser excitation wavelength of 785 nm is favored in biological sensing

applications since the photochemical reactions can be activated and the fluorescence

from adsorbed molecules can interfere the Raman signals in the visible excitation

wavelength.[116] In this work, we try to tune the excitation of LSPR to the wavelength

range around the laser excitation wavelength of 785 nm by combining the STV/PEG-

GNP arrays and the gold nanohole arrays.

Fabrication of GNP-nanohole Arrays

The STV/PEG-GNP arrays on gold nanohole arrays are fabricated by colloidal

self-assembly and metal deposition, as shown in Figure 3-9. First, the gold nanohole

arrays are prepared on a glass substrate based on the colloidal self-assembly for









hexagonally non-close packed arrays and nanosphere lithography. Then, gold

nanoparticles (30 nm) functionalized with streptavidin (STV) and polyethylene glycol

(PEG) chains are assembled on the prepared gold nanohole arrays instead of bare

glass substrate to form hexagonally close-packed arrays by using a flow cell, as shown

in Figure 3-7.

The fabricated STV/PEG-GNP arrays on gold nanohole arrays with different sizes

(330 and 400 nm) are shown in Figure 3-11. Gold nanohole arrays with different sizes

(330 nm and 400 nm) are fully covered with gold nanoparticles (30 m) forming non-

close-packed arrays due to streptavidin (STV) and PEG chains.

Optical and SERS Properties of STV/PEG-GNP Arrays on Nanohole Arrays

The absorption spectra of STV/PEG-GNP arrays with different sized nanoholes

are shown in Figure 3-12b. In the case of 330 nm nanoholes, the excitation of LSPR is

at the wavelength of 551 nm and 746 nm, which are red-shifted from those in the

STV/PEG-GNP arrays (544 nm) and the nanohole arrays (722 nm). In the case of 400

nm nanoholes, the excitation wavelengths of LSPR are also red-shifted to 563 nm and

875 nm. There are delocalized surface plasmon polaritons (SPP) along the flat surface

of periodic nanohole arrays. The localized surface plasmon in gold nanoparticles is

indicated as a Raman peak around the wavelength of 550 nm and the localized surface

plasmon in nanoholes is indicated as a Raman peak from 722 nm to 875 nm. The red-

shifts in the excitation of LSPR indicate the interactions between the excitation of LSPR

in gold nanoparticles and the excitation of LSPR in gold nanoholes. At the wavelength

of 785 nm, STV/PEG-GNP arrays on 330 nm nanohole arrays show a high absorption

plateau, while the STV/PEG-GNP arrays on 400 nm nanohole arrays show a high and

still increasing absorption.









The Raman spectra of STV/PEG-GNP arrays with different sized nanoholes at the

excitation wavelength of 785 nm are shown in Figure 3-13b,c. There is no Raman peak

around 2600 cm-1 (corresponding to v(S-H) stretching and vibration modes), indicating

that there is no analyte (benzenethiol) unbound to the substrate. Both substrates show

much stronger Raman signals than those from nanoparticle arrays. Moreover, the

signal-to-background ratios in both cases are larger than the reported ratios in the

literature, which is favorable in sensing and detecting applications. The STV/PEG-GNP

arrays on nanohole arrays show the good reproducibility in the SERS enhancement in

terms of low standard deviations (~ 10 %) of Raman signals (Table 3-1). The higher

SERS enhancement in 400 nm nanoholes is consistent with the higher absorption in

400 nm nanoholes.

The SERS enhancement factors at different Raman peaks are different even in

the same substrate, as shown in Table 3-1. Reported empirical results show that the

high SERS enhancement occurs when the excitation wavelength of LSPR is between

the excitation and the Raman scattered photons. The difference in the SERS

enhancement can be explained by different wavelengths of Raman scattered photons at

different Raman peaks.

Conclusions

In conclusion, we have developed a self-assembly technology for fabricating gold

nanoparticle arrays on gold nanohole arrays as reproducible and high SERS-active

substrates. The excitation of localized surface plasmon resonance (LSPR) in

nanoparticle arrays and nanohole arrays around the wavelength of excitation laser of

785 nm contribute to high SERS enhancements. Due to a high sensitivity (high signal-

to-background ratios) as well as a good reproducibility (low variations of Raman signal









on different spots and samples), this simple and scalable technology for fabricating gold

nanoparticle arrays on gold nanohole arrays is promising for developing ultra-sensitive

detectors for chemicals and reproducible sensors for chemical and biological molecules.

Materials and Methods

Materials

Monodispersed silica colloids with less than 10 % diameter variation are

synthesized by the Stober method.[129] Ethoxylated trimethylolpropane triacrylate

(ETPTA) monomer is obtained from Sartomer (Exton, PA). The photoinitiator, Darocur

1173 (2-hydroxy-2-methyl-1 -phenyl-1 -propanone), is provided by Ciba Specialty

Chemicals. Streptavidin PEGylated gold nanoparticles (30 nm) are purchased from

Polysciences (Warrington, PA).

Instrumentation

A standard coater (WS-300B-6NPP-Lite Spin Processor, Laurell) is used to spin-

coat colloidal suspensions. The polymerization of ETPTA monomer is carried out on a

Pulsed UV Curing System (RC 742, Xenon). A Unaxis Shuttlelock RIE/ICP reactive-ion

etcher is utilized to remove polymerized ETPTA and PMMA. Scanning electron

microscopy is carried out on a JEOL 6335F FEG-SEM. Raman spectra are measured

with a Renishaw inVia confocal Raman microscope.

Gold Nanohole Arrays on a Glass Slide

Glass slides were cleaned using a standard RCA1 cleaning[130]. A resist PMMA

(MicroChem 950 PMMA A4) was spin-coated on a 1 inch2 glass slide (4000 rpm for 1

min) as a sacrificial layer and then was baked for hardening (180 C for 3 min).

Monolayer non-close-paced silica particles dispersion (400 or 330 nm) was then spin-

coated on PMMA.[131] The final step of the spin coat process is 8000 rpm for 5 min.









After UV-curing poly(ethoxylated trimethylolpropane triacrylate), reactive ion etching

(Unaxis Shuttlelock operating at 5 mTorr oxygen pressure, 15 sccm oxygen, 5 sccm

argon, and 100 W RIE power and 300 W ICP power for 90 s) was performed to etch

both ETPTA and PMMA by using silica nanoparticles as etch masks. A 100 nm thick

gold film was deposited by an electron beam evaporator. Silica nanoparticles were

removed by ultrasonication in ethanol. The PMMA/ETPTA layers were then lifted off in

acetone resulting in gold nanohole arrays on a glass substrate.

STV/PEG-GNP Arrays on Gold Nanohole Arrays

Flow cell was constructed by placing two strips of double-side sticky tape on the

Au nanohole arrays about 7 mm apart and covering with a cover glass. To provide

enough number of gold particles to cover whole surface area, 3 layers of double-side

sticky tape (~ 300 fm) were used to increase the volume of solution (~ 60 pL).

Streptavidin PEGylated gold nanoparticles dispersed in distilled water was flown

through a flow cell. Solutions are pipeted on one side and sucked out the other side by

capillary action. Solvent was then evaporated at room temperature. To remove crystals

formed due to salts in the solution, Au nanoparticles array was rinsed by flowing ample

amount of distilled water through the flow cell.

Absorbance of Substrates

The absorbance of the GNP arrays on gold nanohole arrays is evaluated using

visible-near-IR absorbance measurement with an Ocean Optics HR4000 high resolution

fiber optic UV-visible-near-IR spectrometer.

Raman Spectra Measurements

STV/PEG-GNP arrays on gold nanohole array samples are placed in a 9 mM

solution of benzenethiol in 200-proof-ethanol for 45 min and then rinsed in roughly 25









mL of 200-proof-ethanol for several minutes. The samples are allowed to air-dry for 20

min, after which the Raman spectra are measured. A gold nanoparticles array on bare

glass is used as the control sample for Raman spectra measurements. Raman spectra

are measured with a Renishaw inVia confocal Raman microscope using a 785 nm diode

laser at 6.4 mW with a 20x objective and an integration time of 10 s and a 170 pm2 spot

size. Raman spectra were also measured in 9.77 M benzenethiol contained in a flow

cell at 6.4 mW with an integration time of 10 s.

Calculation of Enhancement Factors

The SERS enhancement factor, Gave, is calculated from data collected using the

confocal Raman microscope. Gave is defined as follows:

I, ,, / N, cmN~Ahl,
ve /N c (3-5)
'bulk Nbulk R bulk

The intensity of the Raman peak obtained at the SERS surface, Isuf, is compared to that

obtained for a solution, Ibulk, of concentration Co. NA is Avogadro's number, a is the

surface area occupied by one adsorbate molecule, R is the roughness factor of the

surface, and h is a parameter defined by the confocal volume of the microscope.[132] h

is measured to be 400 pm with the 20x objective.









Table 3-1. Calculated SERS enhancement factor on the fabricated substrates from
measured data of Raman intensities at 999.2 cm-1 and 1023 cm1.
SERS substrate ~ 999.2 cm- ~ 1023.2 cm1
GNP array 4.59x10b + 1.93x10b 1.56x106 6.52x10b
330 nm nanohole-GNP 2.15x106 + 2.06x105 7.90x106 8.55x105
400 nm nanohole-GNP 2.98x106 2.64x105 1.15x107 + 1.07x106











Electronic States


......... Virtual Energy States


a b c
== == =:=l== == ==
i____ : __


3
2 Vibrational
i Energy States
o


Figure 3-1. Various characteristic energies: a) Rayleigh scattering, b) Stokes Raman
scattering, and c) anti-Stokes Raman scattering.


Dielectric


Figure 3-2. Schematic diagrams of a) a surface plasmon polariton (or propagating
plasmon) on a flat surface and b) a localized surface plasmon on a
nanostructured surface. Adapted from [79].


f++ --- +++







































Figure 3-3. Various SERS substrates: a) Ag film on nanospheres, periodic
nanostructures by b) nanosphere lithography and c) electron-beam
lithography, and d) colloidal aggregates. Adapted from [80, 99].





















* Clcose silver


Figure 3-4. SERS applications: a) in vivo glucose sensing equipment consisted of
SERS spectroscopy, implanted substrate, beam directing optics, and
collection lens and b) identification of cancer genes by Raman labels.
Adapted from [82].


SH











1000 1200 1400 1600
Raman shift (cm-1)


Figure 3-5. Representative Raman spectrum of benzenethiol adsorbed on a SERS
substrate. Adapted from [90].












x10-7
1.5-1-


-0.5 -

-1-


7
6


4


2
E

0


-21
A


-1.5 -1 -0.5 0 0.5 1 1.5
Sxl0-7


Figure 3-6. The SERS enhancement calculated based on finite element methods in the
case of nanoparticle arrays. Adapted from [93].


A :4:I: d
l
.........










PEG


Au / ---

Flow-cell
self-assembly









a 02 plasma
etching









Figure 3-7. Schematic diagram depicting self-assembly of gold nanoparticles using a
flow cell.























Figure 3-8. SEM images of 30 nm gold nanoparticle arrays with a gap between
particles on glass substrates.

























Reactive ion etch


ze D
Au
Silica particles
in ETPTA





R(

r. Au film
I deposit

r 2)


S Streptavidin
'eposit Au nanoparticle


move silica & polymers
A


Figure 3-9. Schematic diagram depicting the fabrication procedures for making GNP-
nanohole arrays.


_V




























Figure 3-10. SEM images of nanohole arrays: a) 330 nm nanohole arrays and b) 400
nm nanohole arrays.







.... ... .........................













Figure 3-11. SEM images of nanohole-GNP arrays: a) 330 nm nanohole arrays
covered by 30 nm gold particles and b) 400 nm nanohole arrays covered by
30 nm gold particles.
.......... ..~::';~'~t
PM a

law. ~~=::













30 nm gold particles.






































































0.5-





0 n-


- --330 nm nanohole & STV-PEG-GNP
400 nm nanchole & STV-PEG-GNP


450 500 550 600 650 700 750 800 850
Wavelengm (nm)


Figure 3-12. Absorption spectra on a) gold nanoparticle (GNP) arrays and nanohole

arrays and b) nanohole arrays covered by gold nanoparticles.


I I .-J


.


1


--
,,


--- -













100000 a) SH

80000

6i 60000

- 40000

20000

0-
500 1000 1500 2000 2500 3000
Raman shift (cm1)


100000-

80000-

60000-

40000-

20000-

n


b)










500 1000 1500 2000 2500 3000
Raman shift (cm1)


500 1000 1500 2000 2500 3000 500 1000 1500 2000 2500 3000
Raman Shift (cm1) Raman Shift (cm )


Figure 3-13. Raman spectra of benzenethiol absorbed on a) flat god surface, b) gold
nanoparticle (GNP) arrays, c) 330 nm nanohole arrays covered by gold
nanoparticles, and d) 400 nm nanohole arrays covered by gold nanoparticles.









CHAPTER 4
BINARY COLLOIDAL CRYSTALS

The electronics revolution sparked by the invention of transistors and the

miniaturization of integrated electronic circuits has affected almost every aspect of our

daily lives. In an effort toward further high-density integration and system performance,

scientists are now turning to light as the information carrier. The travel speed of light in a

dielectric material is greater than that of an electron in a metallic wire.[133] The

bandwidth of dielectric materials is about 3 to 4 orders of magnitude larger than that of

metals.[133] Moreover, light particles (photons) are not as strongly interacting as

electrons, which helps to reduce energy losses. Unfortunately, our ability to control

photons in miniaturized volumes is in many ways in its infancy, compared with our

ability to manipulate electrons.

A new class of optical materials known as photonic crystals (PCs) may hold the

key to continued progress towards all-optical integrated circuits and high-speed optical

computing.[134-137] PCs are periodic dielectric structures (Figure 4-1) with a forbidden

gap (or photonic band gap) for electromagnetic waves, analogous to the electronic band

gap in semiconductors. Photons with energies lying in the photonic band gap (PBG)

cannot propagate through the medium, providing the opportunity to control the flow of

light for photonic information technology. The lattice constant of the artificial crystal must

be comparable to the wavelength of the light passing through the crystal.[136] For

optical communication systems operating at near-infrared wavelengths, the lattice

constant must have dimensions on the submicrometer scale.[138, 139]

Unfortunately, the development and implementation of integrated optical circuits

with photonic crystals have been greatly impeded by expensive and complex









nanofabrication techniques. Electron-beam lithography (EBL) and focused ion-beam

(FIB) are two popular methods in fabricating photonic crystals with arbitrary

geometries.[140, 141] However, attaining high-throughput and large-area fabrication

continues to be a major challenge with these top-down techniques. By contrast, bottom-

up colloidal self-assembly and subsequent templating nanofabrication provide a much

simpler, faster, and inexpensive alternative to nanolithography.[138, 139, 142] A variety

of methods, such as gravity sedimentation,[138, 143] electrostatic repulsion,[144-147]

template-assisted assembly,[148-150] and capillary force induced convective self-

assembly,[139, 151-153] have been developed to create colloidal photonic crystals.

However, current colloidal self-assemblies are only favorable for low volume, laboratory-

scale production. It usually takes days or even weeks to grow a centimeter-size colloidal

crystal.[139, 152, 153] In addition, most of current colloidal self-assembly technologies

are not compatible with mature semiconductor microfabrication, limiting the mass-

production and on-chip integration of practical photonic crystal devices.

Another major issue of current colloidal self-assembly is the limitation on

achievable crystal structures. Although calculations show that nonclose-packed

photonic crystals (e.g., diamond-structured crystal) facilitate the opening of wider

PBGs,[154, 155] the realization of open-structured crystals by self-assembly is

challenging.[156] Binary colloidal photonic crystals composed of particles of two

different sizes have attracted a great deal of recent interest as they are promising to

open wider PBGs.[157-182] For instance, van Blaaderen et al. developed a convective

self-assembly technology to assemble small silica particles on a pre-assembled

colloidal array consisting of larger particles.[176] Similar to the limitations of traditional









colloidal assembly, all available bottom-up methodologies in creating binary colloidal

photonic crystals suffer from the scalability and microfabrication-compatibility issues.

We have recently developed a simple and scalable spin-coating technology that

combines the simplicity and cost benefits of bottom-up colloidal self-assembly with the

scalability and compatibility of standard top-down microfabrication.[183, 184] The spin-

coating technique enables mass-fabrication of wafer-scale (up to 8 inch) colloidal

crystals, which is a length scale nearly two-orders of magnitude larger than that

currently available through other methods. Additionally, the entire crystal is formed

within minutes, as compared to days or even weeks needed to produce a centimeter-

size crystal using other self-assembly techniques. Most important, the spin-coating

technique is compatible with standard microfabrication, allowing for the creation of

complex micropatterns for optical on-chip integration. In this chapter, we developed a

new bottom-up approach to create non-close-packed binary colloidal crystals by using

spin-coated colloidal crystals as structural template.

Results and Discussion

Figure 4-2 shows a schematic outline of a procedure for achieving binary colloidal

crystals. The established spin-coating technique is utilized to generate a wafer-scale

monolayer of hexagonally ordered nonclose-packed silica nanoparticles. The resulting

nanocomposite of silica particles and polymer matrix has a thin polymer wetting layer

(~100 nm) next to the Si substrate, which can still immobilize the silica particles on the

substrate after partial etching of the polymer.[185] The fabricated monolayer of silica

particles (300 nm diameter) is shown in Figure 4-3. In the literature, the close-packed

colloidal crystals were achieved via the spin-coating technique, where the evaporation

rate of solvent in the colloidal dispersion was very high.[186] In the established spin-









coating technique for nonclose-packed colloidal crystals, on the other hand, silica

particles are dispersed in non-volatile monomer (ETPTA). The nonclose-packed

monolayer has the particle center-to-center distance of 1.41 (of particle diameter), which

corresponds to the minimal volume fraction of silica particle in the silica-polymer

nanocomposite.[183]

The dispersions of silica particles in different sizes are subsequently spin-coated

on the prepared monolayer. The prepared monolayers of silica particles are used as

templates for guiding the silica particles in the dispersions into the interstices between

the three neighbor particles in the template layers. The fabricated binary colloidal

crystals are shown in Figure 4-4. In the colloidal crystal consisting of a 300 nm particle

monolayer on a 300 nm particle monolayer, the particle in the second layer is located in

the trap which is the center of interstice between three neighboring particles in the first

template layer as shown in Figure 4-4b. In the colloidal crystal consisting of a 400 nm

particle monolayer on a 300 nm particle monolayer, on the other hand, the particles in

the second layer are not confined in the traps, as shown Figure 4-4d. The larger

particles (400 nm) in the second layer rather form a nonclose-packed array independent

of the first template layer.

In the literature, attempts at localizing small particles in the traps between large

particles were successful. However, attempts at localizing larger particles in the traps

which are the interstices between small particles in the templating layer were not

successful with the convective assembly (but, they were successful in the LS2 close-

packed binary crystal),[176] although it is thermodynamically favorable. In our spin-

coating process with a template layer, the "trapping effect" due to the geometry of









template layer is competing with the centrifugal force due to spinning. In the case of

localizing larger particles, the trapping effect is reduced because the depth of interstice

between particles in the template layer is shallow. Thus, the particle arrays in the

second layer are affected by the centrifugal force, where the minimal volume fraction of

silica particles is achieved.

Several strategies have been reported to improve the ordering in the colloidal

crystals. Enhanced orderings of colloidal particles were successful by steady shearing,

ultrasonication, and oscillatory shearing.[151, 187, 188] Defect-free arrays of nanoholes

in the block-copolymer were achieved by solvent-induced ordering.[189] In this work,

pressure is applied on the surface of shear-aligned silica particles in a ETPTA monomer

matrix to increase the trapping effect. The resulting binary colloidal crystals are shown

in Figure 4-5. In the binary colloidal crystals consisting of a 300 nm particle layer on a

300 nm particle layer, the orderings of particles in the second layer into the traps, which

are interstices between the three neighbor particles in the template layer, are improved,

as shown in Figure 4-5a-d. In addition, the domain boundaries are clearly consisting of

vacancies, while the domains in the binary crystals fabricated without an applied

pressure show gradual change across the domain boundaries. This is due to the

rearrangement of particles in monomer matrix under the applied pressure. There is no

significant difference between 0.2 MPa and 0.33 MPa of pressure. On the other hand,

the binary colloidal crystals consisting of a large particle (400 nm) layer on a small

particle (300 nm) don't exhibit the pressure effect. The large particles form a non-close-

packed array, while they are independent of the first template layer.









The particle size and the thickness of layer in colloidal crystal are controllable.

Figure 4-6a shows the cross-section of the binary colloidal crystals consisting of 400 nm

particles on 345 nm particles. The individual layers are monolayers of 400 nm particles

or 345 nm particles. By repeating the spin-coating process with different sized particles,

the colloidal crystal consisting of many monolayers of different size of particles can be

achieved. The thickness of individual layers can also be controlled with spin-coating

parameters such as spinning speed and time. Binary colloidal crystals consisting of a

400 nm particle monolayer on a 345 nm particle double layer and ternary colloidal

crystals consisting of a 300 nm double layer on a 400 nm double layer which in turn is

on a 345 nm monolayer are shown in Figure 4-6b,c.

Conclusions

Binary colloidal crystals are achieved with a simple, fast, and scalable spin-coating

technique. The thickness of individual layers is easily controlled with spin-coating

parameters. Although the pressure is helpful for the ordering of particles, further

improvement for better orderings of particles in the individual layers is still needed. The

characterization of optical properties in the fabricated binary colloidal crystals is in

investigation.

Materials and Methods

Materials and Substrates

Monodispersed silica colloids with less than 10% diameter variation are

synthesized by the Stober method.[190] Ethoxylated trimethylolpropane triacrylate

(ETPTA) monomer is obtained from Sartomer (Exton, PA). The photoinitiator, Darocur

1173 (2-hydroxy-2-methyl-1 -phenyl-1 -propanone), is provided by Ciba Specialty

Chemicals. The silicon-wafer primer, 3-acryloxypropyl trichorosilane (APTCS), is









purchased from Gelest (Morrisville, PA). Silicon wafer (test grade, n-type, (100)) are

obtained from Wafernet (San Jose, CA) and primed by swabbing APTCS on the wafer

surfaces using cleanroom Q-tips (Fisher), rinsed and wiped with 200 proof ethanol three

times, spin coated with a 200 proof ethanol rinse at 3000 rpm for 1 min, and baked on a

hot plate at 110 oC for 2 min.

Instrumentation

A standard coater (WS-300B-6NPP-Lite Spin Processor, Laurell) is used to spin-

coat colloidal suspensions. The polymerization of ETPTA monomer is carried out on a

Pulsed UV Curing System (RC 742, Xenon). A Unaxis Shuttlelock RIE/ICP reactive-ion

etcher is utilized to remove polymerized ETPTA for releasing shear-aligned colloidal

crystals. Scanning electron microscopy is carried out on a JEOL 6335F FEG-SEM.

Non-close-packed Colloidal Monolayer

The fabrication of wafer-scale, monolayer, non-close-packed colloidal crystal-

polymer nanocomposites is performed according to reference.[ 19] Monodispersed

silica colloids (300 nm) are dispersed in non-volatile ethoxylated trimethylolpropane

triacrylate (ETPTA, Sartomer) monomer (20% volume fraction) and 2 wt% Darocur 1173

is added as photoinitiator. The silica-ETPTA dispersion is dispensed on a 3-

acryloxypropyl trichlorosilane (APTCS)-primed (100) silicon wafer and spin-coated at

8000 rpm for 3 min on a standard spin coater, yielding a hexagonally ordered colloidal

monolayer. The monolayer is then photopolymerized for 4 s using a Pulsed UV Curing

System. To utilize this monolayer as a substrate for spin-coating of the second layer,

the polymer matrix between particle arrays is partially removed using a reactive ion









etcher operating at 40 mTorr oxygen pressure, 40 sccm flow rate, and 100 W RIE

power for 2 min 30 s.

Binary Layer of Hexagonally Non-close-packed Colloidal Structure

20 vol% silica colloid (400 nm)-ETPTA dispersion (including 2 wt% photoinitiator)

is prepared. The silica-ETPTA dispersion is dispensed on a pre-fabricated substrate,

non-closed packed colloidal monolayer and spin-coated at 8000 rpm for 3 min on a

standard spin coater, yielding a hexagonally ordered colloidal binary layer. The binary

layer is then photopolymerized for 4 s using a Pulsed UV Curing System.

For better ordering of silica particles in the second layer by applying a pressure, a

glass slide is placed on top of the binary layer before photopolymerizing and then a

weight (2 or 3.2 kg) is placed on this glass slide for 2 min. The pressure is calculated

from the weight and the area of binary layer. The ETPTA monomer in the binary layer is

then photopolymerized for 4 s using a Pulsed UV Curing System.

Ternary Layer Hexagonally Non-close-packed Colloidal Structure

To utilize the binary layer as a substrate for spin-coating of third layer, the polymer

matrix is partially removed using a reactive ion etcher operating at 40 mTorr oxygen

pressure, 40 sccm flow rate, and 100 W power for 2 min 30 s. The 20 vol% silica-

ETPTA dispersion (including 2 wt % photoinitiator) is dispensed on the prepared

substrate and spin-coated at 8000 rpm for 3 min on a standard spin coater, yielding a

hexagonally ordered colloidal ternary layer. The ETPTA monomer in the ternary layer is

then photopolymerized for 4 s using a Pulsed UV Curing System.
















2


b)

Conduction Band



S Electronic Band Gap

VaI ln Ba

Valence Band
->


(N

3 Photonic Band Gap


C

L.
U-
I F


Wavevector (ka/2j) Wavevector (ka/2j)


Figure 4-1. Photonic crystals: a) photonic crystals in 1-D, 2-D, and 3-D and b) a
photonic band gap compared to an electronic band gap. Adapted from [136].









Silica particle of different size


SO plasma etching
t_


Silica particle


g


k Spin-coating &
Polymerization


0, plasma etching


Figure 4-2. Schematic illustration of the procedure for fabricating binary hexagonal
arrays of silica spheres by using monolayer nonclose-packed colloidal
crystals as substrates.


Figure 4-3. Monolayer of nonclose-packed silica particles (300 nm) fabricated by the
spin-coating technique.


/Polymer




































Figure 4-4. Binary hexagonal arrays of silica spheres: a, b) 300 nm particles array on
300 nm particles array (300/300), and c, d) 400 nm particles array on 300 nm
particles array (400/300).


















































Figure 4-5. Pressure effects on the ordering of particles in the second layer: a, b) 0.2
MPa for 2 min, and c, d) 0.33 MPa for 2 min in 300 nm particles arrays on 300
nm particles arrays (300/300), and e, f) 0.2 MPa for 2 min in 400 nm particles
arrays on 300 nm particles arrays (400/300).






88




















b


I t ?ti~tjV .a *


Figure 4-6. Cross-sections of hexagonal arrays of silica spheres: a, b) binary layer of
400 nm particles array on 345 nm particles array (400/345) and c) ternary
layer of 300 nm particles array on 400 nm particles array on 345 nm particles
array (300/400/345).









CHAPTER 5
BIOINSPIRED, ORGANIC-INORGANIC NANOCOMPOSITES

In nature, hard biological tissues show excellent mechanical properties due to their

unique microstructures (Figure 5-1).[191] Scientists have been inspired by mechanical

design principles found in nature to generate new materials of high mechanical

performance. The nacreous layer of mollusk shells has an intricate brick-and-mortar

nanostructure consisting of 95 vol% brittle aragonite platelets and 5 vol% of soft

biological macromolecules, making the shells exceptionally tough and stiff.[191-196]

Mimicking the unique structure in the nacreous layer has been tried by various bottom-

up self-assembly techniques. Layer-by-layer (LBL) assembly of ceramic-polymer

alternative layers is successful in generating reinforced nanocomposites with aligned

structures.[197, 198] Ice-templated crystallization, spin-coating, gravitational

sedimentation, and centrifugation have been explored to assemble ceramic platelets

into ordered structures.[199-202]

Electrophoretic deposition is a simple, inexpensive, and scalable process that

enables rapid production of thick films over large areas, while LBL assembly is a slow

process.[203-205] In addition, Nanocomposites can be assembled in a single step by

electrophoretic codeposition of colloids and polymers.[206] For the layered

nanocomposites of nanoclays and polymer, the agglomeration of nanoclays should be

avoided for highly aligned structures.

To achieve nanocomposites with a high level of strength and toughness, inorganic

platelets should have high aspect ratio, small thickness, high shear stress or adhesion

with polymer matrix, and high volume fraction (Figure 5-2).[191] Uniform gibbsite

(AI(OH)3) nanoplatelets can be synthesized by hydrolysis of aluminium alkoxide.[207,









208] The aspect ratio of the synthesized gibbsite nanoplatelets (~ 10) is close to that of

natural aragonite (CaCO3) platelets in nacre.[193] The diameter and thickness can be

controlled by seeded growth.[209] Due to the hydroxyl groups on the surface, different

functionalities can be rendered by the chemical modification.[208]

In this work,[210, 211] synthesized gibbsite nanoplatelets are aligned and

assembled by electrophoretic deposition. Inorganic-organic nanocomposites having

nacreous microstructures are achieved by infiltrating monomer into the interstitials

between the assembled nanoplatelets and polymerizing the monomer. The resulting

nanocomposites exhibit significantly improved mechanical properties. By the surface

modifications of gibbsite nanoplatelets, covalent linkages between the inorganic

platelets and organic matrix are facilitated to create further reinforced nanocomposites.

In this study, Huang synthesizes gibbsite nanoplatelets and Lin modifies the surfaces of

gibbsite nanoplatelets and fabricates gibbsite-polymer nanocomposites. My contribution

is characterizing the mechanical properties of the fabricated nanocomposites.

Assembly of Colloidal Nanoplatelets

Synthesized Gibbsite Nanoplatelets

The gibbsite nanoplatelets synthesized from aluminium alkoxides in an acidic

aqueous solution have a hexagonal shape and uniform size, as shown in Figure 5-3a.

The diameter is about 188 40 nm and the measured thickness ranges from 10 to 15

nm, as measured by atomic force microscopy (AFM). The hydroxyl groups on the

surface of gibbsite nanoplatelets can be modified by reacting with 3-

(trimethoxylsily)propyl methancrylate (TPM) via the silane coupling reaction.[212] This

TPM-modification forms dangling acrylate bonds which can be cross-linked with

acrylate-based ethoxylated trimethylolpropane triacrylate (ETPTA) matrix.









Assembly of Gibbsite Nanoplatelets by Electrophoretic Deposition

Gibbsite nanoplatelets are assembled in a parallel-plate sandwich cell under an

electric field. Ethanol is added to the aqueous dispersions to promote colloidal

coagulation on the ITO electrode by reducing the electric double layer thickness of the

particles. In the resulting film, gibbsite nanoplatelets are densely packed and aligned

(Figure 5-3b). It is known that the isoelectric point (IEP) of the gibbsite edges (pH ~ 7)

differs from the IEP on the gibbsite faces (pH ~ 10).[207] Thus, in the suspension, the

nanoplatelets have positively charged faces and neutral edges. This charge distribution

is favorable in alignment of nanoplatelets in parallel to the electrode surface.

Gibbsite-polymer (ETPTA) nanocomposites are made by infiltrating photocurable

monomers into the interstitials between the nanoplatelets in the assembled gibbsite film

and photopolymerizing the monomer matrix.

Mechanical Properties of Nanocomposites

The tensile stress-strain curves for ETPTA, gibbsite-ETPTA, and TPM-modified

gibbsite-ETPTA films are shown in Figure 5-3c. The gibbsite-ETPTA nanocomposite

shows 2-fold higher tensile strength and 3-fold higher Young's modulus compared to

those of pure ETPTA polymer. The TPM-modified gibbsite-ETPTA nanocomposite

displays 4-fold higher tensile strength and 1 order of magnitude higher Young's modulus

due to cross-linking with ETPTA matrix. It is known that the covalent linkage between

the inorganic fillers and the organic matrix determines the mechanical properties of the

nacres composites.[197]









Assembly of Surface-roughened Nanoplatelets

Silica-coated Gibbsite Nanoplatelets

Silica-coated gibbsite nanoplatelets are synthesized by coating a thin shell of sol-

gel silica on the surface of gibbsite nanoplatelets.[213] The amphiphilic

polyvinylpyrrolidone (PVP) macromolecule can be adsorbed onto a broad range of

colloids stabilizing them in water and various nonaqueous solvent and acts as a

coupling agent during this coating process. The TEM image in Figure 5-4a shows

hollow silica nanoplatelets after leaching out gibbsite parts. The silica shell has a

thickness of 10 nm. The silica shells driven by the sol-gel process are much rougher

than the single-crystalline gibbsite nanoplatelets.

Assembly of Silica-coated Gibbsite Nanoplatelets

Silica-coated gibbsite nanoplatelets are assembled in a parallel-plate sandwich

cell under an electric field. Cracks easily form on the silica-coated gibbsite film during

the drying process. Polyethyleneimine (PEI) is added to the bath solution of silica-

coated gibbsite nanoplatelets to solve the cracking issue by increasing the adherence

and strength of the electrodeposited films. PEI is known to act as a particle binder by

adsorbing strongly onto silica at various pH.[214]

Unlike gibbsite nanoplatelets, silica-coated gibbsite-PEl nanoplatelets have

positive charges on both faces and edges due to the PEI macromolecules adsorbed on

the uniform silica shell. There is no electric-field-induced reorientation of silica-coated

gibbsite-PEl nanoplatelets due to the uniform distribution of surface charge. The

formation of polymer-bridges between neighboring particles also leads to the imperfect

alignment of nanoplatelets.[214] Nevertheless, the nanoplatelets still preferentially









aligned since the orientation is energetically favorable under electric field (Figure 5-

4b,inset).

Silica-coated gibbsite-polymer (ETPTA) nanocomposites are made by infiltrating

photocurable monomers into the interstitials between the nanoplatelets in the

assembled gibbsite film under vacuum for a few hours and photopolymerizing the

monomer matrix.

Mechanical Properties of Nanocomposites

The tensile stress-strain curves for ETPTA, gibbsite-ETPTA, and silica-coated

gibbsite-PEI-ETPTA films are shown in Figure 5-4c. The silica coated-gibbsite-PEI-

ETPTA nanocomposite shows 2.5-fold higher tensile strength. This is due to the

presence of PEI macromolecules, which strongly adsorbed on the negatively charged

surface of silica-coated gibbsite. The PEI macromolecules can also interlock with cross-

linked ETPTA backbone.

The silica-coated gibbsite-PEI-ETPTA nanocomposite shows 5-fold larger

elongation compared to that of pure ETPTA. The large elongation is due to the strong

ionic bonding between the PEI macromolecules and the nanoplatelets, and the natural

elasticity of PEI. The surface roughness of nanoplatelets and the rotation of misaligned

nanoplatelets under an applied tensile mode can also be a reason for the large

elongation observed.

Conclusions

We developed a simple and rapid electrodeposition technique for assembling

gibbsite nanoplatelets into organized multilayers. Nanoplatelets with high aspect ratio

(diameter-thickness ratio) are aligned under electric field and the interstitials between

nanoplatelets are infiltrated with polymer generating organic-inorganic nanocomposites









with significantly improved mechanical properties. For further improvement in

mechanical properties of nanocomposites, gibbsite nanoplatelets are surface-treated

(TPM or silica) and assembled into aligned multilayers. This technique is promising to

achieve oriented deposition of a wide range of materials including ceramics, metals,

ceramic-metal, or ceramic-conducting polymer nanocomposites.

Materials and Methods

Materials

Ultrapure water (18.2 MQ cm-1) was used directly from a Barnstead water system.

200-proof ethanol is purchased from Pharmaco Products. Hydrochloric acid (37%),

aluminum sec-butoxide (> 95%), aluminum isopropoxide (> 98%), polyvinylpyrrolidone

(PVP, Mw ~ 40,000), polyethylenimine (PEI, 50 wt% in water, Mw ~ 750,000), and

sodium hydroxide (> 98%) were obtained from Sigma Aldrich. Tetraethyl orthosilicate

(TEOS, > 99%) was purchased from Gelest. Ammonium hydroxide (14.8 N) was

obtained from Fisher Scientific. Ethoxylated trimethylolpropane triacrylate (ETPTA,

SR454) monomer was provided by Sartomer (Exton, PA). The photoinitiator, Darocur

1173 (2-hydroxy-2-methyl-1 -phenyl-1 -propanone), was obtained from Ciba Specialty

Chemicals. Two-part polydimethylsiloxane (PDMS, Sylgard 184) was provided by Dow

Corning. Indium tin oxide (ITO) coated glass substrates with sheet resistance of 8 ohms

were purchased from Delta Technologies. Silicon wafers (test grade, n type, (100)) were

purchased from University Wafer.

Instrumentation

An EG&G Model 273A potentiostat/galvanostat was used for electrophoretic

deposition. Scanning electron microscope (SEM) was carried out on a JEOL 6335F









FEG-SEM. Transmission electron microscope (TEM) and selected area electron

diffraction were performed on a JEOL TEM 2010F. Atomic force microscope (AFM)

was conducted on a Digital Instruments Dimension 3100 unit. A standard spin-coater

(WS-400B-6NPP-Lite spin processor, Laurell) was used to spin-coat ETPTA monomer.

The polymerization of ETPTA was carried out on a pulsed UV curing system (RC 742,

Xenon). A kurt j. Lesker CMS-18 Multitarget Sputter was used for the deposition of Ti

and Au.

Synthesis of Gibbsite Nanoplatelets

The gibbsite nanoplatelets were synthesized by following a published

method.[207] Hydrochloric acid (0.09 M), aluminum sec-butoxide (0.08 M), and

aluminum isopropoxide (0.08 M) were added to 1 L ultrapure water. The mixture was

stirred for 10 days and then heated in a polyethylene bottle in a water bath at 85 C for

72 h. After cooling to room temperature, dispersions of gibbsite nanoplatelets were

centrifuged at 3500 g for 6 h and the sediments are redispersed in deionized water. For

completely removing the unreacted reactants and concentrating the nanoplatelets, this

process was repeated for five times.

Surface Modification of Gibbsite Nanoplatelets with TPM

Gibbsite nanoplatelets were surface-modified with 3-(trimethoxysilyl)propyl

methacrylate (TPM).[215] Prior to adding gibbsite nanoplatelets, 10 mL TPM was mixed

with a 100 mL water-methanol solution (water/methanol volume ratio of 3:1) for 1 hour

in order to fully hydrolyze TPM. Surface modification was then accomplished by adding

100 mL of gibbsite dispersion (ca. 1 vol% aqueous solution) into the hydrolyzed TPM

solution. The suspension was stirred at 40 C for 30 min. The modified nanoplatelets









were washed by repeated centrifugation-redispersion cycles with pure ethanol and

finally concentrated to a stock suspension of 0.045 and 0.035 (g/g) in ethanol.

Coating of Gibbsite Nanoplatelets with Silica

Purified gibbsite nanoplatelets were coated with a thin layer of silica by a two-step

procedure: adsorption of PVP and growth of silica shell via Stober method.[129] PVP

was dissolved in DI water by ultrasonication. Subsequently, 200 mL aqueous solution of

gibbsite nanoplatelets (1 wt%) was mixed with 300 mL PVP solution (10 wt%). Then,

the mixture was stirred for 1 day for the complete adsorption of PVP on the gibbsite

surface. PVP-coated gibbsite nanoplatelets were transferred into ethanol by repetitively

centrifuging the mixture and redispersing the sediments in ethanol three times. The

PVP-modified gibbsite nanoplatelet suspension of 500 mL was mixed with 33 mL

ammonium hydroxide and 1 mL TEOS for the growth of silica shell. After stirring for 4-6

h, silica-coated gibbsite nanoplatelets were transferred into water by centrifuging the

dispersion and redispersing the sediments in DI water.

Electrophoretic Deposition

Electrophoretic deposition of nanoplatelets was performed in a horizontal

sandwich-cell. The bottom and the top of the cell were an ITO working electrode and a

gold counter electrode, respectively, with a PDMS spacer. The active area and cell gap

were 1.5x1.5 cm2 and 2.2 mm, respectively. The bath solutions for gibbsite-ETPTA and

TPM-modified ETPTA were nanoplatelet dispersions in water-ethanol mixtures. The

volumetric ratio of ethanol to the aqueous suspension was 2. The bath solutions for

silica-coated gibbsite-PEI-ETPTA were prepared by mixing 9 mL of 1.5 wt% silica-

coated gibbsite nanoplatelet aqueous solution with 1 mL 1.5 wt% PEI aqueous solution

and ultrasonicating the mixture. A constant voltage of -2.5 V (ITO vs. Au) was applied to









deposit the positively charged nanoplatelets onto the ITO cathode. The electroplated

gibbsite films were rinsed with 200-proof ethanol.

Mechanical Test

Tensile strengths were measured using an Instron model 1122 load frame

upgraded with an MTS ReNew system and equipped with a 500 g load cell at a

crosshead speed of 0.5 mm/min. Testing samples with width of 1.5 mm and thickness

ranging from 30 to 80 pm were adhered on home-made sample holders with a 10 20

mm gap using polyurethane monomer (NOA 60, Norland) as an adhesive and then UV-

cured. The thickness of the tested samples was measured by cross-sectional SEM to

calculate the final tensile strength.





























2pm


Figure 5-1. Hard biological tissues and their microstructures: a) tooth, b) vertebral
bone, c) shell, d) Enamel made of long needle-like crystals with soft protein
matrix, e) dentin and bone made of plate-like crystals embedded in a
collagen-rich protein matrix, and f) nacre made of plate-like crystals with a
very small amount of soft matrix in between. Adapted from [191].


Tension zones


Mineral


Protein matrix


zones


1 4(1- ) 1
E G p 2p2 2OE
p r P E,


0: the volume concentration of mineral
Em: the Young's modulus
G : the shear modulus of protein
p: the aspect ratio of the mineral platelets


Figure 5-2. A model of biocomposites: a) a schematic diagram of staggered mineral
crystals embedded in protein matrix and b) a simplified model showing the
load-transfer mechanism in the mineral-protein composites. Adapted from
[191].

























c) 60


50 ....... JIUU ISL I I" I
cc ETPTA
0-
S40-

S30-

I 20 '-
C-
20

0---
0C



0.00 0.01 0.02 0.03
Strain



Figure 5-3. Nanocomposite of colloidal nanoplatelets: a) TEM image of gibbsite
nanoplatelets, b) SEM image of a free standing gibbsite-ETPTA
nanocomposite, and c) tensile strain-stress curves for plain ETPTA film,
gibbsite-ETPTA nanocomposite, and TPM-modified gibbsite-ETPTA
nanocomposite. Adapted from [210].


100


---.- TPM-modified Gibbsite+ETPTA
A :L L.:4-l -rT A















I'
/


S40



U)
a)
20
aQ
I-


0 -
0.00


100 nm


0.02 0.04 0.06 0.08


0.10


Strain




Figure 5-4. Nanocomposite of surface-roughened nanoplatelets: a) TEM image of acid-
leached silica-coated gibbsite nanoplatelets, b) SEM image of silica-coated
gibbsite-PEI-ETPTA nanocomposite on an ITO electrode, and c) tensile
strain-stress curves for plain ETPTA film, gibbsite-ETPTA nanocomposite,
and silica-coated gibbsite-PEI-ETPTA nanocomposite. Adapted from [211].


101


------ Silica-coated Gibbsite-PEI-ETPTA

.....Gibbsite-ETPTA
--ETPTA


~nr~r'
r ; :
~~F~'









CHAPTER 6
CONCLUSIONS AND OUTLOOK

Functional nanoscale materials mimicking nanostructures found in nature are

investigated in a broad range of fluidic, optical, and mechanical systems.

Self-pumping Membrane

The developed self-pumping membrane harvests chemical energy from the

surrounding fluid and uses it for accelerated mass transport of 0.9 nL cm-2 s-1 across the

membrane. Micro- and macroscopic devices for drug delivery, sensing and purification,

as well as oil recovery and removal may benefit from this technology.

The self-pumping membrane still needs to be improved by optimizing the

electrochemical reaction, the materials of membrane and electrodes, and the geometry

of the membrane. The removal of oxygen gas generated on the electrode surfaces

should be studied to improve pumping efficiency.

Reproducible and Highly SERS-active Substrates

Gold nanoparticle and nanohole combined arrays are developed as reproducible

(low variation of Raman signal on different sites) and highly active SERS (high SERS

EFs and high signal-to-background ratios) substrates via the cost-effective technology.

Due to a high sensitivity as well as a good reproducibility, these SERS substrates are

promising in applications of ultra-sensitive detectors for chemicals and reproducible

sensors for chemical and biological molecules.

Potential Applications as a Hybrid Biosensor

Robust and stable synthetic transport systems and effective detection systems are

essential to solve the problems of poor stability and weak fluorescence in hybrid

biosensors. A biomimetic, self-pumping membrane can be integrated as a robust


102









transport system. Reproducible and highly SERS-active substrates can be integrated as

effective detection systems. SERS sensors are known as label-free detecting systems.

Thus, a simpler configuration of biosensor device is feasible without an analyte tagging

process for detection.

Binary Colloidal Crystals

Binary colloidal crystals consisting of colloidal particles of different sizes are

achieved with a simple, fast, and scalable spin-coating technology. The thickness of

individual layer is easily controlled with spin-coating parameters. Although the pressure

is helpful for ordering of particles, further improvement for better orderings of particles in

the individual layers is still needed. The characterization of optical properties in the

fabricated binary colloidal crystals is in progress.

Organic-inorganic Nanocomposites

Gibbsite-polymer nanocomposites are developed via a simple and rapid

electrodeposition. Gibbsite nanoplatelets having high aspect ratio (> 10) are aligned

under electric field and the interstitials between nanoplatelets. The assembled

nanoplatelets are infiltrated with monomer, and monomer is cured by UV generating

organic-inorganic nanocomposites. The developed gibbsite-ETPTA composite shows 2-

fold higher tensile strength than pure polymer. TPM-modified gibbsite-ETPTA composite

has 4-fold higher tensile strength and silica-coated gibbsite-PEI-ETPTA composite has

5-fold larger elongation.

Although this technique is promising to achieve oriented deposition of a wide

range of materials including ceramics, metals, ceramic-metal, or ceramic-conducting

polymer nanocomposites, the alignment of nanoplatelets and the complete infiltration of


103









polymer matrix between aligned nanoplatelets are still under investigation for better

mechanical performance.

The self-pumping membrane designed and demonstrated in this dissertation will

stimulate the research of converting chemical energy into kinetic energy directly with

high efficiency, especially in micro- and nanodevices. The simple, scalable, and cost

effective spin-coating technique utilized in the fabrication of binary colloidal crystals and

hole-particle arrays in nanoscale can replace expensive and slow top-down techniques.

On the other hand, the simple and fast electrophoretic deposition technique employed in

the assembly of nanoplatelets can be substituted for slow bottom-up techniques. The

functional nanomaterials and techniques discussed in this dissertation represent a

contribution to achieve and control nanostructures via simpler, cheaper, and faster

technologies.


104









APPENDIX A
ELECTROOSMOTIC FLOW

Electroosmotic Flow in One-wall Channel

In the one-wall case, if the ionic concentration is Co and electrical potential, cp,

goes to zero and the potential takes the value, C (zeta potential), at the surface, the ionic

concentration are given by standard expression from statistical physics.[216]


C+ = Co expT i (z)J Co Ze k(z), whereZe < IkBT kT

The assumption (Zep << kBT) is known as the Debye-HCckel approximation.

The charge density, pei, is,

2(Ze)2 Co
Pea 2 o(z) (A-2)
kT

With the Poisson equation (0"= -Pe,/e), the electrical potential, cp, is,


(z)= exp where the Debye-length AD= 2(Z--C (A-3)
2(Ze)2C D0

Electroosmotic Flow in Cylindrical Tube

The Navier-Stokes equation for steady-static cylindrical tube is,


S--a (r = SE-- I r (A-4)
r r Or) r Or Or

Bv eE p
r '-2 -r + C, (A-5)
Or q Or


With the boundary conditions ( O 0 = 0, and [(p = 0, C, = 0), the fluid
Dr Jr,, L r
r=0 r=O

velocity is,

eE
vz = + C (A-6)
77


105









With boundary conditions (v,(r


R) = 0, and ((r


R)= C, C


eEC ), the fluid
'1


velocity is,


v,(z)= eE[o(r)-_]

In the analogy with the one-wall case, the electric potential, cp(r), is,


exp -r+ exp -- cosh r
(r0)= C D{ C A
exp I +exp -R cosh ,


The fluid velocity, Vz(r), is,


v (r) sE


cosh r

cosh 2
ADJ


The average fluid velocity, , is,


(V ) = Z (r)rdr
mR2 f


sE4 2 ARh R.
-.2, R tanh -
7 R KAD


D +


A 2

cosh R
ADJ


where D << R.

The charge density, pei, is calculated from the Poisson equation.


106


(A-7)


(A-8)


(A-9)


cEC
n


(A-10)


R2 ~









2 C sinh sinh

Or r Or or A cosh cA r Cosh
K2 cosh 2 (A-1l1)

cosh sinh

A RD rA D R
D cosh cosh R)


The average charge density, , is calculated,


R cosh< (rsinh r
2z \ R 2c R"D D
P= 2 Perdr = rdr + --dr
2 0 R 0 2 l
AD cosh AD cosh
D D

-2
R2AD cos R D D AD
AD

2 < tanh R
RAD AD

The volume occupied by proton flowing through the cylindrical tube for time t, V, is,

V = vpron t .7irR2 (A-13)

The total charge of proton flowing through the tube for time t, Ctoti, is,

Ctol = Vproton t R2 P ) (A-14)

Then the flowing current, /, is,

I = V roton e2. e) (A-15)
t

The velocity of proton, Vproton, is calculated from the balance between the electric

force and the drag force of proton in the electric field,


107









eE
Vproton 677rproton


In the membrane having cylindrical pores, the porosity of membrane, p, is,

A pr R2 Npore
A A

where Npore is the number of pore and A is the total area of membrane.

The total current flowing through membrane, Itotal, is,


taal = Vproton R2 el) pore
eE 2 tanh( R
eE -R -N tanh -
6roo RANproD ADJ

The fluid velocity, fluid, is,

Z R2. pore E Z R
Vflu=d =(Vz)- V A pore

Total 3 total r p
A 2e tanh R Ae tanh
6z r proton RAD, 1
3 lotal rproton D ( << R)
eA


_ e;TeR2 Nre (- 2) tah R
S 6rproo RD AD


S pore
A
oton RD
Ra
flD


The maximum pumping pressure (counter pressure) is calculated by applying zero

velocity in the Navier-Stokes equation.


r--r ar = VP PejE = 0 (A-20)
r Or Or )


P = p,,Ed


(A-21)


where pel is not constant, but a function of r.

The average pressure,

, is calculated by

= < pei>Ed.


108


(A-16)





(A-17)


(A-18)


(A-19)











< >= -I" Ed
*proton ;R pore


I oal Ed
eE R2
6-R Npore
6 z proton


6zpro~ total d 6zrproton I tota d
R 2eNpore eApor


109


(A-22)








APPENDIX B
SELF-PUMPING FLOW

Flow Rate as a Function of Tracer Velocity

According to Holmes and Vermeulen,[217] the fluid velocity in the narrow

rectangular channel, v(x, y) is,

v 2 2 H 2
V-= -1- 1--y for0<-<- (B-1)
Vmax B H B 3

where vmax is the maximum velocity in the center of the channel and equals the

measured tracer velocity, and the width, B, and the height, H, of the narrow channel are

3.2x10-4 m, 5.1 xl0-5 m, respectively.

The flow rate through the channel, Qchannel, is calculated from the measured tracer

velocity, Vmax.

QchH annel 54 2 H B[ 4m 1- 2 1 54 d2
2 f2... L7 B l)2 2 2 _B2


(B-2)

For a channel of the given dimensions, this yields

Qchnnel = (6.6x10-9 2)ma (B-3)

Conductivity of Working Fluid

The measured slope of 120 pA V-1 for the I-V curve for electroosmotic pumping

(Figure 2-7) implies an ohmic resistance of 8.4 kQ by


R = (B-4)
k pA

where the length of the pores, /, is 1.8x10-5 m, the membrane area, A, is 9x10-5 m2, and

the porosity of the membrane, p, is 12%.


110









We obtain a conductivity, k, of 2.0 x10-4 S m-1 which compares well with the

conductivity of de-ionized water of 9.9x10-5 S m-1.[218]

Flow Rate as a Function of Current

The flow rate through membrane, Qpore, equals to the flow rate through channel,

Channel. The flow through the pore is composed of an electroosmotic and counter-

pressure component

Channel Qpore Qelectroosmotic Qconter pressure (B-5)

According to Lazar and Karger,[40] the flow rate due to electroosmosis,

Qelectroosmotic, is,


Qelectroosmtic c d pore (B-6)
4771

with e as the permittivity and r7 as the viscosity of the working fluid, (as the zeta-

potential, d as the diameter and I as the length of the pore, and U as the applied voltage

and Npore as the number of pores.

Utilizing U = IR together with (B-4) and pA = Td2Npore/4 we obtain


Qelectroosmtic i (B-7)
7/k

Using e = 7.08x10-10 C V-1 m-1 (the permittivity of water at 20 C),[218] (~O= -27

mV,[71] r7 = 1.002x10-3 N m-2 s (the viscosity of water at 20 C),[218] and k = 2.0x10-4 S

m-1, we obtain

Qe1ectroomotic =(97 nL pA-' s1-). I (B-8)

According to Lazar and Karger,[40] the flow rate due to counter pressure, Qcounter-

pressure, is,


111










pore d4N (B-9)
Counter pressure pore d pore (B-9)
1287 pore

with APpore as the pressure differential across the membrane.

Since this pressure is balanced by the pressure differential across the small

channel, APchannel, we can write


Q- channel 4 (B-hanne 4
Qcounterpressure 12877 pore (B- )


According to Holmes and Vermeulen,[217] the flow rate through a rectangular

channel can be approximated by


Channel = (B-11)
127Lc

with the channel length Lc = 6 mm, the channel width B = 320 pm, the channel height H

= 51 pm and the correction factor M given by


M= 1-0.630 +0.052 5 0.9 (B-12)
(mB) B

Inserting (B- 1) into B-(10), and the resulting expression into (B-5), we obtain

E 3d2ApL
Channel k 8cMBHa3 hnnel (B-13)

which simplifies to


Channel = (B-14)
77e + 3d 2ApLc
S81MBH3

Using d = 0.96 pm and /= 18 pm, we obtain


112










Qchannel = 97 nL (2.9 nL A- s-) I (B-15)
(1+ 33)

Flow Rate at Zero Opposing Pressure

The flow rate in the absence of the counter pressure is equal to the flow rate due

to electroosmosis calculated in (B-7) and (B-8). Given the maximum available current of

0.26 pA this translates into a maximum flow rate of 25 nL s-1.

Stall pressure at zero flow. The stall pressure can be calculated by equating (B-7)

and (B-9) to be

321s
Pstall = 3 I (B-16)
d2pAk

For a current of 0.26 pA, this yields a stall pressure of 1.4 Pa.

Consumption of H202 and Decreasing Flow Rate

The 02 generation rate per electrode area, ko2, is

1 I
ko =- (B-17)
2 fAF

where F is the Faraday constant, and f is the fraction of oxygen which originates from

the electrochemical reaction H202 -- 02 + 2 H+ + 2 e- (and not from 2 H202 -- H20 +

02). Paxton et al estimated f = 40 % based on their measurements of 02 generation and

current density for gold/platinum microelectrodes,[56] which implies

ko = 4(2.6 x107 A) = 4x10- mol m2 s (B-18)
2 2-0.4 (96485 C mol )-(9 x 105 m2)

Since 60% of the oxygen molecules require 2 hydrogen peroxide molecules to be

produced and 40% of the oxygen molecules require only one hydrogen peroxide

molecule, the hydrogen peroxide consumption rate is given by


113








kH02 = 1.6ko0 (B-19)

The fraction of the initially available hydrogen peroxide which is consumed after

time t is approximately given by

[H2o2] 1.6k2 At
[Al c (B-20)
[H202]o cpV

where c is the hydrogen peroxide concentration (0.01 wt%), r is the density of the

solution (1 g cm-3) and V is the volume of the solution (4.3 mL), and M is the molecular

weight of hydrogen peroxide (34 g mol-1).

Inserting (B-17) into (B-20) yields

[H2021_ 4 MIt
[H202 5 fFcpV

At the end of the experiment described in Figure 2-8, 270 minutes have passed,

so that

[H202]t 4-(0.2610-6 A)-270 60s (34 g mol )
[H202] 5 0.4.(96485 C mol 1) (10-4) (1000 g L1)- (0.0043 L)

In contrast, the measured decrease in the flow rate and the current is about 20 %.

However, the fraction of oxygen originating from the electrochemical reaction may be

sensitive to the experimental conditions, and only a fraction of the total hydrogen

peroxide in the cell may be locally available to the membrane. This may cause us to

underestimate the hydrogen peroxide depletion. Alternatives, which cannot be ruled out

at this time, are a reduction in the catalytic efficiency of the membrane and partial

clogging of the pores with tracer particles.


114









APPENDIX C
FLOW RATE MEASUREMENT

Length-converting Method

The flow rate is measured by tracing the flow front in test section with a ruler

(Figure C-la).[74] The flow rate is calculated by multiplying the velocity of the flow front

with the cross-section of channel. The error in the flow rate measured by this method is

within 50 nL min-1.

Mass-converting Method

The flow rate is measured with a digital balance (Figure C-1b).[42, 219] The flow

rate is calculated by dividing the change in mass of the reservoir with the density of

fluid. Heads of the head tank and the weighing reservoir are set at the equal height for

equilibrium pressure. The measured flow rate is in the range from to 100 pL s-1.[219]

Brask et al reported the accuracy of the flow rate within 1 pL min-' with a balance of 0.1

mg precision.[42]

Current-monitoring Method

The electroosmotic flow rate is determined by the current-monitoring method

(figure C-1c).[220] When the fluid is driven through a capillary tube from reservoir 1 to

reservoir 2 by the electroosmotic flow, the change in measured current corresponds to

the volume of driven fluid from reservoir 1 to reservoir 2.

Particle Image Velocimetry Method

The flow rate is measured by tracing the marker particles (Figure C-1d).[221, 222]

The two syringes connected to the ends of the channel in the micropump are set at the

equal height for equilibrium pressure. The speed of an air slug in the channel is

measured. The pressure difference due to electroosmotic pumping is almost zero since


115









the liquid level change in the syringe is negligible due to large diameter of the syringe.

This method can measure the flow rate in the range of nL min1.

Concentration-monitoring Method

The flow rate is estimated based on the change in concentrations of dye in

compartment 1 and compartment 2 (Figure C-2). This method is similar to the current-

monitoring method. The concentration is measured with the absorbance or fluorescence

of dye instead of current. To remove back-pressure problems, double membrane-based

electroosmotic pumps are used.

A flow is driven by the first electroosmotic pump from the compartment 1 having

volume, V1, and concentration, C1, of dye to the compartment 2 having volume, V2, and

concentration, C2, of dye. At the same time, a flow is also driven by the second

electroosmotic pump from the compartment 2 to the compartment 1.

It is assumed that the pumping rates of the first and second electroosmotic pumps,

kcl~ c2 and kc2-c1, are identical (k= kcl, c2= kc2,cl) and the diffusion constant of dye, D, is

constant.

The dye flux through the membrane (electroosmotic pump), Jdye, is,

AC2 V2 (C-1)
dy At A

where At is the elapse time and A is the area of membrane.

According to the Fick's second law, the dye flux through the membrane

(electroosmotic pump), Jdye, is,

dC dC dC
Jd =-D d+C k-Dk -D C2 k =-2Dd +(C -C2)k (C-2)
dx dx dr

By combining (C-1) and (C-2), the following relationship is obtained.


116









1+C2,0 V2 V2 C2
+I
Ino I0 1 ) + k t. __ (C-3)
1
C1,

where I, t, Cl,o, C2,o are the thickness of membrane, time, initial concentration in

compartment 1, and initial concentration in compartment 2, respectively.

From the slope in a plot drawn with experimental data, the diffusion constant, D,

and the pumping rate, k, by the electroosmotic pump can be estimated (Figure C-3).


117



















Q'.


c)


Figure C-1. Various methods for measuring the flow rate: a) length-converting method,
b) mass-converting method, c) current-monitoring method, and d) particle
image velocimetry method. Adapted from [42, 74, 220, 221].






k2-C1 kC1 -C2


Figure C-2. The double membrane configuration having compartment 1 and
compartment 2 in the concentration-monitoring method.


118


,,
r.

r re
so~






















0.02


0.01 slope + vl A- +


0.00.
0 10 20 30 40 50 60
Time (min)


Figure C-3. Plot of function, f, with time. The diffusion constant, D, and the pumping
rate, k, by the electroosmotic pump can be estimated from the slope.


119









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BIOGRAPHICAL SKETCH

In-kook Jun grew up in Seoul, Republic of Korea. He received his bachelor's

degree at the Seoul National University in 2003 and his master's degree at the Seoul

National University in 2005. He started his military service in November 1998, and he

was discharged in January 2001. He joined the group of Dr. Henry Hess in the Materials

Science and Engineering Department and the group of Dr. Peng Jiang in the Chemical

Engineering department at the University of Florida, Gainesville. He graduated in the

Summer 2010 after spending four years being educated in materials science and

chemical engineering.


136





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1 BIOINSPIRED, FUNCTIONAL NANOSCALE MATERIALS By IN KOOK JUN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNI VERSITY OF FLORIDA 2010

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2 2010 I n Kook J un

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3 To my family and friends for their unwavering support

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4 ACKNOWLEDGMENTS First and foremost, I thank my mentors and guides, Drs. Henry Hess and Peng Jiang for their bril liance, humility, and patience. Directly and indirectly, they have taught me far more things than what can be articulated in this note of acknowledgement. I would like to thank my committee members Drs. David Norton, Wolfgang Sigmund, Valentin Craciun, and Yiider Tseng for their time, guidance and constructive comments. I thank all the Hess group members: Thorsten Fischer, Rob, Parag, Isaac, Ashu, and Yoli, and former members, Shruti and Krishna. I also thank all the Jiang group members: Stanley, Nick, Will iam, Erik, Tzung hua, and Hongta, and visiting professors Dr. Xuefeng Liu from Jiangnan University and Dr. Satoshi Watanabe from Kyoto University for their constructive feedback and assistance in research. I was ver y fortunate to be a member of both group s Finally, and most importantly, I express my thanks and gratitude to my parents and friends in Korea who have always supported me in spirit along this journey. My parents and friends gave me the courage to pursue my dreams, and encouraged me to be a succ essful researcher with an aggressive attitude in life and work. I am always going to be grateful for having them by my side. Right now, I am so missing my Mom in the Heaven.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF FIGURES ................................ ................................ ................................ .......... 9 LIST OF ABBREVIATIONS ................................ ................................ ........................... 12 ABSTRACT ................................ ................................ ................................ ................... 14 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 16 Bioinspired, Engineered Functional Materials ................................ ......................... 16 Scope of Thesis ................................ ................................ ................................ ...... 17 Self pumping Membranes for Synthetic Transport Systems ............................. 17 Nanoscale SERS Substrates for Effective Detecting Systems ......................... 17 Binary Colloidal Crystals for Optical Systems ................................ ................... 18 Organic inorganic Nanocomposites for Mechanical Systems ........................... 18 2 A BIOMIMETIC, SELF PUMPING MEMBRANE ................................ ..................... 21 Background ................................ ................................ ................................ ............. 21 Electroosmotic Flow ................................ ................................ ......................... 22 Applications of Electroosmotic Micropumps ................................ ..................... 24 Challenges for Electroosmotic Pumps ................................ .............................. 25 Requirements of Self pumping Membr anes ................................ ............................ 25 Electrochemical Systems for the Self pumping Membrane ................................ ..... 26 Compartment less Fuel Cell System for the Self pumping Membrane ................... 27 Results and Discussion ................................ ................................ ........................... 28 Conclusions ................................ ................................ ................................ ............ 31 Materials and Metho ds ................................ ................................ ............................ 32 Membrane Preparation ................................ ................................ ..................... 32 Chamber Design ................................ ................................ .............................. 32 Flow Measurement s ................................ ................................ ......................... 33 3 GOLD NANOPARTICLE NANOHOLE ARRAYS AS SERS SUBSTRATES ........... 41 Background ................................ ................................ ................................ ............. 41 Raman Scattering & Surface Enhanced Raman Scattering (SERS) ................ 41 Mechanisms for SERS Enhancement ................................ .............................. 42 Various Technique s for SERS Substrates ................................ ........................ 44

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6 SERS Applications ................................ ................................ ........................... 45 Challenges for SERS Substrates ................................ ................................ ..... 46 Raman Scattering Intensity Measurement / SERS EFF Calculation ................ 47 Simulation of Effects of Size and Spacing on Electromagnetic Field ................ 49 Particle Self assembly Approach ................................ ................................ ............ 50 Particle Aggregates for SERS ................................ ................................ .......... 50 Self assembled Particles as a SERS Sub strate ................................ ............... 51 STV/PEG GNP Arrays ................................ ................................ ...................... 54 Periodic Nanostructure Approach ................................ ................................ ........... 55 Periodic Nanostructures as SERS Substrates ................................ .................. 55 Periodic Nanostructures from Non close packed Particle Arrays ..................... 57 Nanohole Array s on a Glass Substrate ................................ ............................ 59 STV/PEG GNP Arrays on Nanohole Arrays ................................ ........................... 60 Concept of GNP nanohole Arrays ................................ ................................ .... 60 Fabrication of GNP nanohole Arrays ................................ ................................ 60 Optical and SERS Properties of STV/PEG GNP Arrays on Nanohole Arrays .. 61 Conclusions ................................ ................................ ................................ ............ 62 Materials and Methods ................................ ................................ ............................ 63 Materials ................................ ................................ ................................ ........... 63 Instrumentation ................................ ................................ ................................ 63 Gold Nanohole Arrays on a Glass Slide ................................ ........................... 63 STV/PEG GNP Arrays on Gold Nanohole Arrays ................................ ............ 64 Absorbance of Substrates ................................ ................................ ................ 64 Raman Spectra Measurements ................................ ................................ ........ 64 Calculation of Enhan cement Factors ................................ ................................ 65 4 BINARY COLLOIDAL CRYSTALS ................................ ................................ ......... 77 Results and Discussion ................................ ................................ ........................... 79 Conclusions ................................ ................................ ................................ ............ 82 Materials and Methods ................................ ................................ ............................ 82 Materials and Substrates ................................ ................................ .................. 82 Instrumentation ................................ ................................ ................................ 83 Non close packed Colloidal Monolayer ................................ ............................ 83 Ternary Layer Hexagonally Non close packed Colloidal S tructure .................. 84 5 BIOINSPIRED, ORGANIC INORGANIC NANOCOMPOSITES ............................. 90 Assembly of Colloidal Nanoplatelets ................................ ................................ ....... 91 Synthesized Gibbsite Nanoplatelets ................................ ................................ 91 Assembly of Gibbsite Nanoplatelets by Electrophoretic Deposition ................. 92 Mechanical Properties of Nanocomposites ................................ ...................... 92 Assembly of Surface roughened Nanoplatelets ................................ ...................... 93 Silica coated Gibbsite Nanoplatelets ................................ ................................ 93 Assembly of Silica coated Gibbsite Nanoplatelets ................................ ........... 93 Mechanical Properties of Nanocomposites ................................ ...................... 94 Conclusions ................................ ................................ ................................ ............ 94

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7 Materials and Methods ................................ ................................ ............................ 95 Materials ................................ ................................ ................................ ........... 95 Instrumentation ................................ ................................ ................................ 95 Synthesis of Gibbsite Nanoplatelets ................................ ................................ 96 Surface Modification of Gibbsite Nanoplatelets with TPM ................................ 96 Coating of Gibbsite Nanoplatelets with Silica ................................ ................... 97 Electrophoretic Deposition ................................ ................................ ................ 97 Mechanical Test ................................ ................................ ............................... 98 6 CONCLUSIONS AND OUTLOOK ................................ ................................ ........ 102 Self pumping Membrane ................................ ................................ ....................... 102 Reproducible and Highly SERS active Substrates ................................ ............... 102 Potential Applications as a Hybrid Biosensor ................................ ................. 102 Binary Colloidal Crystals ................................ ................................ ....................... 103 APPENDIX A ELECTROOSMOTIC FLOW ................................ ................................ ................. 105 Electroosmotic Flow in One wall Channe l ................................ ............................. 105 Electroosmotic Flow in Cylindrical Tube ................................ ............................... 105 B SELF PUMPING FLOW ................................ ................................ ........................ 110 Flow Rate as a Function of Tracer Velocity ................................ .......................... 110 Conductivity of Working Fluid ................................ ................................ ............... 110 Flow Rate as a Function of Current ................................ ................................ ...... 111 Flow Rate at Zero Opposing Pressure ................................ ................................ .. 113 C FLOW RATE MEASUREMENT ................................ ................................ ............ 115 Length converting Method ................................ ................................ .................... 115 Mass converting Method ................................ ................................ ....................... 115 Current monitoring Method ................................ ................................ ................... 115 Particle Image Velocimetry Method ................................ ................................ ...... 115 Concentration monitoring Method ................................ ................................ ......... 116 LIST OF REFERENCES ................................ ................................ ............................. 120 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 136

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8 LIST OF TABLES Table page 3 1 Calculated SERS enhancement factor on the fabricated substrates from measured data of Raman intensities at 999.2 cm 1 and 1023 cm 1 .................... 66

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9 LIST OF FIGURES Fig ure page 1 1 Functional nanoscale materials in nature: a) moth lotus leaf.. ................................ ................................ ................................ ........... 20 2 1 Electroosm otic flow in a channel.. ................................ ................................ ...... 34 2 2 Various types of electroosmotic pumps: a) packed column EP, b) porous monolith column EP, c) open channel EP, and d) porous membrane based EP. ................................ ................................ ................................ .................... 34 2 3 The self pumping membrane: a) gold and platinum electrodes deposited on the opposing surfaces of a track etched polycarbonate membrane. b) surface and c) cross section of the track etched membrane. ................................ .......... 35 2 4 The dimensions and photograph of the experimental set up. ............................. 36 2 5 The experimental set up. The membrane is mounted submerged in solution to create a compartment connected to the larger reservoir by the membrane and a narrow channel.. ................................ ................................ ....................... 36 2 6 The electroosmotic pumping membrane. Flow rate and electric current as a fu nction of external voltage in an aqueous solution. The inset shows a rescaled plot of the voltage range from 1 V to + 1 V.. ................................ ....... 37 2 7 Flow rate dependent on current in electroosmotic pumping. In the electroosmotic flow, the measured flow rate increases linearly with increasing measured current. ................................ ................................ .............................. 37 2 8 The self pumping membrane. Flow rate and electric current as a function of time in a 0.01% hydrogen peroxide solution as the gold and platinum electrode are externally connected and disconnected.. ................................ ...... 38 2 9 Scanning electron microscopy images of the membrane after operation sho w tracer particles accumulated on the membrane surface and within the pores, possibly leading to a reduction in pumping efficiency. ................................ ........ 38 2 10 Flow rate dependent on current in the self pumping. In the self pumping flow, the measured flow rate increases linearly with increasing measured current. .... 39 2 11 Calculation of tracer velocity at low flow rates. At a low velocity of tracers, in dividual tracer positions can be accurately determined and tracers can be followed from frame to frame. ................................ ................................ ............. 39

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10 2 12 Calculation of tracer velocity at high flow rates. At a high velocity of tracers tracers moving in the center of the channel appear as streaks while tracers adsorbed to the channel surface appear as dots.. ................................ .............. 40 3 1 Various characteristic energies: a) Rayleigh scattering, b ) Stokes Raman scattering, and c) anti Stokes Raman scattering. ................................ ............... 67 3 2 Schematic diagrams of a) a surface plasmon polariton (or propagating plasmon) on a flat surface and b) a localized surfac e plasmon on a nanostructured surface.. ................................ ................................ ..................... 67 3 3 Various SERS substrates: a) Ag film on nanospheres, periodic nanostructures by b) nanosphere lithography and c) electron beam lithography, and d) colloidal aggregates.. ................................ ........................... 68 3 4 SERS applications: a) in vivo glucose sensing equipment consisted of SERS spectroscopy, implanted substrate, beam directing optics, and collection lens and b) id entification of cancer genes by Raman labels.. ................................ .... 69 3 5 Representative Raman spectrum of benzenethiol adsorbed on a SERS substrate.. ................................ ................................ ................................ ........... 69 3 6 The SERS enhancement calculated based on finite element methods in the case of nanoparticle arrays.. ................................ ................................ .............. 70 3 7 Schematic diagram depicting self assembly of gold nanoparticles using a fl ow cell. ................................ ................................ ................................ .............. 71 3 8 SEM images of 30 nm gold nanoparticle arrays with a gap between particles on glass substrates. ................................ ................................ ............................ 72 3 9 Schematic diagram depicting the fabrication procedures for making GNP nanohole arrays. ................................ ................................ ................................ 73 3 10 SEM images of nanohole arrays: a) 330 nm nanohole arrays and b) 400 nm nanohole arrays. ................................ ................................ ................................ 74 3 11 SEM images of nanohole GNP arrays: a) 330 nm nanohole arrays covered by 30 nm gold particles and b) 400 nm nanohole arrays covered by 30 nm gold particles. ................................ ................................ ................................ ..... 74 3 12 Absorption spectra on a) gold nanoparticle (GNP) arrays and nanohole arrays and b) nanohole arrays covered by gold nanoparticles. .......................... 75 3 13 Raman spectra of benzenethiol absorbed on a) flat god surface, b) gold nanoparticle (GNP) arrays, c) 330 nm nanohole arrays covered by gold nanoparticles, and d) 400 nm nanohole arrays. ................................ .................. 76

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11 4 1 Photonic crystals: a) photon ic crystals in 1 D, 2 D, and 3 D and b) a photonic band gap compared to an electronic band gap.. ................................ ................. 85 4 2 Schematic illustration of the procedure for fabricating binary hexagonal arrays of silic a spheres by using monolayer nonclose packed colloidal crystals as substrates. ................................ ................................ ........................ 86 4 3 Monolayer of nonclose packed silica particles (300 nm) fabricated by the spin coating technique. ................................ ................................ ....................... 86 4 4 Binary hexagonal arrays of silica spheres: a, b) 300 nm particles array on 300 nm particles array (300/300), and c, d) 400 nm particles array on 300 nm particles array (400/300). ................................ ................................ .................... 87 4 5 Pressure effects on the ordering of particles in the second layer: a, b) 0.2 MPa for 2 min, and c, d) 0.33 MPa for 2 min in 300 nm particles arrays on 300 nm particles arrays (300/300). ................................ ................................ ..... 88 4 6 Cross sections of hexagonal arrays of silica spheres: a, b) binary layer of 400 nm particles array on 345 nm particles array (400/345) and c) ternary layer. ..... 89 5 1 Hard biological tissues and their microstructures: a) tooth, b) vertebral bone, c) shell, d) Enamel made of long needle like crystals with soft protein matrix, e) dentin and bone made of plate like crystals. ................................ .................. 99 5 2 A model of biocomposites: a ) a schematic diagram of staggered mineral crystals embedded in protein matrix and b ) a simplified model showing the load transfer mechanism in the mineral protein composites.. ............................ 99 5 3 Nanocomposite of colloidal nanoplatelets: a) TEM image of gibbsite nanoplatelets, b) SEM image of a free standing gibbsite ETPTA nanocomposite, and c) tensile strain stress curves. ................................ ......... 100 5 4 Nanocomposite of surface roughened nanoplatelets: a) TEM image of acid leached silica coated gibbsite nanoplatelets, b) SEM image of silica coated gibbsite PEI ETPTA nanocomposite on an IT O electrode. ............................... 101 C 1 Various methods for measuring the flow rate: a) length converting method, b) mass converting method, c) current monitoring method, and d) particle image velocimetry method .. ................................ ................................ .............. 118 C 2 The double membrane configuration having compartment 1 and compartment 2 in the concentration monitoring method. ................................ .. 118 C 3 Plot of function, f with time. The diffusion constant, D and the pumping rate, k by the electroosmotic pump can be estimated from the slope. ..................... 119

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12 LIST OF ABBREVIATION S ADP A denosine d iphosphate AFM Atomic force microscopy APTCS A cryloxypropyl tri chorosilane ATP A denosine triphosphate CTAB Cetrimonium bromide DI D eionize d DMFC D irect methanol fuel cell DNA Deoxyribonucleic acid EBL Electronic beam lithography ETPTA Ethoxylated trymethyl olpropane triacrylate FEG SEM Field emission gun scanning electron microscopy FEM Finite element method FIB Focused ion beam FITC Fluoresc ein isothiocyanate GNP Gold nanoparticle HIV Human immunodeficiency virus ICP Inductively coupled plasma IEP Isoelectric point ITO Indium tin oxide LB Langmuir Blo dgett LBL Layer by layer LSPR Localized surface plasmon resonance MFON Metal film on nanosphe re NIR Near infrared

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13 NSL Nanosphere l ithography ORC Oxidation reduction cycle PBG P hotonic band gap PC P hotonic crystal PDMS Poly ( dimethylsiloxane ) PEG P oly ( ethylene glycol ) PEI Poly ( ethylenimine ) PEMFC Proton exchange membrane fuel cell PML Perfect matche d layer PMMA Poly ( methyl methacrylate) PNIPAM Poly(N isopropylacrylamide) PVP Polyvinylpyrrolidone RIE Reactive ion etching SEM Scanning electron microscopy SERS S u rface enh anc ed Raman scattering SERS EF Surface enhanced Raman scattering enhancement factor STV Streptavidin TEM Tranmission electron microscopy TEOS Tetraethyl orthosilicate TPM Trimethoxysilyl pro pyl methacrylate UV Ultra violet

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14 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Parti al Fulfillment of the Requirements for the Degree of Doctor of Philosophy BIOINSPIRED, FUNCTIONAL NANOSCALE MATERIALS By In kook Jun August 2010 Chair: Henry Hess Cochair: Peng Jiang Major: Materials Science and Engineering Functional nanomaterials in nature exhibit many unique functions and optical and mechanical superhydrophob ic self cleaning of a lotus leaf. This dissertation is devoted to creating unique and enhanced properties by mimicking such functional materials. We have developed a novel self pumping membrane which does not require an applied volt age. The self pumping membrane harvests chemical energy from a surrounding fluid and uses it for accelerated mass transport across the membrane. A device such as this has promising applications in implantable or remotely operating autonomous devices and me mbrane based purification systems. Reproducible and highly active surface enhanced Raman scattering (SERS) substrates were developed using a bottom up self assembly technology. With their high sensitivity and good reproducibility, the developed nanostructu res (gold nanoparticle and nanohole arrays) as SERS substrates are very promising for applications such as

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15 ultra sensitive detectors for chemicals and reproducible sensors for chemical and biological molecules. Binary colloidal crystals were created using a simple fast, and scalable spin coating technology Although further investigation of the procedure is needed to improve the ordering of particles in the individual layers, the developed assembly technology has a promising outlook in applications such as optical integrated circuits and high speed optical computing Inorganic organic nanocomposites were realized by assembling synthesized gibbsite nanoplatelets using the electrophoretic deposition and infiltration of a monomer followed by polymerization Via surface modifications of gibbsite nanoplatelets, nanocomposites were further reinforced with covalent linkages between the inorganic platelets and organic matrix.

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16 CHAPTER 1 INTRODUCTION In nature, functional nanoscale materials exhibit un ique optical, mechanical and functional properties, usually obtained from hierarchical structures. [1] For example, a of nanoscale nipples are located. [2] The hierarchic al nanoscale nipple arrays generate the enhanced light sensitivity of light craving moths. The gecko has an exceptional ability to climb rapidly up smooth vertical surfaces due to the hierarchical toe structure consisting of a myriad of nanoscale spatulae. [3] The nanoscale hair like structures on micro scale mound like structures protruding from its leaf are responsible for the superhydrophob icity in a Lotus leaf. [4] Bioinspired, Engineered Functional M aterials s are most efficient. Researchers have tried to mimic butterfly, the high strength and toughness of nacre, and the superhydrophobic self cleaning of a lotus leaf. [3 6] Biomolecular motors such as kinesin and myosin are capable of transporting analytes in a biosensor. [7, 8] They gain mechanical energy from hydrolyzing ATP to ADP and inorganic phosphate at high efficiency. Biomolecular motors could potentially replace a pump and a power supply in a smart dust b iosensor; they may be integrated with the housing and communication unit. Biofuel cells, which generate electricity using glucose as fuel from the fluid, have been developed. [9] They gain electrical energy from oxidizing fuel and reducing oxygen. Highly active catalysts for the electrochemical

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17 reactions and electron transfer systems have been investigated to improve the performance. Hydrogen peroxide, hydrazine, ethanol, and methanol have been explored as alternative fuels for the electrochemical r eactions in fuel cell systems to enhance their stability, durability, and power density. Artificial antireflection coatings are used in displays, optical components, and solar cells; however, current antireflection technologies such as quarter wavelength m ultilayer films and nanoporous coatings are expensive and suboptimal. By mimicking the moth eyes, non close packed nipples in the sub 300 nm size range, effective antireflection coatings have been recently developed with a bottom up self assembly technique [10 12] S cope of T hesis Self pumping Membranes for S ynthetic T ransport S ystem s Hybrid biosensors face the fundamental challenge of limited stability due to the ir biological co mponents and the difficulty of obtaining macroscopic observable signals. Conventional transporting systems such as electroosmotic pumps are robust and stable; however, they still require a highly miniaturized pump and power supply. It has been shown that s ome electrochemical reactions can induce mass transport by converting the chemical energy gained from the surrounding fluid into the mechanical energy with no need of an externally applied voltage. In this work, we design such a synthetic transport system and investigate the effectiveness and efficiency in mass transport. Nanoscale SERS Substrates for E ffective D etecting S ystems One of the challenges in biosensors is achieving high sensitivity. Surface enhanced Raman scattering (SERS) can be utilized to am plify the signal. It is well

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18 known that Raman signals dramatically increase when metallic nanostructures with wavelength scale topography are used as SERS substrate. The maximum SERS enhancement has been reported up to 10 14 in nanoparticle aggregates. [13] Moreover, Raman spectroscopy is a label free technique, so a tagging procedure is not necessary. In this work, we fabricate several nanostructures with a bottom up self assembly technique and investigate their capabilities as SERS substrates. Binary Colloidal Crystals for Optical S ystems T he development of integrated optical circuits with photonic crystals ha s been greatly impeded by its reliance on expensive and complex nanofabrication techniques such as el ectron beam lithography (EBL) and focused ion beam (FIB) B ottom up colloidal self assembly and subsequent templating nanofabrication c an provide a much simpler, faster, and inexpensive alternative However, current colloidal self assemblies are limited to a low volume, laboratory scale production. In this work, we investigate a bottom up self assembly approach via a spin coating technolo gy to create non close packed binary colloidal crystals. Organic inorganic Nanocomposites for Mechanical Systems The nacreous layer of mollusk shells has an intricate brick and mortar nanostructure which makes the shells exceptionally tough and stiff. Vari ous bottom up self assembly techniques such as layer by layer (LBL), ice templated crystallization, spin coating, gravitational sedimentation, and centrifugation, have been explored to mimic the nacre structure. In this work, we assemble gibbsite nanoplate lets, having high aspect ratio (diameter thickness ratio) via a simple, inexpensive, and scalable elelctrophoretic deposition We generate organic inorganic nanocomposites by

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19 infiltrating monomer between platelet layers and polymerizing monomer, and investigate their mechanical properties.

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20 Figure 1 1 Functional nanoscale materials in n ature : a) moth eye, b) gecko foot, c) lotus leaf. Adapted from [2, 4, 14 16]

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21 CHAPTER 2 A BIOMIME TI C, SELF PUMPING MEMBRANE Biological membranes accelerate materials exchange across the membrane by active, ATP dependent transport through specialized channel proteins. Similarly, the in synthetic membrane is highly desirable from an engineering point of view, since it would obviate the need for external devices, such as pumps or centrifuges, to drive flow acr oss the membrane. Here, a novel pumping membrane with no need of applied voltage is described. Instead of the externally applied voltage, e lectrodes deposited on the opposing surface of a membrane generate a transmembrane potential from the electrochemical reactions (the hydrolysis of hydrogen peroxide in aqueous solution ) Short circuiting the electrodes permits an ionic current to flow between the electrodes, which in turn creates a flow of about 100 nL cm 2 min 1 Future applications of such self pumpin g membranes may include implantable or remotely operating autonomous devices and membrane based purification systems. B ackground Microfludic devices have advantages for mixing separating, and sensing reagents due to their laminar flows and small diffusio n distances compared to macro scale devices. [17] Microfluidic devices require small volumes of samples (from 100 nL to 10 are disposable due to low production costs, and ar e amenable to high throughput due to processing assays in parallel. Micropumps as fluidic transport system are essential in operating microfluidic devices.

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22 Various Types of M icropumps Micropumps can be divided into two categories according to their opera ting mechanisms: (1) displacement pumps, which exert forces on the working fluid through moving surfaces and (2) dynamic pumps, which add energy to the working fluid by increasing momentum or pressure directly. [18] The displacement pumps usually rely on moving parts such as check valv es, oscillating membranes, or turbines for the constant delivery of a fluid. Diaphragm pumps, which make up the majority of reported displacement micropumps, are actuated by piezoelectric, thermopneumatic, electrostatic, and electromagnetic mechanisms. [19 22] On the other hand, dynamic pumps generate the kinetic energy of the fluid by electroosmosis, electrowetting, thermocapillary, and electrochemical, electrohydrodynamic, and m agnetohydrodynamic mechanisms without moving parts. [23 28] Dynamic pumps are advantageous in micrometer scale devices where the high surface to volume ratio is favorable. E lectr oosmotic F low Electroosmotic micropumps are of significant promise for a wide variety of applications [17, 29, 30] Briefly, surface charges within a channel attract counter ions which experience a force directed along the channel axis when an electric field is applied across the channel (Figure 2 1) The viscous drag between the counter ions and fluid in turn exerts a force on the fluid that is localized at the channel wall, indu cing a plug like flow profile [31] In electrophoresis the soli ds (the particles) are moving in the applied electrical field Moving particles can drag fluid (water) molecules, generating an electrophoretic fluid flow through channels. On the other hand, i n electroosmosis, the solid (the channel wall) is stationary, w hile the fluid is moving in the applied electrical field. When the solid

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23 contacts the liquid, the ions having charge opposite to that of the solid are attracted to the solid forming an electric double layer [30, 32, 33] Electroosmotic flow is induced by an applied electric field to the electric double layer. V arious T ypes of E lectroosmotic M icropumps Based on the pumping elements, there are various types of electroosmotic mi c ropu mps such as packed column, porous monolith pumps, open channel pumps, and membrane based pumps. [29, 34 44] Packed column electroosmotic pumps can produce high pressures (Figure 2 2a). [29, 34, 35] The interstitial spaces between the packed particles are parallel passages for fluid flows. The pumping power closely depends on the size of the packed partic les. In the pumps with large particles, high flow rates can be achieved, while pumping pressures decrease. Physical geometries of the channel such as tortuosity, porosity, and effective pore size and properties of the working fluid are controlled to optimi ze the performance of electroosmotic pumps. A porous monolith column is prepared by polymerizing the column of a monomer a crosslinker, a free radical initiator, and a porogenic solvent (Figure 2 2b). [36, 37] The materials can be an organic polymer or an inorganic silica monolith. The advantage of a porous monolith column is the elimination of frits or filters which are used in packed column electroosmotic pumps. The pore size a nd porosity as well as chemistry and crosslinking density can be easily controlled to achieve the desired flow rate and pressure. Channel electroosmotic pumps use narrow capillaries or microchannels as pumping elements (Figure 2 2c). [38 40] By consisting of hundreds of parallel and small microchannels, both high pumping rates and high pressures can be achieved. Channel

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24 electroosmotic pumps can be easily integrated on a microfluid ic platform by standard microfabrication techniques and are robust and reliable due to the simplicity of design. The pump rate is controlled by the electric field and the size and number of channels. Porous membrane pumps have many channels through the thi n membrane (Figure 2 2d). [41 44] Since the channels are short, a high electric field is achieved with a low applied voltage. Due to the large number of microchannels, it is poss ible to generate high pressures at high flow rates. Supporting frames are often used to secure the robustness of the membrane and to apply voltage as electrodes. Various membranes such as glass, alumina, and polymer are reported to show excellent pumping p erformances. Alumina membranes with highly aligned nanochannels are reported to show high flow velocities at low voltage. Applications of Electroosmotic Micropumps Electroosmotic pumps can transport liquid samples with significant flow rates and pressures. Without moving parts, electroosmotic pumps generate plug like and laminar fluid flows, which can be controlled by applied voltages. The electroosmotic pumps are favorable in microfluidic devices where the surface to volume ratios are high. [45] Due to the unique characteristics of electroosmotic micropumps, many potential applications have been suggested. Electroos motic micropumps have great potential in liquid drug delivery and biological sample assays. [30] Electroosmotic micropumps have been integrated into proton exchange membrane fuel cells (PEMFCs) to remove water from cathodes. [46] Electroosmotic pumps have also been used as fuel delivery systems in direct methanol fuel cells (DMFCs). [47] Compact micropumps having high heat dissipation rates are essential in microelectronics as cooling systems. Electroosmotic mic ropumps made of glass frits have been reported as effective cooling systems. [48]

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25 Challenges for Electroosmotic Pumps Although electroosmotic pumps have great potentials as trans port systems, there are still challenges to overcome for real applications. Nonpolar liquids such as oil are difficult to transport by the electroosmotic pump and the pumping function degrades over time due to the interaction with working fluids. The bubbl es generated on the electrodes can interfere with the pumping function, especially in closed loop fluidic devices. The efficiency of the pump should also be improved. Requirements of Self pumping Membranes To construct a self pumping membrane which generat es a fluid flow by harvesting chemical energy from a fluid and converting the chemical energy into the kinetic energy, there are several requirements to be satisfied. First, the electrochemical reactions on the electrodes deposited on the opposite surface s of a membrane should generate a transmembrane potential. From this point of view, battery systems such as primary cells (e.g. Daniell cell) and secondary cells (e.g. lead acid cell) and fuel cell systems can be candidates. Second, the electrochemical reac tions should generate new ions or protons as products. For example, the protons generated on the anode will drag the fluid flow in the electric field. On the other hand, the transition from Fe 3 + to Fe 2 + ions or protons for dragging the fluid flow. Third, the electrodes should not be changed by the reduction oxidation reactions. In battery systems, the anode themselves dissolve into the solution, while metal ions reduce on the cathode. In this view point, fuel cell systems are preferable since electrodes in fuel cell systems can continue working by being fed with fuels without a change of electrode.

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26 Fourth, the fluid flow should not interfere with the electrochemical reactions. The reactants for the electrochemical reaction (oxidation) on the anode and their products should not interfere with the electrochemical reaction (reduction) on the cathode and candidates for the self pumping membrane. E lectrochemical S y stems for t he S elf pumping M embrane Electrochemical systems utilized in the research on chemotaxis or catalytic nanomotors can offer promising candidate systems for the self pumping membrane satisfying the above requirements. Metal rods or surfaces (Pt, Au /Ni, catalyst, Pt/Au, Ag) in the hydrogen peroxide solution show autonomous movement, rotational motion, and transportation of cargos by ejecting small oxygen bubbles or generating an interfacial tension force or a difference in diffusion coefficient due t o the catalytic decomposition of hydrogen peroxide. [49 53] However, it was proved later that the autonomous movements of Pt/Au nanorods were due to the self electrophoresis. Pro tons generated by the electrochemical reaction of hydrogen peroxide flow in the catalytically induced electric field inducing autonomous movements of nanorods. [54, 55] In furthe r research, Pt/Au or Au/Ag rods in hydrogen peroxide solution have show n an autonomous movement and the speed and direction of movement can be controlled. [56, 57] A convective f luid flow was shown on the Ag/Au patterned surface in hydrogen peroxide solution. [58] Other electrochemical systems have also been investigated for autonomous movement. Glucose oxidase/bilirubin oxidase fibers were propelled in the interface between electrolyte and air due to the electrochemical reactions of glucose and

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27 oxygen. [59] A Pd/Au patterned surface in hydra zine solution showed electroosmotic flow in a catalytically induced electric field. [60] C ompartment less F uel C ell S ystem for the S elf pumping M embrane The electrochemical systems developed in the investigations of chemotaxis and catalytic nanomotors have been applied to compartment less fuel cell systems. In compartment less fuel cells, a differential in the ability of the two electrodes to catalyze the anodic and cathodic reaction enables the creation of an electric potential and removes the need for an ion exchange membrane. A compartment less hydrogen peroxide fuel cell with Au and Ag as catalytic electrodes was developed generating the maximum current density of 2.9 mA cm 2 [61] In this fuel system, the bubble generation on the anode interfered with further electrochemical reactions. While Ni electrodes generated less bubbles, they were also less active for the electrochemical reaction of hydrogen peroxide. Compartment less glucose/oxygen fuel cells have been developed. [9, 62 65] A biofuel cell having an anode functionalized with surface reconstituted glucose oxidase and a cathode modified with cyt ochrome c and cytochrome oxidase generated a maximum power of 4 [62] The current density generated in biofuel cells having an anode immobilized with glucose oxidase and a cathode immobilized with a bilirubin oxidase reached up to 10 mA cm 1 [9] The maximum power density from the biofuel cell having an anode functionalized with glucose dehydrogenase complex and a Pt cathode was 930 nW cm 2 [64] Compartme nt less fuel cells using methanol or ethanol as fuels were also reported. [66, 67] Fuel cells with a nickel hydroxide anode and a silver oxide cathode using a fuel mixture of met hanol and hydrogen peroxide achieved the maximum power

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28 density of 28. cm 2 [66] The maximum power density from fuel cells having an anode of alcoholdehydrongenase associated with carbon nanotubes and a cathode of bilirubin Pt nanoparticle co mposite was recorded at cm 2 [67] Although t here are membrane less fuel cells, these fuel cells separate fuels by laminar flow instead of ion system for self pumping membrane. [68, 69] While glucose is the fuel of choice for most studies, and significant advances have been achieved in the design of enzyme based biofuel cells [70] in this work, hydrogen peroxide is adopted as the fuel to simplify the electrode design and to utilize a combination of platinum and gold electrodes known as effective catalysts for the electrochemical decomposition of hydrogen peroxide to dr ive electroosmotic flows. [56] R esults and D iscussion To construct the self pumping membrane, platinum and gold films of 25 nm thickness (and a 5 nm titanium adhesion layer) are sputter deposited on the opposing surfaces of track etched polycarbonate membranes with a pore diamete thicknes % (Fig ure 2 3, 2 4 ). The platinum electrode and the gold electrode are electrically connected by an external switch. In the proposed self pumping membrane, o n the platinum electrode, hydrogen peroxide decomposes into oxygen, protons, and electrons. The generated electrons flow along the electrical connection, while the generated protons flow through the pores of the membrane under the catalytically induced electric field. Th e se electrons and protons ar e consumed on the gold electrode by combining with hydrogen peroxide to produce water.

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29 The membrane is then integrated into a fluid chamber designed to facilitate the measurement of nL s 1 pumping speeds at near zero backing pressure (Fig ure 2 5 ). Flow rat es are measured by microparticle image velocimetry [71] using fluorescent microspheres ) as tracers. To amplify the flow velocity, the flow is The proper functioning of the experimental setup was validated by providing an external voltage to the memb rane submersed in a solution of water and tracer microspheres. The dependence of electric current and particle velocity as a function of external voltage (Fig ure 2 6 ) followed the behavior expected for the hydrolysis of water, which has a decomposition pot ential difference of 1.23 V [72] Positive numbers in flow rate and current correspond to flow/current from the gold electrode to the platinum electrode, while negative numbe rs signify the opposite direction. The flow rate is calculated from the measured velocities of tracers based on the relationship between maximum velocity and the flo w rate in a rectangular channel. [73] The flow rate increases linearly with increasing voltage across the membrane up to about 1.2 V. At an applied voltage of 1V the flow rate is 0.94 nL s 1 and the current is 1.9 A. For applied voltages above 1.4 V, flow rate and cu rrent increase linearly with increasing voltage wi th slope s of 43 nL s 1 V 1 and 120 V 1 respectively. This implies a conductivity of the working fluid (water and tracer particles) of 2.0 cm 1 which is close to the conductivity of water. Using th e Helmholtz Smoluchowski equation to calculate the electroosmotic flow through the membrane while assuming the zeta potential of polycarbonate [71] to be 27 mV and considering the pressure induced reverse flow caused by the resistance of the

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30 small outlet channel, the flow rate as a function of current can be c alculated. While the calculation shows the observed linear dependence of flow rate on current with a slope of 3 nL A 1 s 1 it also shows that the high resistance of the small detection channel relative to the membrane resistance reduces the net flow thro ugh the membrane about 30 fold relative to the expected electroosmotic flow at zero pressure. In the electroosmotic pump, the observed pumping efficiency varies from 0.4 to 0.5 nL A 1 s 1 (Fig ure 2 7) which is lower than the calculated 3 nL A 1 s 1 Pum ping in the absence of an external voltage is activated by the addition of hydrogen peroxide to the aqueous solution at a concentration of 0.01 wt%. At this low concentration of hydrogen peroxide the formation of gas bubbles at the electrodes is avoided. The platinum and gold electrodes are connected to a switch and an amperemeter. Fluid flow is depende nt on the state of the switch: w hen the switch is closed, flow across the membrane commences f rom platinum to gold (Figure 2 8); w hen the switch is open, th e flow rate is near zero, initially with a small flow resulting from small initial pressure differences. In less than 30 s, the flow rate reaches 0.9 nL s 1 and subseq % after 270 min. This reduction is likely to be the result of a falling hydrogen peroxide concentration due to its consumption at the electrodes or the result of clogging of the pores wi th tracer particles (Figure 2 9 ) The observed flow direction ( from platinum to gold) is consistent with the proposed pumping mechanism. The observed pumping efficiency of 3 nL A 1 s 1 matches the above calculated electroosmotic pumping efficiency (Figure 2 10 ) This agreement

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31 supports the hypothesis that the pumping results from electroosmosis and not from the formation of gas bubbles at the electrodes. The observed current de nsity on the order of 3 mA m 2 at a hydrogen peroxide concentration of 0.01 wt% is in good agreement with 2 at a concentration of 0.006 wt% [56] Using our model of the system, we can estimate a flow rate at zero opposing pressure of 25 nL s 1 and a stall pressure of 1 Pa in the self pumping membrane; both parameters are typical for microfluidic pumps [72] Future improvements to such self pumping membranes can f ocus on the performance of the fuel cell component, the pump component, or their interaction. The fuel cell can be improved by the use of better electrodes and alternative fuels, in particular glucose. Compartment less biofuel cells have reached current de nsities on the order of 10 A m 2 [9, 73] which means that an improvement of four orders of magnitude is possible. The electroosmotic pump can be optimized by increasing the zeta potential, and tuning the hydrodynamic resistance of the pump and outlet, e.g. by adjusting the pore diameter. Miao et al. described an electroosmotic pump which creates a more than ten fold higher flow rate for a given amount of current and membrane area [43] Finally, the current voltage characteristics of the fuel cell power source and the electroosmotic pump should be matched to maximize power conversion efficiency. In the present design the voltage drop across the electroosmotic pump is on the order of 2 mV Thus, the pump uses only a small fraction of the electromotive fo rce provided by the fuel cell. C onclusions Despite its performance limitations, the self pumping membrane described here is a first step towards replicating the ability of biological membranes to harvest chemical

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32 energy from the surrounding fluid and use it for accele rated mass transport across the membrane. Micro and macroscopic devices for drug delivery, sensing and purification, as well as oil recovery and removal may benefit from this technology. M aterials and M ethods M embrane Preparation A titanium adhesion layer (5 nm) and platinum and gold films (25 nm) were deposited on the opposing surfaces of a polycarbonate membrane (pore diameter of 0.96 images, Isopore membrane filter, Millipore, Ire land) with a multi target sputtering system (Kurt J. Lesker CMS 18, Clairton, PA) The membrane was glued to a polycarbonate electrodes were connected to metal wires usin g silver paste (Leitsilber 200 Silver Paint, Ted Pella Inc., Redding, CA). The resistance between the electrodes after manufacturing was measured to be 0. C hamber Design A chambers was patterned with Polydimethylsiloxane (PDMS, Sylgard 184 silicone elastomer kit, Dow Corning Corporation, MI) using a mold. The prepared chamber channel layer and the membrane layer were assembled on glass to form the experimental set up as shown in Fig ure 2 4 Billerica, MA) with yellow green fluorescent microspheres (1 m diameter 0.002 % solid loading, FluoSphere amine modified microsphere, Molecular Probes, Eugene, OR). An external voltage is applied between the electrodes with a DC regulated power supply

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33 (EXTECH instruments, Waltham, MA ). For the self pumping membrane 0.01 wt% H 2 O 2 solution (Aldrich, St. Louis, MO) is added. The platinum and gold electrodes are electrically connected by clamping the metal wires connecting platinum and gold electrodes together. F low M easurements The narrow channel between the two chamb ers was imaged with an Eclipse TE200U epi fluorescence microscope (Nikon, Melville, NY) equipped with an X cite 120 lamp (EXFO, Ontario, Canada), a 10X objective, a cooled CCD camera (Andor iXon, Andor Technology, Windsor, CT), and a FITC filter cube (no. 48001, Chroma Technology Corp, Rockingham, VT). The focal plane was set to the center of the channel where moving tracers were clearly visible. At low flow rates, images were acquired at 100 ms intervals with 20 ms exposure times (Figure 2 11 ) Flow rate s were calculated by measuring the particle displacement after 1 s. At high flow rates, images were acquired at 1 s intervals with 100 ms exposure times (Figure 2 12 ) Flow rates were calculated by measuring the length of the streak generated by the movi ng particle and dividing by the exposure time. The current was measured with an am pere meter (Digital Multimeter 34410A, Agilent Technologies, S anta Clara, CA ).

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34 Figure 2 1. Electroosmotic flow in a channel. Adapted from [74] Figure 2 2. Various type s of electroosmotic pump s: a) packed column EP, b) porous monoli th column EP, c) open channel EP, and d) porous membrane based EP. Adapted from [32, 36, 39, 43]

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35 Figure 2 3 The self pumping membrane : a) g old a nd platinum electrodes deposited on the opposing surfaces of a track etched polycarbonate membrane b) s urface and c) cross section of the track etched membrane imaged by scanning electron microscopy enable measurements of pore diameter, porosity and thick ness of the membrane.

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36 Figure 2 4. The dimension s and photograph of the experimental set up. Figure 2 5. The experimental set up. The membrane is mounted submerged in solution to create a compart ment connected to the larger reservoir by the membrane and a narrow channel. Flow through the membrane forces fluid through the narrow channel where the flow velocity can be measured by particle tracking microscopy.

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37 Figure 2 6 The electroosmotic pumping membrane. Flow rate and electric current as a function of external voltage in an aqueous solution. The inset shows a rescaled plot of the voltage range from 1 V to + 1 V. Error bars show the standard deviation of tracer parti cle velocities. Figure 2 7. Flow rate dependent on current in electroosmotic pumping. In the electroosmotic flow, the measured flow rate increases linearly with increasing measured current.

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38 Figure 2 8. The self pumping me mbrane. Flow rate and electric current as a function of time in a 0.01% hydrogen peroxide solution as the gold and platinum electrode are externally connected and disconnected. Error bars show the standard deviation of tracer particle velocities. Figure 2 9 Scanning electron microscopy images of the membrane after operation show tracer particles accumulated on the membrane surface and within the pores, possibly leading to a red uction in pumping efficiency : a) Au surface of me mbrane and b) cross section of membrane

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39 Figure 2 10 Flow rate dependent on current in the self pumping. In the self pumping flow, the measured flow rate increases linearly with increasing measured current. Figure 2 1 1 Calculation of tracer velocity at low flow rates. At a l ow velocity of tracers, individual tracer positions can be accurately determined and tracers can be followed from frame to frame (time between frames: 100 ms; exposure time: 20 ms). Each velocity data point is the average of the velocity of 10 tracer particles, which is determined by dividing the distance advanced after 10 frames by 1 s.

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40 Figure 2 1 2 Calculation of tracer velocity at high flow rates. At a high veloc ity of tracers tracers moving in the center of the channel appear as streaks while tracers adsorbed to the channel surface appear as dots. The tracer velocity is calculated by dividing the streak length by the exposure time (100 ms). Each velocity data po int is the average of 10 tracer particle velocities.

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41 CHAPTER 3 GOLD NANOPARTICLE NANOHOLE ARRAYS AS S ERS SUBSTRATES Raman spectroscopy is a noninvasive technology that enables label free detection of mole cules. However, the Raman signal is very weak due to the small inelastic Raman scattering cross section. Surface enhanced Raman scattering (SERS) can greatly increase the Raman signal by electromagnetic enhancement and chemical enhancement. The SERS enhanc ement factor was reported to be 10 14 with silver or gold nanoparticle aggregates as SERS substrates. [13] In this range of SERS enhancement, even single molecules can be detected. The SERS substrates should have high enhancements of Raman signal and show reproducible enhancements for sensing or detecting applications. Particle aggregates or fractals show high SERS enhancements, but the reproducibility is poor due to their irregular structures. On the other hand, periodic nanostructures fabricated by lithographical techniques exhibit better reproducibility, but their SERS enhancements reporte d in literature are lower than that of particle aggregates. B ackground R aman S cattering & S urface E nhanced R aman S cattering ( SERS ) Raman spectroscopy is a critical technique for structural analysis of molecules which relies on inelastic scattering of visib le light Raman scattering is attributed to the excitation and relaxation of vibrational modes of a molecule (Figure 3 1) Because different functional groups have different characteristic vibrational energies, the molecular structure s of every molecule ca n be probed by the inelastic Raman scattering. However, since Raman scattering cross sections are typically 14 orders of magnitude smaller than those of fluorescence the Raman signal is several orders of

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42 magnitude weaker than fluorescence emission Thus, the applicability of Raman scattering is restricted to structural analysis [75] However, a dramatically enhanced Raman signal has been obtained with a technique called surface enhanced Raman scattering (SERS). When the scatterer is placed on or near roughened noble metal substrates, the magnitude of the Raman scattering signal can be greatly enhanced. This SERS enhancement of the signal transforms Raman spectroscopy from a structural analytical tool to a structural probe with single molecule sensitivity. [76] M echanisms for SERS Enhancement The mechanism of SERS enhancement remains an active research topic T here are t wo mechanisms which contribut e to the SERS effect: an electromagnetic enhancement and a chemical enhancement. In the chemical enhancement, new electronic states are created from chemisorption between the metal and adsorba te molecules. They serve as resonant intermediate states in Raman scattering. Charge transfer excitations can occur at about half the energy of the intrinsic intramolecular excitations of the adsorbate. The existence of charge transfer state increases the probability of a Raman transition by providing a pathway for resonant excitation. This mechanism is site specific as well as and analyte dependent and contribute s an e nhancement factor of about 100 [77] The electromagnetic enhancement arises from focusing an electromagnetic field via plasmon resonance of the metallic substrate on the metal surface. Surface plasmon polaritons propagate along the m etallic surface and are trapped on the surface because of the resonant interaction between the surface charge oscillation and the electromagnetic field of the light (Figure 3 2a). [78] When ligh t interacts with

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43 nanostructures much smaller than the incident wavelength, surface plasmon polaritons are localized (Figure 3 2b). W hen the localized surface plasmon resonance (LSPR) of nanostructures on a sliver or gold substrate is excited by visible lig ht strong electromagnetic fields are generated due to selective absorption and scattering of the resonant electromagnetic radiation When the analyte molecule is subjected to these intensified electromagnetic fields, the intensity of the inelastic Raman s cattering increases Electromagnetic enhancement contributes an average enhancement factor of over 10,000 [75] The resonant frequency of the conduction electrons in a metallic nanostructure depends on the size, shape, and material of the structure. In the case of a spher ical nanoparticle of radius a that is irradiated by z polarized light of wavelength (where a is much smaller tha n ), by solving M a static approximation, the resulting solution for the electromagnetic field outside the part icle is given by (3 1) where in is the dielectric constant of the metal nanoparticle, and out is the dielectric constant of the external environment. When the dielectric constant of the metal is roughly equa l to 2 out the electromagnetic field is enhanced relative to the incident field. In the case of silver and gold, this condition is met in the visible region of the spectrum [79 ] The enhancement factor for SERS is calculated as

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44 (3 2) The Raman enhancement effect is a result of enhancing both the incident excitation, E out E out 0 ), electromagnetic fields. The overall enhancement scales roughly as E 4 A small increase in the local field produces large enhancements in the Raman scattering. The enhancement factor from experimental measurements is given by the right hand side of equation It is the SERS enhanced Raman intensity, I SERS v ), normalized by the number of molecules bound to the enhancing metallic substrate, N surf divided by the normal Raman intensity, I NRS 0 ), normalized by the number of molecules in the excitati on volume, N vol Various Techniques for SERS S ubstrate s Various SERS substrates have been developed: electrodes roughened by an oxidation reduction cycle (ORC), island films, colloidal nanoparticles, and surface confined nanostructures. ORC roughened elect rodes provide reproducible, in situ SERS substrates with moderate (~ 10 6 ) enhancement factors [75] Metal island film substrates are easy to fabricate and the LSPR wavelength can be factors ach ieved with these films are generally smaller (~ 10 4 10 5 ) than those observed with other SERS substrates. Surface confined nanostructures can be produced by several fabrication schem e s, including electron beam lithography, colloid immob ilization, and soft lithography (Figure 3 3) With substrates fabricated via electron

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45 beam lithography, enhancement factors as large as 10 8 have been achieved by controlling the inter particle spacing [80] Colloidal nanoparticle substrates are well suited to solution phase SERS stu dies. The Kneipp research group probed small (100 150 nm) silver colloid aggregates dosed with crystal violet molecules. The large (10 14 ) single molecule enha n ceme n t was attributed to large electromagnetic fields generated by fractal pattern clusters of silver colloid nanoparticles [13] In colloidal aggregates substrates, there are a sma ll number of hot spot s which occur at the junction between nanoparticles. Theoretical modeling shows the strong electromagnetic field between nanoparticles separated by < 1 nm is due to the surface junction excitation a n d the efficient interaction of the molecular wave function with the wave function of the excited metal surface [81] SERS A pplications SERS holds great potential as an ultra sensitive and selective tool for the identification of biological or chemical agents. The narrow and well resolved bands allow simultaneous detection of multiple analytes As wate r has a very weak Raman signal, investigation of biological samples can also be carried out SERS offers a method for multicomponent or multiplexed detection of low concentration ana l y te s, either by directly revealing the target analyte or by indirectly de tecting the fingerprint of a molecular label. SERS has been applied to the signal transduction mechanism in a prototype for an implantable glucose sensor (Figure 3 4) Glucose was detected and quantified in the physiological range with an accuracy approach ing the requirements as a biomedical device. SERS has also been applied in the detection of trace levels of chemical warfare agents. Silver nanowires have been used as a substrate to detect 2,4 dinitrotoluene, the

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46 most common chemical indicator of buried l andmines and explosives, at a sensitivity of 0.7 pg [82, 83] SERS active molecules have been implemented as labels on the analyte of interest. Raman labels have been used to ide ntify cancer genes, thus avoiding the introduction of undesirable radioactive DNA labels [84] ssDNA coupled with a gold nanoparticle and a SERS label has been used to detect DNA hybridization events. Also multiplexed detection has been used to distinguish he patitis A, hepatitis B, HIV, Ebola, smallpox, and anthrax with a detection limit of 20 fM [85] C hallenges for SERS Substrates The Raman spectroscopy using SERS provides much more information about molecular structure and the local environment in condensed phases than electronic spectroscopy technique s like fluorescence. Minor changes in the orientation of an adsorbate can be d etected as slight variations yields measurable shifts in the locations of Raman spectral peaks. The abrupt decay of t he electromagnetic fields ensures that only adsorbate molecules on or near noble metal substrate are probed. This technique is well suited for analyses performed on molecules in aqueous environment s because water has an extremely weak Raman signal in tensit y. [75] Ho wever, t he inherent limitation of the technique is that the substrate s should be made of silver, gold, or copper. Other materials are not usable unless they are applied as thin coatings on SERS active materials. SERS has limited applicability when the mole cule of interest is not adsorbed directly onto the substrate The primary bottleneck has been the reproducible preparation of well defined, reliable, and stable substrates with a high SERS activity [86] C olloid al substrate s tend to aggregate and thus the molecular surface coverage changes with time Variation in

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47 the fabrication processes used to produce SERS active substrates leads to inconsistent optical properties and discrepa nt enhancement factors. In the currently used nanofabrication techniques, the SERS e nhancement factors can fluctuate by up to an order of magnitude for substrates fabricated with s eemingly identical methodology. Regularly arranged monodi s persed colloidal g old and silver particles on functionalized metal or glass substrates or well ordered nanostructured surfaces produce SERS with good reproducibility and stability [87] As the SERS intensity depends on the excitation of the localized surface plasmon resonance ( LSPR ) it is i mportant to control all of the factors influencing the LSPR namely the size, shape, particle interspacing, and the dielectric environment to maximize signal and ensure reproducibility. Reproducibility can be improve d by creating a long range pattern with sub micrometer periodicity on the substrate. N anosphere lithography (NSL) has excellent control over nanometer scale features by utilizing self assembled polymer nanospheres as vapor deposition masks [88] Electron beam lithography (EBL) on a scanning electron microscope has also been used to produce regular elongated particle arrays for SERS with optimization control based on center to center spacing [89] However, these nanofabrication techniques have relatively lower enhancements than colloidal aggregates. These techniques are also limited by high er cost of fabrication, the availability of equipment, the substantial expertise requ ired and the low surface area of the metal structure. Raman Scattering Intensity Measurement / SERS EFF C alculation To evaluate the performance of prepared metal nanoparticles array as a SERS substrate, benzenethiol is used as a model compound. In addition to the excellent

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48 affinity to gold surfaces, benzenethiol molecules have a large Raman cross section, presumably as a result of chemical enhancement alongside electromagnetic enhancement. [82] Each peak in the Raman spectra corresponds to the vibrational specific mode: the peak at 1572 cm 1 to C C stretching, the peak at 1072 cm 1 to C S stretching, the peak at 1021 cm 1 to in plane ring deformation, and the peak at 997 cm 1 to S H bending, as shown in Figure 3 5. [90] Gold nanoparticles arrays are placed in a 5 mM solution of benzenethiol in 200 proof ethanol for 45 min and then rinsed in 25 ml of 200 pr oof ethanol for several minutes. The samples are allowed to air dry for 20 min, after which the Raman spectra are measured. [91] A flat gold film is sputter deposited on a glass slide using the same deposition condition used for the control sample for Raman spectra. R aman spectra are measured with a Renishaw inVia confocal Raman microscope using a 785 nm diode laser at 15 mW with an integration time of 10 s and a 40 2 spot size. Raman scattering intensity is also measured in an aqueous environment. [ 13] The excitation source is an argon ion laser pumped Ti:sapphire laser operating at 830 nm with a power of about 200 mW at the sample. Dispersion is achieved using a Chromex spectrograph with a deep depletion CCD detector. A water immersion microscope o bjective (363, NA 0.9) is brought into direct contact with a 30 ml droplet of sample solution for both excitation and collection of the scattered light. Scattering volume is estimated with a cylinder of diameter derived from the applied power and the inten sity and length derived from depth of field consideration. The SERS enhancement factors are calculated from data collected using confocal Raman spectrometers in which the surface enhancement, G is defined as follows:

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49 (3 3) The intensity of the Raman peak obtained at the SERS surface, I surf is compared to that obtained for a solution, I bulk of concentration c N A is the surface area occupied by the adsorbate, R is the roughness factor of the surface, and h is a parameter defined by the confocal volume of the spectrometer. [92] S imulation of E ffects of S ize and S pacing on E lectro magnetic F ield If SERS substrate s with various parameters are assessed and optimized by modeling their electromagnetic characteristics, time and effort c an be saved in laboratory preparation and experimental testing. The field enhancement is strongly depen dent on physical parameters such as the surface morphology, the dielectric constants used to perform the modeling, and the excitation conditions F inite element electromagnetic modeling was applied to predict the Raman enhancement with a variety of SERS su bstrates with differently sized, spaced, and shaped morphologies with nanometer dimensions [93] domain, and adhere to the boundary equations at the interface between the media and the scatterer. Finite element methods (FEM) using Comsol Multiphysics software have been employed to provide numerical solutions for each substrate. For a radiation condition, the perfect matched layers (PM L) boundaries method and a low reflection boundary condition are applied. The output of the modeling process is a two dimensional map of the electric field intensity, which can be used to calculate the Raman enhancement G When the polarization of the scattered light is the same

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50 as that of the incident light, the expected electromagnetic enhancement of the Raman signal may be expressed to a first approximation as (3 4) where ) is the total predicted electric field at position r and E inc is the electric field associated with the incoming electromagnetic radiation. P article Self assembly A pproach P article A ggregate s for S ERS Kneipp et al reported that the SERS enhancement of 10 14 was achieved with colloidal silver solution, where silver nanoparticles slightly aggregated. [13] The enhancement was calculated from comparison between the concentration of analyte in the colloidal silver solution and the concentration in control solution (without colloidal silver particles) having the same level of Raman signal. The general method to calculate the enhancement factor (G) will be discussed in the next section. The possibility of single molecule detection was demonstrated by the change in the distribution of Raman si gnal when the average number of analyte is one. [13] Single hemoglobin molecules in a junction between silver nanoparticles could be detected and the maximum SERS enhancement in the hot spot was estimated to be 10 10 [94] The SERS enhancement has an advantage in the applications in an aqueous solution due to very weak Raman signal of water. The SERS imaging in a living cell was successful by using colloidal gold particles. [95] The biological molecules including adenine, L cysteine, L lysine, and L histidine, were successfully detected by using gold nanoparticle aggregates as SERS substrates, where the enhancement was 10 7 10 9 in bulk solution. [96]

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51 The hot spot of high enhancement, w hich is generally located in a junction between particles, is sensitive to the spacing between particles as well as the wavelength and polarization of the excitation laser, as shown in Figure 3 6. [76, 93, 97] The SERS enhancement up to 10 14 in a junction between Ag coated particles was reported by calculation based on the extended Mie theory (the classical electromagnetic theory of spherical particles). [94] According to th e mechanism for the electromagnetic enhancement, the localized surface plasmon resonance (LSPR) is excited to generate a high SERS enhancement when electromagnetic radiation is incident upon substrates with the same wavelength. [75] To generate a high SERS enhancement, th e wavelength of electromagnetic radiation was tuned to the excitation of LSPR with different laser sources. Alternatively, the excitation of LSPR, which is indicated as a peak in the extinction spectrum, was tuned to the wavelength of electromagnetic radia tion with using various size and shape of particle and controlling the interparticle spacing. The plasmon absorption was tuned by using different sizes of gold nanoparticles in aqueous solution. [98] By assembling silver nanoparticles with DNA bases (adenine, guanine, cytosine, and thymine) or surface modifying gold nanoparticles wi th ATP, the interparticle spacings between particles in particles aggregates were controlled so that the plasmon absorption could be tuned. [99, 100] S elf assembled P article s as a SERS S ubstrate For a SERS substrate, a three dimensional multilayered film with assembled gold nanoparticles was prepared by using the Langmuir Blodgett (LB) method. [90] The Raman signals from self assembled particles as a SERS substrate were ~ 10 7 higher

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52 than those from the control (a bare substrate). The enhancements depended on the Raman peak as well as the particle size and the film thickness. Periodically order ed arrays of nanoparticles were devised to produce reproducible SERS enhancement. To predict or estimate the SERS enhancement on the ordered array of nanoparticles and understand the effects of parameters such as the particle size, shape, and interparticle spacing, several models were designed to solve the T matrix method, RLC circuit analogy were employed to solve the electromagnetic field on nanoparticle arrays. [93, 101, 102] It is worthwhile noting that the maximum value of enhancement (G max ) obtained from the hot spot should be distinguished from the average value of enhancement (G ave ) calc ulated over the entire surface of the substrate. The maximum enhancement factor was used to emphasize the capability of single molecule detection, for example, in [13] while the average enhancement factor was used to demonstrate the paper. [90] Since the portions of hot spot in the entire surface of substrate are much less than 1 %, the average enhancement (G ave ) is several orders lower than the maximum enhancement factor (G max ). [103] The enhancement factors are maximized at the wavelengths of the incident light which are close to the excitation of the localized surface plasmon resonance (LSPR). The maximum enhancement factor (G max ) on periodic arrays of silver nanoparticles calculated based on the Finite element methods was increased up to 10 8 with decreasing separation between particles. [93] Based on the T matrix theory, the

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53 maximum calculated enhancement factor (G max ) on one dimensional arrays of silver nanoparticle s was calculated to be 10 9 [101] The average enhancement factor (G ave ) on two dimensional square arrays of gold nanospheres calculated based on the RLC circuit model is about 10 8 where G ave depends on the ratio of interparticle spacing and particle size. [102] Controlling the interparticle spacing and particle size is important to achieve a high enhancement factor (G ave ) in terms of generating large area of hot spots in a junction between particles. Periodically ordered a rrays of nanoparticles functionalized with surfactant molecules were assembled to control the interparticle spacing between particles. Gold nanoparticles of 50 nm diameter functionalized with cetytrimethylammonium bromide (CTAB) were self assembled to gene rate hexagonally close packed monolayer with the interparticle spacing of 8 nm. [104] The average enhancement factors (G ave ) were calculated to be up to 10 8 at the near infrared (785 nm) excitation by comparing rat ios of the SERS peak intensities to the corresponding unenhanced signals from neat analyte films. Close packed arrays of silver nanoparticles of 20 nm capped with surfactant molecules of oleic acid and oleylamine were assembled on the poly (N isopropylacry lamide) (PNIPAM) film, where the interparticle spacing could be controlled by altering the temperature since PNIPAM is a temperature sensitive polymer. [105] Controlling the interparticle spacing from less than 4 nm to 24 nm, brought the plasmo n resonance peak closer to the laser excitation wavelength, generating larger Raman signals of an analyte. Two dimensional hexagonal close packed arrays of gold nanoparticles functionalized with resorcinarene tetrathiol had interparticle spacing less than 1 nm. [106, 107] The average enhancement factors (G ave )

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54 were determined using peak integration ratios of the SERS peak intensities to the corresponding unenhanced signals from ne at analyte films. The SERS enhancement factors (G ave ) of 10 7 with NIR excitation were shown in large particle arrays. Most investigation has been carried out based on the direct relationship between extinction/absorption and SERS enh ancement. On the other hand, Lu et al claim that the connection between extinction/absorption and SERS enhancement is indirect and qualitative at best since the spatial distribution of collective resonances should be considered. [108] Bulk like resonances (proportional to the volume) have a large contribution to absorption, while surface like resonances (proportional to the surface) have a large contribution to SERS enhancement. For example, a high SERS enhancement was observed at the wavelength of excitation laser where t here is no resonance in the absorption/extinction. STV/PEG GNP A rrays In this work, gold nanoparticle (30 nm) functionalized with streptavidin and polyethylene glycol (PEG) chain are assembled into hexagonally close pac ked arrays using a flow cell, as shown in Figure 3 7. The size of streptavidin immobilized on the nanoparticles is about 4 nm. [109] Double sided tapes are assembled as spacers o n a clean glass slide and a thin glass slip is covered to make a flow cell. Streptavidin/PEGylated gold nanoparticles (STV/PEG GNP) solution monodispersed in distilled water is flown through the flow cell. Solutions are pipeted from one side of the flow ce ll and sucked out from the other side by capillary action. After 5 minutes, rinsing is done by flowing enough volume of DI water to remove excessive gold particles and the remaining salts in the solution. Solvent is then evaporated at the room temperature generating STV/PEG GNP arrays on a glass slide.

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55 The self assembled STV/PEG GNP arrays are shown in Figure 3 8. They are non close packed arrays due to spacers such as streptavidin and PEG. The interparticle spacing corresponds to two fold of the streptavidin size (8 nm). Most part of the surface are covered with a monolayer of particle arrays, though some areas are not covered with particles. In the absorption spectrum, there is a peak at the excitation wavelength of 544 nm indicati ng the excitation of localized surface plasmon resonance (LSPR), as shown in Figure 3 12a. The SERS spectrum at the excitation wavelength of 785 nm is shown in Figure 3 13a. Benzenethiol is used as an analyte as it has a good affinity to gold and forms a m onolayer on gold surface. The SERS enhancement factor (G ave ) is about 5 10 5 which is calculated from the SERS peak intensity at the Raman shift of 999.2 cm 1 (S H bending + in plane ring deformation mode). [110] Although the wavelength of the excitation laser (785 nm) is not close to the excitation of LSPR (544 nm), there is a high SERS enhancement of 5 10 5 P eriodic N anostructure A pproach Periodic Nanostructures as SERS S ubstrates To generate reproducible SERS substrates, periodic nanostructures have been fabricated by various micro/nanofabrication techniques. Silver nanoparticle arrays were produced by electro beam lithography. [111] The i mportant factors in SERS enhancements such as particle size, shape, and interparticle spacing could be controlled. Triangular silver nanoparticle arrays were fabricated by nanosphere lithography (NSL). [112] The excitation wavelength of localized surface plasmon resonance (LSPR) was controlled with various dimensions of th e nanoparticles and the

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56 wavelength of the excitation laser was tuned with tunable laser systems. According to the systematic investigation, the SERS enhancement was maximized when the excitation wavelength of LSPR was located between the wavelength of the excitation laser and the wavelength of the Raman scattered photon by the analyte molecules. The SERS enhancement factor (G ave ) was calculated up to be 10 8 Hexagonally ordered arrays of nanovoids as SERS substrates were fabricated by self assembly of sacr ificial nanospheres and electrochemical deposition of gold. [113 115] The fabricated nanostructures had either gold flat surfaces or corrugated surfaces depending on the thicknes s of gold film. Surface plasmon polaritons propagate along the flat gold surface and scatter at the rims of shallow dishes forming delocalized Bragg modes. On the highly corrugated surface with nanovoids, surface plasmon polaritons were localized in the na novoids forming localized Mie modes. The Bragg plasmon mode depends on the incident angle and sample orientation, while the Mie plasmon mode depends on the geometry of nanostructure. Maximum SERS enhancements occurred when the excitation laser is incident at the wavelength of the excitation of LSPR on the substrate and the Raman scattered photon is also coincide with the excitation of LSPR. [92, 115] The measured SERS enhancement factor (G ave ) was 7 10 8 on a highly corrugated substrate with nanovoids of 350 nm diameter, where the wavelengths of both excitation and Raman scattered photons were in the absorbance peak (corresponding to the excitation of LSPR). It was claimed that it w as not possible to achieve completely uniform SERS enhancements over whole Raman scattering modes, because the wavelengths of Raman scattered were all different and could not be matched to the excitation of LSPR all together.

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57 The excitation of localized surface plasmon resonance (LSPR) could be tuned to near infrared (NIR) wavelength, by controlling the nanovoid size and the film thickness. [116] The use of NIR laser sources can be favored since the photochemical reactions can be activated and the fluorescence from adsorbed molecules can interfere with the Raman signals. The SERS enhan cement factor of 3 10 6 was obtained at the NIR of 1064 nm. Three dimensional nanostructures were fabricated for reproducible and highly SERS active substrates. [117, 118] Inverse opal films fabricated by self assembling a binary mixture of sacrificial latex microbeads and gold nanoparticles showed stable and reproducible SERS enhancements. The alumina membranes decorated with gold nanoparticles as SERS substrates had advantages of efficient light interaction on the wall of cylindrical pores with minimal absorption and scattering. The SERS enhancement factor (G ave ) was calculated to be 10 6 Periodic N anostructure s from Non close packed Particle A rrays Various non close packed arra ys of nanopillars, nanodots, nanoholes, nanovials, and nanovoids, could be achieved based on the hexagonally non close packed nanoparticle arrays fabricated by a colloidal self assembly. [119 123] Concentrated silica nanoparticles (20 vol%) dispersed in ethoxylated trimethyllolpropane triacrylate (ETPTA) monomer were spin coated on a silicon wafer. Then, ETPTA monomer is rapidly photopolymerized to immobilized silica particles on the substrate. After removing the polymer matrix by an oxygen plasma etching, colloidal monolayer of hexagonally non close packed silica particles is fabricated. The interparticle spacing between particles is about 1.4 times of the particle diameter, which is explained by keeping a

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58 minimal volume fraction of silica particle in ETPTA matrix due to the centrifugal force during spin coating. [119] Periodic nanopyramid arrays are fabricated using the non close packed nanopartic le arrays as deposition masks during Cr deposition. [91] The silica particles are removed by dissolving in hydrofluoric acid aqueous solution, generating Cr nanohole arrays on silicon wafer. Inverted pyramid arrays of silicon are produced by wet etching in KOH solutio n, where Cr nanohole arrays play the role of etching masks. After removing Cr layer with a Cr etchant and depositing gold with thickness of 500 nm, the deposited gold layer is transferred onto a glass substrate using a polyurethane adhesive. The SERS enhan cement on the fabricated nanopyramid arrays is 7 10 5 Instead of depositing gold and transferring onto a glass substrate, polymer replication can be used to generate nanopyramids of ETPTA and, by deposition of a gold layer, gold coated nanopyramid arrays a re fabricated. [124] The SERS enhancement is improved to 10 8 due to the unbroken tips of nanopyramid. The strong concentration of electr omagnetic field near sharp tips of nanopyramids is contributed to the high SERS enhancement. The charged analytes can be concentrated to the surface of nanopyramid arrays under externally applied electric field, strengthening the Raman signals. [125] The metal film over nanosphere (MFON) as a SERS substrate is fabricated by depositing gold on the hex a gonally non close packed array of silica nanoparticles without a polymer etching process. [126] Contrary to conventional MFON substrates, the fabricated MFON consists of gold islands of 10 nm and gaps of less than 10 nm. Gold islands form only on the polymer wetting layer during the deposition of gold layer. The

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59 SERS enhancement is up to 10 8 due to the delocalized Bragg plasmon modes along the periodic nipple structure and of the localized Mie plasmon mod es in gold islands. Disordered arrays of gold half shells as SERS substrates are fabricated by depositing gold on non close packed arrays of silica nanoparticles, transferring to a glass substrate, and removing the silica particles in HF solution. [127] The strong concentration of electromagnetic field near the sharp edge of half shell and hot spots between half shells contribute to the measured high SERS enhancement (G ave ) of 10 10 On the other hand, a maximum SERS enhancement factor (G max ) of larger than 10 10 in the hot spot near sharp tips of gold nanocrescent moons was reported. [128] N anohole A rrays on a Glass S ubstrate In this work, nanohole arrays are fabricated based on the colloidal self asse mbly for hexagonally non close packed arrays and the nanosphere lithography, as shown in Figure 3 9. To generate nanoparticle arrays on glass slides instead of silicon wafers, PMMA is spin coated as a sacrificial layer. The concentrated silica nanoparticle s dispersed in ETPTA monomer are spin coated on a PMMA coated glass slide generating hexagonally non close packed arrays of nanoparticles. After photopolymerization of ETPTA to immobilize particles on the substrate, ETPTA and PMMA are etched by using nanop articles arrays as an etching mask. Silica nanoparticles are removed by ultrasonication in ethanol and the remaining ETPTA and PMMA are removed in acetone, generating nanohole arrays on a glass substrate. The fabricated gold nanohole arrays with different sizes (330 and 400 nm) are shown in Figure 3 10. They are hexagonally non close packed arrays of nanoholes. In the absorption spectrum of 330 nm nanoparticles arrays, there is a peak at the excitation wavelength of 722 nm, indicating the excitation of loca lized surface plasmon

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60 resonance, as shown in Figure 3 12a. In the case of 400 m nanoparticles arrays, a peak is present at the excitation wavelength of 816 nm. These excitations of LSPR are close to the used excitation wavelength of 785 nm. STV/PEG GNP A rr ays on N anohole A rrays C oncept of GNP nanohole A rrays In the absorption spectra of STV/PEG GNP arrays, 330 nm gold nanohole arrays fabricated in our study, and 400 nm nanohole arrays, the excitation of localized surface plasmon resonance are located at the wavelengths of 544 nm, 722 nm, and 816 nm, respectively. To maximize the SERS enhancement, the excitation of LSPR should be between the wavelengths of the excitation and the Raman scattered photons, according to literatures. Thus, there are two ways to ma ximize the SERS enhancement such as tuning the excitation of LSPR to the wavelengths of excitation and the Raman scattered photons by controlling the geometry of the substrates and tuning the laser excitation wavelength by using tunable laser systems. The laser excitation wavelength of 785 nm is favored in biological sensing applications since the photochemical reactions can be activated and the fluorescence from adsorbed molecules can interfere the Raman signals in the visible excitation wavelength. [116] In this work, we try to tune the excitation of LSPR to the wavelength range around the laser excitation wavelength of 785 nm by combining the STV/PEG GNP arrays and the gold nanohole arrays. Fabrication of GNP nanohole A rrays The STV/PEG GNP arrays on gold nanohole arrays are fabricated by colloidal self assembly and metal deposition as shown in Figure 3 9. First, the gold nanohole arrays are prepared on a glass substrate based on the colloidal self assembly for

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61 hexagonally non close packed arrays and nanosphere lithography. Then, gold nanoparticles (30 nm) f unctionalized with streptavidin (STV) and polyethylene glycol (PEG) chains are assembled on the prepared gold nanohole arrays instead of bare glass substrate to form hexagonally close packed arrays by using a flow cell, as shown in Figure 3 7. The fabricat ed STV/PEG GNP arrays on gold nanohole arrays with different sizes (330 and 400 nm) are shown in Figure 3 11 Gold nanohole arrays with different sizes (330 nm and 400 nm) are fully covered with gold nanoparticles (30 m) forming non close packed arrays du e to streptavidin (STV) and PEG chains. Optical and SERS Prop erties of STV/PEG GNP Arrays on Nanohole A rrays The absorption spectra of STV/PEG GNP arrays with different sized nanoholes are shown in Figure 3 12b. In the case of 330 nm nanoholes, the excitat ion of LSPR is at the wavelength of 551 nm and 746 nm, which are red shifted from those in the STV/PEG GNP arrays (544 nm) and the nanohole arrays (722 nm). In the case of 400 nm nanoholes, the excitation wavelengths of LSPR are also red shifted to 563 nm and 875 nm. There are delocalized surface plasmon polaritons (SPP) along the flat surface of periodic nanohole arrays. The localized surface plasmon in gold nanoparticles is indicated as a Raman peak around the wavelength of 550 nm and the localized surfac e plasmon in nanoholes is indicated as a Raman peak from 722 nm to 875 nm. The red shifts in the excitation of LSPR indicate the interactions between the excitation of LSPR in gold nanoparticles and the excitation of LSPR in gold nanoholes. At the waveleng th of 785 nm, STV/PEG GNP arrays on 330 nm nanohole arrays show a high absorption plateau, while the STV/PEG GNP arrays on 400 nm nanohole arrays show a high and still increasing absorption.

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62 The Raman spectra of STV/PEG GNP arrays with different sized nano holes at the excitation wavelength of 785 nm are shown in Figure 3 13b,c. There is no Raman peak around 2600 cm 1 (corresponding to v(S H) stretching and vibration modes), indicating that there is no analyte (benzenethiol) unbound to the substrate. Both su bstrates show much stronger Raman signals than those from nanoparticle arrays. Moreover, the signal to background ratios in both cases are larger than the reported ratios in the literature, which is favorable in sensing and detecting applications. The STV/ PEG GNP arrays on nanohole arrays show the good reproducibility in the SERS enhancement in terms of low standard deviations (~ 10 %) of Raman signals (Table 3 1). The higher SERS enhancement in 400 nm nanoholes is consistent with the higher absorption in 4 00 nm nanoholes. The SERS enhancement factors at different Raman peaks are different even in the same substrate as shown in Table 3 1 Reported empirical results show that the high SERS enhancement occurs when the excitation wavelength of LSPR is betwe en the excitation and the Raman scattered photons. The difference in the SERS enhancement can be explained by different wavelengths of Raman scattered photons at different Raman peaks. C onclusions In conclusion, we have developed a self assembly technology for fabricating gold nanoparticle arrays on gold nanohole arrays as reproducible and high SERS active substrates. The excitation of localized surface plasmon resonance (LSPR) in nanoparticle arrays and nanohole arrays around the wavelength of ex citation laser of 785 nm contribute to high SERS enhancements. Due to a high sensitivity (high signal to background ratios) as well as a good reproducibility (low variations of Raman signal

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63 on different spots and samples), this simple and scalable technolo gy for fabricating gold nanoparticle arrays on gold nanohole arrays is promising for developing ultra sensitive detectors for chemicals and reproducible sensors for chemical and biological molecules. Materials and M ethods M aterials Monodispersed silica col loids with less than 10 % diameter variation are synthesized by the Stober method. [129] Ethoxylated trimethy lolpropane triacrylate (ETPTA) monomer is obtained from Sartomer (Exton, PA). The photoinitiator, Darocur 1173 (2 hydroxy 2 methyl 1 phenyl 1 propanone), is provided by Ciba Specialty Chemicals. Streptavidin PEGylated gold nanoparticles (30 nm) are purchas ed from Polysciences (Warrington, PA). I nstrumentation A standard coater (WS 300B 6NPP Lite Spin Processor, Laurell) is used to spin coat colloidal suspensions. The polymerization of ETPTA monomer is carried out on a Pulsed UV Curing System (RC 742, Xenon) A Unaxis Shuttlelock RIE/ICP reactive ion etcher is utilized to remove polymerized ETPTA and PMMA. Scanning electron microscopy is carried out on a JEOL 6335F FEG SEM. Raman spectra are measured with a Renishaw inVia confocal Raman microscope. Gold N anoh ole A rray s on a G lass S lide Glass slides were cleaned using a standard RCA1 cleaning [130] A resist PMMA (MicroChem 950 PMMA A4) was spin coated on a 1 inch 2 glass slide (4000 rpm for 1 min) as a sacr ificial layer and then was baked for hardening (180 C for 3 min). Monolayer non close paced silica particles dispersion (400 or 330 nm) was then spin coated on PMMA [131] The final step of the spin coat pr ocess is 8000 rpm for 5 min.

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64 After UV curing poly(ethoxylated trimethylolpropane triacrylate), reactive ion etch ing (Unaxis Shuttlelock operating at 5 mTorr oxygen pressure, 15 sccm oxygen, 5 sccm argon an d 1 00 W RIE power and 300 W ICP power for 90 s) wa s performed to etch both ETPTA and PMMA by using silica nanoparticles as etch masks. A 100 nm thick gold film was deposited b y a n electron beam evaporator. Silica nanoparticles were removed by ultrasonication in ethanol. The PMMA /ETPTA layer s were then lif ted off in acetone resulting in gold nanohole arrays on a glass substrate. STV/PEG GNP A rray s on Gold N anohole A rray s Flow cell was constructed by placing two strips of double side sticky tape on the Au nanohole arrays about 7 mm apart and covering with a cover glass. To provide enough number of gold particles to cover whole surface area, 3 layers of double side sticky tape (~ 300 ) were used to increase the volume of solution (~ 60 L ). Streptavidin PEGylated gold nanoparticles dispersed in distilled wa ter was flown through a flow cell. Solutions are pipeted on one side and sucked out the other side by capillary action. Solvent was then evaporated at room temperature. To remove crystals formed due to salts in the solution, Au nanoparticles array was rins ed by flowing ample amount of distilled water through the flow cell. A bsorbance of Substrates The absorbance of the GNP arrays on gold nanohole arrays is evaluated using visible near IR absorbance measurement with an Ocean Optics HR4000 high resolution fib er optic UV visible near IR spectrometer. R aman S pectra M easure m ents STV/PEG GNP arrays on gold nanohole array samples are placed in a 9 mM solution of benzenethiol in 200 proof ethanol for 45 min and then rinsed in roughly 25

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65 mL of 200 proof ethanol for s everal minutes. The samples are allowed to air dry for 20 min, after which the Raman spectra are measured. A gold nanoparticles array on bare glass is used as the control sample for Raman spectra measurements. Raman spectra are measured with a Renishaw inV ia confocal Raman microscope using a 785 nm diode laser at 6.4 mW with a 20 objective and an integration time of 10 s and a 2 spot size. Raman spectra were also measured in 9.77 M benzenethiol contained in a flow cell at 6.4 mW with an integration t ime of 10 s. C alculation of E nhancement F actors The SERS enhancement factor, G ave is calculated from data collected using the confocal Raman microscope. G ave is defined as follows: (3 5) The intensity of the Raman peak obtained at the SERS surface, I surf is compared to that obtained for a solution, I bulk of concentration C N A is the surface area occupied by one adsorbate molecule, R is the roughness factor of the surface, and h is a parameter defined by the confocal volume of the microscope. [132] h is measured to be 400 m with the 20 objective

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66 Table 3 1. Calculated SERS enhancement factor on the fabricated substrates from measured data of Raman intensities at 999.2 cm 1 and 1023 cm 1 SERS substrate ~ 999.2 cm 1 ~ 1023.2 cm 1 GNP array 4.5910 5 1.9310 5 1.5610 6 6.5210 5 330 nm nanohole GNP 2.1510 6 2.0610 5 7.9010 6 8.5510 5 400 nm nanohole GNP 2.9810 6 2.6410 5 1.1510 7 1.0710 6

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67 Figure 3 1. Various characteristic energies: a) Rayleigh s catteri ng, b) Stokes Raman s cattering, and c) a nti Stokes Raman s cattering Figure 3 2. Schematic diagrams of a) a surface plasmon polariton (or propagating plasmon) on a flat surface and b) a localized surface plasmon on a nanostructured surface Adapted fro m [79]

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68 Figure 3 3. Various SERS substrates: a) Ag film on nanospheres periodic nanostructures by b) n anosphere lithography a n d c) e lectron beam lithography, and d) c ol loidal aggregates. Adapted from [80, 99]

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69 Figure 3 4. SERS applications: a) in vivo glucose sensing equipment consisted of SERS spectroscopy, implanted substrate, beam directing optics, and collection lens and b) identification of cancer genes by Raman labels. Adapted from [82] Figure 3 5. Representative Raman spectrum of benzenethiol adsorbed on a SERS substrate Adapted from [90]

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70 Figure 3 6. The SERS enhancement calculated based on finite element methods in the case of na noparticle arrays Adapted from [93]

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71 Figure 3 7. Schematic diagram depicting self assembly of gold nanoparticles using a flow cell.

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72 Figure 3 8. SEM images of 30 nm gold nanoparticle arrays with a gap between particles on glass substrates.

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73 Figure 3 9. S chematic diagram depicting t he fabrication procedures for making GNP nanohole arrays.

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74 Figure 3 10. SEM images of nanohole arrays : a) 330 nm nanohole array s and b) 400 nm nanohole arrays Figure 3 11. SEM images of nanohole GNP arrays : a) 330 nm nanohole arrays covered by 30 nm gold particles and b) 400 nm nanohole arrays covered by 30 nm gold particles.

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75 Figure 3 12. Absorption spectra on a) gol d nanoparticle (GNP) arrays and nanohole arrays and b) na nohole arrays covered by gold nanoparticles.

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76 Figure 3 13. Raman spectra of benzenethiol absorbed on a) flat god surface, b ) gold nanoparticle (GNP) arrays, c ) 330 nm nanohole arrays covered by gold nanoparticles, and d ) 400 nm nanohole arrays covered by gold nanoparticles.

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77 CHAPTER 4 BINARY COLLOIDAL CRY STALS The electronics revolution sparked by the invention of transistors and the miniaturization of integrated electronic circuits has affe cted almost every aspect of our daily lives. In an effort toward further high density integration and system performance, scientists are now turning to light as the information carrier. The travel speed of light in a dielectric material is greater than tha t of an electron in a metallic wire [133] The bandwidth of dielectric materials is about 3 to 4 orders of magnitude larger than that of metals [133] Moreover, light particles (photons) are not as strongly interacting as electrons, which helps to reduce energy losses. Unfortunately, our ability to control photons in miniaturized volu mes is in many ways in its infancy, compared with our ability to manipulate electrons. A new class of optical materials known as photonic crystals (PCs) may hold the key to continued progress towards all optical integrated circuits and high speed optical c omputing [134 137] P Cs are periodic dielectric structures (Figure 4 1) with a forbidden gap (or photonic band gap) for electromagnetic waves, analogous to the electronic band gap in semiconductors. Photons with energies lying in the photonic band gap (PBG) cannot propagate through the medium, providing the opportunity to control the flow of light for photonic information technology. The lattice constant of the artificial crystal must be comparable to the wav elength of the light passing through the crystal [136] For optical communication systems operating at near infrared waveleng ths, the lattice constant must have dimensions on the submicrometer scale [138, 139] Unfortunately, the development and implementation of integrated optical circuits with photonic crystals have been greatl y impeded by expensive and complex

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78 nanofabrication techniques. Electron beam lithography (EBL) and focused ion beam (FIB) are two popular methods in fabricating photonic crystals with arbitrary geometries. [140, 141] However, attaining high throughput and large area fabrication continues to be a major challenge with these top down techniques. By contrast, bottom up colloidal self assembly and subsequent templating nanofabrication provide a much simpler, faster, and inexpensive alternative to nanolithography [138, 139, 142] A variety of methods, such as gravity sedimentation [138, 143] electrostatic repulsion [144 147] template assisted assembly [148 150] and capillary force induced convective self asse mbly [139, 151 153] have been developed to create colloidal photonic crystals. However, current colloidal self assemblies are only favorable for low volume, laboratory scale production. It usually take s days or even weeks to grow a centimeter size colloidal crystal [139, 152, 153] In addition, most of current colloidal self assembly technologies are not compatible with mature semiconductor microfab rication, limiting the mass production and on chip integration of practical photonic crystal devices. Another major issue of current colloidal self assembly is the limitation on achievable crystal structures. Although calculations show that nonclose packed photonic crystals (e.g., diamond structured crystal) facilitate the opening of wider PBGs [154, 155] the realization of open structured crystals by self assembly is challenging [156] Binary colloidal photonic crystals compos ed of particles of two different sizes have attracted a great deal of recent interest as they are promising to open wider PBGs [157 182] For instance, van Blaaderen et al. developed a convective self assembly technology to assemble small silica particles on a pre assembled colloidal array consisting of larger particles [176] Similar to the limitations of traditional

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79 colloidal assembly, all available bottom up methodologies i n creating binary colloidal photonic crystals suffer from the scalability and microfabrication compatibility issues. We have recently developed a simple and scalable spin coating technology that combines the simplicity and cost benefits of bottom up colloi dal self assembly with the scalability and compatibility of standard top down microfabrication [183, 184] The spin coating technique enables mass fabrication of wafer scale (up to 8 i nch) colloidal crystals, which is a length scale nearly two orders of magnitude larger than that currently available through other methods. Additionally, the entire crystal is formed within minutes, as compared to days or even weeks needed to produce a cen timeter size crystal using other self assembly techniques. Most important, the spin coating technique is compatible with standard microfabrication, allowing for the creation of complex micropatterns for optical on chip integration. In this chapter, we deve loped a new bottom up approach to create non close packed binary colloidal crystals by using spin coated colloidal crystals as structural template. R esults and D iscussion Figure 4 2 shows a schematic outline of a procedure for achieving binary colloidal cr ystals. The established spin coating technique is utilized to generate a wafer scale monolayer of hexagonally ordered nonclose packed silica nanoparticles. The resulting nanocomposite of silica particles and polymer matrix has a thin polymer wetting layer (~100 nm) next to the Si substrate, which can still immobilize the silica particles on the substrate after partial etching of the polymer. [185] The fabricated monolayer of silica particles (300 nm diameter) is shown in Figure 4 3. In the literature, the close packed colloidal crystals were achieved via the spin coating technique, wher e the evaporation rate of solvent in the colloidal dispersion was very high. [186] In the established spin

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80 coating technique for nonclose packed colloidal crystals, on the other hand, silica particles are dispersed in non volatile monomer (ETPTA). The nonclose packed monolayer has the particle center to center distance of 1.41 (of particle diameter), which corresponds to the minimal volume fraction of silica particle in the silica polymer nanocomposite. [183] The dispersions of silica particles in different sizes are subsequently spin coated on the prepared monolayer. The prepared monolayers of silica particles are used as templates for guiding the silica particles in the dispersions into the interstices between the three neighbor particles in the template layers. The fabricated binary colloidal crystals are shown in Fi gure 4 4. In the colloidal crystal consisting of a 300 nm particle monolayer on a 300 nm particle monolayer, the particle in the second layer is located in the trap which is the center of interstice between three neighboring particles in the first template layer as shown in Figure 4 4b In the colloidal crystal consisting of a 400 nm particle monolayer on a 300 nm particle monolayer, on the other hand, the particles in the second layer are not c onfined in the traps, as shown Figure 4 4d. The larger particles (400 nm) in the second layer rather form a nonclose packed array independent of the first template layer. In the literature, attempts at localizing small particles in the traps between large particles were successful. However, attempts at localizing larger particles in the traps which are the interstices between small particles in the templating layer were not successful with the convective assembly (but, they were successful in the LS 2 close packed binary crystal), [176] although it is thermodynamically favorable. In our spin coat

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81 template layer is competing with the centrifugal force due to spinning. In the case of localizing larger particles, the trapping effect is reduced because the depth of intersti ce between particles in the template layer is shallow. Thus, the particle arrays in the second layer are affected by the centrifugal force, where the minimal volume fraction of silica particles is achieved. Several strategies have been reported to improve the ordering in the colloidal crystals. Enhanced orderings of colloidal particles were successful by steady shearing, ultrasonication, and oscillatory shearing. [151, 187, 188] Defect free arrays of nanoholes in the block copolymer were achieved by solvent induced ordering. [189] In this work, pressure is applied on the surface of she ar aligned silica particles in a ETPTA monomer matrix to increase the trapping effect. The resulting binary colloidal crystals are shown in Figure 4 5. In the binary colloidal crystals consisting of a 300 nm particle layer on a 300 nm particle layer, the o rderings of particles in the second layer into the traps which are interstices between the three neighbor particles in the template layer, are improved as shown in Figure 4 5a d In addition, the domain boundaries are clearly consisting of vacancies, whi le the domains in the binary crystals fabricated without an applied pressure show gradual change across the domain boundaries. This is due to the rearrangement of particles in monomer matrix under the applied pressure. There is no significant difference be tween 0.2 MPa and 0.33 MPa of pressure. On the other hand, the binary colloidal crystals consisting of a large particle (400 nm) layer on a small close packed array while they are independent of the first template layer.

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82 The particle size and the thickness of layer in colloidal crystal are controllable. Figure 4 6a shows the cross section of the binary colloidal crystals consisting of 400 nm particles on 345 nm particles. T he individual layers are monolayers of 400 nm particles or 345 nm particles. By repeating the spin coating process with different sized particles, the colloidal crystal consisting of many monolayers of different size of particles can be achieved. The thick ness of individual layers can also be controlled with spin coating parameters such as spinning speed and time. Binary colloidal crystals consisting of a 400 nm particle monolayer on a 345 nm particle double layer and ternary colloidal crystals consisting o f a 300 nm double layer on a 400 nm double layer which in turn is on a 345 nm monolayer are shown in Figure 4 6b,c. C onclusions Binary colloidal crystals are achieved wi th a simple fast, and scalable spin coating techn ique. The thickness of individual lay ers is easily controlled with spin coating parameters. Although the pressure is helpful for the ordering of particles, further improvement for better orderings of particles in the individual layers is still needed. The characterization of optical propertie s in the fabricated binary colloidal crystals is in investigation. M aterials and M ethods M aterials and S ubstrates Monodispersed silica colloids with less than 10 % diameter variation are synthesized by the Stober method. [190] Ethoxylated trimethylolpropane triacrylate (ETPTA) monomer is obtained from Sartomer (Exton, PA). The photoinitiator, Darocur 1173 (2 hydroxy 2 methyl 1 phenyl 1 propanone), is provided by Ciba Specialty Chemicals. The silicon wafer primer, 3 acryloxypropyl trichorosilane (APTCS), is

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83 purchased from Gelest (Morrisville, PA). Silicon wafer (test grade, n type, (100)) are obtained from Wafernet (San Jose, CA) and primed by swabbing APTCS on the wafer surfaces using cleanroom Q tips (Fisher), rinsed and wiped with 200 proof ethanol three times, spin coated wit h a 200 proof ethanol rinse at 3000 rpm for 1 min, and baked on a hot plate at 110 C for 2 min. I nstrumentation A standard coater (WS 300B 6NPP Lite Spin Processor, Laurell) is used to spin coat colloidal suspensions. The polymerization of ETPTA monomer i s carried out on a Pulsed UV Curing System (RC 742, Xenon). A Unaxis Shuttlelock RIE/ICP reactive ion etcher is utilized to remove polymerized ETPTA for releasing shear aligned colloidal crystals. Scanning electron microscopy is carried out on a JEOL 6335F FEG SEM. N on close packed C olloidal M onolayer The fabrication of wafer scale, monolayer, non close packed colloidal crystal polymer nanocomposites is performed according to reference. [119] Monodispersed silica colloids ( 300 nm) are dispersed in non volatile ethoxylated trimethylolpropane triacrylate ( ETPTA Sartomer) monomer (20% volume fraction) and 2 wt% Darocur 1173 is added as photoinitiator. The silica ETPTA dispersion is dispensed on a 3 acryloxypropyl trichlorosila ne (APTCS) primed (100) silicon wafer and spin coated at 8000 rpm for 3 min on a standard spin coater, yielding a hexagonally ordered colloidal monolayer. The monolayer is then photopolymerized for 4 s using a Pulsed UV Curing System. To utilize this monol ayer as a substrate for spin coating of the second layer, the polymer matrix between particle arrays is partially removed using a reactive ion

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84 etcher operating at 40 mTorr oxygen pressure, 40 sccm flow rate, and 100 W RIE power for 2 min 30 s. B inary L ayer of H exagonally N on close packed C olloidal S tructu r e 20 vol% silica colloid (400 nm) ETPTA dispersion (including 2 wt% photoinitiator) is prepared. The silica ETPTA dispersion is dispensed on a pre fabricated substrate, non closed packed colloidal monola yer and spin coated at 8000 rpm for 3 min on a standard spin coater, yielding a hexagonally ordered colloidal binary layer. The binary layer is then photopolymerized for 4 s using a Pulsed UV Curing System. For better ordering of silica particles in the se cond layer by applying a pressure, a glass slide is placed on top of the binary layer before photopolymerizing and then a weight (2 or 3.2 kg) is placed on this glass slide for 2 min. The pressure is calculated from the weight and the area of binary layer. The ETPTA monomer in the binary layer is then photopolymerized for 4 s using a Pulsed UV Curing System. T ernary L ayer H exagonally N on close packed C olloidal S tructure To utilize the binary layer as a substrate for spin coating of third layer, the polymer matrix is partially removed using a reactive ion etcher operating at 40 mTorr oxygen pressure, 40 sccm flow rate, and 100 W power for 2 min 30 s. The 20 vol% silica ETPTA dispersion (including 2 wt % photoinitiator) is dispensed on the prepared substrate a nd spin coated at 8000 rpm for 3 min on a standard spin coater, yielding a hexagonally ordered colloidal ternary layer. The ETPTA monomer in the ternary layer is then photopolymerized for 4 s using a Pulsed UV Curing System.

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85 Figure 4 1. Photonic c rystals : a ) photonic crystals in 1 D, 2 D, and 3 D and b) a photonic band gap compared to an electronic band gap. Adapted from [136]

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86 Figure 4 2. Schematic illustration of the procedure for fabricating binary hexagonal arrays of silica sph eres by using monolayer nonclose packed colloidal crystals as substrates Figure 4 3. Monolayer of nonclose packed silica particles (300 nm) fabricated by the spin coating technique.

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87 Figure 4 4. Binary hexagonal arrays of silica spheres: a, b) 3 00 nm particles array on 300 nm particles array (300/300), and c, d) 400 nm particles array on 300 nm particles array (400/300)

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88 Figure 4 5. Pressure effects on the ordering of particles in the second layer: a, b) 0.2 MPa for 2 min, and c, d) 0.3 3 MPa for 2 min in 300 nm particles arrays on 300 nm particles arrays (300/300), and e, f) 0.2 MPa for 2 min in 400 nm particles arrays on 300 nm particles arrays (400/300)

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89 Figure 4 6. Cross sections of hexagonal arrays of silica spheres: a, b) binar y layer of 400 nm particles array on 345 nm particles array (400/345) and c) ternary layer of 300 nm particles array on 400 nm particles array on 345 nm particles array (300/400/345)

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90 CHAPTER 5 BIOIN S PIRED, ORGANIC INORGANIC NANOCOMPOS ITES In nature, har d biological tissues show excellent mechanical properties due to their unique microstructures (Figure 5 1). [191] Scientists have been inspired by mechanical design principles found in natu re to generate new materials of high mechanical performance. The nacreous layer of mollusk shells has an intricate brick and mortar nanostructure consisting of 95 vol% brittle aragonite platelets and 5 vol% of soft biological macromolecules, making the she lls exceptionally tough and stiff. [191 196] Mimicking the unique structure in the nacreous layer has been tried by various bottom up self assembly techniques. Layer by layer (LBL) assembly of ceramic polymer alternative layers is successful in generating reinforced nanocomposites with aligned structures. [197, 198] Ice templated cryst allization, spin coating, gravitational sedimentation, and centrifugation have been explored to assemble ceramic platelets into ordered structures. [199 202] Electrophor etic deposition is a simple, inexpensive, and scalable process that enables rapid production of thick films over large areas, while LBL assembly is a slow process. [203 205] In addition, Nanocomposites can be assembled in a single step by electrophoretic codeposition of colloids and polymers. [206] For the layered nanocomposites of nanoclays and polymer, the agglomeration of nanoclays should be avoided for highly aligned structures. To achieve nanocomposites wit h a high level of strength and toughness, inorganic platelets should have high aspect ratio, small thickness, high shear stress or adhesion with polymer matrix, and high volume fraction (Figure 5 2). [191] Uniform gibbsite (A l(OH) 3 ) nanoplatelets can be synthesized by hydrolysis of aluminium alkoxide. [207,

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91 208] The aspect ratio of the synthesized gibbsite nanopl atelets (~ 10) is close to that of natural aragonite (CaCO 3 ) platelets in nacre. [193] The diameter and thickness can be controlled by seeded growth. [209] Due to the hydroxyl groups on the surface, different functionalities can be rendered by the chemical modification. [208] In this work [210, 211] synthesized gibbsite nanoplatelets are aligned and assembled by electrophoretic deposition. Inorganic organic nanocomposites having nacreous microstructures are achie ved by infiltrating monomer into the interstitials between the assembled nanoplatelets and polymerizing the monomer. The resulting nanocomposites exhibit significantly improved mechanical properties. By the surface modifications of gibbsite nanoplatelets, covalent linkages between the inorganic platelets and organic matrix are facilitated to create further reinforced nanocomposites. In this study, Huang synthesize s gibbsite nanoplatelets and Lin modif ies the surface s of gibbsite n anoplatelets and fabricates gibbsite polymer nanocomposites My contribution is characterizing the mechanical properties of the fabricated nanocomposites. A ssembly of C olloidal N anoplatelets Synthesized Gibbsite Nanoplatelets The gibbsite nanoplatelets synthesized from aluminium alk oxides in an acidic aqueous solution have a hexagonal shape and uniform size, as shown in Figure 5 3a. The diameter is about 188 40 nm and the measured thickness ranges from 10 to 15 nm, as measured by atomic force microscopy (AFM). The hydroxyl group s on the surface of gibbsite nanoplatelets can be modified by reacting with 3 (trimethoxylsily)propyl methancrylate (TPM) via the silane coupling reaction. [212] This TPM modifica tion forms dangling acrylate bonds which can be cross linked with acrylate based ethoxylated trimethylolpropane triacrylate ( ETPTA ) matrix.

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92 Assembly of Gibbsite Nanoplatelets by Electrophoretic Deposition Gibbsite nanoplatelets are assembled in a parallel plate sandwich cell under an electric field. Ethanol is added to the aqueous dispersions to promote colloidal coagulation on the ITO electrode by reducing the electric double layer thickness of the particles. In the resulting film, gibbsite nanoplatelets a re densely packed and aligned (Figure 5 3b). It is known that the isoelectric point (IEP) of the gibbsite edges (pH ~ 7) differs from the IEP on the gibbsite faces (pH ~ 10). [207] Thus, in the suspension, the nanoplatelets have positively charged faces and neutral edges. This charge distribution is favorable in alignment of nanoplatelets in parallel to the elect rode surface. Gibbsite polymer (ETPTA) nanocomposites are made by infiltrating photocurable monomers into the interstitials between the nanoplatelets in the assembled gibbsite film and photopolymerizing the monomer matrix. Mechanical Properties of Nanocomp osites The tensile stress strain curves for ETPTA, gibbsite ETPTA, and TPM modified gibbsite ETPTA films are shown in Figure 5 3c. The gibbsite ETPTA nanocomposite shows 2 fold higher tensile strength and 3 those of pure ETPTA polymer. The TPM modified gibbsite ETPTA nanocomposite displays 4 due to cross linking with ETPTA matrix. It is known that the covalent linkage between the inorganic fi llers and the organic matrix determines the mechanical properties of the nacres composites. [197]

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93 A ssembly of S urface roughened N anopl a telets Silica coated Gibbsite Nanoplatelets Si lica coated gibbsite nanoplatelets are synthesized by coating a thin shell of sol gel silica on the surface of gibbsite nanoplatelets. [213] The amphiphilic polyviny lpyrrolidone (PVP) macromolecule can be adsorbed onto a broad range of colloids stabilizing them in water and various nonaqueous solvent and acts as a coupling agent during this coating process. The TEM image in Figure 5 4a shows hollow silica nanoplatelet s after leaching out gibbsite parts. The silica shell has a thickness of 10 nm. The silica shells driven by the sol gel process are much rougher than the single crystalline gibbsite nanoplatelets. Assembly of Silica coated Gibbsite Nanoplatelets Silica coa ted gibbsite nanoplatelets are assembled in a parallel plate sandwich cell under an electric field. Cracks easily form on the silica coated gibbsite film during the drying process. Polyethyleneimine (PEI) is added to the bath solution of silica coated gibb site nanoplatelets to solve the cracking issue by increasing the adherence and strength of the electrodeposited films. PEI is known to act as a particle binder by adsorbing strongly onto silica at various pH. [214] Unlike gibbsite nanoplatelets, silica coated gibbsite PEI nanoplatelets have positive charges on both faces and edges due to the PEI macromolecules adsorbed on the uniform silica shell. There is no electric f ield induced reorientation of silica coated gibbsite PEI nanoplatelets due to the uniform distribution of surface charge. The formation of polymer bridges between neighboring particles also leads to the imperfect alignment of nanoplatelets. [214] Nevertheless, the nanoplatelets still preferentially

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94 aligned since the orientation is energetically favorable under electric field (Figure 5 4b ,inset ). Silica coated gibbsite p olymer (ETPTA) nanocomposites are made by infiltrating photocurable monomers into the interstitials between the nanoplatelets in the assembled gibbsite film under vacuum for a few hours and photopolymerizing the monomer matrix. Mechanical Properties of Nan ocomposites The tensile stress strain curves for ETPTA, gibbsite ETPTA, and silica coated gibbsite PEI ETPTA films are shown in Figure 5 4c. The silica coated gibbsite PEI ETPTA nanocomposite shows 2.5 fold higher tensile strength. This is due to the prese nce of PEI macromolecules, which strongly adsorbed on the negatively charged surface of silica coated gibbsite. The PEI macromolecules can also interlock with cross linked ETPTA backbone. The silica coated gibbsite PEI ETPTA nanocomposite shows 5 fold larg er elongation compared to that of pure ETPTA. The large el o ngation i s due to the strong ionic bonding between the PEI macromolecules and the nanoplatelets, and the natural elasticity of PEI. The surface roughness of nanoplatelets and the rotation of misaligned nanoplatelets under an applied tensile mode can also be a reason for the large elongation observed. Conclusions We developed a simple and rapid electrodeposition technique for assembling gibbsite nanoplatelets i nto organized multilayers. Nanoplatelets with high aspect ratio ( diameter thickness ratio) are aligned under electric field and the interstitials between nanoplatelets are infiltrated with polymer generating organic inorganic nanocomposites

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95 with significan tly improved mechanical properties. For further improvement in mechanical properties of nanocomposites, gibbsite nanoplatelets are surface treated (TPM or silica) and assembled into aligned multilayers. This technique is promising to achieve oriented depos ition of a wide range of materials including ceramics, metals, ceramic metal, or ceramic conducting polymer nanocomposites. M aterials and M ethods M aterials Ultrapure water (18.2 M cm 1 ) was used directly from a Barnstead water system. 200 proof ethanol is purchased from Pharmaco Products. Hydrochloric acid (37%), aluminum sec butoxide ( 95%), aluminum isopropoxide ( 98%) polyvinylpyrrolidone (PVP, M w ~ 40,000), polyethylenimine (PEI, 50 wt% in water, M w ~ 750,000), and sodium hydroxide ( 98%) were obta ined from Sigma Aldrich. Tetraethyl orthosilicate (TEOS, 99 %) was purchased from Gelest. Ammonium hydroxide (14.8 N) was obtained from Fisher Scientific. Ethoxylated trimethylolpropane triacrylate (ETPTA SR454 ) monomer was provided by Sartomer (Exton, PA). The photoinitiator, Darocur 1173 (2 hydroxy 2 methyl 1 phenyl 1 propanone), was obtained from Ciba Specialty Chemicals. Two part polydimethylsiloxane (PDMS, Sylgard 184) was provided by Dow Corning. Indium tin oxide ( ITO ) coated glass substrates with sheet resistance of 8 ohms were purchased from Delta Technologies. Silicon wafers (test grade, n type, (100)) were purchased from University Wafer. I nstrumentation An EG&G Model 273A potentiostat/galvanostat was used for electrophoretic deposition. Scannin g electron microscope (SEM) was carried out on a JEOL 6335F

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96 FEG SEM. Transmission electron microscope (TEM) and selected area electron diffraction a were performed on a JEOL TEM 2010F. Atomic force microscope (AFM) was conducted on a Digital Instruments Dim ension 3100 unit. A standard spin coater (WS 400B 6NPP Lite spin processor, Laurell) was used to spin coat ETPTA monomer. The polymerization of ETPTA was carried out on a pulsed UV curing system (RC 742, Xenon). A kurt j. Lesker CMS 18 Multitarget Sputter was used for the deposition of Ti and Au. S ynthesis of G ibbsite N anoplatelets The gibbsite nanoplatelets were synthesized by following a published method. [207] H ydrochloric acid (0.09 M), aluminum sec butoxide (0.08 M), and aluminum isopropoxide (0.08 M) were added to 1 L ultrapure water The mixture was stirred for 10 days and then heated in a polyethylene bott le in a water bath at 85 C for 72 h. After cooling to room temperature, dispersions of gibbsite nanoplatelets were centrifuged at 3500 g for 6 h and the sediments are redispersed in deionized water. For completely removing the unreacted reactants and conc entrating the nanoplatelets, this process was repeated for five times. S urface M odification of G ibbsite N anoplatelets with TPM Gibbsite nanoplatelets were surface modified with 3 (trimethoxysilyl)propyl methacrylate (TPM) [215] Prior to adding gibbsite nanoplatelets, 10 mL TPM was mixed with a 100 mL water methanol solution (wa ter/methanol volume ratio of 3:1) for 1 hour in order to fully hydrolyze TPM. Surface modification was then accomplished by adding 100 mL of gibbsite dispersion (ca. 1 vol% aqueous solution) into the hydrolyzed TPM solution. The suspension was stirred at 4 0 C for 30 min. The modified nanoplatelets

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97 were washed by repeated centrifugation redispersion cycles with pure ethanol and finally concentrated to a stock suspension of 0.045 and 0.035 ( g/g ) in ethanol. Coating of Gibbsite Nanoplatelets with Silica Purif ied gibbsite nanoplatelets were coated with a thin layer of silica by a two step procedure: adsorption of PVP and growth of silica shell via Stober method. [129] PVP was dissolved in DI water by ultrasonication. Subsequently, 200 mL aqueous solution of gibbsite nanoplatelets (1 wt%) was mixed with 300 mL PVP solution (10 wt%). Then, the mixture was stirred for 1 d ay for the complete adsorption of PVP on the gibbsite surface. PVP coated gibbsite nanoplatelets were transferred into ethanol by repetitively centrifuging the mixture and redispersing the sediments in ethanol three times. The PVP modified gibbsite nanopla telet suspension of 500 mL was mixed with 33 mL ammonium hydroxide and 1 mL TEOS for the growth of silica shell. After stirring for 4 6 h, silica coated gibbsite nanoplatelets were transferred into water by centrifuging the dispersion and redispersing the sediments in DI water. E lectrophoretic D eposition Electrophoretic deposition of nanoplatelets was performed in a horizontal sandwich cell. The bottom and the top of the cell were an ITO working electrode and a gold counter electrode, respectively with a P DMS spacer The active area and cell gap were 1.51.5 cm 2 and 2.2 mm respectively The bath solution s for gibbsite ETPTA and TPM modified ETPTA were nanoplatelet dispersions in water ethanol mixture s T he volumetric ratio of ethanol to the aqueous suspens ion was 2. The bath solutions for silica coated gibbsite PEI ETPTA were prepared by mixing 9 mL of 1.5 wt% silica coated gibbsite nanoplatelet aqueous solution with 1 mL 1.5 wt% PEI aqueous solution and ultrasonicating the mixture. A constant voltage of 2 .5 V (ITO vs. Au) was applied to

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98 deposit the positively charged nanoplatelets onto the ITO cathode. T he electroplated gibbsite films were rinsed with 200 proof ethanol M echanical T est T ensile strength s were measured using an Instron model 1122 load frame upgraded with an MTS ReNew system and equipped with a 500 g load cell at a crosshead speed of 0.5 mm/min. Testing samples with width of 1.5 mm and thickness ranging from 30 to 80 m were adhered on home made sample holders with a 10 20 mm gap using polyu rethane monomer (NOA 60, Norland) as an adhesive and then UV cured. The thickness of the tested samples was measured by cross sectional SEM to calculate the final tensile strength.

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99 Figure 5 1. Hard biologica l tissues and their microstructures: a) tooth, b) vertebral bone, c) shell d) Enamel made of long needle like crystals with soft protein matrix, e) dentin and bone made of plate like crystals embedded in a collagen rich protein matrix, and f) nacre made o f plate like crystals with a very small amount of soft matrix in between. Adapted from [191] Figure 5 2. A model of biocomposites : a ) a schematic diagram of staggered mineral crystals embedded in protein matrix and b ) a simplified model showing the load transfer mechanism in the mineral protein composites. Adapted from [191]

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100 Figure 5 3. Nanoco m posite of colloidal nanoplatelets : a) TEM image of gibbsite nanoplatelets, b) SEM image of a free standing gibbsite ETPTA nanocomposite, and c) tensile strain stre ss curve s for plain ETPTA film, gibbsite ETPTA nanocomposite, and TPM modified gibbsite ETPTA nanocomposite. Adap ted from [210]

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101 Figure 5 4. Nanoco m posite of surface roughened nanop latelets : a) TEM image of acid leached silica coated gibbsite nanoplatelets, b) SEM image of silica coated gibbsite PEI ETPTA nanocomposite on an ITO electrode, and c) tensile str ain stress curve s for plain ETPTA film, gibbsite ETPTA nanocomposite, and sil ica coated gibbsite PEI ETPTA nanocomposite. Adapted from [211]

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102 CHAPTER 6 CONCLUSIONS AND OUTL OOK Functional nanoscale materials mimicking nanostructures found in natu re are investigated in a broad range of fluidic, optical, and mechanical systems. S elf pumping M embrane The developed self pumping membrane harvest s chemical energy from the surrounding fluid and uses it for accelerated mass transport of 0.9 nL cm 2 s 1 ac ross the membrane. Micro and macroscopic devices for drug delivery, sensing and purification, as well as oil recovery and removal may benefit from this technology. The self pumping membrane still needs to be improved by optimizing the el ectrochemical reaction, the materials of membrane and electrodes, and the geometry of the membrane. The removal of oxygen gas generated on the electrode surfaces should be studied to improve pumping efficiency. R eproducible and H ighly SERS active Su bstrate s Gold nanoparticle and nanohole combined arrays are developed as reproducible (low variation of Raman signal on different sites) and highly active SERS (high SERS EF s and high signal to background ratio s ) substrates via the cost effective technology Due to a high sensitivity as well as a good reproducibility these SERS substrates are pr omising in applications of ultra sensitive detectors for chemicals and reproducible sensors for chemical and biological molecules. Potential Applications as a Hybrid Biosensor Robust and stable synthetic transport systems and effective detection systems ar e essential to solve the problems of poor stability and weak fluorescence in hybrid biosensors. A biomimetic, self pumping membrane can be integrated as a robust

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103 transport system. Reproducible and highly SERS active substrates can be integrated as effectiv e detection systems. SERS sensors are known as label free detecting systems. Thus, a simpler configuration of biosensor device is feasible without an analyte tagging process for detection. B inary C olloidal C rystals Binary colloidal crystals consisting of c olloidal particles of different sizes are achieved wi th a simple fast, and scalable spin coating technology The thickness of individual layer is easily controlled with spin coating parameters. Although the pressure is helpful for ordering of particles, f urther improvement for better orderings of particles in the individual layers is still needed. The characterization of optical properties in the fabricated binary colloidal crystals is in progress. O rganic inorganic N anocomposites Gibbsite pol ymer nanocomposites are developed via a simple and rapid electrodeposition. Gibbsite nanoplatelets having high aspect ratio (> 10) are aligned under electric field and the interstitials between nanoplatelets The assembled nanoplate lets are infiltrated wi th monomer, and monomer is cured by UV generating or ganic inorganic nanocomposites The developed gibbsite ETPTA composite shows 2 fold higher tensile strength than pure polymer TPM mo dified gibbsite ETPTA composite has 4 fold higher tensile strength and silica coated gibbsite PEI ETPTA composite has 5 fold larger elongation. Although this technique is promising to achieve oriented deposition of a wide range of materials including ceramics, metals, ceramic metal, or ceramic conducting polymer nanocomposites, the alignmen t of nanoplatelets and the complete infiltration of

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104 polymer matrix between aligned nanoplatelets are still under investigation for better mechanical performance. The self pumping membrane designed and demonstrated in this dissertation will stimulate the re search of converting chemical energy into kinetic energy directly with high efficiency, especially in micro and nanodevices. The simple, scalable, and cost effective spin coating technique utilized in the fabrication of binary colloidal crystals and hole particle arrays in nanoscale can replace expensive and slow top down techniques. On the other hand, the simple and fast electrophoretic deposition technique employed in the assembly of nanoplatelets can be substituted for slow bottom up techniques. The fun ctional nanomaterials and techniques discussed in this dissertation represent a contribution to achieve and control nanostructures via simpler, cheaper, and faster technologies.

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105 APPENDIX A ELECTROOSMOTIC FLOW E lectroosmotic F low in O ne wall C hannel In the one wall case, i f the ionic concentration is C 0 and electrical potential goes to zero and the potential takes the value, (zeta potential), at the surface, the ionic concentration are given by standard expression from statistical physics. [216] (A 1) The assumption ( << k B T ) is known as the Debye Hckel approximation. The charge density el is, (A 2) With the Poisson equation ( ), the electrical potential is where the Debye length (A 3) E lectroosmotic F low in Cylindrical Tube The Navier Stokes equation for steady static cylindrical tube is, (A 4) (A 5) With the boundary conditions ( C 1 = 0 ), the fluid velocity is, (A 6)

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106 With boundary conditions ( ), t he fluid velocity is, (A 7) In the analogy with the one wall case, the electric potential, (r), is (A 8) The fluid velocity v z (r) is, (A 9) The average fluid velocity < v z >, is, (A 10) where The charge density el is calculated from the Poisson equation.

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107 (A 11) The average charge densi ty < el >, is calculated, (A 12) The volume occupied by proton flowing through the cylindrical tube for time t V is, (A 13) The total charge of proton flowing through the tube for time t C total is, (A 14) Then the flowing current I is, (A 15) The velocity of proton v proton is calculated from the balance between the electric force and the drag force of proton in the electric field

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108 (A 16) In the membrane having cylindrical pores, the porosity of membrane p is (A 17) where N pore is the number of pore and A is the total area of membrane. The total current flowing through membrane I tota l is, (A 18) The fluid velocity v fluid is, (A 19) The maximum pumping pressure (counter pressure) is calculated by applying zero velocity in the Navier Stokes equat ion. (A 20) ( A 21) where el is not constant, but a function of r The average pressure, < P >, is calculated by < P > = < el > Ed.

PAGE 109

109 (A 22)

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110 APPENDIX B SELF PUMPING FLOW F low R ate as a F unction of T racer V elocity According to Holmes and Vermeulen [217] the f luid velocity in the narrow rectangular channel, v ( x y ) is ( B 1) where v max is the maximum velocity in the center of the channel and equals the measured tracer velocity, and the width B and the height, H of the narrow channel are 3.210 4 m, 5.110 5 m, respectively. The flow rate through the channel, Q channel is calculated from the measured tracer velocity, v max ( B 2) For a channel of the given dimensions, this yields ( B 3) C ondu ctivity of W orking F luid The measured slope of 120 V 1 for the I V curve for electroosmotic pumping (Fig ure 2 7 ) implies an ohmic resistance of 8.4 by ( B 4) where the length of the pores, l is 1.810 5 m, the membrane area, A is 910 5 m 2 and the porosity of the membrane, p is 12 %

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111 W e obtain a conductivity, k of 2.0 10 4 S m 1 which compares well with the conduct ivity of de ionized water of 9.910 5 S m 1 [218] F low R ate as a F unction of C urrent The flow rate through membrane, Q pore equals to the flow rate through channel, Q channel The flow through the pore is composed of an electroosmotic and counter pressure component ( B 5) According to Lazar and Karger, [40] the flow rate due to electroosmosis, Q electroosmotic is ( B 6) with as the permittivity and as the viscosity of the working fluid, as the zeta potential, d as the diameter and l as the length of the pore, and U as the applied voltage and N pore as the number of pores. Utilizing U = IR together with ( B 4 ) and pA = d 2 N pore /4 we obtain ( B 7) Using = 7.0810 10 C V 1 m 1 (the permittivity of water at 20 ) [218] 27 mV [71] = 1.00210 3 N m 2 s (the viscosity of water at 20 ) [218] and k = 2.010 4 S m 1 we obtain ( B 8) According to Lazar and Karger, [4 0] the flow rate due to counter pressure, Q counter pressure is

PAGE 112

112 ( B 9) with P pore as the pressure differential across the membrane. Since this pressure is balanced by the pressure differential across the small channel P channel we can write ( B 10) According to Holmes and Vermeulen [217] the flow rate through a rectangular channel can be approximated by ( B 11) with the channel length L c = 6 mm, the channel width B = 3 H M given by ( B 12) Inserting ( B 11) into B (10), and the resulting expression into ( B 5), we obtain ( B 1 3 ) which simplifies to ( B 1 4 ) Using d and l obt ain

PAGE 113

113 ( B 1 5 ) F low R ate at Z ero O pposing P ressure The flow rate in the absence of the counter pressure is equal to the flow rate due to electroosmosis calculated in ( B 7) and ( B 8). Given the maximum available current of this translates into a maximum flow rate of 25 nL s 1 Stall pressure at zero flow The stall pressure can be calculated by equating ( B 7) and ( B 9) to be ( B 1 6 ) For a current of 0. this yields a stall pressure of 1.4 Pa. C onsumption of H 2 O 2 and D ecreasing F low R ate The O 2 generation rate per electrode area, k O2 is (B 17) where F is the Faraday constant, and f is the fraction of oxygen which originates from the electrochemical reaction H 2 O 2 2 + 2 H + + 2 e (and not from 2 H 2 O 2 2 O + O 2 ). Paxton et al estimated f = 40 % based on their measuremen ts of O 2 generation and current density for gold/platinum microelectrodes, [56] which implies (B 18) Since 60% of the oxygen molecules require 2 hydrogen peroxide molecules to be produced and 40% of the oxygen molecules requi re only one hydrogen peroxide molecule, the hydrogen peroxide consumption rate is given by

PAGE 114

114 (B 19) The fraction of the initially availab le hydrogen peroxide which is consumed after time t is approximately given by (B 20) where c is the hydrogen peroxide concentration (0.01 wt%), r is the density of the solution (1 g cm 3 ) and V is the volume of the solution (4.3 mL), and M is the molecular weight of hydrogen peroxide (34 g mol 1 ). Inserting (B 17) into (B 20) yields (B 21) At the end of the experiment described in Figure 2 8, 270 minutes have passed, so that (B 22) In contrast, the measured decrease in the flow rate and the current is ab o ut 20 %. However, the f raction of oxygen originating from the electrochemical reaction may be sensitive to the experimental conditions, and only a fraction of the total hydrogen peroxide in the cell may be locally available to the membrane. This may cause us to underestimate the hydrogen peroxide depletion. Alternatives, which cannot be ruled out at this time, are a reduction in the catalytic efficiency o f the membrane and partial clogging of the pores with tracer particles.

PAGE 115

115 APPENDIX C FLOW RATE MEASUREMEN T Length converting M ethod The flow rate is measured by tracing the flow front in test section with a ruler (Figure C 1a). [74] The flow rate is calculated by multiplying the velocity of the flow front with the cross section of channel. The error in the flow rate measured by this method is within 50 nL min 1 Mass converting M ethod The flow rate is measured with a digital balance (Figure C 1b). [42, 219] The flo w rate is calculated by dividing the change in mass of the reservoir with the density of fluid. Heads of the head tank and the weighing reservoir are set at the equal height for equilibrium pressure. The measured flow rate is in the range from1 to 1 [219] Brask et al reported the accuracy of the flow rate within 1 with a balance of 0.1 mg precision. [42] C urrent monitoring M ethod The electroosmotic flow rate is determined by the current monitoring method (figure C 1c). [220] When the fluid is driven through a capillary tube from reservoi r 1 to reservoir 2 by the electroosmotic flow, the change in measured current corresponds to the volume of driven fluid from reservoir 1 to reservoir 2. P article I mage V elocimetry M ethod The flow rate is measured by tracing the marker particles (Figure C 1 d). [221, 222] The two syringes connected to the ends of the channel in the micropump are set at the equal height for equilibrium pressure. The speed of an air slug in the channel is measured. The pressure difference due to electroosmotic pumping is almost zero since

PAGE 116

116 the liquid level change in the syringe is negligible due to large diameter of the syringe. This method can measure the flow rate in the range of nL min 1 C oncentration monitoring M ethod The flow rate is estimated based on the change in concentrations of dye in compartment 1 and compartment 2 (Figure C 2). This method is similar to the current monitoring method. The concentration is measured with the absorban ce or fluorescence of dye instead of current. To remove back pressure problems, double membrane based electroosmotic pumps are used. A flow is driven by the first electroosmotic pump from the compartment 1 having volume, V 1 and concentration, C 1 of dye t o the compartment 2 having volume, V 2 and concentration, C 2 of dye. At the same time, a flow is also driven by the second electroosmotic pump from the compartment 2 to the compartment 1. It is assumed that the pumping rates of the first and second electr oosmotic pumps, k c1 c2 and k c2 c1 are identical (k= k c1 c2 = k c2 c1 ) and the diffusion constant of dye, D is constant. The dye flux through the membrane (electroosmotic pump), J dye is, (C 1) where t is the elapse time and A is the area of membrane. (electroosmotic pump), J dye is, (C 2) By combining (C 1) and (C 2), the following relationship is obtained.

PAGE 117

117 (C 3) where l t C 1,0 C 2,0 are the thickness of membrane, time, initi al concentration in compartment 1, and initial concentration in compartment 2, respectively. From the slope in a plot drawn with experimental data, the diffusion constant, D and the pumping rate, k by the electroosmotic pump can be estimated (Figure C 3)

PAGE 118

118 Figure C 1. Various methods for measuring the flow rate : a) length converting method, b) mass converting method, c) current monitoring method, and d) particle image velocimetry method Adapted from [42, 74, 220, 221] Figure C 2. The double membrane configuration having compartment 1 a n d compartment 2 in the concentration monitoring method

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119 Figure C 3. Plot of function, f with time. T he diffusion constant, D and the pumping rate, k by the electroosmotic pump can be estimated from the slope

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136 BIOGRAPHI C AL SKETCH In kook Jun grew up in Seoul, Republic of Korea degree at the Seoul National University in 200 3 and his master degree at the Seoul National University in 200 5 He started his military service in November 1998 and he was discharged in January 200 1 H e joined the group of Dr. Henry Hess in the Materia ls Science and Engineering Department and the group of Dr. Peng Jiang in the Chemical Engineering department at the University of Florida, Gainesville. He graduated in the Summer 20 10 after spending four years being educated in material s science and chemical engineering