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. 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.
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. Nevertheless, the nanoplatelets still preferentially
Photonic band gap
Proton exchange membrane fuel cell
Perfect matched layer
Reactive ion etching
Scanning electron microscopy
Surface enhanced Raman scattering
Surface enhanced Raman scattering enhancement factor
Tranmission electron microscopy
Trimethoxysilyl propyl methacrylate
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.
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. 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. In the established spin-
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. The
bandwidth of dielectric materials is about 3 to 4 orders of magnitude larger than that of
metals. 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. 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
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.
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
Materials and Methods
Materials and Substrates
Monodispersed silica colloids with less than 10% diameter variation are
synthesized by the Stober method. 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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* 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 .
1000 1200 1400 1600
Raman shift (cm-1)
Figure 3-5. Representative Raman spectrum of benzenethiol adsorbed on a SERS
substrate. Adapted from .
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.
3 electroosmotic flow L
-80 -60 -40 -20 0 20 40 60 80
Figure 2-7. Flow rate dependent on current in electroosmotic pumping. In the
electroosmotic flow, the measured flow rate increases linearly with increasing
ow rate ^ 1.0 0 Pt Au 2
current .1 0 9 P t Au
S0 .0 ........................ .. .. .. .. .. ..........
-0.5 A -1 P
1.0 Au Pt 00 0
-1.0 -0.5 0.0 0.5
............... ... ..... .. ..O .. .. ... .. ..................
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, while the average enhancement factor was used to demonstrate the
effectiveness of the nanostructure as a SERS substrate, for example, in Jung's
paper. 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).
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. Based on the T-matrix theory, the
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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. 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. 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.
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. 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
Reactive ion etch
r. Au film
'eposit Au nanoparticle
move silica & polymers
Figure 3-9. Schematic diagram depicting the fabrication procedures for making GNP-
0 50 100 150 200 250
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.
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. 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.
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-
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
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
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.
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t7 3+2 20
I 05 5 l t 8
,I 0 20I
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-
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
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. 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.
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.
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. Colloidal substrates tend
to aggregate and thus the molecular surface coverage changes with time. Variation in
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.
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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.
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
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.
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-
Channel Qpore Qelectroosmotic Qconter pressure (B-5)
According to Lazar and Karger, the flow rate due to electroosmosis,
Qelectroosmtic c d pore (B-6)
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)
Using e = 7.08x10-10 C V-1 m-1 (the permittivity of water at 20 C), (~O= -27
mV, r7 = 1.002x10-3 N m-2 s (the viscosity of water at 20 C), and k = 2.0x10-4 S
m-1, we obtain
Qe1ectroomotic =(97 nL pA-' s1-). I (B-8)
According to Lazar and Karger, the flow rate due to counter pressure, Qcounter-
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. 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. 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
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. 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. 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.
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
a Au electrode Pt electrode
H2,O, 2H' 2e 2H.O H,O, -0 O, 2H- + 2e
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.
-1.5 -1 -0.5 0 0.5 1 1.5
Figure 3-6. The SERS enhancement calculated based on finite element methods in the
case of nanoparticle arrays. Adapted from .
A :4:I: d
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
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.
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
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.
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
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. 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
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.
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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
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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. 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
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. 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 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.
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
208] The aspect ratio of the synthesized gibbsite nanoplatelets (~ 10) is close to that of
natural aragonite (CaCO3) platelets in nacre. The diameter and thickness can be
controlled by seeded growth. Due to the hydroxyl groups on the surface, different
functionalities can be rendered by the chemical modification.
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. This
TPM-modification forms dangling acrylate bonds which can be cross-linked with
acrylate-based ethoxylated trimethylolpropane triacrylate (ETPTA) matrix.
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.
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. Electroosmotic micropumps have
been integrated into proton exchange membrane fuel cells (PEMFCs) to remove water
from cathodes. Electroosmotic pumps have also been used as fuel delivery systems
in direct methanol fuel cells (DMFCs). 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.
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 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
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). 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). The excitation wavelength of localized surface plasmon
resonance (LSPR) was controlled with various dimensions of the nanoparticles and the
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. 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 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 . 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.
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
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
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
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
In the membrane having cylindrical pores, the porosity of membrane, p, is,
A pr R2 Npore
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)
_ e;TeR2 Nre (- 2) tah R
S 6rproo RD AD
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
where pel is not constant, but a function of r.
The average pressure,
, is calculated by
= < pei>Ed.
infiltrating monomer between platelet layers and polymerizing monomer, and investigate
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Materials and Methods................................ ............... 95
M aterials................................................. ............... 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
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
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).
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
Synthesis of Gibbsite Nanoplatelets
The gibbsite nanoplatelets were synthesized by following a published
method. 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). 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
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).
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. 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. 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. The
Raman signals from self-assembled particles as a SERS substrate were ~ 107 higher
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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
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
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
aligned since the orientation is energetically favorable under electric field (Figure 5-
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
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
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
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, 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. 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. 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 to be -27
mV and considering the pressure-induced reverse flow caused by the resistance of the
Flow Rate as a Function of Tracer Velocity
According to Holmes and Vermeulen, 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
QchH annel 54 2 H B[ 4m 1- 2 1 54 d2
2 f2... L7 B l)2 2 2 _B2
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)
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%.
< >= -I" Ed
*proton ;R pore
I oal Ed
6 z proton
6zpro~ total d 6zrproton I tota d
R 2eNpore eApor
TABLE OF CONTENTS
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
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
Figure 3-8. SEM images of 30 nm gold nanoparticle arrays with a gap between
particles on glass substrates.
magnitude weaker than fluorescence emission. Thus, the applicability of Raman
scattering is restricted to structural analysis.
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.
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.
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). When light interacts with
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. 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.
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
LIST OF ABBREVIATIONS
Atomic force microscopy
Direct methanol fuel cell
Electronic beam lithography
Ethoxylated trymethylolpropane triacrylate
Field emission gun scanning electron microscopy
Finite element method
Focused ion beam
Human immunodeficiency virus
Inductively coupled plasma
Indium tin oxide
Localized surface plasmon resonance
Metal film on nanosphere
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.
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
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.
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. 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.
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. 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.
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ultra-sensitive detectors for chemicals and reproducible sensors for chemical and
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
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.
maximum calculated enhancement factor (Gmax) on one-dimensional arrays of silver
nanoparticles was calculated to be 109. 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
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. 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. 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)
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Qchannel = 97 nL (2.9 nL A- s-) I (B-15)
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
Pstall = 3 I (B-16)
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
ko =- (B-17)
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, 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
G = CNAo-hIurf
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.
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.
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
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.
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
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
The enhancement factor for SERS is calculated as
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. 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
0.01 slope + vl A- +
0 10 20 30 40 50 60
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.
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IP"w 5 urr
Figure 1-1. Functional nanoscale materials in nature: a) moth-eye, b) gecko's foot, c)
lotus leaf. Adapted from [2, 4, 14-16].
0.05 0.10 0.15 0.20 0.25
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
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
Deprotonaled / i iiini
Silariol groups Flow
Figure 2-1. Electroosmotic flow in a channel. Adapted from .
+ electrodes All depth=20pm
S M ianlhic pumping
An-dc C Clak, Ac"ic hwsing
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].
Output EOF / -v ,r
- I *3UU. l
7 5-530pm ;
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].
S Electronic Band Gap
VaI ln Ba
3 Photonic Band Gap
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 .
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.
C+ = Co expT i (z)J Co Ze k(z), whereZe <
The assumption (Zep << kBT) is known as the Debye-HCckel approximation.
The charge density, pei, is,
Pea 2 o(z) (A-2)
With the Poisson equation (0"= -Pe,/e), the electrical potential, cp, is,
(z)= exp where the Debye-length AD= 2(Z--C (A-3)
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
vz = + C (A-6)
In nature, functional nanoscale materials exhibit unique optical, mechanical, and
functional properties, usually obtained from hierarchical structures. For example, a
moth's eye has many hexagonally shaped facet lenses on which highly ordered arrays
of nanoscale nipples are located. 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. The nanoscale hair-like structures on
micro-scale mound-like structures protruding from its leaf are responsible for the
superhydrophobicity in a Lotus-leaf.
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. They gain electrical energy from
oxidizing fuel and reducing oxygen. Highly active catalysts for the electrochemical
50 ....... JIUU ISL I I" I
I 20 '-
0.00 0.01 0.02 0.03
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 .
---.- TPM-modified Gibbsite+ETPTA
A :L L.:4-l -rT A
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.
30 nm gold particles.
2010 In-Kook Jun
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
Monodispersed silica colloids with less than 10 % diameter variation are
synthesized by the Stober method. 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).
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. 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. The final step of the spin coat process is 8000 rpm for 5 min.
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. h
is measured to be 400 pm with the 20x objective.
Silica particle of different size
SO plasma etching
k Spin-coating &
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
Au / ---
a 02 plasma
Figure 3-7. Schematic diagram depicting self-assembly of gold nanoparticles using a
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
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), although it is thermodynamically favorable. In our spin-
coating process with a template layer, the "trapping effect" due to the geometry of
oxygen. A Pd/Au patterned surface in hydrazine solution showed electroosmotic
flow in a catalytically induced electric field.
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.  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
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. 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. 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.
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
1+C2,0 V2 V2 C2
Ino I0 1 ) + k t. __ (C-3)
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).
beam lithography, enhancement factors as large as 108 have been achieved by
controlling the inter-particle spacing.
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. 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.
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
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
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.
An EG&G Model 273A potentiostat/galvanostat was used for electrophoretic
deposition. Scanning electron microscope (SEM) was carried out on a JEOL 6335F
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
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. 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 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