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Nanofabrication Based on Self-Ordered Porous Anodic Alumina

Permanent Link: http://ufdc.ufl.edu/UFE0041622/00001

Material Information

Title: Nanofabrication Based on Self-Ordered Porous Anodic Alumina
Physical Description: 1 online resource (136 p.)
Language: english
Creator: Park, Dooho
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: alumina, nanofabrication, nanotube
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: NANOFABRICATION BASED ON SELF-ORDERED POROUS ANODIC ALUMINA Self-ordered porous nanostructures have attracted a lot of attention because they can be used as starting materials in both the top-down and bottom-up approaches. The typical dimension of pores can be varied from a few nanometers to many micrometers. The highly-ordered patterns can be transferred to desired materials by the top-down method. In the bottom-up approaches, the porous substrates can be used as a template to fabricate nanomaterials with narrow size distribution. One of the most widely used porous substrates is self-ordered porous anodic alumina (PAA). This dissertation describes the control over the morphology of PAAs and their applications in both top-down and bottom-up approaches for preparing nanostructures such as the template synthesis and the pattern transfer, respectively. First, a highly self-ordered porous anodic alumina has been fabricated by the anodization of Al in appropriate acidic solutions. It exhibits narrow distributions in pore size, pore density, porosity and pore depth. Different types of PAAs were prepared for the template synthesis and the pattern transfer, respectively. For the former, PAA was attached on Al and the pore size was controlled by the post etching treatment. For the latter, free standing PAA?s were prepared and used as a mask. Two different pore sizes (~80 nm and ~50 nm) were prepared under the different anodization voltages. The thickness of PAA showed a good linear relationship with the anodization time. In the template synthesis study, the channel shape of PAA was tuned from straight-through type to branched one. Bifurcated silica nano tubes with a control over the length of each segment were fabricated from the template. In addition, straight-through silica nano test tubes were filled with gold by using the gold electroless plating. The aspect ratio of gold cores was controlled by the inner space of silica test tubes. The optical properties of gold-filled silica nano test tubes were investigated. In the pattern transfer study, the PAA pattern was successively transferred onto silicon via an Ar plasma etch method. The pore depth linearly increased as the etch time increased. The porous silicon substrates were used as platforms in Laser Desorption/Ionization Mass Spectrometry. The effect of pore depth on the ionization efficiency was investigated. Significant improvement in the ionization was observed by increasing the pore depth from 10 nm to 30 nm.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Dooho Park.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Martin, Charles R.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2012-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041622:00001

Permanent Link: http://ufdc.ufl.edu/UFE0041622/00001

Material Information

Title: Nanofabrication Based on Self-Ordered Porous Anodic Alumina
Physical Description: 1 online resource (136 p.)
Language: english
Creator: Park, Dooho
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: alumina, nanofabrication, nanotube
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: NANOFABRICATION BASED ON SELF-ORDERED POROUS ANODIC ALUMINA Self-ordered porous nanostructures have attracted a lot of attention because they can be used as starting materials in both the top-down and bottom-up approaches. The typical dimension of pores can be varied from a few nanometers to many micrometers. The highly-ordered patterns can be transferred to desired materials by the top-down method. In the bottom-up approaches, the porous substrates can be used as a template to fabricate nanomaterials with narrow size distribution. One of the most widely used porous substrates is self-ordered porous anodic alumina (PAA). This dissertation describes the control over the morphology of PAAs and their applications in both top-down and bottom-up approaches for preparing nanostructures such as the template synthesis and the pattern transfer, respectively. First, a highly self-ordered porous anodic alumina has been fabricated by the anodization of Al in appropriate acidic solutions. It exhibits narrow distributions in pore size, pore density, porosity and pore depth. Different types of PAAs were prepared for the template synthesis and the pattern transfer, respectively. For the former, PAA was attached on Al and the pore size was controlled by the post etching treatment. For the latter, free standing PAA?s were prepared and used as a mask. Two different pore sizes (~80 nm and ~50 nm) were prepared under the different anodization voltages. The thickness of PAA showed a good linear relationship with the anodization time. In the template synthesis study, the channel shape of PAA was tuned from straight-through type to branched one. Bifurcated silica nano tubes with a control over the length of each segment were fabricated from the template. In addition, straight-through silica nano test tubes were filled with gold by using the gold electroless plating. The aspect ratio of gold cores was controlled by the inner space of silica test tubes. The optical properties of gold-filled silica nano test tubes were investigated. In the pattern transfer study, the PAA pattern was successively transferred onto silicon via an Ar plasma etch method. The pore depth linearly increased as the etch time increased. The porous silicon substrates were used as platforms in Laser Desorption/Ionization Mass Spectrometry. The effect of pore depth on the ionization efficiency was investigated. Significant improvement in the ionization was observed by increasing the pore depth from 10 nm to 30 nm.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Dooho Park.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Martin, Charles R.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2012-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041622:00001


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1 NANOFABRICATION BASED ON SELF ORDERED POROUS ANODIC ALUMINA By DOOHO PARK A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010

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2 2010 D ooho P ark

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3 To my father, Jaekwan Park, my mother, Seonok Lee, my father in law, Soonkee Moon, my mother in law, Jongrye An, my wife, Heh Young Moon and my daughter, Suebin Park

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4 ACKNOWLEDGMENTS I would like to sincerely thank Dr. Charles Martin for his patience in my progress, encouragement and guidance in my research. To say nothing of providing lots of advice on my resear ch, Dr. Martin taught me how to approach science and improve my scientific communication skills. Dr. Myungchan Kang should be thanked for his great efforts in training me in Atomic Force Microscope (AFM), ion plasma etching machine and other analytical tec hniques and instrumentation to fabricate nanowell arrays. I should thank Dr. Weihong Tan, Dr. David Powell, and Basri Gulbakan for a great collaboration of Mass Spectrometery project. I would like to thank Dr. Lane Baker for ATR FTIR and his suggestions through the project and Dr. Youngseon Choi for their insightful advice on my research. I should also thank Dr. Jon D. Stewart and Dr. Jillian Perry for a help about the fabrication of si lica nano test tubes. I wish also to thank all other committee, Dr. Donn Dennis, and Dr. Charles Y. Cao for their valuable time. Dr. Nicolo Omenetto, Mario Caicedo, Dr. Hitomi Mukaibo, and Gregory W. Bishop were always willing to discuss about my project and give me suggestions. Daniel Edward Shelby, Otonye Braide, Jonathan Alan Merten, Funda Tongay, group member should be thanked for having been unanimously supportive and sharing their knowledge. Dr. Ben Smith, Cindi Marsh Lori Clark, and Antoinette Knight are supposed to be thanked for th e i r assistance. I have to say thank my friends Youngseok Kim and Younghwan Namgung for their warm friendship during my life in Gainesville. I would like to sincerely thank my father, Jaekwan Park, my mot her, Seonok Lee, my father in law, Soonk ee Moon and my mother in law, Jongrye An. I would not succeed without their tremendous support

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5 Finally, a special thank goes to my wife Heh Young Moon for sharing all my concerns and supporting me physically and me ntally and my daughter, Suebin Park for b ring us a beautiful mind and happiness

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURES ................................ ................................ ................................ ........ 10 ABSTRACT ................................ ................................ ................................ ................... 15 CHAPTER 1 NANOFABRICATION ................................ ................................ ............................. 17 Porous Nanostructures in Nanofabrication ................................ ............................. 17 Bottom Up Approach: Template Synthesis Based on Porous Membranes ............. 18 Track Etch Membranes ................................ ................................ .................... 19 Porous Alumina Membranes ................................ ................................ ............ 19 Top Down Approach: Pattern Transfer Using Porous Masks ................................ 20 2 FABRICATION OF POROUS ANODIC ALUMINA ................................ ................. 24 Self Ordered Porous Anodic Alumina (PAA) Membrane ................................ ......... 24 Electrochemical Growth Cell for Anodic Oxidation of Aluminum ...................... 24 Morphology of Porous Anodic Alumina (PAA) F ilm ................................ .......... 25 Nonporous and porous alumina ................................ ................................ 25 Morphology of porous anodic alumina ................................ ....................... 25 Mechanism ................................ ................................ ................................ ....... 26 Highly Ordered Pores ................................ ................................ ....................... 29 Detachment of the Alumina Film ................................ ................................ ...... 30 Experimental ................................ ................................ ................................ ........... 31 Materials ................................ ................................ ................................ ........... 31 Preparation of a Nanop orous Anodic Alumina Membrane ................................ 31 Results and Discussion ................................ ................................ ........................... 32 Control over a Pore Size of PAA attached to Alumium Foil .............................. 32 Characteristics of Free Standing Alumina Membranes ................................ .... 34 Conclusion ................................ ................................ ................................ .............. 35 3 FABRICATION of BIFURCATED SILICA NANO TEST TUBES WITH A CONTROL OVER EACH SEGMENT ................................ ................................ ...... 52 Silica Nano Test Tubes ................................ ................................ ........................... 52 Branched Channels in PAA ................................ ................................ .................... 53 Experime ntals ................................ ................................ ................................ ......... 53 Materials ................................ ................................ ................................ ........... 53 Fabrication of Branched Channels in PAA ................................ ....................... 53

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7 Fabrication of Free Branched Silica Nano Test Tubes ................................ ..... 54 Silica Surface Sol Gel Process ................................ ................................ ......... 54 Removing Top Silica Layer using Argon Plasma Etch Method ......................... 54 Results and Discussion ................................ ................................ ........................... 55 Control over the Dimension of Each Segment in Branched Channels of PAA 55 Control over the Thickness of Silica Layer of SiNTTs ................................ ...... 56 Optimization of Argon Plasma Etch Cond ition to Remove Top Silica Surface .. 56 Characterization of Bifurcated SiNTTs ................................ ............................. 57 Fine Control over a Formation Position of Branches in Three Block Branched Channels of PAA ................................ ................................ ........... 58 Conclusion ................................ ................................ ................................ .............. 60 4 GOLD FILLED SILICA NANO TEST TUBES ................................ .......................... 80 Gold Nanoparticles Used in Nanomedicinal and Nanomedical Applications .......... 80 Synthesis of Gold Nanorods for Nanomedicinal and Nanomedical Applications .... 82 Silica Coated Gold Nanorods Based on Chemical Synthesis ........................... 82 Synthesis of Gold Filled Silica Nano Test Based on the Template Synthesis .. 83 Experimentals ................................ ................................ ................................ ......... 85 Materials ................................ ................................ ................................ ........... 85 Amine Functionalized Silica Surface on the Porous Alumina Membrane ......... 85 Electroless Plating of Amine Functionalized SiNTTs ................................ ........ 85 Plating Time Dependence of the Morphology of Gold Cores of Au SiNTTs ..... 86 Fabrication of Au SiNTTs with the Different Aspect Ratios of Gold Cores ....... 87 Vis NIR Absorbance Measurements ................................ ................................ 87 Electron Microscopy ................................ ................................ ......................... 87 Results and Discussion ................................ ................................ ........................... 88 Plating Time Dependence of the Morphology of the Gold Core of Au SiNTTs ................................ ................................ ................................ .......... 88 Free Au SiNTTs Prepared at Different Plating Times ................................ ....... 89 A Control over the Aspect Ratios of Gold Cores within Au SiNTTs .................. 90 Optical Properties of Au SiNTTs with Different Aspect Ratios of Gold Cores .. 91 Conclusion ................................ ................................ ................................ .............. 91 5 PATTERN TRANSFER: POROUS SILICON FOR LASER DESORBSION IONIZATION MASS SPECTROMETRY ................................ ............................... 105 Motivation ................................ ................................ ................................ ............. 105 Experimental ................................ ................................ ................................ ......... 107 Materials ................................ ................................ ................................ ......... 107 Preparation of P orous A lumina M ask ................................ ............................. 107 Fabrication of the N anowell A rrays on S ilicon S ubstrates .............................. 107 Characterization of P orous S urfaces ................................ .............................. 108 Mass Spectrometry ................................ ................................ ........................ 109 R esults and D iscussion ................................ ................................ ......................... 109 Fabrication of P orous S ilicon S ubstrates ................................ ........................ 109 Characterization of alumina mask ................................ ............................ 109

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8 M orphology of porous silicon substrates ................................ .................. 110 Mass S pectrometry A nalyses ................................ ................................ ......... 110 C onc lusion ................................ ................................ ................................ ............ 113 6 CONCLUSION ................................ ................................ ................................ ...... 125 LIST OF REFERENCES ................................ ................................ ............................. 127 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 136

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9 LIST OF TABLES Table page 2 1 Characteristics of free standing alumina membranes prepared under the different anodization conditions. ................................ ................................ ......... 51 3 1 Control parameters for each anodization step to control over t he dimension of each segment of branched channels in PAA ................................ .................. 76 3 2 Control parameters for the optimization of argon plasma etch conditi on to efficiently remove the top silica surface layer ................................ ..................... 77 3 3 Length of each segment of bifurcated SiNTTs. The silica tubes were fabricated using PAA template grown under the Type A conditions with different 35 V anodi zation times. L1, L2, L3, and Lt represent the length of the segment L1, L2, L3, and Lt in Figure 3 1. ................................ ..................... 78 3 4 Length of each segment of branched channels in PAA templates prepared under the Type B conditions with different 35 V anodization times. L 1 L 2 L 3 and L t represent the length of the segment L 1 L 2 L 3 and L t in Figure 3 9B. ...... 79

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10 LIST OF FIGURES Figure page 1 1 Nanofabrication process based on highly ordered porous anodic alumina: (A) Bottom up approach; (B) Top down approach ................................ .................... 23 2 1 A picture of porous alumina with self ordered nanohole arrays over a large area: it can be used as a template or a mask. ................................ .................... 37 2 2 Electrochemical growth cell for anodic oxidation of aluminum. ........................... 38 2 3 Structure of anodic alumina. A) Schematic of the cross section of anodized alumina membrane. Metallic Al, on the bottom, is covered by an impervious of thickness t b and then by a hexagonal array of pores of diameter D p B) SEM image of top view of alumina membrane: a honeycomb structure consisting of a close packed array of hexagonal units called cells, each containing a central hole. ................................ ................................ ........... 39 2 4 Schematic for a two step anodizing process. A) the self organization of porous alumina during the first anodizing process; B) removal of porous alumina layer, leaving an uniform indentation in the underlying aluminum; C) initiation of hole formation in second anodizing; D) an alumina with a highly ordered pore after the second anodization. ................................ ........................ 40 2 5 SEM Images of the morphology of a highly ordered porous alumina template prepared by the two step anodization, followed by the acid treatment for widening pores: (A) top view a nd (B) tilted view. ................................ ................ 41 2 6 SEM images of top view of alumina membranes anodized at 35 V after immersing in 10 wt% H 3 PO 4 for differe nt periods: (A) 1 min, (B) 20 min, and (C) 40 min. ................................ ................................ ................................ .......... 42 2 7 A plot of pore widening rate of alumina membrane anodized at 35 V by the immersion in 10 wt% H 3 PO 4 ................................ ................................ .............. 43 2 8 SEM images of top view of alumina membranes anodized at 50 V before (A) and after immersing in 10 wt% H 3 PO 4 for different periods: (B) 42 min and (C) 61 min. ................................ ................................ ................................ .......... 44 2 9 A plot of pore diameter of alumina membrane anodized at 50 V vs. the immersion time in 10 wt% H 3 PO 4 ................................ ................................ ...... 45 2 10 Fabrication process of free standing alumina membrane: (A) the anodization voltage was stepped down to 0 V; (B) Immersing it in phosphoric acid solution to liberate. ................................ ................................ ............................. 46

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11 2 11 SEM images of both surfaces of a free standin g alumina membrane anodized at 50 V: (A) outer surface facing toward electrolyte solution and (B) barrier layer surface facing toward aluminum foil. ................................ .............. 47 2 12 SEM images of outer surface of free standing alumina films prepared under the two different anodization voltages, separately: (A) 35 V and (B) 50 V. ......... 48 2 13 SEM images of cross sectional view of free standing alumina membranes prepared under the different anodization conditions: anodized at 50 V for 7 min (A), 15 min (B), 18 min(C), and 25 min (D); anodized at 35V for 60 min (E). ................................ ................................ ................................ ...................... 49 2 14 A plot of thickness of alumina membranes anodized at 50 V vs. the anodization time: growth rate=0.081 m/min; R=0.99939. Error bars represent a standard de viation achieved from more than 10 of thickness measurements. ................................ ................................ ................................ ... 50 3 1 Dimension of fragments of bifurcated silica nano test tubes wh ich can be controlled by the anodization time at each voltage step: Step 1, and Step 2, and Step 3 represent the anodization at 50 V, voltage reduction period, and anodization at 35 V, respectively. ................................ ................................ ....... 62 3 2 Fabrication process of Y branched channel in alumina template. ...................... 63 3 3 Fabrication process of free bifurcated silica nano test tubes: (A) Silica layer replicated the shape of branched channels in PAA by using silica surface sol gel, (B) top silica layer was removed by a argon plasma etch metho d, and (C) the alumina template was removed after immersed in 10 wt% phosphoric acid. ................................ ................................ ................................ .................... 64 3 4 SEM images of tilted view of PAA with branched channels prepared under the conditions with a 35 V anodization time of 22 min (Table 3 1). Three segments were created by the anodization at 50 V (A), voltage reduction step (B), and at 35 V (C), respectively. ................................ ................................ ....... 65 3 5 SEM images of tilted view of PAA with branched channels. The length of branches increases as the 35 V anodization period increases: (A) 10 min, (B) 22 min, and (C ) 60 min, respectively. ................................ ................................ 66 3 6 TEM images of representative silica nano test tubes prepared via different number of cycles of surface sol gel process: (A) 2 cycles, (B) 3 cycles, (C) 4 cycles, and (D) 5 cycles. ................................ ................................ ..................... 67 3 7 A plot of growth rate of silica layer as the number of silica layers increases: the growth rate is 5.94 nm/layer and the correlation factor is 0.99613. .............. 68 3 8 SEM images o f top view of silica coated alumina template after different argon plasma treatments: (A) 100 W 10 Pa 30 s, (B) 100 W 10 Pa 60 s, (C)

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12 140 W 10 Pa 30 s, (D) 140 W 10 Pa 60 s, (E) 140 W 20 Pa 60 s, and (F) 140 W 20 Pa 20 s. (G) Silica coated alumina template without any argon plasma treatment. ................................ ................................ ............................... 69 3 9 SEM images of bifurcated SiNTTs prepared under the type A with a 35 V anodization time of 40 min (A). Image (B), (C), and (D) represent close up view of nano tube segments replicating the junction grown between 50 V anodization and the voltage reduction step (i), between the voltage reduction step and 35 V anodization (ii), and continuous 35 V anodization step (iii), respectively. ................................ ................................ ................................ ........ 70 3 10 SEM images of Type A bifurcated silica nano test tubes (SiNTTs) with different 35 V anodization period: 5 min (A and B), 20 min (C), and 40 min (D). Image B represents the close up view of the red box in image A. The arrows indicate the initial positions of branches. ................................ ................. 71 3 11 (A) A plot of dimension of each segment in bifurcated SiNTTs using PAA template grown under Type A condition with different 35 V anodization times. L1, L2, L3, and Lt represent the length of the segment L 1 L 2 L 3 and L t in Figure 3 1. (B) A linear fitting plot of t otal length L t in figure (A): Total length = 24.4 35V anodization time + 3814 (R = 0.98717) ................................ ............ 72 3 12 SEM images of bifurcated SiNTTs prepared under the Type A anodization condition with a 35 V anodization time of 5 min. (B) A close up view of red box in figure A. The arrows indicate the starting position of branches. ............... 73 3 13 SEM images of tilted view of branched channels in PAA prepared under the Type B anodization conditions with a 35 V anodization time of 22 min. (B) Close up view of the red box in figure A : the arrows indicate branch starting and third segment, respectively. ................................ ................................ ......... 74 3 14 A plot of dimension of each segment in branched channels of PAA template grown under Type B conditions with different 35 V anodization times (A). L1, L2, L3, and Lt represent the length of the segment L 1 L 2 L 3 and L t in the diagram (B). ................................ ................................ ................................ ........ 75 4 1 Fabrication process of amine functionalized silica nano test tubes on the porous alumina template. (A) T he pores of two step anodized alumina membrane were widened in 10 wt% H 3 PO 4 solution. (B) SiNTTs were formed on the porous alumina membrane by using the surface sol gel method. (C) The silica surface was modified with amine functional groups using APTS. ...... 93 4 2 Schematic diagram of the electroless plating procedure used to deposit gold into amine functionalized SiNTTs. (A) The amine fu nctionalized SiNTTs were immersed in a SnCl 2 formed on the surface of membrane by the surface redox reaction between Ag + and Sn 2+ (C) Au nanoparticles were created by the second surface

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13 redox reaction between Au + and Ag. The Au nanoparticles served as autocatalysts for the continuous reduction of Au + with formaldehyde as the reducing agent. Diagram (A) to (C) represent the close up view of the blue box in diagram (D). ................................ ................................ ............................. 94 4 3 Fabrication procedure of free Au SiNTTs. (A) The silica layer on the rim surface was removed by the Ar plasma etch method. (B) Silica layer was modified wit h amine functional groups. (C) Another argon etch process was conducted to remove the amine functional groups on the rim surface. (D) Gold electroless plating was accomplished for the desired time. (E) Alumina template was removed by the immersion in 10 wt % H 3 PO 4 .............................. 95 4 4 SEM images of tilted view of PAA templates with Au SiNTTs prepared by different plating times: (A) 42 min, (B) 90 mi n, (C) 150 min, and (D) 1170 min (19 h 30 min). Bright areas represent gold. ................................ ........................ 96 4 5 Schematic diagram of the morphology of Au cores within amine functionalized SiNTTs after the electroless plating for different periods: (A) 42 min, (B) 90 min and 150 min, and (C) 1170 min. ................................ ................ 97 4 6 SEM images of free Au SiNTTs prepared under the plating time of 1170 min (19 h 30 min). (B) and (D) shows a close up view of the box in (A) and (C), respectively. The arrows and bright areas indicate silica layers (pale layer) and gold co res of Au SiNTTs, respectively. ................................ ........................ 98 4 7 TEM images of free Au SiNTTs prepared under the different plating times: (A) 25 min, (B D) 6 5 min, and (E G) 1170 min (19 h 30 min). (C) and (D) exhibit a close show a close ........................ 99 4 8 TEM images of free Au SiNTTs prepared using the PAA template anodized at 50 V for 3 min. The gold electroless plating was conducted for 21h. The image (B) and (C) exhibit a close image (A), respectively. Red solid arrows and blue dot arrows i ndicate Au SiNTTs sitting upright and lying down, respectively. ................................ ......... 101 4 9 TEM images of free Au SiNTTs prepared using the PAA template anodized at 50 V for 6 min. The gold electroless plating was conducted for 21h. ............ 102 4 10 TEM images of free Au SiNTTs prepared usi ng the PAA template anodized at 50 V for 10 min. The gold electroless plating was conducted for 21h. The image (B) exhibits a close up view of the box area in the image (A). ............... 103 4 11 Vis NIR absorption spectra of Au SiNTTs with different aspect ratios of gold cores. Black solid line, red dot line, and green dash line represent the spectra from gold cores with an aspect ratio of 8.8, 6. 1, and 2.1, respectively. ............ 104

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14 5 1 Scanning electron micrographs of the alumina mask: Top view of (A) outer surface and (B) barrier layer surface, and (C) cross section. ............................ 115 5 2 Tapping mode AFM imgaes of nanowell arrays on silicon substrates prepared by means of Ar ion plasma etching for: (A) 5 min, (B) 15 min, and (C) 25 min. ................................ ................................ ................................ ........ 116 5 3 Nanowell depth vs etch time (The solid line is the linear regression best fit line (slope = 2.0 nm /min; R = 0.99932). ................................ ........................... 117 5 4 Workflow for the preparation of nanowell arrays and MS analysi s. .................. 118 5 5 Mass spectra of adenosine acquired on 10 nm (top) and 30 nm deep (bottom) nanowells and DIOS surface. ................................ ............................. 119 5 6 Mass spectra of Pro Leu Gly acquired on 10 nm (top) and 30 nm deep (bottom) nanowells. ................................ ................................ .......................... 120 5 7 Mass spectra of [des Arg9] bradykinin,on 10 nm (top) and 30 nm deep (bottom) nanowells. ................................ ................................ .......................... 121 5 8 Comparison of atmospheric pressure and vacuum regime laser desorption ionization on 30 nm deep nanowell arrays. A: Adenosine, B: Pro Leu Gly, C: [Des Arg9] bradykinin ................................ ...... 122 5 9 Plot of signal intensity versus amount of Pro Leu Gly using 30 nm deep nanowell arrays ................................ ................................ ............................... 124

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15 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 NANOFABRICATION BASED ON SELF ORDERED POROUS ANODIC ALUMINA By Dooho Park A ug ust 2010 Chair: Charles R. Martin Major: Chemist ry Self ordered porous nanostructures have attracted a lot of attention because they can be used as starting materials in both the top down and bottom up approaches. The typical dimension of pores can be varied from a few nanometers to many micrometers The highly ordered pattern s can be transferred to desired materials by the top down method. In the bottom up approaches the porous substrates can be used as a template to fabricate nanomaterials with narrow size distribution One of the most widely used p orous substrates is self ordered porous anodic alumina (PAA). This dissertation describes the control over the morphology of PAAs and their applications in both top down and bottom up approach es for preparing nanostructures such as the template synthesis a nd the pattern transfer, respectively First, a highly self ordered porous anodic alumina has been fabricated by the anodization of Al in appropriate acidic solutions. It exhibits narrow distribution s in pore size, pore density, porosity and pore depth. Different type s of PAA s w ere prepared for the template synthesis and the pattern transfer respectively. For the former, PAA was attached on Al and the pore size was controlled by the post etching treatment. For the d and used as a mask. Two different pore sizes

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16 ( ~80 nm and ~50 nm ) were prepared under the different anodization voltage s The t hickness of PAA showed a good linear relationship with the anodization time. In the template synthesis study, the channel sha pe of PAA was tuned from straight through type to branched one. B ifurcated silica nano tubes with a control over the length of each segment were fabricated from the template. In addition, straight through silica nano test tubes were filled with gold by using the gold electroless plating. The aspect ratio of gold cores was controlled by the inner space of silica test tubes. The optical properties of gold filled silica nano test tubes were in vestigated. In the pattern transfer study, the PAA pattern was successively transferred onto silicon via an Ar plasma etch method. The pore depth linearly increased as the etch time increased. The porous silicon substrates were used as platform s in Laser Desorption/Ionization Mass Spectrometr y The effect of pore depth on the ionization efficiency was investigated. Significant improvement in the ionization was observed by increasing the pore depth from 10 nm to 3 0 nm.

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17 CHAPTER 1 NANOFABRICATION Porous Nanostructures in Nanofabrication Numerous major advances in research and technology over the last two decades have been made possible by the successful development of nanostructures made of metals 1 4 insulators 5 9 and semiconductors 10 12 Nanostructures are objects that have one, t wo or three dimensions in the sub micrometer to nanometer regime. These nanostructures have attracted a lot of attention in analytical chemistry because of th eir applications as nanoparticle based sensing 13 17 and platforms for sensing 3, 18, 19 and separation 20, 21 For example, the strongly enhanced surface p lasmon resonance (SPR) scattering from Au nanoparticles makes them useful as bright optical tags for molecular specific biological imaging and detection using simple dark field optical microscopy 13, 16 U ltrasmall sensors or electrodes might be used to communicate with cells and to form the basis for min imally invasive diagnostic systems 22 The small, often molecular, size of the nanotubes prepared show ed selective ion transport 4 and a separation of enantiomers of a chiral drug 21 One significant diff iculty with nanostructures is how to prepare them. One can distinguish two approaches: top down and bottom up. In the top down approach, objects of ever smaller dimensions are carved out of larger objects. This approach is frequently used in the semiconductor industry where advanced lithography aided by specific steps such as selective oxidation has shrunk the typical dimensions to well below 1 m. The bottom up approach consists of growing small objects to their desired size and shape. Thi s is usually accomplished by chemical means.

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18 Self ordered porous na n ostructures have attracted a lot of attention because they combine many of the advantages of the top down and bottom up approaches. The typical dimension can be varied from a few nanomete rs to many micrometers. P orous substrates can be used as template s to fabricate nanomaterials with narrow distribution in dimensions because their pore size and depth have narrow distribution (Figure 1 1A) Moreover, t he highly ordered pattern can be trans ferred into the desired materials by top down method (Figure 1 1B) One of the most widely used self ordered porous materials has been self ordered porous anodic alumina membranes. Central to the non lithographic nano fabrication method favored here is th e utilization of an alumina nanopore membrane as either the growth template or the etch mask for the formation of the proposed nanostructures. The alumina nanopore array is formed by anodization of high purity aluminum under certain carefully controlled an odization conditions. The nanopores can self organize into a highly ordered array of uniform pores T he pore diameter, the period, and the array size are determined by the anodization conditions Bottom Up Approach: Template Synthesis Based on Porous Memb ranes Martin group pioneered one of the most useful bottom up approach es called t emplate synthesis. This method entails synthesizing the desired material within the pores of a membrane. Porou s membranes offer a facile manner to handle and manipulate nanom aterials without the use of highly specialized equipment. Further more homogeneous pores ensure homogenous nanomaterials, a characteristic that is often not easily achieved at these small scales. 3 The template method has proved to be a versatile approach for preparing nanomaterials. Nanotubes and nanowires composed of metals, semiconductors, insulators, polymers, conducting polymers, and various

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19 composites of these materials have been prepared. 1, 3, 23, 24 Appropriate membranes can be purchased commercially, or fabricated The most often used membranes for the template synthesis method are track etch membranes and porous alumina membranes. Track Etch Membranes Membra nes prepared by the track etch procedure are created by bombarding (or energy particles, creating damage tracks. The damage tracks are then chemically developed (etched) to pro duce pores. 25 A variety of membrane materials are compatible with this technique; however, polymer films have shown the greatest utility. Porous poly(carbon ate), poly(ethylene terephthalate), and poly(imide) membranes are all commonly produced with this method. Track etch membranes can be obtained from commercial sources or fabricated using tracked material s The pore density is controlled by the fluence of i mpinging particles during the tracking process and can range from a single pore to millions of pores cm 2 Pore dimensions can be controlled by development conditions, including pH, temperature, and time. Through the use of anisotropic etching techniques, the geometry of the pore etched in a tracked membrane can be controlled. The geometry of the pore can have a strong influence on the transport properties of the membrane. Porous Alumina Membranes Alumina membranes are obtained through the electrochemical growth of a thin, porous layer of aluminum oxide from aluminum metal in acidic solutions. 26 29 Membranes of this type may be obtained commercially with a variety of pore sizes, or can be grown using well established procedures. Pores with dimensions from 5 to 200 n m can be obtained in mm thick membranes. The pores created are normally arranged in a hexagonally packed array. Pore densities can be as high as 10 11 pores cm 2

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20 Top Down Approach: Pattern Transfer Using Porous Masks The pattern transfer process es for nanostructuring are divided into two separate parts: writing and replication 30, 31 Writing and replication are usually different kinds of processes with different characteristics, and they also emphasize different aspects of nanofabrication. They both con nect to the idea of a master: that is, a structure that provides or encodes a pattern to be replicated in multiple copies. Writing nanostructures ion beam of electrons or ions. Replication of nanostructures is a process of pattern transfer in which the information for example, the shape, morphology, structure, and pattern present on a master is transferred to a functional material in a single step, rapidly, inexpensively and with high fidelity. Most techniques used for writing nanostructures de novo are either too slow or too expensive to be used for mass production. Replication of a master should provide an economical and convenient route to multiple copies of the nanos tructures written on the master. A combination of high precision writing of masters with low cost replication of these masters seems to provide the most practical protocol for nanomanufacturing: high resolution, high cost fabrication techniques can be used to make masters in one set of materials; these structures can then be replicated in a low cost process in other materials. The cost of the master can be an insignificant part of the overall cost structure if it is used to make many replicas. One of high precision writing techniques is e beam lithography. Current e beam writing machine s can readily write one or a hundred or even thousands of nanodot patterns in e beam photoresist with great accuracy and without extraneous efforts. 32 However, when a sk ed to write hundreds of millions of nanodots, the process would take an extraordinary amount of time and also be limited by the microscope field of view if

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21 stitching errors are to be avoided. Also, this does not account for complex prox imity effect issues when deciding on exposure times which can dramatically alter the dimensions of the desired patterns especially ( about 100 nm or less ) With the PAA templating method, billions of nanodots of uniform size and spacing can be formed in par allel often in one or a few steps. The fabrication of nanopores is a relatively simple process. T he nanopore array pattern is replicated conveniently through etching, chemically and/or physically in the substrate resulting in lateral superlattices of pores One procedure we often follow employs the PAA nanopore array membrane as an etch mask. 33 36 After the template has been formed and the barrier layer is removed, the highly ordered PAA film is placed onto the desired substrate. Finally, d ry etch method, such as plasma etching, ion milling or react ive ion etching, is used to transfer the pattern to the substrate. Nankao et al. were amongst the first to report the formation of nanopore arrays in semiconductors using the AAO templating metho d in 1999 37 GaAs and InP nanopore arrays were formed using reactive beam etching te chnique with Br 2 N 2 gases at elevated substrates temperatures. Nanopores of up to 1 m in depth were etched, however, as noted by the authors, the holes at this depth partly collapsed. According to the ir studies etching rates inside the pores are dramatic ally reduced to the tune of one order of magnitude slower as the thickness increases An argument is made for reducing the thickness of the alumina template to as thin as possible if deep nanopores are desired with it still being possible to physically han dle. Micro loading effects were also seen as 60 nm diameter pores were 10% shallower in depth than 80 nm diameter pores.

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22 In transferring the PAA pattern onto the substrate, reactive ion etching (RIE) is typically used due to its directive etching nature, its smooth etched surface morphology, high etch rates, and vertical sidewalls. In this process, any gap between the template and the substrate should be minimized to ideally a direct contact so as to prevent unwanted etch ing along the gap. To circumvent the challenges in placing the ultra thin PAA film onto the receiving substrate, an Al film can be sputtered or evaporated onto the substrate as done by Crouse et al. 38 After evaporating 2 m of Al onto a conducting Si substrate, anodization using similar recipes proceeded. One limitation to this process is that the substrate must be conducting as the anodization procedure requires the voltage drop on the Al surface. After anodization, the PAA film was thinned from 2 m to 300 nm by a 1 h argon ion milling procedure to make the subsequent RIE pattern transfer to Si substrate easier. The nanopore alumina can be separated from the Al base and further processed into a free standing membrane of nanopores with pores on both the top and bottom open. The free standing alumina membranes are useful as a mask for fabricating a variety of highly ordered nanostructures. 6, 7, 33 35, 37 The advantages of the non lithographic fabrication using self organized, highly ordered porous anodized alumin a template include: U niform pore diamet er adjustable from 20 to 200 nm U niform por e periodicity from 50 to 400 nm H ighest packing density 10 9 10 10 /cm 2 due to its hexagonal symmetry T he pore diameter, period, and array size variable over ranges that are beyond the reach of standard e beam lithography with the PAA template pattern comple tely replicated in dimensionali ty onto the receiving substrate

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23 L arge area and low cost P rocess is not material specific ranging from oxide to semiconductors to metals to polymer s D oes not need a cleanroom process Figure 1 1. Nanofabrication process based on highly ordered porous anodic alumina: ( A) Bottom up approach; ( B) Top down approach

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24 CHAPTER 2 FABRICATION OF POROU S ANODIC ALUMINA Self Ordered Porous Anodic Alumina (PAA) Membrane The protection or decoration of Al surfaces by anodization has been used commercially since 1923. Self due to their high pore density 39 41 and their potential use for masking 6, 7, 33 35, 37 information storage 42 and template s for nanomaterial synth esis 1 3, 7, 11, 12, 39, 43 45 Relative to other porous media, such as polycarbonate and polyethylene terephthalate (PET) membranes, well fabricated anodic alumina membranes possess a much higher pore density ( up to about 10 11 pores/cm 2 ) 42 and a narrower distribution of pore diameters. Figure 2 1 demonstrates a successful fabrication of self ordered p orous alumina over a large area. The very thin film on the left side is a free standing alumina membrane detached from the aluminum foil on the right side. The squares indicate that the size of alumina membrane (left square) is same as that of the anodized area in aluminum template ( right square: shiny area). An alumina membrane is commonly used as a template when it is attached to the aluminum foil For a mask application, it is usually detached from the aluminum foil Electrochemical Growth Cell for Ano dic Oxidation of Aluminum Figure 2 2 shows our current setup for anodic oxidation of aluminum. S tainless steel is used as the cathode of the cell. Both cathode and anode are immersed in aqueous acid electrolyte and voltage is supplied by a variable power supply. The geometry of cathode and anode is important for uniform alumina growth. Our system uses a cylindrical cathode that surrounds the aluminum anode to assure homogenous

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25 current density on both sides of the aluminum plate. The pore channels of a good PAA membrane are grown mostly parallel to each other and perpendicular to the surface of the membrane. Morphology of Porous Anodic Alumina ( PAA ) F ilm Nonporous and porous alumina Two forms of anodic aluminum oxide exist, the nonporous barrier oxide and the porous oxide 46, 47 When Al is anodized in neutral or basic solutions (pH > 5), a flat, layer forms. 48 When Al is anodized in an acid electrolyte, usually phosphoric acid (H 3 PO 4 ) 49 sulfuric acid (H 2 SO 4 ) 27 or oxalic acid (H 2 C 2 O 4 ) 50 deep pores can form. The bottom of each pores also consists of a thin 100 nm thick) over the metallic Al surface. M orphology of porous anodic alumina In porous film, the diameter and density of the pores depends on p H, anodization voltage, temperature, growing time and choice of acid. The diameter varies between 5 and 200 nm 51 and the length of the pore channel is also adjustable from sub micrometer to tens of micrometers. The geometry of cross section of anodic porous alumina is schematically represented in Figure 2 3 A. Metallic Al, on the bottom, is covered by an impervious b and then by a hexagonal arra y of pores of diameter D p 46, 47 In Figure 2 3 B, Scanning Electron Microscope ( SEM ) image shows the top view of th e alumina membrane. It indicates a honeycomb structure consisting of a close packed array of hexagonal units called cells, each containing a central hole. The cell size, which is equivalent to the hole interval, is determined by the applied voltage used fo r the anodization; the cell size has a good linear relationship with the applied voltage.

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26 The value of the cell size divided by the applied voltage is approximately 2.5 nm/V. The hole size is dependent on the electrolyte composition, temperature, period of anodization as well as applied voltage. The hole size is also controlled by the pore widening treatment by dipping the porous alumina in an appropriate acid solution after the anodization. The cell size usua lly ranges from 10 to 500 nm and the hole size from 5 to 400 nm depending on the anodizing and post anodizing conditions. The depth of holes (thickness of the oxide films) has a good linear relationship with the period of the anodization. 52 Mechanism Anodic porous alumina has been studied in detail over the last five decades 27, 46, 47 In the anodization process, an electrical circuit is established between a cathode and a thin film of aluminum which serves as the anode, according to the following reaction: w here G is the standard Gibbs free energy change. During the anodization, initially a planar barrier film forms followed by pore development leading to the formation of the relatively regular porous anodic film, which thickens in time In general, ordering is always due to repulsive or attractive forces between the pores leading in two dimensions to the hexagonal lattice. However, the physical and chemical mechanism s of self ordering are still under investigation. Several mechanisms for pore growth in PAA have been proposed. 47, 53 56 Thomson, Wood and co workers proposed that pore nucleation is due to a cracking and self healing of the oxide layer atop preexisting ridges on the Al surface and that this

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27 forms a barrier layer of nonuniform thickness 55 mechanism for the formation of alumina membrane 47 During formation at constant current density, pore initiation occurs by the merging of locally thickening oxide regions, which seem related to the substructure of the substrate, and the consequent concentration of current into the residual thin areas. The pores grow in diameter and c hange in number until the steady state morphology is established. In addition, the steady state barrier layer thickness and pore diameter are observed to be directly proportional to the formation voltage. It becomes evident that the barrier layer thickness decided largely by an equilibrium established between oxide formation in the barrier layer and field assisted dissolution (probably thermally enhanced) at the pore bases, determines the cell and pore sizes by a simple geometrical mechanism. The formatio n of the porous channels is most likely enhanced by the applied electric field for anodization, as observed in alumite. Considering a typical three layer configuration (Al Al oxide anodizing solution) for the anodization process, both the transport of oxyg en containing anions (O 2 /OH ) from the solution/oxide interface to oxide/Al interface and the migration of Al 3+ from Al/oxide interface to oxide/solution interface would be enhanced by the applied electric field. The transport of O 2 /OH contributes to th e oxide formation at the interface between Al and Al oxide, while the migration of Al 3+ to the solution leads to the dissolution of Al. Since the electric field strength across the channel bottom (i.e., barrier layer) is much greater than that across the channel wall, the Al dissolution rate at the bottom is far greater than that at the wall, resulting in perpendicular growth of the channel with a high aspect ratio.

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28 Li and coworkers also provided a possible mechanism for pore formation 53 During porous type film (PTF) formation, hydrogen ions and the electric field at the oxide/electrolyte interface must play important roles. (1) PTF usually starts from some Al surface with pits formed at lattice These Al pits will be covered by an air formed oxide, so that both the metal/oxide and the oxide/electrolyte interfaces are locally curved. (2) As anodization starts the electric field at the oxide/electrolyte interface, Al 3+ ions form at the metal/oxide interface Al (s) Al 3+ (oxide) + 3e (2 1) and migrate into the oxide layer. (3) At the oxide/electrolyte interface the water splitting reaction H 2 O(l) 3H + (aq) + O 2 (oxide) (2 2) occurs and is rate determining. The oxide O 2 ions migrate, due to the electric field, within the oxide from the oxide/solution interface toward the metal/oxide interface, to form Al 2 O 3 (4) The protons can locally dissolve more oxide: Al 2 O 3(s) + 3H + (aq) Al 3+ (aq) + H 2 O (l) (2 3) (5) Hydronium ions can also migrate toward the cathode, where they leave the electrolysis cell as H 2 gas, completing the circuit: 3H + (aq) + 3e H 2(g) (2 4) (6) The hexagonal ordering of the pores is not yet explained, but the mobility of ions within the barrier oxide and of Al atoms within the metal may explain why pores can rearrange dynamically and why linear domain grow th with time is possible.

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29 Highly Ordered Pores During the initial stage of film growth, the pores are somewhat various in shape and packing, and there is some heterogeneity in size. This is a result of the pores nucleating from random pits and valleys on the aluminum surface 53 The degree of the ordering of the hole configuration at the surface of the anodic porous alumina is low because the holes develop randomly at the initial stage of the anodization as the top layer of Figure 2 4 A indicates. To improve the ordering of the surface side of the anodic porous alumina two step anodization is effectively adopted. The process involves two separate anodization processes (Figure 2 4 ): the first anodization p rocess consists of a long period anodization to form the highly ordered hole configuration at the oxide/Al interface and the second anodization is performed after the removal of the oxide formed in the first anodization step. After the removal of the oxide an array of highly ordered dimples was formed on the Al, and these dimples can act as initiation sites for the hole development in the second anodization. This process generates an ordered hole array throughout the entire oxide layer. This process can al so be applied for the preparation of a porous alumina mask used for several types of nanofabrication techniques in which the ordered straight through holes are essential. Figure 2 5 demonstrates a highly ordered porous anodic alumina (PAA) prepared by the two step anodization, followed by immersing in 10 wt% H 3 PO 4 solution. Top view image (Figure 2 5A) show s a two dimensional lateral superlattice structure. The tilted view (Figure 2 5B) exhibits straight through pores with a uniform size in diameter throug h the whole channel. In addition, it also shows that t he pore channels of a PAA membrane are mostly parallel to each other and perpendicular to the surface of the membrane.

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30 Detachment of the Alumina Film One problem is encountered when using this alumina film as a mask. We need to detach the PAA membrane from the aluminum substrate. After the anodization process, there is always an alumina barrier layer between the bottom of the pore and the unanodized base aluminum that serves as the electrochemical anode (Figure 2 2 ). This insulating barrier layer is usually a thickness of 10 100 nm. It can thus block direct electrical and chemical contact between the substances in the pore channels and the base conducting substrate in applications such as dc electrodepos ition of nanowires and field emission devices. The anodized PAA layer is firmly attached to the base aluminum, making it difficult to obtain free standing PAA membranes for some applications. A common practice to overcome th is problem is to etch away the b ase aluminum substrate with saturated or 2% HgCl 2 and then etch away the exposed barrier layer with ~6 wt % H 3 PO 4 36 This does not damage the alumina. This method works best if thin foils of aluminum are used to grow the aluminum films but one aluminum foil is consumed to produce one alumina membrane. In another approach, the barrier layer may be thinned by gradually r educing the anodizing voltage 57 After the film reaches the desired thickness, the bias voltage is progressively stepped down in 5% decrements, causing perforation of the barrier layer. 49 Because the pore size is determined by the applied voltage, the pores at the barrier layer branch to smaller diameters As the voltage steps down, the current decreases. Like the pore size, the thickness of the b arrier layer is reduced with decreased anodization voltage. The voltage reduction process is carried out through many branching cycles until the barrier layer is very thin Anodization is stopped, and

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31 the alumina is then immersed in an acid (H 2 SO 4 or H 3 PO 4 ) etchant solution. The etchant solution quickly dissolves the thin barrier and the alumina is detached from the Al substrate. This Al substrate can be anodized to produce a PAA membrane again. Tian, et al have used a reverse bias voltage to penetrate th e oxide barrier layer of a PAA film and detach the film from the underlying aluminum metal in one step 58 Because both approaches above induce isotropic chemical etching of alumina film, they both inevitably enlarge the pore size. This method can penetrate the barrier la yer without significantly enlarging the original pore size of the anodized PAA film. This chapter entails a method for controlling the morphology of two different type s of PAA such as Al 2 O 3 /Al (alumina attached to aluminum foil ) and free standing alumina. The Al 2 O 3 /Al and free standing alumina were prepared for template synthesis and pattern transfer applications, respectively. T he pore size and length of PAA were controlled E xperimental Materials Aluminum foil (99.99%) was obtained from Alfa Aesar, and o xalic acid, o p hosphoric acid, chromium trioxide, sulfuric acid, methanol, and ethanol were obtained from Fisher. Preparation of a Nanoporous Anodic Alumina Membrane alumina membrane with highly ordered nanopore s. 59 Briefly, the Al foil was first annealed in air at 400 C for 1 h. Al foil was then electropolished at 15 V in a solution that was 95 wt% H 3 PO 4 5 wt % H 2 SO 4 and was 0.20 M CrO 3 ; the cathode was a Pb plate. After rinsing with distilled water, the polished Al foil was anodized at 50 V or 35 V in 5 wt% aqueous oxalic acid at

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32 1.5 C for 1 h; the cathode was a stainless steel plate (Figure 2 2 ). This resulted in the formation of a thin nanopore alumina fil m on the surface of the Al foil. This precursor film was dissolved away in an aqueous solution that was 0.2 M in CrO 3 and 0.4 M in H 3 PO 4 at 80 C resulting in a highly ordered pattern of scallops on the Al substrate (Figure 2 4 C ). In the second step, this textured Al substrate was anodized at 50 V or 35 V in the 5 wt % oxalic acid solution under the same condition s as the first anodization step. This yields the desired ordered nanopore alumina film. The thickness of alumina (pore length) was controlled by c hanging the anodization time. The pore diameter of two step anodized alumina template was adjusted with a treatment with 10 wt% H 3 PO 4 solution for a desired period. The pore diameter was monitored using SEM images as the acid treatment period increased. In order to achieve a free standing PAA, t he voltage reduction technique was then used to detach the alumina film from the underlying Al surface. This entails gradually ( 3 V / 2 min ) reducing the anodizing voltage to 0 V, followed by immersing the membrane in a 10 wt% H 3 PO 4 solution. The resulting free standing nanopore alumina membrane has two distinct surfaces : the barrier layer surface, which faced the substrate Al foil during the anodization and the outer surface which faced the electrolyte solution T he pore size and thickness of free standing PAA were investigated using SEM images. Results and Discussion Control over a Pore Size of PAA attached to Alumium Foil The pore size and pore density of PAA can be determined by the anodization voltage In addit ion, the pore size can be adjusted further by using acid treatment after

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33 the anodizat ion. Two different types of PAA were prepared under two different anodization voltages such as 50 V and 35 V. SEM images in Figure 2 6 describe the top view of alumina me mbranes anodized at 35 V after immersing in 10 wt% H 3 PO 4 for different periods such as 1 min (A), 20 min (B), and 40 min (C). The pore size increased as the acid treatment period increased while the pore density remained constant. Figure 2 7 shows a plot of pore size vs the acid treatment time. More than 2 00 pores were used to obtain the average pore size T he error bar represents a standard deviation from them. The pore size increased linearly with a narrow distribution as a function of etch time (widenin g rate=0.77 nm in diameter/min; R=0.9999) The pore size before the acid treatment (y intercept value) was determined to be 20.5 nm in diameter. Th e result s prove that the pore size can be adjusted to a desired size. Figure 2 8 A shows top view of alumina membranes anodized at 50 V (pore size = 24.7 nm in diameter) The membrane was immersed in 10 wt% H 3 PO 4 to control the pore size. The pore size was increased with an increased immersion time as seen in Figure 2 6 while the pore densit y remained consistent The order of pore lattice from the alumina membrane anodized in oxalic acid at 50 V was greater than that from alumina membranes anodized in oxalic acid at 35 V. A plot in F igure 2 9 demonstrates a linear relationship between the pore size of alumina pr epared at an anodization voltage of 50 V and the phosphoric acid etch time (widening rate=0.95 nm in diameter/min; R=0.99468). The error bars represent a standard deviation obtained from more than 400 of measurements. The plot indicates that the pore size was well controlled by adjusting the acid treatment time as shown

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34 from 35 V anodized alumina membranes. The plot also exhibits a narrow distribution in pore size while the etch time increases. Characteristics of Free Standing Alumina Membranes In order to prepare various types of alumina masks, free standing alumina membranes were prepared via the voltage reduction method. The schematic diagram in Figure 2 10 illustrates the fabrication process of the voltage reduction method used in this projec t. First, an alumina membrane attached to aluminum was prepared at a constant voltage by the two step anodization method. After a desired anodization period, the anodization voltage was then stepped down to 0 V in order to create a perforation on the barri er layer (Figure 2 10A) Finally, a free standing alumina was obtained by the immersion of the template in a phosphoric acid solution (Figure 2 10B). Figure 2 11 shows the morphology of both outer surface and barrier layer side of a free standing alumina. The outer surface (Figure 2 11A) was facing toward the electrolyte solution and barrier layer (Figure 2 11B) was facing toward aluminum foil. The outer surface illustrates a well arrayed pattern with a narrow distribution in pore size. The barrier layer si de also shows a narrow distribution in pore size. The SEM images proved that both sides of alumina mask were open and available as a mask. Figure 2 12 exhibits representative SEM images of alumina masks prepared at an anodization voltage of 35 V (A) and 50 V (B), separately. The pore density of each mask was 1.7 10 10 and 9.210 9 pores/cm 2 respectively. The pore diameter for each membrane was 523 and 815 nm, respectively. The pore density of 35 V anodized alumina membrane was twice than that of 50 V anodized alumina membrane. The pore size of 35 V anodized alumina film was about two third s of that of 50 V anodized one.

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35 Both types of membranes have a relative standard deviation of about 6%, showing a narrow distribution in diameter. The cross section al morphology of alumina masks was characterized based on SEM images (Figure 2 13). Figure 2 13B shows clearly that all pores are straight through and parallel to each other. The thickness of free standing alumina masks prepared under the different anodiza tion conditions was investigated. At an anodization voltage of 50 V, alumina films were grown for 7 min (A), 15 min (B), 18 min (C), and 25 min (D) The thickness was increased as the anodization period was prolonged. The thickness of the alumina anodized at 35 V for 60 min (E) was 2.9 m. The plot of thickness of alumina films prepared at an anodization voltage of 50 V against the anodization period demonstrates that the thickness of alumina masks can be adjusted by changing the anodization time. The thic kness was increased linearly with an increased anodization time (growth rate=0.081 m/min; R=0. 99939) The error bars represent a standard deviation obtained from more than 10 measurements, showing a narrow distribution in thickness. Table 2 1 shows a sum mary of characteristics of free standing alumina membranes prepared under the different anodization conditions. Overall, the pore size, porosity, and pore density were determined by setting up a constant anodization voltage. The thickness was controlled by adjusting anodization time. Conclusion This chapter entails a method to control over the morphology of alumina membrane s The porous alumina membrane has a narrow distribution in pore size and thickness (pore length). The pore size and pore density were controlled by changing the

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36 anodization voltage. The pore size was also adjusted by the acid treatment after anodization, showing a linear increase in d iameter as the etch time increased. The thickness of alumina films was increased as the anodization time was prolonged Therefore, the morphology of alumina membranes for the template synthesis (as a template) and the pattern transfer method (as a mask) wa s investigated and successively controlled.

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37 Figure 2 1. A p icture of porous alumina with self ordered nanohole arrays over a large area: it can be used as a template or a mask

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38 Figure 2 2 Electrochemical growth cell for anodic oxidation of aluminum. Variable Power Supply + Electrolyte Stainless steel cathode Aluminum anode

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39 Figure 2 3 Structure of anodic alumina. A) Schematic of the c ross section of anodized alumina membrane. Metallic Al, on the bottom, is covered by an impervious b and then by a hexagonal array of pores of diameter D p B) SEM image of top view of alumina membrane: a honeycomb structure consisting of a close packed array of hexagonal units called cells, each containing a central hole. Cell Pore B) D p Barrier layer: t b Al Al 2 O 3 A)

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40 Figure 2 4 Schematic for a two step anodizing process. A) the self organization of porous alumina during the first anodizing process; B) removal of porous alumina layer, leaving an uniform indentation in the underlying aluminum; C) initiation of hole formation in s econd anodizing; D) an alumina with a highly ordered pore after the second anodiz ation C) B) D) A) Al Al 2 O 3

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41 Figure 2 5. SEM Images of the morphology of a highly ordered porous alumina template prepared by the two step anodization, followed by the acid treatment for wid ening pores: (A) top view and (B) tilted view.

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42 Figure 2 6. SEM images of top view of alumina membranes anodized at 35 V after immersing in 10 wt% H 3 PO 4 for different periods: (A) 1 min, (B) 20 min, and (C) 40 min.

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43 Figure 2 7. A plot of pore widening rate of alumina membrane anodized at 35 V by the immersion in 10 wt% H 3 PO 4

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44 Figure 2 8. SEM images of top view of alumina membranes anodized at 50 V before (A) and after immersing in 10 wt% H 3 PO 4 for different period s: (B) 42 min and (C) 61 min.

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45 Figure 2 9. A plot of pore diameter of alumina membrane anodized at 50 V vs. the immersion time in 10 wt% H 3 PO 4

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46 Figure 2 10. Fabrication process of free standing alumina membrane: (A) the anodiz ation voltage was stepped down to 0 V; (B) Immersing it in phosphoric acid solution to liberate.

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47 Figure 2 11. SEM images of both surfaces of a free standing alumina membrane anodized at 50 V: (A) outer surface facing toward electrolyte solution and (B) barrier layer surface facing toward aluminum foil.

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48 Figure 2 12. SEM images of outer surface of free standing alumina films prepared under the two different anodization voltages, separately: (A) 35 V and (B) 50 V.

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49 Figure 2 13. SEM images of cross sectional view of free standing alumina membranes prepared under the different anodization conditions: anodized at 50 V for 7 min (A), 15 min (B), 18 min(C), and 25 min (D); anodized at 35V for 60 min (E)

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50 Figure 2 14. A plot of thickness of alumina membranes anodized at 50 V vs. the anodization time: growth rate=0.081 m/min; R=0.99939. Error bars represent a standard deviation achieved from more than 10 of thickness measurements.

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51 Table 2 1. Characteristics of free standing alumina membranes prepared under the different anodization conditions.

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52 CHAPTER 3 FABRICATION OF BIFURCATED SILICA NA NO TEST TUBES WITH A CONTROL OVER EACH SEGMENT Silica Nano Test Tubes Martin and his colleagues have pioneered a technology, called template synthesis for preparing monodisperse nanotubes of nearly any size and composed of nearly any material 1 3, 6, 9, 44, 60 One of their synthesized m aterials was a new type of nanostructure the nano test tube 6 9, 43 As the name implies, these are tubular nanostructures, open on one end and closed on the other. These nanotubes have distinct inner and outer surfaces that can be differentially functionalized 6, 9 The biofunctionalization of the outer tube surfaces allowed for selective targeting of the tubes to breast cancer cells 6 Th ese tubes have been used in the drug deliver y system as delivery vehicles or bio sensing applications 14, 61 63 To our knowledge, there has been no re port about three block branched silica nano test tubes which were fabricated by the template synthesis method It is expected to be an interesting structure of silica nano test tube s (SiNTTs) We believe that these types of silica nano test tubes could be used in biosensing and separation tools. In this Chapter the dimension of each segment of branched silica test tubes were finely controlled by changing the anodization time at each voltage step (Figure 3 1). Their shapes were adjusted by preparing the bra nched channel with different dimensions. Silica surface sol gel method was applied to make a replica of the branched PAA. The silica la yer on the rim surface of PAA was successively removed by an Ar plasma etch method. Free branched silica test tubes were collected by filtration with polycarbonate filtration membrane after the immersion in 10 wt % H 3 PO 4 solution. The structure of branched SiNTTs was investigated by SEM.

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53 Branched Channels in PAA In recent years, branched channels with various controllable mo rphology inside PAA membranes were fabricated by lowering the anodizing voltage suddenly, and they had been proven to be promising templates to fabricate nanostructures of high complexity. 64, 65 The growth mechanism of branched channels is mainly based on the equal area model by Li et al. 64 Weakness of this model is also found while trying to explain the formation of tree like morphology of branched channels. Shuoshuo et al. proposed a competitive growth mechanism for branched channels inside PAA membranes. 66 At an abrupt lower anodization voltage, two types of competition which originate from the non uniform thinning of stem channel barrier layers result in the tree like morphology of branched channels. Furthermore, competitive growth can be suppressed or eliminated by thinning the barrier layers of stem channels before the second step anodization. Experimentals Materials Aluminum foil (99.99%) was obtained from Alfa Aesar, and silicon te trachloride, and carbon tetrachloride were purchased from Sigma Aldrich. Oxalic acid, p hosphoric acid, chromium trioxide, sulfuric acid, methanol, and ethanol were obtained from Fisher. Fabrication of B ranched C hannels in PAA First, straight through pores in PAA were created at an anodization voltage of 50 V for 10 min (Step 1) as described before. The pores were widened in 10 wt% phosphoric acid for 40 min (Figure 3 2A) After rinsing with about 500 mL of water, the aluminum foil with PAA was set back in the anodization set up. The power supply was turned on and the applied voltage was reduced from 50 V to 35 V at two different rates ( Step 2:

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54 Figure 3 2B) At an applied voltage of 35 V, the anodization was conducted for the desired time ( Step 3: Figure 3 2C) The porous alumina was rinsed with about 500 mL of water a nd stored in a desiccator until used. Fabrication of Free Branched Silica Nano Test Tubes Silica surface sol gel method was used to make a replica of branched channels in PAA (Figure 3 3A). The silica layer on the rim surface was then removed via a argon plasma etch method (Figure 3 3B). Finally, b ranched SiNTTs were liberated by dissolving the PAA template in 10 wt% H 3 PO 4 solution (Figure 3 3C) Silica S urface S ol Ge l Process All experiments have been conducted under the acrylbox with a flow of N 2 gas as previously described 6, 67 Branched PAA w as immersed in a mixture of SiCl 4 /CCl 4 (25 mL/3.5 mL) for 2 mins The PAA was then rinsed in CCl 4 CCl 4 /Methanol (volume ratio = 1:1), and ethanol for 10 mins 2 mins, and 5 mins respectively. The treated PAA was then dried with a gas of N 2 The dried template was immersed in water and meth a nol for 5 mins and 2 mins, respectively. Finally, it was dried with a gas of N 2 This cycle was repeated until the desired thickness of silica layer was obtained. Removing Top Silica Layer using Argon Plasma Etch Method To efficiently liberate branched SiNTTs, the top silica layer on the rim surface of porous alumina was removed th rough an Ar plasma etch. Sputtering efficiency of RIE IC from Samco Inc. was investigated by controlling t he following parameters: power and time. The power values were 100 and 140 W separately and the run times were 30 and 60 sec separately The Ar flux rate and pressure was kept consistent at 12 sccm and 10 Pa respectively.

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55 Results and Discussion Control over the Dimension of Each Segment in Branched Channels of PAA In order to control the length of each segment (L 1 L 2 and L 3 ) of branched channels in PAA, the period at each anodization step was controlled as Table 3 1 shows. L 1 L 2 and L 3 in Figure 3 1, respectively. First, for a control over the dimension of top segment (L 1 in Fiugre 3 1), 50 V anodization time was adjusted. In order to control the length of second segment (L 2 Figure 3 1), t wo different types of branched SiNTTs were prepared. Type A has long voltage reduction time from 50 V to 35 V, while Ty pe B has a short period. In each type of SiNTTs, the length of branches (L 3 Figure 3 1) grown at an anodization of 35 V varies as the anodization period changes. Figure 3 4 shows a SEM image of tilted view of branched channels in PAA. The sample was pre pared under the conditions of Type B with a 35 V anodization time of 22 min. The channels grow clearly into two branches In F igure 3 4, branched channels have three distinct segments grown at each anodization step: 50 V ( Step 1: Figure 3 4 A) voltage reduction step ( Step 2: Figure 3 4 B) and 35 V ( Step 3: Figure 3 4 C) respectively. Uniform pore size and channel length were obtained Bifurcated channels in PAA were fabricated with a combination of anodization voltages (Figure 3 4) while straight throu gh channels were grown at a steady anodization voltage (Figure 3 4A). T he length of branches was controlled by changing the 35 V anodization period (Figure 3 5). The channel length increases as the period increases. The length of L 1 and L 2 was kept constan t while the length of branches varied. The pore size and length of L 1 and L 2 are same as one another. Their dimensions were well controlled by changing the anodization period.

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56 Control over the T hickness of S ilica L ayer of SiNTTs The thickness of silica layer was controlled by repeating the surface sol gel process Figure 3 6 showed TEM images of representative SiNTTs prepared via different number of cycles of surface sol gel process. The thickness of wall in SiNTTs linearly increa sed as the number of layers increased. Based on TEM images, the growth rate of silica layer thickness vs number of cycles was achieved with a value of 5.94 nm/layer (R = 0.99613) as shown in Figure 3 7. The thickness of silica layer of SiNTTs was adjusted by repeating a certain number of cycles of surface sol gel process. Optimization of A rgon P lasma E tch C ondition to R emove To p S ilica Sur face The etch feature of argon plasma was adjusted to efficiently remove the top silica layer by changing the parameters such as power, pressure, and time. Figure 3 8 showed the top view of silica layer coated alumina template after different argon plasma treat ments. Figure 3 8G describes a surface morphology of silica coated alumina template without any argon plasma treatment. As the power increases from 100 W (Figure 3 8A) to 140 W (Figure 3 8C) at higher power, top silica layer was removed clearly to show a clear boundary between in ner layer and rim layer. However, even though at low power (100 W), if the plasma etch time is long enough, the silica layer was clearly removed (Figure 3 8B). At too much long etch time, the silica layer was desorbed and readsorbe d to show smaller pores (Figure 3 8D) than ones at short etch time (Figure 3 8C). At higher pressure (Figure 3 8E) the anisotropic etch feature was diminished to create sputtering effect showing the result similar to Figure 3 8D. However, even at high pressure, the top silica layer was efficiently removed at a short etch time (20 sec, Figure 3 8F).

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57 In table 3 2 the plasma etch conditions for eff icient removal of silica layer are the followings: at 100 W and 10 Pa for 60 sec, at 140 W and 10 Pa for 30 sec, and at 140 W and 20 Pa for 20 sec. Therefore, in order to liberate silica nano test tubes, silica coated PAA templates were etched under the fo llowing parameters: an Argon flux rate of 12 sccm, a power of 140 W, a pressure of 10 Pa, and etch time of 30 sec. Characterization of Bifurcated SiNTTs Figure 3 9 shows a liberated bifurcated SiNTT obtained after the filtration of the solution with pol ycarbonate filter. The PAA template for the SiNTT was grown under the Type A with 35 V anodization time of 40 min. The shape is a replica of the branched channel in PAA. Figure 3 9 B C, and D show the close up view of the junction area between each segment in a bifurcated SiNTT The boundary was clear enough to distinguish one from the other. Bifurcated SiNTTs with a control over the length of branches were fabricated using the PAA templates which were grown under the Type A with different 35 V anodization times such as 5, 20, and 40 min (Figure 3 10 A, C, and D, respectively). The arrows indicate the starting point of branches. The branch length was 1140 (A), 1480 (C), and 2700 nm (D), respectively. The length increases as the 35 V anodization time increases. More than 30 of bifurcated SiNTTs prepared under the Type A anodization condition with different 35V anodization period were investigated to measure a dimension of each segment. Figure 11 shows a plot of dimension of each segment in branch ed SiNTTs vs the anodization time at 35V. The first segment dimension L1 (black square) from 50V anodization step was kept constant. The second segment dimension L2 (red circle) from voltage reduction step was also kept constant because the voltage reducti on rate was kept constant. The third segment dimen sion L3 (blue triangle) from

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58 35 V anodization voltage step was increased linearly as the anodization period increased. Therefore, the total length of silica nanotubes was increased as the 35 V anodization i ncreased. However, large standard deviation from L2 and L3 was an issue for a fine control over the dimension of each segment. The standard deviation was greater than 25% of the average value. Since the SiNTTs were replicas of branched channels in PAA, th e results resulted from the channel shape of PAA. We believe that large standard deviation was created from the voltage reduction step. According to a linear fitting plot of total length vs 35V anodization time (Figure 3 11B), the average length of channel s grown before 35V anodization step must be a y intercept value at a 35V anodization time of 0 min. Therefore the length of channels grown before 35V anodization time was about 3800 nm. The fitting plot gives a growth rate of 24 nm/min at an anodization v oltage of 35 V. Table 3 3 shows the dimension of each segment reflecting the plot in Figure 3 12. The sum of L1 and L2 (between 2500 and 3000 nm) showed a value less than 3800 nm. This means that the starting position of branches must be created during the voltage reduction period. Figure 3 12 shows representative images for bifurcated SiNTTs grown under the Type A anodization with a 35V anodization time of 5 min. According to Figure 3 11B, the len gth of branches must be about 125 nm. However, the branch length is much greater than 125 nm. These images support the conclusion above. Fine Control over a Formation Position of Branches in Three Block Branched Channels of PAA Under the Type A anodizat ion condition, the length of second and third segments was controlled with a large standard deviation because many branched channels were

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59 created randomly during the voltage reduction step. In order to have a fine control over a dimension of each segment i n three block bifurcated SiNTTs, the voltage reduction step was optimized. The anodization condition used for the optimization was the Type B set of anodization parameters in Table 3 1. In order to reduce a chance to create branches, the voltage reduction rate was increased. Figure 3 13 show representative images for PAA templates prepared under the Type B anodization condition with a 35 V anodization time of 22 min. In Figure 3 nd third segment, respectively. The images showed that the branch creation positions were uniformly controlled. demonstrate a uniform control over the length of the second segment. The dimension of ea ch segment of branched channels in PAA anodized under the Type B conditions with different 35 V anodization period was analyzed based on SEM images. Figure 3 14 exhibits a plot of dimension of each segment vs 35V anodization period. Each data point and err or bar represent an average value and standard deviation value obtained from more than 30 channels respectively The segment L 1 and L 2 along different 35 V anodization time were kept constant in length with a narrow distribution. The length of both segment L 3 and whole channel L t increased linearly with an increased 35 V anodization time with similar slopes ( a slope of L 3 = 22.5 nm/min and L t = 23.2 nm/min) Therefore, these data supported a statement that the branches were created at the same locat ions as shown in Figure 3 13. Table 3 4 shows the length of each segment of branched channels in PAA templates grown under Ty pe B anodization conditions. The standard deviation values

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60 from L2 and L3 in Table 3 3 are much lower than those from Table 3 3 T he dimension of each segment of three block branched channels in PAA was fine controlled under the Type B and ization conditions. In addition, L1 values in Table 3 3 are lower than those in Table 3 2 because the 50 V anodization time was reduced in the Type B anodization. However, the standard deviation of L1 values from both Types anodization was very low, showing a precise control during 50 V anodization step. The silica replicas of these well controlled PAA templates will be fabricated for a fine control of three block silica nano test tubes in the future. Conclusion This chapter demonstrated the fabrication methods of three block bifurcated silica nano test tubes. In order to control their structure, branched channels of PAA have been used as a templat e. The f irst segment was formed during the 50 V anodization step. The second segment was created during the voltage reduction step from 50V to 35V. The last segment was developed during the 35 V anodization step. The silica surface sol gel method was emplo yed to form replicas of branched channels. The optimized argon plasma etch method was applied to efficiently remo ve top silica layer without affecting the morphology of silica tubes. Free three block bifurcated silica test tubes were successively collected to analyze. Based on SEM images of free bifurcated SiNTTs, many branches started forming during the voltage reduction step when the voltage reduction rate was too slow. Using a short period of voltage reduction step, the uniform starting positions of bran ched channels in PAA were successively achieved. The precisely controlled PAA templates can be used to effectively govern the structure of three block silica nano test tubes. These three block silica tubes could provide an

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61 interesting nanostructure. We bel ieve that the se types of tubes could be used in biosensing and separation applications

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62 Figure 3 1. Dimension of fragments of bifurcated silica nano test tubes which can be controlled by the anodization time at each voltage step : Step 1, and Step 2, and Step 3 represent the anodization at 50 V, voltage reduction period, and anodization at 35 V, respectively.

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63 Figure 3 2. Fabrication process of Y branched channel in alumina template

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64 Figure 3 3. Fabrication process of free bifurcated silica nano test tubes : (A) Silica layer replicated the shape of branched channels in PAA by using silica surface sol gel, (B) top silica laye r was removed by a argon plasma etch method, and (C) the alumina template was removed after immersed in 10 wt% ph osphoric acid.

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65 Figure 3 4. SEM images of tilted view of PAA with branched channels prepared under the conditions with a 35 V anodization time of 22 min (Table 3 1). Three segments were created by the anodization at 50 V (A), voltage reduction step (B), and at 35 V (C), respectively.

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66 Figure 3 5. SEM images of tilted view of PAA with branched channels. The length of branches increases as the 35 V anodization period increases: (A) 10 min, (B) 22 min, and (C) 60 min, respectively.

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67 Figure 3 6. TEM images of representative silica nano test tubes prepared via different number of cycles of surface sol gel process: (A) 2 cy cles, (B) 3 cycles, (C) 4 cycles, and (D) 5 cycles. A B D C 200 nm 200 nm 200 nm 200 nm

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68 Figure 3 7. A plot of growth rate of silica layer as the number of silica layers increases: the growth rate is 5.94 nm/layer and the correlation factor is 0.99613.

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69 Figure 3 8. SEM images of top view of silica coated alumina template after different argon plasma treatments: (A) 100 W 10 Pa 30 s, (B) 100 W 10 Pa 60 s, (C) 140 W 10 Pa 30 s, (D) 140 W 10 Pa 60 s, (E) 140 W 20 Pa 60 s, and (F) 140 W 20 Pa 20 s. (G) Silica coated alumina template without any argon plasma treatment.

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70 Figure 3 9 SEM images of bifurcated SiNTTs prepared under the type A with a 35 V anodization time of 40 min (A). Image (B), (C), and (D) represent close up view of nano tube segments replicating the junction grown between 50 V anodization and the voltage reduction step (i), between the voltage reduction step and 35 V anodization (ii), and continuous 35 V anodization step (iii), respectively.

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71 Figure 3 10. SEM images of Type A bifurcated silica nano test tubes (SiNTTs) with different 35 V anodization period: 5 min (A and B), 20 min (C), and 40 min (D). Image B represents the close up view of the red box in image A. The arrows indicate the initial positions of branches.

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72 Figure 3 11. (A) A plot of dimension of each segment in bifurcated SiNTTs using PAA template grown under Type A condition with different 35 V anodization times. L1, L2, L3, and Lt represent the length of the segment L 1 L 2 L 3 and L t in Figure 3 1. ( B ) A linear fitting plot of total length L t in figure (A): Total length = 24.4 35V anodization time + 3814 (R = 0.98717)

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73 Figure 3 12. SEM images of bifurcated SiNTTs prepared under the Type A anodization condition with a 35 V anodization time of 5 min. (B) A close up view of red box in figure A. The arrows indicate the starting position of branches.

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74 Figure 3 13. SEM images of tilted view of branched channels in PAA prepared under the Type B anodization conditions with a 35 V anodization time of 22 min. (B) Close up view of the red box in figure A: the arrows indicate branch starting and third segment, respectively

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75 Figure 3 14. A plot of dimension of each segment in branched channels of PAA template grown under Type B conditions with different 35 V anodization times (A). L1, L2, L3, and Lt represent the length of the segment L 1 L 2 L 3 and L t in the diagram (B).

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76 Type Step 1 Anodization time at 50V Immersion time in 10% (w/w) H 3 PO 4 Step 2 Reduction time from 50V to 35V Step 3 Anodization time at 35V Type A 21 min 30 s 40 min 50 min 5 min 21 min 30 s 40 min 50 min 20 min 21 min 30 s 40 min 50 min 40 min Type B 11 min 30 s 40 min 2 min 30 s 10 min 11 min 30 s 40 min 2 min 30 s 22 min 11 min 30 s 40 min 2 min 30 s 60 min Table 3 1. Control parameter s for each anodization step to control over the dimension of each segment of branched channels in PAA

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77 Power ( W) Pressure (Pa) Etch Time (sec) Top silica layer 100 10 30 No change 100 10 60 Removed 140 10 30 Removed 140 10 60 Readsorbed 140 20 20 Removed 140 20 60 Readsorbed Table 3 2. Control parameters for the optimization of argon plasma etch condition to efficiently remove the top silica surface layer

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78 Type 35V anodization time L1 L2 L3 Lt Type A 5 min 1490 62 1490 382 1150 523 3970 198 20 min 1380 60 1160 262 1710 304 4260 168 40 min 1340 52 1250 412 2280 357 4800 122 Table 3 3 Length of each segment of bifurcated SiNTTs. The silica tubes were fabricated using PAA template grown under the Type A conditions with different 35 V anodization times. L1, L2, L3, and Lt represent the length of the segment L1, L2, L3, and Lt in Figure 3 1.

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79 Type 35 V anodization time L1 L2 L3 Lt Type B 10 min 720 15 120 16 280 20 1140 20 22 min 740 15 140 25 580 30 1440 29 60 min 730 14 120 10 1410 12 2300 19 Table 3 4 Length of each segment of branched channels in PAA templates prepared under the Type B conditions with different 35 V anodization times. L 1 L 2 L 3 and L t represent the length of the segment L 1 L 2 L 3 and L t in Figure 3 9B.

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80 CHAPTER 4 GOLD FILLED SILICA NANO T EST TUBES Gold N anoparticles Used in Nanomedicinal and Nanomedical Applications The strongly enhanced tunable absorption and scattering of noble metal nanocrystals has made them a novel class of optical and spectroscopic tags for b iological sensing 68, 69 and imaging 13 and biomedical the rapeutics 70, 71 In noble metals, the coherent collective oscillation of electrons in the conduction band induces large surface electric fields which greatly enhance the radiative properties of gold and silver nanoparticles when they interact with resonant electromagnetic radiation. 72 This makes the absorption cross section of these nanoparticles orders of magnitude stronger than that of the most strongly absorbing molecules 73 and the light scattering cross section orders of magnitude more intense than that of organic dye s. 74 Thus, these particles act as excellent sensors and novel contrast agents for optical detection due to their enhanced absorption and scatt ering, respectively. In addition, when it is realized and the strongly absorbed radiation is converted efficiently into heat on a picoseconds time domain due to electron 75 their potential use in photothermal therapy becomes obvious. The use of nanoparticles in medicine is one of the important directions that nanotechnology is taking at this time. They have be en attractive tools in the applications of drug delivery 76 cancer cell diagnostics 68, 77 and therapeutics 71, 78, 79 The scattering properties of gold nanospheres have been used for cancer cell imaging using confocal microscopy and simple dark field microscopy. 13 Recently, photothermal therapy using the absorption properties of antibody conjugated gold nanoshells 80 and solid gold

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81 nanospheres 70 has been demonstrate d to selectively kill cancer cells, leaving the healthy cells unaffected. To use long wavelength laser irradiation that penetrates tissue for in vivo photothermal treatment (650 900 nm), 81 the absorption band of the nanoparticles has to be in the near infrared (NIR) region. The absorption band of core shell particles has been tuned by adjus ting the ratio of the thickness of the gold shell to the diameter of the silica core (about 120 nm in diameter) and thus enables photothermal therapy in this region. Carbon nanotubes absorb naturally in this region and have been proposed as near infrared t herapy agents. 82 It is important to mention that surface plasmon field enhancement of the absorption of nanorods is predicted to be the strongest of all the different shap es of gold nanoparticles O ne can not only change the abosption and scattering wavelength from visible to the NIR region b ut also increase their absorbance and scattering cross sections. El sayed and his colleagues demonstrated the potential use of gold na norods as a novel contrast reagent for dual molecular imaging using simple dark field microscopy and selective photothermal therapy of cancer cells using a near infrared low energy continuous wave (cw) laser. 71, 78 They have developed a synthetic method to enable the conjugation of the nanorods to antipidermal growth factor receptor (anti EGFR) antibodies. Solid gold nanorods have several advantages over other photothermal contrast agents. The synthesis of gold nanorods with various aspect ratios, which enable tunable absorption wavelength in the NIR region is quite simple and well established. 73, 83 The appropriate size of the nanorods is quite small and is potentially useful in applications such as drug delivery and gene therapy. In addition, the biosafety

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82 of metallic gold is well known and they have been used in vivo since the 1950s 84 and recently the noncytotoxicity of gold nanoparticles in human cells has been studied in detail by Wyatt et al. 85 Synthesis of Gold Nanorods for Nanomedicinal and Nanomedical Applications These gold nanorods (AuNRs) were usually produced by the seeding approach. The s eed mediated synthesis can provide high aspect ratios. 86 G old seeds are usually made by chemical reduction of a gold salt with a strong reducing agent such as NaBH 4 These seeds, serving as the nucleation sites for nanorods, are then added to a growth solution of gold salt with a weak reducing agent such as ascorbic acid and hexadecyltrimethylammonium bromide. The aspect ratios of the gold nanorods can be controlled by varying the amount of gold seeds with respect to the gold precursor. Silica C oated G old N anorods B ased on C hemical S ynthesis In order to use the AuNRs in medicin al or medical applications, their surfaces must first be functionalized with biomolecules w ithout altering their stability in solution. For example, thiol modified single stranded DNA (ssDNA) can be immobilized onto the surface of gold nanoparticles (NPs) in a single step displacement reaction of electrostatically absorbed citrate anions. These DNA modified NPs, first reported by Mirkin et al. 87 and Alivisatos et al. 88 have been studied extensively for the detection and identification of oligonucleot ides. The straightforward thiol attachment chemistry is made possible by the anionic character of the NP surface due to the presence of the citrate. However gold nanorods produced by the methods developed by either Murphy 86 or El Sayed 89 have a net positive surface charge due to the presence of an adsorbed monolayer of the surfactant, hexadecyltrimethyl ammo nium bromide (CTAB), on the nanorod surface. Thus, the thiol chemistry used to modify AuNRs is very difficult when

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83 employed for the attachment of ssDNA to surfactant coated gold nanorods. The reasons for this are that the high density of the surfactant mon olayer decreases the access of the thiol modified ssDNA to the nanorod surface, and the negatively charged phosphate backbone of the ssDNA interacts with the positively charged CTAB molecules; the net result is typically a rapid aggregation and precipitati on of the gold nanorods from solution. Corn and et al. reported an alternative strategy for preparing ssDNA functionalized gold nanorods based on a multistep process in which the gold nanorods are first modified with a thin silica film, and then the ssDNA is attached to the silica shell via an aldehyde coupling reaction. Coating the nanorods with 3 5 nm of silica improves their solubility and stability. They demonstrated that the DNA functionalized silica coated gold nanorods can be used to greatly enhance the sensitivity of surface plasmon resonance imaging (SPRI) measurements of DNA hybridization adsorption onto DNA microarrays. Synthesis of Gold Fille d Silica Nano Test B ased on the T emplate S ynthesis Martin and his colleagues have developed the synthesis method of gold nano rods using nanoporous membranes, called template synthesis Two different deposition methods have been used: the electrodeposition and the electroless deposition. The former method entailed the electrochemical deposition of gold within the pores of nanoporous polycarbonate or alumina membranes. 1 3, 44, 45, 60, 90 92 The diameter of the gold nanorod is determined by the pore diameter of the template membrane, while the length of the nanorod can be controlled through the amount of gold deposited within the pores of the membrane. Martin et al. have also utilized an electroless method to prepare metal nanotubues/nanowires that span the entire thickness of the template membrane. 4,

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84 93, 94 electrochemical method needs a metal film on one side of the membrane serving as a working electrode 3 In addition, during th e electroless plating, metal deposition proceeds from the pore wall itself rather than from a base layer of metal. Therefore, the template can have only one open end. As described in Chapter 3, silica nano test tubes (SiNTTs) have been successively proved to be useful delivery vehicles with disti n ct inner and outer surfaces that can be differentially functionalized. 6, 9, 14, 62 Mo reover, the inner space of SiNTTs can be used as a template for the deposition of gold The shape of gold nanomaterials is determined by the shape of inner space of silica test tubes. The diameter and length of silica test tubes have been well controlled. 14, 61 63, 95 This ability can provide a control over an aspect ratio of gold core nanomaterials. This chapter describes an alternative strategy for preparing silica coated gold nanorods /nanotubes that is, gold filled silica nano test tubes (Au SiNTTs) based on the template synthesis method using the electroless plating of silica coated alumina template. The silica coated alumina template was prepared by the surface sol gel method. Since the silica layer wa s a non condu ctive layer, the electroless plating method was chosen to fill gold into the SiNTTs. The diameter and length of SiNTTs w ere well controlled to provide an ability to govern the structures of gold cores. The optical properties of silica coated gold nanorods were investigated by using the absorption spectrometry.

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85 Experimentals Materials Aluminum foil (99.9998%) was obtained from Alfa Aesar and s ilicon tetrachloride and tetrachloro carbon were purchased from Sigma Aldrich. Oxalic acid, o p hosphoric acid, chromium trioxide, sulfuric acid, methanol, and ethanol were obtained from Fisher. (3 Aminopropyl)triethoxy silane (APTS; gelest) and Gold plating solution (Oromerse SO Part B Technic Inc ) w ere used as received Amine Functionalized Silica Surface on t he Porous Alumina Membrane Alumina membranes were prepared by the two step anodization as described in Chapter 2. The membrane was immersed in a 10 wt% H 3 PO 4 solution for the desired time to widen the pores (Figure 4 1A). SiNTTs were formed by us ing the su rface sol gel method as described in Chapter 3 (Figure 4 1B) The silica surface was then modified with amine functional groups using APTS as described previously (Figure 4 1C) 5 Electroless Plating of Amine Funct ion alized SiNTTs The electroless plating procedure described previously 4, 93, 94 was applied to deposit gold on the amine functionalized surface of SiNTTs (Figure 4 2 ). In order to activate silica surfaces, the silica layer was first modified with amine functional groups. The amine functionalized silica surface was immersed into a SnCl 2 solution (called, sensitized) which results in deposition of Sn 2+ onto the silica coated membrane surfaces (Figure 4 1A). The membrane surface s w ere then swabbed with cotton swabs soaked in ethanol. The sensitized membrane was immers ed into a AgNO 3 solution, and a surface redox reaction occurs (eq 4 1), which yields nanoscopic metallic Ag particles on the membrane surfaces (Figure 4 1B) species adsorbed to the membrane surfaces and species disso lved in solution,

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86 respectively.) The membrane surfaces were then swabbed with cotton swabs soaked in ethanol. Sn 2+ surf + 2Ag + aq Sn 4+ aq + 2Ag surf (4 1) The membrane surfaces is then immersed into a commercia l gold plating solution and a second surface redox reaction occurs, to yield nanoscopic Au nanoparticles on the surfaces (eq 4 2). Ag surf + Au + aq + aq + Au surf (4 2) These surface bound Au nanoparticles are good autocatalysts for the reduction of Au + to Au using formaldehyde as the reducing agent. As a result, Au deposition began at the pore walls. Plating Time Dependence of the Morphology of Gold Cores of Au SiNTTs Alumina membranes were prepared by the anodization at 50 V for 10 min. The membranes were immersed in 10 wt% H 3 PO 4 solution for 40 min. A cycle of the surface sol gel method was applied to the alumina membranes. The silica layer on the rim surface was removed by the Argon plasma etch for 1 min ( Figure 4 3A: 140 W, 12 sccm of Ar(g), and 10 Pa). The silica layer was modified with APTS silanes (Figure 4 3B) and the amine functional group s on the rim surface w ere then removed by another Argon plasma etch for 10 sec ( Figure 4 3C: 140 W, 12 sccm of Ar(g), and 10 Pa). Gold was deposited onto the SiNTTs with amine functional groups by the electroless plating

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87 method for a desired time as described above (Figure 4 3D) The morphology of gold filled SiNTTs embedded in PAA template was investigated by SEM. Moreover, the Au SiNTTs prepared above were liberated by immersing the template in 10 wt% H 3 PO 4 solution overnight (Figure 4 3E) The alumina template was completely dissolved away. The free Au SiNTTs were collected by the vacuum filtration with 100 nm or 30 nm polycarbonate membrane The tubes were dispersed into ethanol and water for TEM characterization and VIS NIR absorbance measurements, respectively. Fabrication of Au SiNTTs with the Different Aspect Ratio s of Gold Cores In order to control the aspect ratio of gold cores of Au SiNTTs, alumina template was anodized at 50 V for 3 min, 6 min, and 10 min, respectively. All of them were treated in the same way s as desc ribed above before the gold electroless plating. Gold was simultaneously deposited into three different types of SiNTTs with different aspect ratios by using the electroless plating for 19 h. The dimensions in diameter and length were characterized by SEM and TEM. Vis NIR Absorbance Measurements The free Au SiNTTs with three different aspect ratios of gold cores such as 2.1, 6.1, and 8.8, respectively were dispersed into PBS buffer by using vo rt ex. The absorption measurements were made with Agilent 8453 UV Visible spectrophotometer using a 500 L black quartz cuvette. Electron Microscopy Scanning Electron Microscopy (SEM) was used to measure the diameter and length of Au SiNTTs as well as their morphology Images were obtained using a Hitachi S4000 FE SEM Transmission Electron Microscopy (TEM Hitachi H 7000 ) was used to

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88 measure the thickness of silica layer as well as the diameter and length of Au SiNTTs The diameter, length, and thickness were calculated by analyzing the SEM/TEM images with the Image J software (Wayne Rasband, NIH, USA) Results and Discussion Plating Time Dependence of the Morphology of the Gold Core of Au SiNTTs Figure 4 4 shows representative SEM images of tilted view of PAA template with Au SiNTTs prepared by different plating times such as 42 min (A), 90 min (B), 150 min (C), and 1170 min (D). The bright areas represent gold. At a short plating time, gold nanoparticles were formed uniformly along the inner surface of SiNTTs (Figure 4 4A). This demonstrated that the nucleation of Au nanoparticles occurred simultaneously along the wall of SiNTTs. At a plating time of 90 min (Figure 4 4B) and 150 min (Figure 4 4C), the Au nanoparticles grew along the wall to make a connection with each other as well as the creation of more Au nuclear na noparticles The length of gold cores was same as that of pores of SiNTTs. The plating was conducted further until 19 h 30 min (Figure 4 4D). At a long plating time, the gold cores became solid like nanorods. These results demonstrate that Au was selective ly deposited within SiNTTs to make replicas of inner structure of SiNTTs. The results demonstrate the deposition mechanism described in Figure 4 5. At a short plating time, Au nuclea r nanoparticles uniformly form along the wall surfaces of SiNTTs (Figure 4 5A). As SiNTTs were further plated, t he gold nanoparticles were connected due to their growth and creation of more nuclei (Figure 4 5B). At a long plating time, gold is completely filling the SiNTTs (Figure 4 5C)

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89 Free Au SiNTTs Prepared at Different P lating Times Au SiNTTs were liberated by dissolving the alumina template in a H 3 PO 4 solution. Figure 4 6 shows free Au SiNTTs prepared under the gold plating time of 19h 30 min. The SEM images show that Au SiNTTs were completely liberated from the alumina template. The images (B) and (D) show a close up view of the box in image (A) and (C). The arrows and bright areas indicate silica layers (pale layer) and gold cores of Au SiNTTs, respectively. The shape of gold cores is a replica of inner structure of SiN TTs. More than 30 of Au SiNTTs were used to analyze the size in diameter and length. The gold cores have a narrow distribution in diameter (71 6.7) and length (490 27) These results demonstrate that Au SiNTTs with a uniform size in diameter and length were successively liberated from the alumina template. The morphology of gold cores within Au SiNTTs prepared under the different gold plating times w as investigated by TEM. Figure 4 7 shows representative TEM images of Au SiNTTs with different plating t imes: 25 min (A), 65 min (B D), and 1170 min (E G). At a short plating time (A: 25 min), since gold nanoparticles have strong connection with each other as shown in Figure 4 5A, they ha d weak bond to the SiNTT surface. Therefor e, many gold nanoparticles were released from the tubes, showing few nanoparticles within SiNTTs. At a plating time of 65 min (Figure 4 7B), the gold cores remained within SiNTTs. The structure of gold core s w as a nanotube with a porous wall as described in Figure 4 5B TEM images in Figure 4 7C and D indicate a close 7B. The images demonstrate that gold nanoparticles completely coated through the pores of SiNTTs to make a strong conne ction with each other At a plating time of 19h 30 min (Figure 4 7E), the gold cores became more solid, as described in Figure 4 5C. The TEM i mages

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90 in Figure 4 7F and G exhibit a close clearly proved that SiNTTs with a s olid gold nanorod were achieved at a long plating time. A Control over the Aspect Ratios of Gold Cores within Au SiNTTs In order to control the aspect ratios of gold cores within Au SiNTTs, the diameter of the porous alumina template was kept constant and the anodization times (3 min, 6 min, and 10 min) were changed to prepare three different sizes in length of pores. Figure 4 8 exhibits representative TEM images of free Au SiNTTs prepared using the PAA template anodized at 50 V for 3 min. The template was plated with gold for 21 h. The Au SiNTTs were well dispersed (Figure 4 8A). The image (B) and (C) exhibit a close SiNTTs were found to si t upright (red solid arrows) and other A u SiNTTs were lying down (blue dot arrows) because the shape was close to a sphere (aspect ratio=2.1). The dimension of length (110 13 nm) and diameter (53 5 nm) of gold cores were analyzed using TEM images. The thickness of silica layer was about 9 nm, wh ich is a little greater than that of a single cycle of silica surface sol gel process as shown in Figure 3 7. Figure 4 9 describes the representative TEM images of free Au SiNTTs prepared using the PAA template by anodizing at 50 V for 6 min. The gold elec troless plating was processed for 21 h. Most of gold cores became solid to completely fill the inner space of SiNTTs. The length and diameter of gold cores were determined to be 340 18 and 56 4 nm, respectively. The aspect ratio was found to be 6.1. The thickness of silica layer was determined to be about 8 nm. The PAA template prepared by anodizing at 50 V for 10 min (10min PAA) was also plated under the same plating conditions as described above. Figure 4 10 display the morphology of free Au SiNTTs

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91 using 10min PAA. Figure 4 10B indicate a close up view of the box area in Figure 4 10A. Most of inner spaces of SiNTTs were filled with gold. The length and diameter were measured to be 560 32 and 64 9 nm, respectively. The aspect ratio was determined to b e 8.8. The thickness of silica layer was about 8 nm. Optical Properties of Au SiNTTs with Different Aspect Ratios of Gold Cores The liberated Au SiNTTs with different aspect ratio s of gold cores prepared by plating for 21 h were dispersed into PBS buffer for Vis NIR absorbance measurements. Figure 4 11 shows the Vi s NIR absorptioin spectra from gold cores with an aspect ratio of 8.8 (black solid line), 6.1 (red dot line), and 2.1 (green dash line). All of the a bsorption spectra from different types of Au SiNTTs exhibit a strong band around 530 nm which indicates a transverse mode of gold nanorods. 96 The short Au SiNTTs showed a longitudinal mode band around 730 nm (green dash line). Since the other types of Au SiNTTs has a high aspect ratio of gold cores, their longitudinal modes are expected to be at longer than 1100 nm. 96, 97 Therefore, two other types of Au longitudinal mode band within a wavelength window from 400 nm to 1100 nm. In order to investigate the optical properties of these types of Au SiNTTs, a Vis NIR spectrometry with a PbS detector, which can detect a light up to 3000 nm, is expected to be used in the future. Conclusion This chapter entails the fabrication method of a new type of nanomaterials, that is, gold filled silica nano test tubes. The gold was selectively deposited into SiNTTs. The SiNTTs were completely filled with gold at the long gold plating time. The aspect r atio of gold cores was well controlled by adjusting the inner space of SiNTTs. The optical properties of Au SiNTTs will be further investigated in the future. Ultimately, their

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92 feasibility as a photothermal therapeutic material or photo response drug deliv ery vehicle will be investigated.

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93 Figure 4 1. Fabrication process of amine functionalized silica nano test tubes on the porous alumina template. (A) The pores of two step anodized alumina membrane were widened in 10 wt% H 3 PO 4 solution. (B) SiNTTs were formed on the porous alumina membrane by using the surface sol gel method. (C) The silica surface was modified with amine functional groups using APTS.

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94 Figure 4 2 Schematic diagram of the electroless plating procedure use d to deposit gold into amine functionalized SiNTTs (A) The amine functionalized SiNTTs w ere immersed in a SnCl 2 formed on the surface of membrane by the surface redox reaction between Ag + and Sn 2+ (C ) Au nanoparticles were created by the second surface redox reaction between Au + and Ag. The Au nanoparticles served as autocatalysts for the continuous reduction of Au + with formaldehyde as the reducing agent. Diagram (A) to (C) represent the close up vie w of the blue box in diagram (D).

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95 Figure 4 3. Fabrication procedure of free Au SiNTTs. (A) The silica layer on the rim surface was removed by the Ar plasma etch method. (B) Silica layer was modified with amine functional groups. (C) Another argon et ch process was conducted to remove the amine functional groups on the rim surface. (D) Gold electroless plating was accomplished for the desired time. (E) Alumina template was removed by the immersion in 10 wt% H 3 PO 4

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96 Figure 4 4. SEM images of tilted view of PAA templates with Au SiNTTs prepared by different plating times: (A) 42 min, (B) 90 min, (C) 150 min, and (D) 1170 min (19 h 30 min). Bright areas represent gold.

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97 Figure 4 5. Schematic diagram of the morphology of Au cores within amine fu nctionalized SiNTTs after the electroless plating for different period s: (A) 42 min, (B) 90 min and 150 min, and (C) 1170 min

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98 Figure 4 6. SEM images of free Au SiNTTs prepared under the plating time of 1170 min (19 h 30 min). (B) and (D) shows a cl ose up view of the box in (A) and (C), respectively. The arrows and bright areas indicate silica layers (pale layer) and gold cores of Au SiNTTs, respectively.

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99 Figure 4 7. TEM images of free Au SiNTTs prepared under the different plating times: (A ) 25 min, (B D) 65 min, and (E G) 1170 min (19 h 30 min). (C) and (D) exhibit a close show a close

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100 Figure 4 7. Continued.

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101 Figure 4 8. TEM images of free Au SiNTTs prepared using the PAA template anodized at 50 V for 3 min. The gold electroless plating was conducted for 21h. The image (B) and (C) exhibit a close image (A), respectivel y. Red solid arrows and blue dot arrows indicate Au SiNTTs sitting upright and lying down, respectively.

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102 Figure 4 9. TEM images of free Au SiNTTs prepared using the PAA template anodized at 50 V for 6 min. The gold electroless plating was conducted for 21h.

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103 Figure 4 10. TEM images of free Au SiNTTs prepared using the PAA template anodized at 50 V for 10 min. The gold electroless plating was conducted for 21h. The image (B) exhibits a close up view of the box area in the image (A).

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104 Figure 4 1 1 Vis NIR absorption spectra of Au SiNTTs with different aspect ratios of gold cores. Black solid line, red dot line, and green dash line represent the spectra from gold cores with an aspect ratio of 8.8, 6.1, and 2.1, respe ctively.

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105 CHAPTER 5 PATTERN TRANSFER: PO ROUS SILICON FOR LASER DESORBSION IONIZATION M ASS SPECTROMETRY Motivation Matrix assisted laser desorption/ionization (MALDI) has been utilized in the study of large biomolecules, such as peptides, proteins, carbohydrates and DNA 98 The broad success of MALDI MS is rela ted to the ability of the matrix to incorporate and transfer energy to an analyte, thereby ionizing it in a very soft manner 99 101 However, MALDI is complicated because the presence of the matrix often causes significant background ion intensity in the low mass range (<500 m / z ) and the heterogeneous cocrystallization of the matrix and analyte result s in poor shot to shot reproducibility 8, 102 104 As a complementary alternative, Siuzdak and co workers have developed desorptio n/ionization on silicon (DIOS) using nanoporous silicon substrates for the detection of small peptides and antiviral drugs without any mass interference due to matrix ions 103, 105, 106 In DIOS MS, the analytes in solution are directly deposited on a nanoporous silicon surface without a chemical matrix. Compared to MALDI, this approach provides simplified sample preparation, more uniform surfaces, a nd less background noise at masses below 600 Da. Although the DIOS mechanism is not fully understood, porous silicon was found to be an effective medium for desorbing compounds and generating intact ions in the gas phase 103, 106, 107 It is suggested that porous silicon effectively absorbs and transfers the energy from the UV irradiation laser to the adsorbed analytes, while also protecting these molecules from fragmentati on caused by direct laser exposure 107, 108 Most existing DIOS platforms are prepared by electrochemical anodization of crystalline silicon necessitating the use of toxic HF solution 109 In addition, it is difficult to

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106 obtain reproducible surfaces with the same nanostructures, because pore size and pore density are not well controlled by chemical etching 104, 110 To overcome this issue, Vertes et.al reported the use of silicon microcolumn arrays and silicon nanopost arrays used as alternative DIOS substrates and showed that structu re specific fragmentation can be also be induced 111, 112 In another study He et al. have used electron beam lithography (EBL) 104 and nanosphere lithography 111 to investigate the impact of the surface morphology of porous silicon on analyte desorption and ionization. Electron beam lithography allows accurate fabrication of sub 100 nm features, but the serial nature of the technique makes it very slow. In the nanosphere technique, the self assembly can be very time consum ing, and long range order is still not achieved 113 Self ordered porous alumina substrates have also been used as useful platforms for laser desorption/ionization MS 114 116 Because Wada et al. found that surface electroconductivity was one of the requirements for a porous alumina chip to be effective for laser induced desorption/ionization, the surface was coated with gold or platinum 115 Masuda et al. suggested that the porous alumina layer beneath the surface metal probably acted as a thermal insulator 116 This chapter focuses on the fabrication of porous silicon by a n on lithographic method using a self ordered porous alumina membrane and its application for laser desorption ionization mass spectrometry. The self ordered pore pattern of the alumina membrane is transferred into the silicon substrate via plasma etching me thod, resulting in rapid, low cost and reproducible fabrication without the use of toxic chemicals. Pore size and porosity are comparable to that of the alumina mask and the pore depth can be controlled in the range of 10 nm to 50 nm by changing the etch t ime. The porous silicon

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107 platforms with 10 nm and 30 nm depth nanowells were used for laser desorption/ionization (LDI) MS detection of biomolecules. To the best of our knowledge laser desorption ionization at this depth regime has not been reported befor e and the submicrometer porosity limit to induce ionization is yet unclear. The utility of nanowell arrays under atmospheric pressure and vacuum regimes were studied. Experimental Materials Aluminum foil (99.99%) was obtained from Alfa Aesar. Single side p olished P/Boron type silicon wafers, having {100} orientation, low resistivity (0.01 52525 m thickness and 100 mm diameter were obtained from Silicon Valley Microelectronics, Inc. (Santa Clara, CA). Phosphoric acid, sulfuric acid, chromium tr ioxide, oxalic acid and hydrogen peroxide were obtained from Fisher Scientific. Triton X 100, adenosine, Pro Leu Gly tripeptide and [ Des Arg 9 ] bradykinin were obtained from Sigma Aldrich. Stock solutions (500 pmol/L) of the analytes were prepared by disso lving each compound in 1:1 acetonitrile:aqueous 0.1 % trifluoroacetic acid solution. Pre paration of P orous A lumina M ask The alumina mask was an inhouse prepared nanopore alumina film. The electrochemical anodization method used to prepare these films has b een described previously in Chapter 2 Fabrication of the N anowell A rrays on S ilicon S ubstrates Silicon wafers were cleaned in a piranha solution (3:1 (v/v) 98% H 2 SO 4 : 30% H 2 O 2 ) for 30 min. The silicon substrates were rinsed with deionized water, followed by drying under a stream of nitrogen. The alumina mask was mounted on top of the silicon wafer with the barrier layer side of the mask facing up using a drop of Triton X 100

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108 so lution as an adhesive. The masked substrate was then inserted into the vacuum chamber of a reactive ion etching apparatus (Samco model RIE 1C). Ar plasma was used to etch the substrate surface through the mask with the following etching parameters (13.56 MHz, 100 W, 10 Pa, 12 sccm Ar flow). The etch time was varied (5, 10, 15, 20, and 25 min, respectively) in order to determine the etch rate of silicon under these conditions. The alumina mask was removed by the use of sonication in water for 10 min. The si licon substrates were rinsed with nanopure water and ethanol followed by drying with a stream of nitrogen gas. Porous silicon substrates were stored in ethanol and they were dried by N 2 flow just prior to mass spectrometry experiments. Char acterization of P orous S urfaces Tapping mode Atomic Force Microscopy (AFM) was used to examine the morphology and depth of the nanowell array on silicon substrates. In order to investigate the etch rate, three porous silicon samples were prepared at each etch time. Averag e depth of wells (at least 450 nanowells) for each etching time was obtained from three different positions of each sample. AFM experiments were performed using a Multimode AFM with a NanoScope IIIa controller (Digital Instruments, Santa Barbara, CA) and M PP 11200 Si tips (Veeco, manufacture suggested tip radius is r < 10 nm). Height images were obtained in the tapping mode under dry N 2 gas. The nanowell depth was assessed using the profile analysis made by the WSxM image analysis package (Nanotec Electroni ca, Madrid, Spain). A Hitachi S4000 FE SEM was used to obtain the pore diameter and the thickness of the alumina mask. Both the top view and the cross section of the alumina membrane were investigated. All samples were coated with an Au/Pd film by a Desk II Cold Sputter instrument (Denton Vacuum, LLC).

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109 Mass Spectrometry Mass spectra were acquired using an Agilent 6210 MSD time of flight (TOF) mass spectrometer configured for ESI (Agilent Technologies Inc., Santa Clara, CA, USA). Laser desorption/ionization experiments were conducted by installing an external MassTech atmospheric pressure/matrix assisted laser desorption ionization pulse dynamic focusing (AP/MALDI PDF) ion source. The results were analyzed using Analyst software. Mass spectrometry experimen ts in the vacuum regime were conducted using an ABI 4700 MALDI TOF TOF proteomics analyzer (Applied Biosystems, Framingham, MA) equipped with an Nd:YAG laser (355 nm, 3 7 ns pulses) which was operated in the reflector, positive ion mode with an accelerati on voltage of 20 kV. The laser was operated at a fixed fluence at 200 Hz. The laser firing pattern was set ected with Data Explorer 4.0. R esults and D iscussion Fabrication of P orous S ilicon S ubstrates Characterization of alumina mask Figure 5 1 shows representative SEM images of the mask prepared as described in an earlier publication 35 The pores were formed in a hexagonally packed array (Figure 1A). Image J software (Wayne Rasband, NIH, USA) was used to characterize the morphology of the alumina membrane: pore diameter (805 nm); pore density ((9.30.3) 10 9 pores/cm 2 ); porosity (482 %). These films have two distinct surfaces. Figure 5 1A shows the surface that faced the solution during the anodization, while Fig ure 5 1B shows the surface that faced the Al substrate during the anodization. The

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110 which must be removed during processing. In Figure 5 1C the cross section of the film indicates that the thickness was 2.3 m and that the pores were cylindrical. M orpholo gy of porous silicon substrates In Figure 5 2, AFM images show the surface characteristics of the silicon nanowell array: density (6.010 9 nanowells/cm 2 ), diameter (937 nm), and porosity (44%). According to surface characteristics from AFM images, the pore pattern of the alumina mask was successively transferred into silicon substrates. The pore size, pore density and porosity of porous s ilicon are compatible to those of alumina mask. The pore diameter showed a narrow distribution. Moreover, as shown in Figure 5 3, as the etching time was increased the depth of nanowells increased linearly while the array pattern remained unchanged. Mass S pectrometry A nalyses Porous silicon substrates were prepared by etching for 5 min (depth = 10 nm) and 15 min (depth = 30 nm) because of less variation in depth. Silicon nanowells were spotted with 0.2 L of 500 pmol/ L solutions of adenosine, Pro Leu Gly tripeptide and bradykinin, and the samples were dried in room temperature for 30 min prior to analyses. A schematic of the entire process is shown in Figure 5 4. The mass spectra of adenosine acquired on 10 nm and 30 nm surfaces (Figure 5 5) clearly show that an increase in etching time provides a significant improvement in the ion signal. Analyses with the 10 nm deep pores were repeated with concentrations from 100 pmol to 500 pmol and with various laser attenuations, but no significant improvement was ob served. In a recent report on the thermochemistry of adenosine 117 it was shown that a significant amount of energy has to be imparted to induce

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111 protonation. The authors reported the proton affinity of adenosine as 9791 kJ/mol in the gas phase by using nano ESI MS. In the light of these findings, we can hypothesize that for the surface etched to a depth of 1 0 nm, the surface induced heating does not provide sufficient energy to desorb the molecule from the surface and ionize it. However, when the etching time was increased and the same experiment was conducted using 30 nm deep nanowells, significant ion signa ls for protonated adenosine [M+H] + before (268.1054) and adenine after (136.0594) loss of the ribose group were observed. After this experiment, analyses on the 10 nm and 30 nm surfaces were extended to two different compounds: Pro Leu Gly ([M+H] + = 285.1 921) and [ Des Arg 9 ] bradykinin( ([M+H] + = 904.4691). Similar to adenosine, 0.2 L of 500 pmol/ L of Pro Leu Gly and [ Des Arg 9 ] bradykinin were spotted on 10 nm deep and 30 nm deep nanowells and the attenuation of the laser was fixed. The mass spectra are shown in Figures 5 6 and 5 7. The mass spectra for the two compounds clearly show that the ion signals significantly increase wi th increasing pore depth. This result led us to think about the events associated with laser desorption/ionization and the effect of increased nanostructuring on the ion signals. Siuzdak et al 118 reported that the surface area is the key factor. However, Alimpiev et al 119 demonstrated that the submicrometer surface porosity, irrespective of the substrate material, is the most important effect fo r desorption/ionization. In the most similar report to our work Okuno et al 115 demonstrated that silicon wafers rough e ned with sandpaper can act as a surface for laser desorption. A n increase in surface porosity enhances ion signals via multiple

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112 effects: The surface is resupplied with analyte after a laser pulse, and a local environment for the laser induced field is provided to facilitate physical separation of preformed analyte ion s from their counterions. Also, the heat capacity and heat conductivity are decreased rapid heating of the surface occurs, resulting in effective sublimation of analytes by laser irradiation as well as effective absorption of the laser light. Recently Hsu et a l. 110 showed that laser desorption ionization can occur even on metals. They showed that although ion signals greatly improve with surface roughness ionization can occur even on a commercial non porous aluminum film. Our results are in good agreement with these previous findings. Significant dimer and trimer signals are observed in Figures 5 6 and 5 7. Since the ionization event occurs at atmospheric pressure, it is possible that ion ion reactions occur after the desorption event, and that dimerization and trimerization are favored. Desorption ionization on porous silicon is mostly carried out in MALDI i nstruments operating under vacuum regime and only a few reports exist about atmospheric pressure DIOS and to date no comparision yet exists. Since the concentrations of the analyte solutions are low, this is probably not due to the concentration effect. T o investigate this factor, a vacuum MALDI instrument was used both to check the effectiveness of the surfaces in vacuum conditions and to make a comparison between atmospheric pressure and vacuum MALDI regimes. Comparison of atmospheric pressure and vacuum spectra, shown in Figure 5 8, indicates that a significant difference exists between atmospheric pressure and vacuum regimes. Because the dimer and trimer signals did not appear in vacuum regime, these peaks were due in part to atmospheric pressure condi tions.

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113 Because of the much larger signal to noise ratios, the nanowells with 30 nm pores were used for quantitative measurements. A calibration plot was prepared by spotting varying amounts of Pro Leu Gly tripeptide onto 30 nm deep nanowells, and spectra were obtained at fixed laser power with all other experimental variables constant. As shown in Figure 5 9, within the 50 pmol 500 pmol range, the signal intensity correlates with the increased concentration in a reasonably linear fashion. Huikko et.al 120 used atmospheric pressure DIOS to analyze drugs and reported sensitivities ranging i n between 1 fmol and 20 pmol. Even though our substrates do not have deep pores like conventional DIOS substrates a similar sensitivity range has been achieved. C onclusion The applicability of nanowell silicon substrates for desorption/ionization on silico n mass spectrometry has been demonstrated. The pore pattern of an alumina mask has been successively transferred onto silicon substrates by means of Ar ion plasma etch. This method has several advantages: (1) elimination of toxic HF; (2) reproducibility (u niform pore size, pore density, and porosity); and (3) low cost and fast production. Nanowell depth is linearly related to etch time. The etch rate was determined to be 2.0 nm/min with about 2 m thick PAA mask. The pore depth requirement for inducing lase r desorption ionization has not been well addressed before and our results show that very little porosity (10 nm) is sufficient to induce ion formation and the ion signals significantly improve with the change of the pore depth. We also showed that a reaso nable level of sensitivity has been achieved with 30 nm deep nanowells. The mechanism of ion formation on nanowell arrays is still not clear but we attribute the ion formation to unique thermal and optical properties of silicon. We are in the process of s ystematically investigating the effects of pore size, pore depth and pore

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114 density by changing the PAA mask and by controlling the aspect ratio of the nanowells using different substrates. Our initial findings suggest that nanowell arrays are suitable subst rates for laser desorption ionization of biomolecules and can be used for the analysis of small molecules such as metabolites.

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115 Figure 5 1. Scanning electron micrographs of the alumina mask: Top view of (A) outer surface and (B) barrier layer surface, an d (C) cross section

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116 Figure 5 2. Tapping mode AFM imgaes of nanowell arrays on silicon substrates prepared by means of Ar ion plasma etching for: (A) 5 min, (B) 15 min, and (C) 25 min.

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117 Figure 5 3. Nanowell depth vs etch time (The solid line is the linear regression best fit line (slope = 2.0 nm /min; R = 0.99932).

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118 Figure 5 4. Workflow for the preparation of nanowell arrays and MS analysis

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119 Figure 5 5. Mass spectra of adenosine acquired on 10 nm (top) and 30 nm deep (bottom) nanowells and DIOS surface

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120 Figure 5 6. M ass spectra of Pro Leu Gly acquired on 10 nm (top) and 30 nm deep (bottom) nanowells.

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121 Figure 5 7. Mass spectra of [des Arg9] bradykinin,on 10 nm (top) and 30 nm deep (bottom) nanowells

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122 Figure 5 8. Comparison of atmospheric pressure and vacuum regime laser desorption ionization on 30 nm deep nanowell arrays A: Adenosine, B: Pro Leu Gly, C: [Des Arg9] bradykinin B A

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123 Figure 5 8. Continued. C

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124 Figure 5 9. Plot of signal intensity versus amount of Pro Leu Gly using 30 nm deep nanowell arrays

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125 CHAPTER 6 CONCLUSION In this dissertation self ordered porous anodic alumina has been used for the nanofabrication. The PAA has been utilized for the nanofabrication by using two methods: the template synthesis and the pattern transfer. In order to make PAA films useful for these approaches a highly self ordered porous anodic alumina (PAA) has been fabricated by the anodization of Al in appropriate acidic solutions. It has a narrow distribution in pore size, pore density, porosity and pore depth. Different type s of PAA w ere prepared for bottom up approach (template synthesis) and top down approach (pattern transfer) respectively. For the former, PAA was attached on Al and the pore size was controlled by the post etching pore sizes were ~80 nm and ~50 nm under the different anodiz ation voltage. Thickness of PAA showed a good linear relationship with the anodization time. In the template synthesis study, the shape and size of nanomaterials were governed by controlling the structure of PAA template. For example, the channel shape o f PAA has been tuned from straight through type to branched one. Three block b ifurcated silica nano tubes with various length s of each segment were fabricated from the template. In addition, a new type of nanomaterial such as gold filled silica nano test tubes was synthesized. The aspect ratio of gold cores was well controlled by adjusting the inner structure of SiNTTs. The optical properties of Au SiNTTs will be further investigated to examine the ir feas ibility as photothermal therapeutic nanomaterials o r photo response delivery vehicles.

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126 In the pattern transfer study, the PAA pattern was successively transferred onto silicon substrates via plasma etch method. The porous silicon substrates enhanced the ion signals as the depth increased. In the future, the morphology effect on the ionization will be further investigated using these porous silicon substrates as a platform for Laser Desorption/Ionization Mass Spectrometer.

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127 LIST OF RE FERENCES (1) Martin, C. R. Chemistry of Materials 1996 8 1739 1746. (2) Foss, C. A.; Hornyak, G. L.; Stockert, J. A.; Martin, C. R. Journal of Physical Chemistry 1994 98 2963 2971. (3) Martin, C. R. Science 1994 266 1961 1966. (4) N ishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995 268 700 702. (5) Kang, M. C.; Trofin, L.; Mota, M. O.; Martin, C. R. Analytical Chemistry 2005 77 6243 6249. (6) Buyukserin, F.; Medley, C. D.; Mota, M. O.; Kececi, K.; Rogers, R. R.; Tan, W. H.; Martin, C. R. Nanomedicine 2008 3 283 292. (7) Buyukserin, F.; Kang, M. C.; Martin, C. R. Small 2007 3 106 110. (8) Hillebrenner, H.; Buyukserin, F.; Kang, M.; Mota, M. O.; Stewart, J. D.; Martin, C. R. Journal Of The American Chemical Society 2006 128 4236 4237. (9) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N. C.; Nevanen, T. K.; Soderlund, H.; Martin, C. R. Journal Of The American Chemical Society 2002 124 11864 11865. (10) Huimeng, W.; Haizhen, Z.; Jiaqi, Z.; Shuo, Y.; Chen, L.; Cao, Y. C. Angewandte Chemie International Edition 2008 47 3730 3734. (11) Lakshmi, B. B.; Dorhout, P. K.; Martin, C. R. Chemistry of Materials 1997 9 857 862. (12) Lakshmi, B. B.; Patrissi, C. J.; Martin, C. R. Chemistry of Materials 1997 9 2544 2550. (13) El Sayed, I. H.; Huang, X.; El Sayed, M. A. Nano Letters 2005 5 829. (14) He, B.; Son, S. J.; Lee, S. B. Analytical Chemistry 2007 79 5257.

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136 BIOGRAPHICAL SKETCH Dooho Park was born in Kimhae in South Korea (ROK). When he was five years old, he moved to Kangnung, a beautiful city famous for being surrounded by beautiful beach and mountains. He graduated a high school in Kangnung. He entered Korea University in Seoul in 1992 and then graduated with a Bachelor of Science degree in Chemistry in 1999. He continued studying bioinorganic chemistry at Korea University to pursue a Master of Science degree from 1999 to 2001. After his graduation, Dooho worked at Korea University as a General Chemistry Coordinator and a part time instruc tor for General Chemistry Laboratory from 2001 to 2003. He entered University of Florida and joined then Dr. Martin s group in 2003.