SELF-ORGANIZED NANOPOROUS MATERIALS FOR CHEMICAL SEPARATIONS AND CHEMICAL SENSING by BIPIN PANDEY B.S., Truman State University, 2007 AN ABSTRACT OF A DISSERTATION submitted in partial fulfillment of the requirements for the degree DOCTOR OF PHILOSOPHY Department of Chemistry College of Arts and Sciences KANSAS STATE UNIVERSITY Manhattan, Kansas 2013 Abstract Self-organized nanoporous materials have drawn a lot of attention because the uniform, highly dense, and ordered cylindrical nanopores in these materials provide a unique platform for chemical separations and chemical sensing applications. Here, we explore self-organized nanopores of PS-b-PMMA diblock copolymer thin films and anodic gallium oxide for chemical separations and sensing applications. In the first study, cyclic voltammograms of cytochrome c on recessed nanodisk-array electrodes (RNEs) based on nanoporous films (11, 14 or 24 nm in average pore diameter; 30 nm thick) derived from polystyrene-poly(methylmethacrylate) diblock copolymers were measured. The faradic current of cytochrome c was observed on RNEs, indicating the penetration of cytochrome c (hydrodynamic diameter ≈ 4 nm) through the nanopores to the underlying electrodes. Compared to the 24-nm pores, the diffusion of cytochrome c molecules through the 11- and 14-nm pores suffered significantly larger hindrance. The results reported in this study will provide guidance in designing RNEs for size-based chemical sensing and also for controlled immobilization of biomolecules within nanoporous media for biosensors and bioreactors. In another study, conditions for the formation of self-organized nanopores of a metal oxide film were investigated. Self-organized nanopores aligned perpendicular to the film surface were obtained upon anodization of gallium films in ice-cooled 4 and 6 M aqueous H2SO4 at 10 V and 15 V. The average pore diameter was in the range of 18 ~ 40 nm, and the anodic gallium oxide was ca. 2 µm thick. In addition, anodic formation of self-organized nanopores was demonstrated for a solid gallium monolith incorporated at the end of a glass capillary. Nanoporous anodic oxide monoliths formed from a fusible metal will lead to future development of unique devices for chemical sensing and catalysis. In the final study, surface chemical property of self-organized nanoporous anodic gallium oxide is explored through potentiometric measurements. The nanoporous anodic and barrier layer gallium oxide structures showed slow potentiometric response only at acidic pH (≤ 4), in contrast to metallic gallium substrates that exhibited a positive potentiometric response to H+ over the pH range examined (3-10). The potentiometric response at acidic pH probably reflects some chemical processes between gallium oxide and HCl. SELF-ORGANIZED NANOPOROUS MATERIALS FOR CHEMICAL SEPARATIONS AND CHEMICAL SENSING by BIPIN PANDEY B.S., Truman State University, 2007 A DISSERTATION submitted in partial fulfillment of the requirements for the degree DOCTOR OF PHILOSOPHY Department of Chemistry College of Arts and Sciences KANSAS STATE UNIVERSITY Manhattan, Kansas 2013 Approved by: Major Professor Takashi Ito Copyright BIPIN PANDEY 2013 Abstract Self-organized nanoporous materials have drawn a lot of attention because the uniform, highly dense, and ordered cylindrical nanopores in these materials provide a unique platform for chemical separations and chemical sensing applications. Here, we explore self-organized nanopores of PS-b-PMMA diblock copolymer thin films and anodic gallium oxide for chemical separations and sensing applications. In the first study, cyclic voltammograms of cytochrome c on recessed nanodisk-array electrodes (RNEs) based on nanoporous films (11, 14 or 24 nm in average pore diameter; 30 nm thick) derived from polystyrene-poly(methylmethacrylate) diblock copolymers were measured. The faradic current of cytochrome c was observed on RNEs, indicating the penetration of cytochrome c (hydrodynamic diameter ≈ 4 nm) through the nanopores to the underlying electrodes. Compared to the 24-nm pores, the diffusion of cytochrome c molecules through the 11- and 14-nm pores suffered significantly larger hindrance. The results reported in this study will provide guidance in designing RNEs for size-based chemical sensing and also for controlled immobilization of biomolecules within nanoporous media for biosensors and bioreactors. In another study, conditions for the formation of self-organized nanopores of a metal oxide film were investigated. Self-organized nanopores aligned perpendicular to the film surface were obtained upon anodization of gallium films in ice-cooled 4 and 6 M aqueous H2SO4 at 10 V and 15 V. The average pore diameter was in the range of 18 ~ 40 nm, and the anodic gallium oxide was ca. 2 µm thick. In addition, anodic formation of self-organized nanopores was demonstrated for a solid gallium monolith incorporated at the end of a glass capillary. Nanoporous anodic oxide monoliths formed from a fusible metal will lead to future development of unique devices for chemical sensing and catalysis. In the final study, surface chemical property of self-organized nanoporous anodic gallium oxide is explored through potentiometric measurements. The nanoporous anodic and barrier layer gallium oxide structures showed slow potentiometric response only at acidic pH (≤ 4), in contrast to metallic gallium substrates that exhibited a positive potentiometric response to H+ over the pH range examined (3-10). The potentiometric response at acidic pH probably reflects some chemical processes between gallium oxide and HCl. Table of Contents List of Figures ................................................................................................................................. x List of Tables ............................................................................................................................... xvi Acknowledgements ..................................................................................................................... xvii Dedication ..................................................................................................................................... xx List of Abbreviations ................................................................................................................... xxi Chapter 1 - Introduction .................................................................................................................. 1 1.1 Objectives and Motivation .................................................................................................... 1 1.2 Background/Literature review .............................................................................................. 3 1.2.1 Self-Organized Nanoporous Materials .......................................................................... 3 1.2.2 Block-copolymer based nanoporous materials .............................................................. 3 1.2.2.1 Fabrication of block-copolymer thin films ............................................................. 5 1.2.2.2 Characterization of block-copolymer thin films ..................................................... 5 1.2.3 Brief history of block copolymers ................................................................................. 5 1.2.4 PS-b-PMMA thin films .................................................................................................. 6 1.2.5 Anodic metal oxide based nanoporous materials ........................................................... 9 1.2.5.1 Anodic porous alumina ......................................................................................... 10 1.2.5.1.1 Mechanisms of Nanoporous Anodic Oxide Formation ................................. 11 1.2.5.1.2 Long-range pore ordering of anodic porous alumina .................................... 12 1.2.5.1.3 Pores initiation and porous oxide growth ...................................................... 13 1.2.5.1.4 Current-time curves ....................................................................................... 14 1.2.5.1.5 Dependence of pore parameter on anodizing voltage and electrolyte concentration ................................................................................................................. 15 1.2.5.2 Anodic titanium oxide........................................................................................... 16 1.2.5.3 Other anodic metal oxides .................................................................................... 18 1.3 Gallium and its electrochemistry ........................................................................................ 19 1.4 Potentiometric Studies on Metal Oxide Electrodes ............................................................ 21 1.5 Dissertation Outline ............................................................................................................ 24 References ................................................................................................................................. 26 vi Chapter 2 - Analytical Techniques used in the Characterization of Nanoporous Materials ......... 34 2.1 Spectroscopic Ellipsometry1-3 ............................................................................................. 34 2.2 Energy-Dispersive X-ray Spectroscopy4 ............................................................................ 37 2.3 Cyclic Voltammetry5-8 .......................................................................................................