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Electrochemical Detection Optimized for Capillary Liquid Chromatographic Determination of Neuroactive Compounds

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Electrochemical Detection Optimized for Capillary Liquid Chromatographic Determination of Neuroactive Compounds ELECTROCHEMICAL DETECTION OPTIMIZED FOR CAPILLARY LIQUID CHROMATOGRAPHIC DETERMINATION OF NEUROACTIVE COMPOUNDS by Xiaomi Xu Bachelor of Science, Xiamen University, 2001 Master of Science, Xiamen University, 2004 Master of Science, University of Pittsburgh, 2008 Submitted to the Graduate Faculty of the Kenneth P. Dietrich School of Arts and Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2011 UNIVERSITY OF PITTSBURGH THE KENNETH P. DIETRICH SCHOOL OF ARTS AND SCIENCES This dissertation was presented by Xiaomi Xu It was defended on November 10, 2011 and approved by Stephen G. Weber, Professor, Department of Chemistry Adrian C. Michael, Associate Professor, Department of Chemistry Shigeru Amemiya, Associate Professor, Department of Chemistry John Kitchin, Associate Professor, Department of Chemical Engineering Dissertation Advisor: Stephen G. Weber, Professor, Department of Chemistry ii Copyright © by Xiaomi Xu 2011 iii ELECTROCHEMICAL DETECTION OPTIMIZED FOR CAPILLARY LIQUID CHROMATOGRAPHIC DETERMINATION OF NEUROACTIVE COMPOUNDS Xiaomi Xu, PhD University of Pittsburgh, 2011 Miniaturization of the electrochemical flow cell detector is critical to improving the performance of a capillary liquid chromatography-electrochemical detection (LC-ECD) system. Fabricating the spacer in the flow cell using a novel photosensitive polymer coating offers the convenience of controllable thickness and easy assembling, which leads to improvement in detector performance. The advantage of the thin polymer coating as the spacer is demonstrated by comparing the performance with that of a traditional free standing spacer. A dual working electrode electrochemical detector increases detection selectivity based on the difference in the electrochemical reversibility of analytes. Carbon fiber/epoxy composite materials are commercially available in various shapes and sizes at very low cost. Using tube and rod composites, a ring-disk electrode with a 20 µm gap between the ring and the disk is successfully fabricated through a unique two-step insulation-adhesion process. The narrow gap is favorable for mass transfer in the generator-collector experiment. When assembled in an electrochemical radial-flow cell detector, the dual electrode gives high collection efficiency (0.8 for a chemically reversible species), which enhances the detection selectivity. Carbon electrodes can also be replaced with naphthalenethiol modified gold electrodes. The modification monolayer suppresses the background current from Au while giving detectable current for biological analytes with sensitivity comparable to a glassy carbon electrode. iv Analyzing microdialysates sampled from the brain area of interest using HPLC is the most popular method among neuroscientists to measure extracellular neurotransmitters. An optimized capillary UHPLC-ECD system is successfully applied to the measurement of serotonin basal concentration in microdialysis samples and the analytical system can be coupled with microdialysis for on-line measurements. v TABLE OF CONTENTS PREFACE ................................................................................................................................. XVI 1.0 INTRODUCTION ........................................................................................................ 1 1.1 LOW-VOLUME ELECTROCHEMICAL FLOW CELL .............................. 3 1.1.1 Background ...................................................................................................... 3 1.1.2 Statement of the problem ................................................................................ 6 1.2 ELECTRODE MATERIAL AND ITS SURFACE MODIFICATION .......... 7 1.2.1 Background ...................................................................................................... 7 1.2.2 Statement of the problem ................................................................................ 9 1.3 DUAL OR MULTIPLE WORKING ELECTRODE ECD ........................... 10 1.3.1 Background .................................................................................................... 10 1.3.2 Statement of the problem .............................................................................. 11 1.4 RESEARCH OVERVIEW ............................................................................... 12 2.0 OPTIMIZATION OF AN ELECTROCHEMICAL FLOW CHANNEL FOR CAPILLARY LC USING A NOVEL PHOTOSENSITIVE POLYMER COATING ......... 14 2.1 INTRODUCTION ............................................................................................. 14 2.2 EXPERIMENTAL ............................................................................................. 18 2.2.1 Chemicals and materials ............................................................................... 18 2.2.2 Fabrication of the photoresist spacer ........................................................... 19 vi 2.2.3 Characterization instrumentations .............................................................. 20 2.2.4 Capillary liquid chromatography ................................................................ 20 2.2.5 Electrochemical flow cell............................................................................... 21 2.2.6 Homogenate sample preparation ................................................................. 21 2.2.7 Theoretical simulation ................................................................................... 22 2.3 RESULT AND DISCUSSION .......................................................................... 23 2.3.1 Optimization of the electrochemical flow cell detector .............................. 23 2.3.2 Fabrication of a photoresist spacer in a flow cell and its surface topography .................................................................................................................. 26 2.3.3 Characterization of the mechanical properties of the photoresist film .... 31 2.3.3.1 Depth-sensing indentation (DSI) method – photoresist coating ..... 32 2.3.3.2 Dynamic mechanical analysis (DMA) method - the photoresist free standing film ....................................................................................................... 38 2.3.4 Application of the photoresist film as a spacer in a miniaturized electrochemical flow cell detector following capillary liquid chromatography ... 42 2.3.5 Theoretical simulation of the flow channel ................................................. 48 2.4 CONCLUSION .................................................................................................. 53 3.0 CARBON FIBER/EPOXY COMPOSITE RING-DISK ELECTRODE: FABRICATION, CHARACTERIZATION AND APPLICATION TO ELECTROCHEMICAL DETECTION IN CAPILLARY HIGH PERFORMANCE LIQUID CHROMATOGRAPHY ............................................................................................. 54 3.1 INTRODUCTION ............................................................................................. 55 3.2 EXPERIMENTAL ............................................................................................. 57 vii 3.2.1 Chemicals and materials ............................................................................... 57 3.2.2 Carbon Fiber / Epoxy Ring-disk Electrode Fabrication ............................ 58 3.2.3 Microscopy characterization ........................................................................ 59 3.2.4 Capillary liquid chromatography and electrochemical detection ............. 60 3.3 RESULTS AND DISCUSSION ........................................................................ 61 3.3.1 Carbon fiber/epoxy composite electrode characterization ........................ 61 3.3.2 Carbon fiber composite ring-disk electrode fabrication and characterization .......................................................................................................... 65 3.3.3 Carbon fiber composite ring-disk electrode detection of peptides following capillary HPLC .......................................................................................................... 68 3.4 CONCLUSION .................................................................................................. 72 3.5 ACKNOWLEDGEMENTS .............................................................................. 72 3.6 SUPPORTING INFORMATION .................................................................... 72 4.0 MODIFICATION OF A GOLD ELECTRODE FOR BIURET-BASED PEPTIDE AMPEROMETRIC DETECTION ......................................................................... 75 4.1 INTRODUCTION ............................................................................................. 75 4.2 EXPERIMENTAL ............................................................................................. 76 4.2.1 Reagents .......................................................................................................... 76 4.2.2 Electrode pretreatment and thiol adsorption ............................................. 77 4.2.3 Cyclic voltammetry ........................................................................................ 77 4.3 RESULTS AND DISCUSSION ........................................................................ 78 4.3.1 Electrochemistry of modified Au electrode ................................................. 78 4.3.2 Capillary liquid chromatography ................................................................ 83 viii 4.3.3 Stability of 2-naphthalenethiol modification monolayer ..........................
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