Electrochemistry at single-walled carbon nanotube networks

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Abstract

The field of carbon nanotube (CNT) electrochemistry has received increasing attention from the scientific community since 1996, when the first CNT electrode displaying electrocatalytic properties was reported. However, the large majority of the field functions on the assumption that the sidewalls of CNTs, both multi-walled (MWNTs) and single-walled (SWNTs), are electrochemically inert and only edgeplane- like defects, open ends and catalytic nanoparticles (NPs) are responsible for the impressive electrochemistry observed at CNT electrodes. This thesis aims to elucidate the fundamental electrochemical properties of SWNTs. Random, highly interconnected 2D SWNT networks (either as <1% or ~ 100% surface coverage) grown using catalysed chemical vapour deposition (cCVD) on an insulating support are used throughout. Unlike other growth methods, cCVD results in clean SWNTs, without the need for further purification and with limited metal NP content. Chemical functionalisation of <1% SWNT networks indicates that the main effect of introducing defects (either as sidewall addends or open ends) is a dramatic decrease in the conductivity of the SWNT network, with repercussions on the electrochemical performance of SWNTs. Disk-shaped ultramicroelectrodes (UMEs) fabricated using pristine <1% SWNT networks shows superb electrochemistry for simple outersphere redox couples and superior characteristics over solid conventional UMEs, such as lower background currents and faster response times. Importantly, reversible cyclic voltammetry (CV) obtained under high mass transport conditions suggests that the SWNT sidewall is more electroactive than previously thought. In addition, electrochemical impedance spectroscopy (EIS) coupled with SWNT UMEs is shown to be a powerful method for determining all parameters of an electrochemical reaction in a single experiment. EIS also offers tremendous promise as a detection strategy for sensing applications. The use of a thin-layer cell (TLC) configuration, generating high mass transport rates to the SWNTs, allows for the quantitative determination of electron transfer (ET) kinetics at SWNTs. The value obtained for the rate of ET again provides strong evidence that the SWNT sidewall must be electrochemically active. While <1% SWNT networks are suitable for low concentration electrochemical detection and for the investigation of ET kinetics at SWNTs, a 100% CNT material, or CNT mat, is preferred for the detection of challenging redox molecules, such as dopamine, known to foul the surface of conventional carbon electrodes. CNT mat UMEs can be used for the electrochemical detection of dopamine at micro-molar concentrations in in-vivo mimics, with no decrease in electrode performance after extensive use, outperforming any other carbon electrode material currently available.

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