Article pubs.acs.org/JPCC
Electroanalysis at Single Gold Nanowire Electrodes Karen Dawson, Amélie Wahl, Richard Murphy, and Alan O’Riordan* Tyndall National Institute, University College Cork, Lee Maltings, Cork, Ireland S Supporting Information *
ABSTRACT: In this work, we report the fabrication and in-depth electrochemical analysis of discrete gold nanowire electrodes for use in electrochemical applications. The single nanowire electrodes were fabricated using a hybrid E-beam/photolithography approach at silicon substrates, providing electrodes with well-defined and reproducible dimensions. Following fabrication, nanowire devices were characterized by electrical and electrochemical techniques. Low electrical resistances with typical linear Ohmic responses were observed from fully packaged electrode devices. Finite element diffusion domain simulation studies were undertaken to explore analyte mass transport to nanowire electrodes at a variety of scan rates. Simulation results suggested that radial analyte diffusion profiles to nanoelectrodes should be present at fast scan rates. This behavior was confirmed experimentally where cyclic voltammograms obtained in ferrocenemonocarboxylic acid were observed to be steady-state, with high measurable currents (nA) and sigmoidal up to 1000 mV s−1. Nanowire electrodes had very low capacitance, ∼ 37 ± 6 nF cm−2 per nanowire, 3 orders of magnitude lower than that typically achieved by ultramicroelectrodes. The electrochemical responses of nanowires, in model redox mediators, were excellently described by Butler−Volmer kinetics. The nanowire electrodes are applied to reproducible determination of heterogeneous electron transfer-rate constants, k0, for three key model redox analytes, FcCOOH, Fe(II)(CN)64−, and Ru(NH3)63+.
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kinetic studies,3 which is the driving research interest in nanoscale electroanalysis. As a result, a range of nanoelectrode devices and device arrays with a variety of geometries and design layouts have been reported in the literature. These are typically categorized as (i) discrete nanodisk electrodes, (ii) nanoelectrode ensembles, or (iii) nanoelectrode arrays. Discrete nanodisk electrodes have been fabricated by sequential etching of a microwire to a fine cone followed by insulation of all but the cone tip,4,6,8 deposition of carbon in a capillary tube,9 or pulling of glass capillaries containing sealed microwires to form disk electrdes.7,10 Although discrete nanodisks have been used to probe nanoscale electrochemical phenomena, major disadvantages associated with these electrodes include the difficulty and length of time for fabrication, leaking of electrolyte though the seals, and the extremely low measurable currents (10−100 pA) that may be achieved with them. Such small currents pose problems when the measured current is to be used as the analytical signal in an electroanalytical application.
INTRODUCTION Advances in nanotechnology have enabled rapid developments in analytical science in terms of sensor design, fabrication, and performance.1,2 This is particularly true in electrochemistry, where the advent of nanoscale electrodes has opened up new research domains and application opportunities.3−5 Nanoelectrodes may be defined as electrodes that have a critical dimension in the nanoscale, i.e.,
