Electroanalysis at single gold nanowire electrodes

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, k ⁰, for three key model redox analytes, FcCOOH, Fe(II)(CN) ₆ ^4-, and Ru(NH ₃) ₆ ³⁺.