Electrochemical Behavior of a Gold Nanoring Electrode Microfabricated on a Silicon Micropillar

We report on the microfabrication and characterization of a gold nanoring electrode (Au NRE) patterned on top of a silicon (Si) micropillar. An NRE of 165 +/- 10 nm in width was micropatterned on 4.6 +/- 1 mu m diameter x 17.5 +/- 2.5 mu m long Si micropillar with an intervening 50 nm thick hafnium oxide insulating layer. Scanning electron microscopy and energy dispersive spectroscopy data confirmed excellent cylindricality of the micropillar with vertical sidewalls and a completely intact layer of Au NRE encompassing the entire micropillar perimeter. The electrochemical behavior of the Au NRE was characterized by a steady-state cyclic voltammo-gram with extremely high signal-to-noise ratios of 2500 and charging currents as small as 1.5 +/- 0.3 pA. A " semicircle spectrum" from the Nyquist plot indicates a kinetically-controlled voltammetric current response, which is unique to nanoscale electrodes. The variations in the exchange current values, which are dependent on the NRE area and standard rate constant, place a greater emphasis on the isotropic gold wet etching process that defined the geometry of the Au NRE. Circuit fitting of the electrochemical impedance spectra suggests that the NRE geometry can be varied from inlaid to nanotrench with depths controllable by the etching process. This resulted in differing stead-state voltammetric currents and interfacial properties in terms of charge transfer resistance, constant phase element and trench resistance values. The applicability of Au NREs to electrochemical sensing is demonstrated by detecting lead, a neurotoxin at 100 ppb levels. Also, by surface-modification with multi-walled carbon nanotubes, dopamine, a neurochemical implicated in various brain disorders, is detected at a sensitivity as low as 100 nM with 1000-fold selectivity versus common interferents. The flexibility of the microfabrication approach allows for the creation of multiple NREs of controllable width and nanometer spacing on a single micropillar. This capability and in particular where the NRE is micropatterned on three-dimensional microstructures as will be reported herein, facilitates unique electroanalytical capabilities such as intracellular electrochemistry and highly multiplexed detection for emerging biological applications.