Microfabrication and Characterization of Gold Nanoring Electrodes on Silicon Micro Pillar for Neurochemical Sensing

Abstract

Nanoelectrodes provide unique size-dependent properties such as enhanced analyte transport, low charging currents and high signal-to-noise ratio (SNR), which allow them to be utilized as a sensitive detector in several areas of electrochemistry. In recent times, there is a tremendous interest in the use of nanoscale band, disk and hemispherical electrodes for novel electroanalytical applications. One major drawback is that they do not provide a 3D geometry that are more suitable for interrogating cells and tissues because of the difficulty in fabricating such nanoelectrodes onto to a 3D platform. Besides, they are bulky and lack multiplexing and multimodal capability. In this work, we report the fabrication and characterization of a gold nanoring nanoelectrode (Au NRE) that is 165±10 nm wide and micropatterned onto a 4.6±1 µm diameter 17.5±2.5 µm long silicon (Si) micropillar with an intervening 50 nm thick hafnium oxide insulating layer. The electrochemical behavior of the Au NRE was characterized by a steady-state cyclic voltammogram with extremely high SNR of 2500 and charging currents as small as 1.5±0.3 pA. A kinetically controlled voltammetric current response, which is unique to nanoscale electrodes, is indicated by a “semicircle spectrum” from the Nyquist plot. Suggested by circuit fitting of the electrochemical impedance spectra, the NRE geometry can be varied from inlaid to nanotrench with depths controllable by the etching process, which results in differing stead-state voltammetric current 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 interferences.