(Invited) Fast-Scan Nanogap Voltammetry at Double-Carbon-Fiber Ultramciroelectrodes

Abstract

In this presentation, we discuss theory [1] and experiment [2] of double-carbon-fiber (CF) ultramicroelectrodes (UMEs) with nanometer-wide gaps to voltammetrically obtain high time resolution, signal-to-background ratio, and kinetic selectivity for dopamine (DA) against ascorbic acid (AA). The novel nanogap-based electrodes are fabricated by using a pair of ~7 µm-diameter CFs, which are inserted into a double-bore glass capillary, heat-pulled, and milled by focused-ion-beam technology to yield ~50 µm-long double-cylinder UMEs separated by a nanometer-wide gap without extensive nanolithography or nanoscale electrode positioning required for other nanogap electrodes. Double-CF UMEs are characterized electrochemically by investigating the redox cycling of the Ru(NH3)6 3+/2+ couple across a nanogap between voltammetric generator and amperometric collector electrodes to achieve quasi-steady states at fast scan rates of 100 V/s, which is ~1000 times faster than reported with any nanogap-based electrochemical cell. Importantly, the transient background of the amperometric collector response is suppressed ~100 times in comparison with that of the voltammetric generator response to enable the quantitative analysis of the collector response against the cycled generator potential without background subtraction. Finite element analysis of oxidative amperometric responses to Ru(NH3)6 2+ at the collector electrode at up to 100 V/s yield consistent gap widths of ~0.18 µm and a standard electron-transfer rate constant of 0.9 cm/s. Moreover, nanogap-mediated redox cycling can be initiated by DA oxidation at the generator electrode to largely improve the DA selectivity of the collector response against AA, which is also oxidized at the generator electrode to immediately and irreversibly produce a redox-inactive species, thereby minimizing the corresponding collector response. The probe-type design of double-CF UMEs as well as their high DA selectivity against AA is attractive for in-vivo applications. [1] Pathirathna, P.; Balla, R. J.; Amemiya, S. “Simulation of fast-scan nanogap voltammetry at double-cylinder ultramicroelectrodes.” J. Electrochem. Soc., Focus Issue on the Brain and Electrochemistry Honoring R. Mark Wightman and Christian Amatore, 2018, 165, G3026. [2] Pathirathna, P.; Balla, R. J.; Amemiya, S. “Nanogap-based electrochemical measurements at double-carbon-fiber ultramicroelectrodes.” Anal. Chem. 2018, 90, 11746.