Nanogap-Based Electrochemical Measurements at Double-Carbon-Fiber Ultramicroelectrodes

Abstract

Electrochemical measurements with unprecedentedly high sensitivity, selectivity, and kinetic resolution have been enabled by a pair of electrodes separated by a nanometer-wide gap. The fabrication of nanogap electrodes, however, requires extensive nanolithography or nanoscale electrode positioning, thereby preventing the full exploration of this powerful method in electrode design and application. Herein, we report the simple fabrication of double-carbon-fiber ultramicroelectrodes (UMEs) with nanometer-wide gaps not only to facilitate nanogap-based electrochemical measurements, but also to gain high time resolution, signal-to-background ratio, and kinetic selectivity for dopamine against ascorbic acid. Specifically, ~7 μm-diameter carbon fibers 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. The redox cycling of the Ru(NH3)63+/2+ couple across a nanogap between voltammetric generator and amperometric collector electrodes reaches quasi-steady states at fast scan rates of 100 V/s as demonstrated experimentally and even 1000 V/s as predicted theoretically. The transient background of the amperometric collector response is suppressed ~100 times in comparison with that of the voltammetric generator response. Nanogap voltammograms based on the collector response against the cycled generator potential are quantitatively analyzed without background subtraction to reproducibly yield nanogap 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 dopamine oxidation at the generator electrode to largely improve the dopamine selectivity of the collector response against ascorbic acid, which is also oxidized at the generator electrode to immediately and irreversibly produce a redox-inactive species.

Review: new insights into optimizing chemical and 3D surface structures of carbon electrodes for neurotransmitter detection

“… 133 The Amemiya group reported a double carbon-fiber microelectrode system, with a nanogap in between them, for the detection of dopamine selectively over ascorbic acid, because the irreversible ascorbic acid is redox-inactive at the collector electrode. 136 This electrode has not been used in vivo, but may offer advantages of limited fouling and high selectivity. 137 Scan rate is an important factor in collector/generator systems.…”

“… 133 At faster scan rates, the plateau gradually changes into peaks. 136 Collector-generator systems are useful for fluidic devices, but have not been implemented in vivo yet.…”

“… 64 Future studies should also take advantage of redox cycling of dopamine in thin layer cells, which should also promote dopamine detection over ascorbic acid, which is less reversible. 136 , 137 …”

Versatile Tools for Understanding Electrosynthetic Mechanisms

“…Amemiya and co-workers recently simulated and experimentally investigated fast-scan voltammetry at nanogaps between two UMEs, which allows for electrochemical analysis without the need for background correction. 33,195 More recently, a convolution-based method was developed for subtraction of the differential capacitive contributions to the measured current. 193 This background correction method is especially important for in vivo measurements, in which environmental changes surrounding the electrode can occur.…”

Ultrasensitive Electrochemistry by Radical Annihilation Amplification in a Solid–Liquid Microgap

“…The study of electrochemistry in thin layers and nanogaps has elucidated important fundamental properties of heterogeneous reactions, such as adsorption , and heterogeneous reaction rates, even at the single-molecule level. In these experiments, two electrodes are placed very close to one another.…”