Simulation of Fast-Scan Nanogap Voltammetry at Double-Cylinder Ultramicroelectrodes

Suppression of Resistive Coupling in Nanogap Electrochemical Cell: Resolution of Dual Pathways for Dopamine Oxidation

A nanogap cell involves two working electrodes separated by a nanometer-wide solution to enable unprecedented electrochemical measurements. The powerful nanogap measurements, however, can be seriously interfered with by resistive coupling between the two electrodes to yield erroneous current responses. Herein, we employ the nanogap cell based on double carbon-fiber microelectrodes to suppress resistive coupling for the assessment of intrinsic current responses. Specifically, we modify a commercial bipotentiostat to compensate the Ohmic potential drop shared by the two electrodes through the common current pathway with a fixed resistance in the solution. Resistive coupling through both non-Faradaic and Faradaic processes is suppressed to eliminate erroneous current responses. Our approach is applied to investigate the mechanism of dopamine oxidation at carbon-fiber microelectrodes as important electrochemical sensors for the crucial neurotransmitter. Resistive coupling is suppressed to manifest the intrinsic current responses based on the oxidation of both adsorbed and non-adsorbed forms of dopamine to the respective forms of dopamine-o-quinone. The simultaneous dual oxidation pathways are observed for the first time and can be mediated through either non-concerted or concerted mechanisms of adsorption-coupled electron transfer. The two mechanisms are not discriminated for the two-electron oxidation of dopamine because it can not be determined whether the intermediate, dopamine semi-quinone, is adsorbed on the electrode surface. Significantly, our approach will be useful to manifest intrinsic current responses without resistive coupling for nanogaps and microgaps, which are too narrow to eliminate the common solution resistance by optimizing the position of a reference electrode.

Versatile Tools for Understanding Electrosynthetic Mechanisms

Electrosynthesis is a popular, green alternative to traditional organic methods. Understanding the mechanisms is not trivial yet is necessary to optimize reaction processes. To this end, a multitude of analytical tools is available to identify and quantitate reaction products and intermediates. The first portion of this review serves as a guide that underscores electrosynthesis fundamentals, including instrumentation, electrode selection, impacts of electrolyte and solvent, cell configuration, and methods of electrosynthesis. Next, the broad base of analytical techniques that aid in mechanism elucidation are covered in detail. These methods are divided into electrochemical, spectroscopic, chromatographic, microscopic, and computational. Technique selection is dependent on predicted reaction pathways and electrogenerated intermediates. Often, a combination of techniques must be utilized to ensure accuracy of the proposed model. To conclude, future prospects that aim to enhance the field are discussed.

Review-Recent Advances in Nanosensors Built with Pre-Pulled Glass Nanopipettes and Their Applications in Chemical and Biological Sensing

Nanosensors built with pre-pulled glass nanopipettes, including bare or chemically modified nanopipettes and fully or partially filled solid nanoelectrodes, have found applications in chemical and biological sensing via resistive-pulse, current rectification, and electrochemical sensing. These nanosensors are easily fabricated and provide advantages through their needle-like geometry with nanometer-sized tips, making them highly sensitive and suitable for local measurements in extremely small samples. The variety in the geometry and layout have extended sensing capabilities. In this review, we will outline the fundamentals in fabrication, modification, and characterization of those pre-pulled glass nanopipette based nanosensors and highlight the most recent progress in their development and applications in real-time monitoring of biological processes, chemical ion sensing, and single entity analysis.

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

The carbon-fiber microelectrode has been used for decades as a neurotransmitter sensor. Recently, new strategies have been developed for making carbon electrodes, including using carbon nanomaterials or pyrolyzing photoresist etched by nanolithography or 3D printing. This review summarizes how chemical and 3D surface structures of new carbon electrodes are optimized for neurotransmitter detection. There are effects of the chemical structure that are advantageous and nanomaterials are used ranging from carbon nanotube (CNT) to graphene to nanodiamond. Functionalization of these materials promotes surface oxide groups that adsorb dopamine and dopants introduce defect sites good for electron transfer. Polymer coatings such as poly(3,4-ethylenedioxythiophene) (PEDOT) or Nafion also enhance the selectivity, particularly for dopamine over ascorbic acid. Changing the 3D surface structure of an electrode increases current by adding more surface area. If the surface structure has roughness or pores on the micron scale, the electrode also acts as a thin layer cell, momentarily trapping the analyte for redox cycling. Vertically-aligned CNTs as well as lithographically-made or 3D printed pillar arrays act as thin layer cells, producing more reversible cyclic voltammograms. A better understanding of how chemical and surface structure affects electrochemistry enables rational design of electrodes. New carbon electrodes are being tested in vivo and strategies to reduce biofouling are being developed. Future studies should test the robustness for long term implantation, explore electrochemical properties of neurotransmitters beyond dopamine, and combine optimized chemical and physical structures for real-time monitoring of neurotransmitters.

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

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.