Past, present and future contributions of microelectrodes to analytical studies employing voltammetric detection. A review

Abstract

Traditionally, electroanalytical studies employing voltammetric techniques with conventionally sized solid electrodes (e.g., 1–5 mm radius disc) have been applied in a relatively small range of conducting solvent (electrolyte) media under transient (time-dependent) conditions. In this review it is emphasized that many of the limitations associated with the use of conventional voltammetric electrodes have either been removed or greatly minimized via the use of microelectrodes (e.g., ⩽25 µm radius disc). A major feature of microelectrode voltammetry is the ready access to near steady-state conditions (radial diffusion) where the influence of the ohmic (iR) drop is considerably reduced relative to that associated with the transient response (linear diffusion). Additionally, under near steady-state conditions, electrochemically derived charging current terms are negligible. Consequently, provided that the difficulties in fabricating and maintaining microelectrodes with high quality seals and electrical contacts can be overcome, then close to the analytically ideal response may be achieved with simple two-electrode (microelectrode and reference electrode) instrumental configurations and applied d.c. waveforms. The need for more complicated potentiostatic forms of instrumentation and relatively sophisticated pulse, square-wave or alternating current techniques is, therefore, reduced. In order to emphasize the wide range of electrochemically exotic media that are now voltammetrically accessible via the use of microelectrodes, representative studies without deliberately added electrolyte (dilute electrolyte), in high resistance organic solvents such as toluene and heptane, in supercritical fluids, at very low temperatures and in the gaseous and solid-state phases are reviewed. Additionally, the very small physical size of a microelectrode sensing element enables their use in precise locations as is often required when monitoring biologically important molecules in single cells or in situations where high spatial resolution is required. Other advantages related to the small physical size of microelectrodes are also reviewed, although it is noted that the determination of oxygen in muscle tissue with a microelectrode was first reported more than 50 years ago, so that in one sense microelectrode voltammetry is not fundamentally new. The review concludes with a discussion of the advantages and disadvantages of microelectrode voltammetry.