Ultramicroelectrode

An Ultramicroelectrode (UME) is a working electrode used in a three electrode system. The small size of UME give them relatively large diffusion layers and small overall currents. These features allow UME to achieve useful staedy-state conditions and very high scan rates (V/s) with limited distortion. UME where developed independently by Wrightmann in the Fleischmann around 1980. [Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications. New York: John Wiley & Sons, 2nd Edition, 2000.]

Structure

UME are often defined as an electrode which is smaller than the diffusion layer achieved in a readily accessed experiment. A working definition is an electrode that has at least one dimension (the critical dimension) smaller than 25 μm. Platinum UME with a radius of 5 μm are commercially available and electrodes with critical dimension of 0.1 μm have been made. Electrodes with even smaller critical dimension have been reported in the literature but exist mostly as proofs of concept. The most common UME is a disk shaped electrode created by embedding a thin wire in glass, resin, or plastic. The resin is cut and polished to expose a cross section of the wire. Other shapes such as wires and rectangles have also been reported.

Application

Linear region

Every electrode has a range of scan rates called the linear region. The response to a reversible redox couple in the linear region is a "diffusion controlled peak" which can be modeled with the Cottrell equation. The upper limit of the useful linear region is bound by an excess of changing current combined with distortions created from large peak currents and associated resistance. The charging current scales linearly with scan rate while the peak current, which contains the useful information, scales with the square root of scan rate. As scan rates increase the relative peak response diminishes. Some of the charge current can be mitigated with RC compensation and/or mathematically removed after the experiment. However the distortions resulting from increased current and the associated resistance can not be subtracted. These distortions ultimately limit the scan rate for which an electrode is useful. For example, a working electrode with a radius of 1.0 mm is not useful for experiments much greater than 500 mV/s.

Moving to an UME drops the currents being passed and thus greatly increases useful sweep rate up to 10⁶ V/s. These faster scan rates allow the investigation of electrochemical reaction mechanisms with much higher rates than can be explored with regular working electrodes. By adjusting electrode size of the working electrode an enormous kinetic range can be studies. For UME only the very fast reactions can be studies through peak current since the linear region only exists for UME at very high scan rates.

Steady-state region

At scan rates slower than those of the linear region is a region which is mathematically complex to model and rarely investigated. At even slower scan rates there is the steady-state region. In the steady-state region linear sweeps traces display reversible redox couple as steps rather than peaks. These steps can readily be modeled for meaningful data.

To access the steady-state region the scan rate must be dropped. As scan rates are slowed the relative currents also drop at a given point reducing the reliability of the measurement. The low ratio of diffusion layer volume to electrode surface area means regular stationary electrodes can not be dropped low enough before their current measurements become unreliable. In contrast the ratio of diffusion layer volume to electrode surface area is much higher for UME. When the scan rate of UME is dropped it quickly enters the steady-state regime at useful scan rates. Even though UME supply small total currents their steady-state currents are high relatively compared to regular electrodes.

References