On Practical Aspects of Single-Entity Electrochemical Measurements with Hot Microelectrodes

When a 10s-100s MHz frequency alternating current (ac) waveform is applied to a disk ultramicroelectrode (UME) in an electrochemical cell, one achieves what is known as a hot microelectrode, or a hot UME. The electrical energy generates heat in an electrolyte solution surrounding the electrode, and the heat transfer leads to formation of a hot zone with the size comparable to the electrode diameter. In addition to heating, ac electrokinetic phenomena generated by the waveform include dielectrophoresis (DEP) and electrothermal fluid flow (ETF). These phenomena can be harvested to manipulate the motion of analyte species and achieve significant improvements in their single-entity electrochemical (SEE) detection. This work evaluates various microscale forces observable with hot UMEs in relation to their utility to improve the sensitivity and specificity of the SEE analysis. Considering only mild heating (with a UME temperature increase not exceeding 10 K), the sensitivity of the SEE detection of metal nanoparticles and bacterial (Staph. aureus) species is shown to be strongly affected by the DEP and ETF phenomena. The conditions have been identified, such as the ac frequency and supporting electrolyte concentration, that can lead to orders-of-magnitude enhancement of the frequency of analyte collisions with a hot UME. In addition, even mild heating is expected to result in up to four times increase in the magnitude of blocking collisions' current steps, with similar outcomes expected for electrocatalytic collisional systems. The findings presented here are thought to provide guidance to researchers wishing to adopt hot UME technology for SEE analysis. With many possibilities still open, the future of such a combined approach is expected to be bright.