Bioelectrochemistry, Local Electrochemistry on Living Cells
In general, structurally small electrodes with characteristic dimensions in the lower micro- or even nanometer range are suitable for performing measurements in microenvironments and at microscopically small objects. From the electrochemical point of view, miniaturisation of the electroactive surface area has the advantage that it effectively lowers the electrode’s double-layer capacitance and hence is fastening the response time. At the same time, an enhanced rate of mass transport is observed as diffusion of redox species to and from a microelectrode surface evolves (hemi-) spherically rather than planar. Compared to the characteristics of larger electrodes, the current response of voltammetric microelectrodes is therefore not strongly affected by parasitic capacitive charging currents, an effect facilitating measurements with an improved signal-to-noise ratio and allowing high scan rates. Finally, the nA to pA currents typically passing through the electrochemical cell when using a microelectrode as working electrodes (WE) are leading to a substantial reduction in the Ohmic (iR) potential drop and practically undisturbed voltammetry is achievable even in highly resistive media as for instance organic solvents or aqueous systems of low ionic strength. Over the past, well-working microelectrodes of various geometries (disk, line, cylinder, ring, and arrays of these) and electrode materials (i.e. Au, Pt, Pt/Ir, C) with sizes ranging from several micrometers to just a few nanometers have been made available. Most common are disk-shaped microelectrodes fabricated from small-diameter noble metal wires or conductive carbon fibres sealed into glass and/or insulating polymers. They are routinely used for studying the kinetics of electron transfer reactions, find elegant application as miniaturized detectors in small-volume sample solutions, the endings of capillary separation columns and flow injection analysers and are employed for the amperometric detection of neurotransmitter release from single living cells and as transducers for miniaturized biosensors. A laser puller for the fabrication of ultra-microelectrodes was proposed. Due to the fast and reproducible local heating of a glass capillary together with an inserted metal wire the metal wire is pulled simultaneously with the glass leading to a drastic decrease of its diameter and a simultaneous tight seal of the metal within the glass capillary. A method for the fabrication and polishing of nanometer-sized disk-shaped microelectrodes was developed which is now readily available.