Instruments microelectrode probes

Both SRET and SVET perform potentiometric measurements with micrometer-sized quasi-reference electrodes (typically platinum micro-cones electrochemically coated with platinum black) held precisely above the sample surface. So far these commercial instruments have not been designed to accommodate ion-selective microelectrodes. However, some groups have reported the use of home-made instruments to probe the concentration of specific ions (see Sec. V.B). 

Another important future direction is in the use of microelectrodes and microelectrode arrays. They are often easier to manipulate by the inexperienced, and instrumentation is simpler. They can be used in highly resistive dirty media where conventional electrodes may be unuseable and are able to probe localized concentrations. Composite electrodes42, of which carbon paste is an example, if conveniently prepared, can act as microelectrode assemblies. In a more general sense, lithographic and... 

A particularly interesting approach for probing electrode reactions at the micro-level has arisen from the combination of microelectrodes and the accurate control instrumentation associated with scanning probe microscopies. 

The advantages of microelectrodes are exploited in many different areas of electroanalysis, with the enviromnental, food quality assurance, and biomedical applications being the most active. Portability, simplified instrumentation, and label-free detection are some of the key advantages of electrochemistry over spectroscopy. In particular, microelectrodes open up the possibility of probing redox processes in small sample volumes or physically small spaces. For example, the redox properties of sample volumes as small as a few picoliters have been interrogated. Experiments of this kind are possible because, as described by Eq. 5, for solution phase reactants, the depletion layer thickness, d, depends on the experimental timescale ... 

The instrumentation and theory for the basic SECM experiment are discussed in detail elsewhere in this book, so only a very brief description is provided here. SECM involves the movement of a very small amperometric microelectrode (usually a disk microelectrode of a few micrometers or smaller radius, referred to as the tip or the probe) near the surface of a substrate immersed in an electrolyte containing at least one redox-active species (a mediator). The two most common modes of operation are the feedback mode and the generator-collector mode. 

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