Theory SECM operation

The precise positioning capabilities, which make high spatial resolution possible, give the SECM an important edge over other electrochemical techniques employing UMEs [20]. For example, the SECM can pattern the substrate surface, visualize its topography, and probe chemical reactivity on the micrometer or nanometer scale. Here, we briefly survey the fundamentals of various modes of the SECM operation and then focus on more recent advances in SECM theory and applications. 

The treatment of the two-phase SECM problem applicable to immiscible liquid-liquid systems, requires a consideration of mass transfer in both liquid phases, unless conditions are selected so that the phase that does not contain the tip (denoted as phase 2 throughout this chapter) can be assumed to be maintained at a constant composition. Many SECM experiments on liquid-liquid interfaces have therefore employed much higher concentrations of the reactant of interest in phase 2 compared to the phase containing the tip (phase 1), so that depletion and diffusional effects in phase 2 can be eliminated [18,47,48]. This has the advantage that simpler theoretical treatments can be used, but places obvious limitations on the range of conditions under which reactions can be studied. In this section we review SECM theory appropriate to liquid-liquid interfaces at the full level where there are no restrictions on either the concentrations or diffusion coefficients of the reactants in the two phases. Specific attention is given to SECM feedback [49] and SECMIT [9], which represent the most widely used modes of operation. The extension of the models described to other techniques, such as DPSC, is relatively straightforward. 

The quantitative scanning electrochemical microscopic (SECM) theory has been developed for various regimes of measurements and electrochemical mechanisms. Different operating modes of the SECM, for example, feedback and generation/collection (G/C) modes, steady-state and transient... 

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. 

---