Hydrodynamic/microelectrode methods

The properties and applications of microelectrodes, as well as the broad field of electroanalysis, have been the subject of a number of reviews. Unwin reviewed the use of dynamic electrochemical methods to probe interfacial processes for a wide variety of techniques and applications including various flow-channel methods and scanning electrochemical microscopy (SEM), including issues relating to mass transport (1). Williams and Macpherson reviewed hydrodynamic modulation methods and their mass transport issues (2). Eklund et al. reviewed cyclic voltammetry, hydrodynamic voltammetry, and sono-voltammetry for assessment of electrode reaction kinetics and mechanisms with discussion of mass transport modelling issues (3). Here, we focus on applications ranging from measnrements in small volumes to electroanalysis in electrolyte free media that exploit the uniqne properties of microelectrodes. 

As discussed in Section 6, sonication may be used to increase the rate of mass transport at steady state, essentially by providing convection to a macroelectrode. The time-scale of 1 ms is comparable with the smallest time-scale attainable using a wall jet electrode. Both methods provide a way of achieving microelectrode-like mass transport rates at large electrodes, though the well-defined hydrodynamics of the wall jet make this the better choice if enhanced mass transport is the sole objective. 

Field flow fraction (FFF) describes a method for separating particles based on combining a deterministic force with hydrodynamic separation. A typical configuration is shown in Fig. 11. The system consists of a channel with interdigitated microelectrodes patterned on the bottom substrate. Particles are introduced into the system and when the field is switched on they experience nDEP, moving to equilibrium positions which are defined according to the balance of dielectrophoresis (DEP) and gravity (buoyancy). Different types of particles move to different equilibrium positions in the... 

In-situ spectroelectrochemical techniques are covered by chapters on infrared, Raman EPR, ellipsometry, electroreflectance, and photocurrent spectroscopy. Ex-situ UHV experiments are treated in a separate chapter. New electrochemical directions are described in chapters on hydrodynamic methods, channel electrodes, and microelectrodes. A final chapter covers computing strategies for the on-line accumulation and processing of elec-trochemial data. 

Finally, among occasionally chosen approaches in analysis of samples with heavy metal ions, one can quote, for example, simultaneous determination with metalloid(s), detection in flowing streams (FIA and HPLC see references (69-71) and references therein), use of hydrodynamic mode with rotated disc electrodes (RDEs) for improvement of accumulation (see references (11, 22, 23) in Table 5.1), array of microelectrodes and some indirect methods like an orientation assay for natural waters heavily polluted with mercury, when the measurement relies upon the decrease of the signal of a special marker—here, the [B(C5H6)4] anion—whose quite sensitive oxidation is suppressed by concurrent reaction with Hg V ... 

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