Microelectrodes edge effect

The UMDE evinces strong edge effects or very uneven current densities along the radius of the disk. It shares this problem with all the other flat microelectrodes such as microbands or rings. The exception of course are the hemispherical or hemicylindrical microelectrodes, which have no effective edges and behave as half of a sphere or cylinder, and also deeply recessed disks or bands, which approach the shrouded types. 

However, the edge effect is maximized on microelectrodes. If the radius of the inlaid disk is only 10 cm, the influence of the first term in the brackets of Eq. (III. 1.4) is smaller than 10% if the measurement time is longer than 2 s (for D =... 

One of the characteristics of microelectrodes is an enhanced rate of mass transport of reactants and products to and from the electrode surface. Nonlinear diffusion prevails at microelectrodes two-dimensional diffusion at microcylinder or microband electrodes and three-dimensional diffusion at microspherical and microdisk electrodes. The enhanced mass transport at microelectrodes, which is often called the edge effect, causes increased current density and results in steady-state current responses at sufficiently slow potential sweep rates in potential sweep voltammetry, as will be illustrated in section 3. 

The steady-state iso-concentration lines, deduced from the numerical treatment are plotted in Fig. 5-2. The effect of the x diffusion term (d2c/dx2) of Eq. (5-2) is important at the leading edge of the microelectrode when this term is dropped, all the isoconcentration lines start from this same point. 

Fig. 2 Reversible voltammograms for (top row) a diffusing redox couple reacting at a planar macroelectrode at which the entire surface is interactive (bottom row) a diffusing couple reacting at a microelectrode, or a macroelectrode at which most of the surface is blocked to protein interaction. Theoretical voltammograms are shown at the center, while the right hand side shows actual results obtained for cytochrome c at a polished pyrolytic graphite edge plane (top) or basal plane electrode, (bottom) showing the effect of the density of interactive sites on the electrode.

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