Amperometry at microelectrodes

A recent review (25) has discussed the electroanalytical application of carbon fibre microelectrodes. These electrodes, constructed from single carbon fibres, 

Indeed, a sphere or a hemisphere is a good model for the diffusion zone that surrounds these electrodes, and because of this enhanced mass transport to the electrode, a steady-state current is rapidly achieved after a potential pulse is applied to the electrode. 

A number of other features follow from the relatively large size of the diffusion zone when compared to the electrode radius. One of these is quite significant to biosensors based on enzyme systems. The large diffusion zone radidly dilutes the products of the electrode reaction, thus catalytic mechanisms of the type shown in Table 8.2, mechanisms 3-5, are not perceived at microelectrodes, unless the rate constant for the reaction is fast. It can be shown that for a reversible electrode reaction, the ratio of the catalytic current (i kc) to the faradaic current obtained in the absence of the homogeneous reaction is given by 

Although the diffusion zone is large in comparison to the electrode surface, it is small compared to the zone around a conventional electrode. This small size means that the faradaic current obtained at microelectrodes enjoys comparative immunity from the effects of convection in the bulk solution. In flowing streams the current is independent of flow rate. 

Decreased effects of capacitance and resistance at microelectrodes, coupled with the high mass transport rates, has enabled electrochemical measurements to be made in cells containing highly resistive solutions. In fact, provided that there is a sufficient quantity of free carrier charge on the surface of the insulator in contact with the microelectrode to charge the double layer, microelectrodes can be used to make direct electrochemical measurements in the gas phase (26). 

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Amatore, C., Arbault, S., Erard, M. 2008. Triangulation mapping of oxidative bursts released by single fibroblasts by amperometry at microelectrodes. AwaZ. Chem. 80 9635-9641. 

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