Electrochemistry in electrochemical cells of sub-microlitre volume

Advances in technology have permitted the miniaturization of many electrochemical systems [2]. In particular, application of modern fabrication techniques permits the manufacture of electrochemical cells of extremely small volume, Fig. 16.2. The first step in this direction was probably the development of thin-layer electrochemical cells. These contain large (2-3 mm) electrodes in a rectangular cell of dimensions typically 2 cm x l cm but of small height produced by a spacer of thickness 2-100 xm, which corresponds to a cell volume of 4-200 p.L [3]. Cell volumes have been progressively reduced, so that now measurements often have to be made with microelectrodes. 

One approach is to use free droplets on surfaces which contain one or all three electrodes [4-10], Fig. 16.2(a). A droplet of solution (5-10 p,L) can be placed on the substrate which contains the electrode and the other electrodes positioned within the solution from above. In some cases the droplet can be pulled across the surface, by judicious movement of the 

For the probing of these microenvironments, appropriately-sized electrochemical sensors and electrodes are necessary, often of nanometre linear dimension, unless they can be incorporated in the walls of the electrochemical cell. By etching, disc microelectrodes with radii of as small as 2 nm have been fabricated [15] nevertheless, problems of interelectrode reproducibility can be large at such miniature electrodes. 

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