Basis of signal generation

To be employed as a chemical sensor the CME must be capable of producing an electrical signal which is related to the concentration of the analyte. (An excellent study of the mechanism of electrode transport in modified electrodes 

Voltammetric signal generation at a conventional clean metal electrode surface depends on 

The overall electron transfer process is much more complicated that at a clean metal surface. Following electron exchange at the surface, the charge must be transported in some way from the substrate surface to the modified boundary. This may occur via a charge hopping process (self-exchange reactions) or some limited diffusion procedure. The process may also rely on diffusion of a counter-ion in the opposite direction to balance the charge. Obviously then the properties of the counter-ion, in particular the mobility. 

Electron transfer may occur in much the same way as in a redox polymer with electron hopping from site to site. Alternatively, if the analyte species are not rigidly held in the modifier, they may diffuse to the electrode surface. 

Whether electron hopping or molecular diffusion takes place will depend on which of these processes is faster. In some cases the centres may diffuse short distances to allow electron hopping to occur. This argument was substantiated by previous workers (65) who measured d-d distances of 2.5-3.4 nm for Fe(CN)5 . For electron self-exchange to occur, distances of 0.92 nm are required. Where the response is limited by the rate of charge transfer, a small charge carrier additive may prove useful. For example, a ferro-ethylenediaminetetra-acetic acid additive has been used (66). It is most likely that the mechanism of transport will vary with analyte concentration and also with the morphology of the polymer (67). 

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Figure 18.9 Detection on the basis of signal generated upon touching object with PLLA fiber.

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