Introductory Bioelectronics by Ronald R. Pethig & Stewart Smith

Author:Ronald R. Pethig & Stewart Smith
Language: eng
Format: epub
Publisher: Wiley
Published: 2012-08-20T04:00:00+00:00

5.4.2 Amperometry

The most common class of biosensor operates as an amperometric device, where measurement is made of the current arising from an electrode reaction involving an electroactive analyte. The choice of electrode material is an important consideration for such devices. In general, as we have seen, when the potential of an electrode is moved from an equilibrium state towards more negative potentials, the chemical species that will be reduced first is the oxidant in the redox couple with the least negative (or more positive) standard reduction potential Eo. Figure 5.17 depicts the relative situations for inert platinum and gold electrodes immersed in an aqueous solution containing iron and copper ions with dissolved carbon dioxide. The approximate locations of the Fermi levels for platinum and gold are based on the work function values given in Figure 5.8. As the potential of a platinum electrode is lowered to less positive potentials with respect to the hydrogen electrode reference level, the first species reduced will be Fe3+, since the Eo of the Fe3+/Fe2+ couple is the least negative (i.e. most positive), followed by Cu2+ and CO2. When the potential of an electrode is made progressively more positive, the chemical species that will be oxidised first is the reductant in the redox couple of least positive (or more negative) Eo. Thus, for a gold electrode in an aqueous solution containing Cu, Fe2+ and CO2, copper will be the first to be oxidised as its potential is made more positive, followed by Fe2+. The associated series of electrode current peaks produced by these electrochemical reactions are shown schematically in Figure 5.18. The dotted curves in Figure 5.18 represent the currents that would be observed if the potential had been increased slowly in small incremental steps, rather than as a relatively fast potential ramp.

Figure 5.17 The standard reduction potentials for some electron transfer reactions, referenced against the standard hydrogen electrode (SHE). The approximate locations of the Fermi levels (vs. SHE) are shown for a platinum and gold electrode.

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