Fourier transform large-amplitude alternating current cyclic voltammetry of surface-bound azurin

The technique of Fourier transform large-amplitude alternating current (ac) cyclic voltammetry has been applied to a study of the electrochemistry of surface-bound azurin (Az) at a paraffin-impregnated graphite electrode. With the methodology used, the total current, as well as the dc component, and the ac harmonics can be obtained from a single experiment. For the dc (and fundamental harmonic cases), the background current dominates the voltammetry, so that quantitatively useful data only can be obtained after employment of an empirical background correction procedure. In contrast, the capacitance current does not contribute to the second and higher harmonic voltammograms. Furthermore, the higher harmonic Faradaic currents are greatly amplified when large-amplitude sinusoidal perturbations are employed as an alternative to the traditionally used small-amplitude ac methods. Results obtained from numerical simulations are presented for a quasi-reversible process (Butler-Volmer model) in order to illustrate the advantages of the higher harmonic ac method, relative to dc cyclic voltammetry. The almost complete background current rejection and high sensitivity of large-amplitude techniques are confirmed experimentally in second- and third-harmonic ac studies with an adsorbed azurin thin film and the surface process, Az[Cu(II)]((surface)) + e(-) reversible arrow Az[Cu(I)]((surface)). However, as is the case with dc cyclic voltammetry, readily detected nonideal behavior places restrictions on the straightforward use of Butler-Volmer theory for quantitative evaluation of the kinetics and thermodynamics of this so-called model surface-confined process.