Using the pulsed nature of staircase cyclic voltammetry to determine interfacial electron-transfer rates of adsorbed species

Staircase cyclic voltammetry (SCV) is the digital counterpart of analog cyclic voltammetry (CV). However, when the redox-active species is adsorbed at the electrode surface, the voltammetric peak shapes (width, height, area, and to a lesser extent the reduction potentials) obtained with SCV can be very different from those of CV, even when small potential steps are used. Like analog CV, SCV provides a straightforward method to estimate and subtract the background and charging currents from the desired Faradaic current, while the pulsed nature of SCV provides the time-dependent decay of the Faradaic current, similar to chronoamperometry. Thus, electron-transfer rate constants can be directly measured as a function of applied potential, and no a priori model is required. An SCV equivalent of the square wave "quasi-reversible maximum" of observed peak height versus sampling moment and step size is predicted. The SCN response can only become independent of potential step size and similar to CV at high scan rates (v > 10 l(0)E(step)), if the current is sampled at half the step interval. The applicability of SCV to studies of redox centers in proteins is illustrated for the two-electron oxidation/reduction of yeast cytochrome c peroxidase, adsorbed at a pyrolytic graphite edge-plane electrode.