Large-amplitude fourier transformed alternating current voltammetry : investigations involving solution-phase and surface-confined electrochemistry

The large-amplitude Fourier transformed (FT) alternating current (ac) cyclic voltammetric technique has been employed in the investigation of problems related to solution-phase and surface-confined electrochemistry. The impact of electrode surface heterogeneity, redox properties of surface-confined proteins and ruthenium dioxide (RuO2), and electrode reaction mechanisms have been investigated by analysis of extensive data sets available from the aperiodic dc component and fundamental, second, third and higher harmonic components that are readily accessible from this electrochemical technique. An individually addressable gold array electrode has been fabricated that allows data to be analyzed for each individual electrode and then summed. Results are compared to those found when all electrodes are electrically connected. Outcomes imply that random levels of surface inhomogeneity may contribute to the overall voltammetric response obtained from a gold electrode. Studies on dual electrode materials reveal that heterogeneity cannot be detected when the rate constant for the two regions lie in the reversible regime. At the other extremity, where very large differences in rate constant occur, it is possible to detect fully resolved voltammetric process. This scenario is highlighted by noting the kinetic selectivity of the ac technique towards the fast electron transfer of the ferricyanide, [Fe(CN)6]3-/4- process that occurs at edge plane defects sites present in a highly ordered pyrolytic graphite electrode, when slow electron transfer occurs at the dominant basal plane surface. A new analytical approach has been introduced where measurements based on rate constant (peak to peak separation from dc component), double layer capacitance (fundamental harmonic) and ac higher harmonic peak currents from FT ac voltammetry provide estimates of the percentage of edge plane defect sites. The heterogeneity effect is very pronounced in the voltammetry of the blue copper protein, azurin, immobilized on pure and mixed alkanethiols modified gold electrodes. A systematic evaluation based on peak current magnitude versus frequency profiles, and also peak broadening in individual harmonic suggests the presence of the kinetic and/or thermodynamic dispersion. The background current rejection feature of second and higher harmonics, and the high sensitivity towards the faradaic current is useful for examining the state of heme in myoglobin molecules incorporated into a didodecyldimethylammonium bromide (DDAB) film that is adhered to a pyrolytic graphite electrode. Studies on the [3Fe-4S] and [4Fe-4S] iron-sulfur redox centres present in ferredoxin variants that differ in the level of separation in their reversible potentials have been undertaken by high order ac harmonics. Theory-experiment comparisons suggest that these cases are non-interacting, and also suggest the presence of kinetic or/and thermodynamic dispersion contributes to the voltammetry of these surface-confined ferredoxin variants. Underlying faradaic processes present in RuO2 in acidic medium that give rise to the pseudocapacitance effect are accessible through measurement of the higher harmonic components. Differences in redox behavior of hydrated and non-hydrated forms of RuO2 are revealed. A first order chemical reaction following a charge transfer step has been studied by ac voltammetry. Simulations that include all the relevant electrochemical parameters as well as uncompensated resistance and background current are compared with the experimental data to quantitatively elucidate the details of the reaction mechanism.