Kinetic separation of amperometric sensor responses

Abstract

The electrochemical behaviour of adriamycin and quinizarin monolayers, which are adsorbed on mercury microelectrodes and are in contact with aqueous electrolyte solutions, were studied by means of cyclic voltammetry and high-speed chronoamperometry. When the solution pH is below 6, reduction of the quinone moieties is a rapid, electrochemically reversible, process that is consistent with a nearly ideal two-electron, two-proton redox reaction involving a surface-confined redox couple. The potential dependence of the redox composition follows the Nernst equation with the expected theoretical slope. The adsorption thermodynamics follow the Langmuir isotherm over the concentration range 2 × 10^–8 to 2 × 10^–5 mol l^–1. Limiting surface coverages, Γ_s of (1.1 ± 0.1)× 10^–10 and (1.3 ± 0.1)× 10^–10 mol cm^–2 and energy parameters, β, of (4.5 ± 0.3)× 10⁵ and (6.1 ± 0.5)× 10⁵ I mol^–1 were observed for adriamycin and quinizarin monolayers, respectively. Microsecond time-scale chronoamperometry was used to probe both the rate of heterogeneous electron transfer to the adsorbed anthraquinone moieties and their surface coverages. Standard heterogeneous electron transfer rate constants, k°, as measured at a solution pH of 3.5, are (3.1 ± 0.2)× 10⁴ and (1.0 ± 0.1)× 10³ s^–1 for adriamycin and quinizarin, respectively. The formal potentials of adriamycin and quinizarin are almost identical. Therefore, binary monolayers, formed by simultaneous adsorption of both anthraquinones exhibit only a single voltammetric peak. In these circumstances, traditional electroanalytical techniques cannot be used to determine the surface coverages of the individual species. However in potential step experiments, three single exponential current decays are separated on a microsecond time-scale. These decays correspond to double-layer charging and heterogeneous electron transfer to the adriamycin and quinizarin redox centres, respectively. This kinetic separation of the Faradaic responses allows the surface coverages of the individual components within the monolayer to be determined. Despite their identical formal potentials, the concentrations of the two anthraquinones in solution were determined by combining information about heterogeneous kinetics and adsorption thermodynamics.