Quasi-Steady-State Voltammetry of Rapid Electron Transfer Reactions at the Macroscopic Substrate of the Scanning Electrochemical Microscope

摘要

We report on novel theory and experiment for scanning electrochemical microscopy (SECM) to enable quasi-steady-state voltammetry of rapid electron transfer (ET) reactions at macroscopic substrates. With this powerful approach, substrate potential is cycled widely across the formal potential of a redox couple while the reactant or product of a substrate reaction is amperometrically detected at the tip in the feedback or substrate generation/tip collection mode, respectively. The plot of tip current versus substrate potential features the retraceable sigmoidal shape of a quasi-steady-state voltammogram although a transient voltammogram is obtained at the macroscopic substrate. Finite element simulations reveal that a short tip–substrate distance and a reversible substrate reaction (except under the tip) are required for quasi-steady-state voltammetry. Advantageously, a pair of quasi-steady-state voltammograms is obtained by employing both operation modes to reliably determine all transport, thermodynamic, and kinetic parameters as confirmed experimentally for rapid ET reactions of ferrocenemethanol and 7,7,8,8-tetracyanoquinodimethane at a Pt substrate with ∼0.5-μm-radius Pt tips positioned at 90 nm–1 μm distances. Standard ET rate constants of ∼7 cm/s were obtained for the latter mediator as the largest determined for a substrate reaction by SECM. The various potential applications of quasi-steady-state voltammetry are also proposed.