Toward More Reliable Measurements of Electron-Transfer Kinetics at Nanoelectrodes: Next Approximation

Steady-state voltammetry at nanoelectrodes and scanning electrochemical microscopy (SECM) have recently been used to measure kinetics of several rapid heterogeneous electron transfer (ET) reactions. One problem with those experiments was that the dependence of the shape of the steady-state voltammogram on kinetic parameters becomes weak when the reaction rate approaches the diffusion limit. The possibility to fit the same experimental voltammogram using different combinations of the standard rate constant, transfer coefficient, and standard potential results in significant uncertainties in extracted parameter values. In this article, the reliability of the kinetic analysis was improved by obtaining steady-state voltammograms with both oxidized and reduced forms of redox species initially present in solution. Additional improvements were attained by characterizing the nanoelectrode geometry with the atomic force microscope and using water with a very low level of organic contaminants (TOC ≤ 1 ppb). This approach was used to re-evaluate the ET rate constants measured for several electroactive species, including ferrocene, ferrocenemethanol, 7,7,8,8-tetracyanoquinodimethane (TCNQ), and ferrocyanide at Pt electrodes. The obtained standard rate constants are higher than the values measured earlier at Pt and Au nanoelectrodes but comparable to those obtained in recent nanogap/SECM experiments.