From Semi-Infinite to Thin-Layer Diffusion─Effects of Finite Mass Transport on the Electrochemical Response of Redox Probes: Implications for Electroanalytical Measurements.

Electrochemistry in confined environments, that is, involving experimental configurations with spatial restrictions that affect the overall mass transport, is becoming a very attractive way of carrying out electroanalytical measurements for sensing, especially for the so-called thin-layer (TL) configuration, which ideally allows the complete conversion of the analytes under study in small volumes and short times. To improve the understanding of this kind of situation, general expressions for the current-potential-time and charge-potential-time responses of charge transfer processes taking place under finite diffusion conditions with two different configurations (no mass renovation, bounded diffusion; and effective mass renovation, unbounded diffusion) are discussed in this work. By using these expressions, it is possible to establish accurate limits for the attainment of TL conditions and to conclude that for bounded conditions, the charge is a more adequate quantity for electroanalytical purposes. For unbounded conditions, stationary currents and charges varying linearly with time are obtained. The TL behavior is more easily reached for unbounded conditions, and the sensitivity of the measurements will be greater due to the dependence of the charge and current responses on the inverse of the length of the diffusion field, which leads to an amplification of the responses. Moreover, for experimental TL cells with a known value of the length of the diffusive field, this type of measurement allows us to easily develop absolute electroanalytical methods. The application of this formalism to the oxidation of a metallic complex under both configurations is presented, and practical values for the operating parameters are also discussed.