This Demonstration shows the current at the tip as a function of distance in scanning electrochemical microscopy (SECM) feedback mode. Three cases are shown: negative feedback, positive feedback with infinite kinetics, and conducting substrate with finite kinetics.
In SECM, a redox reaction occurs at the surface of the tip (microelectrode): .
This reaction is controlled by the diffusion of the redox mediator. When the tip is moved forward toward the substrate, there are three possible feedback modes:
1. The substrate is insulating: the current of the tip decreases when approaching the substrate because diffusion of redox to the tip is blocked. This is negative feedback.
2. The substrate is conductive: the current of the tip increases when approaching the substrate because of regeneration of the redox reaction at the conductive substrate. In this case, the reaction is assumed to follow infinite kinetics.
3. The substrate is conductive with finite kinetics: In this case, if the rate is slow, the conductive substrate behaves as if it were insulating. Positive feedback reverses to negative. If the reaction is fast, we have a positive feedback with infinite kinetics.
The approximate solution is given in C. Wei, A. J. Bard, and M. V. Mirkin, "Scanning Electrochemical Microscopy. 31. Application of SECM to the Study of Charge Transfer Processes at the Liquid/Liquid Interface," J. Phys. Chem., 99(43), 1995 pp. 16033–16042.