redoxmediator

A redox mediator is a small molecule or complex that undergoes reversible oxidation and reduction to shuttle electrons between a redox-active center, such as an enzyme or substrate, and an electrode or another redox partner. They are used when direct electron transfer between an enzyme and an electrode is slow or energetically unfavourable, or when a defined potential is required.

Mediators can be soluble, diffusing in solution to carry electrons from the enzyme to the electrode, or immobilized within an electrode matrix to enable rapid and repeatable cycling. The mediator’s redox potential is chosen to efficiently accept electrons from the donor and donate them to the acceptor, while remaining stable and reversible under operating conditions.

Common redox mediators include ferricyanide/ferrocyanide, ferrocene and its derivatives, phenazine methosulfate, methylene blue, and menadione. The choice depends on compatibility with the enzyme, operating pH, and the potential window of the electrode system.

In sensors and biofuel cells, mediators enable enzymatic electron transfer and can reduce overpotential, increasing current and sensitivity. They also extend operation to conditions where direct electron transfer is not feasible. Immobilized mediators can improve stability and reusability.

Limitations include mediator leakage or degradation, possible interference from other redox-active species, and costs. Stability, toxicity, and consistent redox potential are important considerations, and researchers balance these factors by using immobilization strategies or alternative redox couples.

Redox mediators are distinct from enzyme cofactors or prosthetic groups; mediators shuttle electrons between two partners rather than participating directly in the enzymatic reaction itself.