Abstract: A biosensor for the detection and determination of the concentration of toxins by use of enzyme inhibition. Inhibition biosensors are affected by non-specific denaturation and substrate utilization which both result in a limited operational lifetime. These problems are mitigated by providing in an environment an enzyme which is oxidized by hydrogen peroxide the oxidized enzyme being reduced by an electron transfer agent, such as ferrocene, which is itself oxidized in the process. The electron transfer agent is capable of regeneration back to the reduced state and the extend of electron transfer regeneration gives a measure of enzyme inhibition by toxin. Electro-chemical technique allows for the generation of hydrogen peroxide from oxygen in aqueous media and the reduction of oxidized electron transfer agent. Immobilization of the enzyme to an electrode increases efficiency while potentially reducing denaturation. The biosensor can be used for the environmental determination of toxins like cyanide.