Electrochemical detection of specific binding species

The presence of an analyte in a sample, said analyte being a member of a specific being pair consisting of a ligand and an anti-ligand, is detected through the electrical signal produced by a chemical species with peroxidase activity which during the course of the assay becomes immobilised at or near a surface of the electrode. In particular, an immunoassay is based on the finding that peroxidase activity can be measured electrochemically at a polarised electrode when the peroxidase is immobilised to the electrode surface via an antibody-antigen bridge. The electrode may be carbon, graphite or a conductive polymer such as polypyrrole. The chemical species with peroxidase activity may be horseradish peroxidase, cytochrome c peroxidase, microperoxidase, fungal peroxidase or hemin derivatives such as protoferrihaem or deuteroferrihaem. The species may become immobilised on the electrode by reaction with a specific binding partner which is immobilised by adsorption or covalent bonding either to the electrode material or to one or more non-conductive materials that are intimately mixed with the electrode material.