Figure 1: Schematic representation of an electrochemical immunoassay using enzyme (a) or GNP (b) labeled secondary antibodies in an amperometric sandwich methodology. Target presence is quantified by the electrochemical activity brought to the electrode surface by the label. In the most common enzymic case signal is through the reduction of generated H2O2 or through mediated electrochemistry of the enzyme itself [33] (mediator not shown). The signal readout in GNP labeled antibody assay is the reduction of a prior oxidised GNP (Oxidation: Au0 +4Cl−4→AuCl−4 + 3e−. Reduction: AuCl−4 + 3e−→Au0 +4Cl− [38,39].