Immunoassays are based on the molecular recognition occurring between an antigen and its antibody. They can be set up in a variety of formats (sandwich or competitive assays). The main differences between them are the immobilised species (antibody or target analyte), the number of experimental steps involved, and in which order the different reagents are exposed to the surface. The choice of the format depends on the molecular size of the analyte, the availability of reagents and the cost. Radioimmunoassays (RIA), fluorescence immunoassays (FIA) and enzyme immunoassays (EIA) are well established in clinical diagnostics. For the development of hand held devices which can be used for point of care measurements, electrochemical immunoassays are optimal alternative to existing immunochemical tests.
In this chapter, the current status of research in electrochemical immunoassays is considered. Primary attention was focused on label-free and enzyme-labelled immunosensors, and the analytical performance of these devices are discussed. Moreover, the use of magnetic beads as new materials for immunoassays coupled with electrochemical transduction is also described. Examples of such devices used for the detection of some biomarkers in clinical analysis are reported.