Electrochemical immunoassay potentiometric detection

Electrochemical immunoassays include a wide variety of devices based on the coupling of immunological reactions with electrochemical transduction. All of them involve the immobilization of an immunoreagent component on the surface of the electrode transducer. Electrochemical detection is based on the direct intrinsic redox behavior either of an analyte species or of some reporter molecule. For the detection no expensive equipment is needed, with the measurement of either a simple current or a voltage charge. Different electrochemical detection strategies are used, but ampero-metric detection is most widely used. Potentiometric and conductometric detection are applied in different assays as well. 

Potentiometric, capacitive, and amperometric transducers have been used for direct and indirect electrochemical immunoassays. However, due to their fast detection, broad linear range, and low detection limit, amperometric transducers are preferred. 

Instead of immobilizing the antibody onto the transducer, it is possible to use a bare (amperometric or potentiometric) electrode for probing enzyme immunoassay reactions (42). In this case, the content of the immunoassay reaction vessel is injected to an appropriate flow system containing an electrochemical detector, or the electrode can be inserted into the reaction vessel. Remarkably low (femtomolar) detection limits have been reported in connection with the use of the alkaline phosphatase label (43,44). This enzyme catalyzes the hydrolysis of phosphate esters to liberate easily oxidizable phenolic products. 

In contrast to direct immunosensors, indirect immunosensors, using as a label an enzyme [307-309] or an ionophore [310], have been more developed. This technique is derived from well established and widely utilized enzyme immunoassays evaluated by photometric methods. Enzyme immunoassay with electrochemical detection (EEIA) represents an important innovation of EIA by the replacement of optical detection with voltammetric or potentiometric measurements. It may also be adapted to flow-through systems [311, 312]. 

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