Electrochemical immunoassay direct immunosensor

There are mainly three types of transducers used in immunosensors electrochemical, optical, and microgravimetric transducers. The immunosensors may operate either as direct immunosensors or as indirect ones. For direct immunosensors, the transducers directly detect the physical or chemical effects resulting from the immunocomplex formation at the interfaces, with no additional labels used. The direct immunosensors detect the analytes in real time. For indirect immunosensors, one or multiple labeled bio-reagents are commonly used during the detection processes, and the transducers should detect the signals from the labels. These indirect detections used to need several washing and separation steps and are sometimes called immunoassays. Compared with the direct immunosensors, the indirect immunosensors may have higher sensitivity and better ability to defend interference from non-specific adsorption. 

Currently, immunosensors provide a powerful tool for monitoring biospecific interactions in real time or for deriving information about the binding kinetics of an immunoreaction or the structure/function relationships of molecules. Amperometric, potentiometric, conductive, and impedimetric transducers have been applied in direct and indirect electrochemical immunoassays. 

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]. 

Historically, the immunoassay variant—the enzyme-linked immunosorbent assay (ELISA)—is a format where the enzyme label is followed optically as a change of absorbance, fluorescence or luminescence. The electrochemical immunoassays employ an electrode to measure the electroactive product released from a biocatalytic reaction of the label enzyme in this case, the immunoassay and electrode reaction occur at different surfaces. These concepts finally inspired various types of immunosensors where the immunorecognition event proceeds directly at the electrode surface. Thus, the electrochemical immunosensor is obtained when the immunorecognition element (antibody, antigen, hapten) becomes immobilised on the surface of the electrode as a transducer. The assays can be realised in the following formats ... 

Immunosensors have been designed which use both direct and indirect immunoassay technology to detect specific analytes within a minute or less in a variety of matrices (see Fig. 9). Indirect immunosensors may employ ELA, FLA, or CLIA principles whereby enzyme-, fluorophore- or chemiluminescent-labeled analyte competes with the target (nonlabeled) analyte for binding sites on the immobilized antibody. Unbound (free) labeled analyte is then quantitated using an electrochemical, optical, or electromechanical transducer and compared to the amount of target analyte in the sample. 

Electrochemical immunosensors based on screen-printed electrodes have recently been applied to the detection of environmental pollutants such as PCBs, PAHs, pesticides [17-20] and of important molecules in clinical and food field [21-23]. In this case, the screen-printed electrodes are both the solid-phase for the immunoassay and the electrochemical transducers antibody or antigen molecules are directly immobilised at the sensor surface (transducer) and one of these species is enzyme-labelled in order to generate an electroactive product which can be detected at the screen-printed electrode surface. 

Labeled immunosensors are derived from the immunoassay technology. This type of sensor, like the fluorescence, is expensive, and makes real-time measurements impossible. Immunosensors for direct, label-free measurements of various markers are attractive, as they also provide real-time monitoring. Optical immunosensors have been the most studied, but electrochemical immunosensors might offer at least the same detection range and provide a less-complicated instrumentation. 

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