Immunosensors analytical reagent

Figure 2. Immunosensor Configuration. Reagent is released from the polymer reservoirs into the reaction chamber where a competition reaction takes place with analyte diffusing in from the bulk solution.

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. 

This complex subsequently reacts with other reagents in the solution to produce either an insoluble dimer product, which also adsorbs on the AW device surface, or a massive ion (l3 was used) that inserts into an ionic binding site in the surface film. Thus, the areal mass increase per bound analyte is significantly amplified. While these and other immunosensor detection schemes can involve rather complex reagent and/or buffer systems, the relative advantages of piezoelectric sensors in terms of cost, speed, and safety make them attractive alternatives to radioimmunoassay and other standard assay techniques. 

The schematic of the flow-injection immunofiltration assay system is shown in Fig. l.b. The system has the following main components the immunosensor assembly (locked components as shown in Fig. l.a), a peristaltic pump, a five-way valve with flow injector and a mixing chamber, reagents and waste vessels, analyte supplier and an electrochemical interface which consists of a dedicated calibrated potentiostat, an LC display and analog data output. The electrochemical interface is used to provide the working potential for the immunosensor and to process the output signal. A manual jack is used for moving the holder to lock with the immunocolumn and the adapter (motion shown by arrows in Fig. l.a). 

In essence, this technique allows for the detection of surface binding interactions in real time without the use of labels. Immunosensors that rely on these principles are commercially available from Biacore AB (formerly Pharmacia Biosensor). These are based on the popular BIACORE 3000, BIACORE 2000 and BIACORE probe instruments in conjunction with disposable sensor chips, which may be purchased with affinity capture chemistries in place (e.g. SA5 Sensor Chips with streptavidin bearing surfaces for capture of biotinylated compounds) [92]. These devices incorporate micro-fluidic systems for delivery of reagents required for analysis and are capable of measuring affinities and kinetics of antibody-antigen interactions in addition to analyte concentrations, which can be determined to the picomolar range [93]. 

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