Figure 1.8 shows the four typical assay formats used: (a) direct binding, (b) sandwich assays, (c) displacement assays, and (d) replacement assays (Lukosz, 1991). The figure shows the different forms of immunoassays that may be used either with integrated optical (IO) or surface plasmon resonance (SPR) biosensors.
(a) Direct binding The analyte in solution binds directly to the receptor on the surface, and the analyte-receptor complex is monitored. For example, if an antigen in solution binds to the antibody immobilized on the biosensor surface, we are looking at an immunoassay format. If a DNA in solution binds directly to a complementary DNA immobilized on a biosensor surface, we looking at a hybridization assay.
(b) Sandwich assay When low molecular weight antigens (haptens, Ag) are to be analyzed, one often resorts to sandwich assays. This is because the
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