Immunosensors affinity interactions

Besides attempts to construct direct potentiometric immunosensors, other approaches are under investigation (piezoelectric, optical, etc.) and to date seem to be more acceptable[27]. An affinity electrode based on mutual interaction of a dye and protein has been constructed by Lowe (Fig. 15, [306]). He immobilized an anionic triazine dye (Cibacron Blue F 3G-A) to an oxidized metal electrode. This affinant specifically binds human serum albumin from plasma. The response of this albumin electrode was linear up to 15 g of human serum albumin per ml and faster than for the immunologically sensitized metal electrode [304]. 

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]. 

Ideally, an immunosensor would be a reusable device, since the reversibiHty of the antigen—antibody interaction may allow regenerating the immobilized component. A high-affinity constant and a labile immobilized antibody, sensitive to the harsh conditions often required, make regeneration of the surface difficult to realize, limiting practical application of immunosensors to single-use devices (Reymond et al., 2007). In this chapter, we will present the basics of different types of immunosensors and their applications to in situ or point-of-care diagnosis. 

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