IntroductionIn the fields of analytical and physical chemistry, medical diagnostics and biotechnology there is an increasing demand of highly selective and sensitive analytical techniques which, optimally, allow an in real-time direct monitoring with easy to use, reliable and miniaturized devices.Biomolecular interactions such as: antigen-antibody, pathogen detection, cell adhesion, adsorption and hybridization of oligonucleotides, characterization of adsorbed proteins, DNA & RNA interactions with complementary strands and detection of bacteria and viruses, among others, are typical applications in these areas.Conventionally, analytical methods include different techniques depending on the application.For instance, for low molecular weight pollutants detection, gas and liquid chromatography are classical techniques.These techniques precise of sophisticated sample pre-treatment: extraction of crude sample with large amounts of organic solvent, which is expensive and needs to be discarded; precolumn filtration and extensive purification (De Kok et al., 1992).Due to these shortcomings the analysis of a large number of samples may be both cost and time prohibitive (Ahmad et al., 1986).Immunoassays for low molecular weight compounds (pesticides, industrial chemical pollutants, etc.) have already gained a place in the analytical benchtop as alternative or complementary methods for routine classical analysis as they are simple, fast, inexpensive, and selective as well as highly sensitive although, in general, not as much as chromatographic techniques.Immunoassays are able to detect specifically one target analyte in a complex sample.Moreover, immunoassays can be performed on portable devices, irrespective of centralized laboratories, which turn them into a suitable tool for quantification analysis in on-line applications.These techniques are based on the interaction of one antigen (analyte) with an antibody which recognizes it in a specific way.Currently, Enzyme Linked ImmunoSorbent Assay (ELISA) and Immunosensors are the most popular immunoassays.In ELISAs the detection of the analyte is always indirect because one of the immunoreagents is labeled.In immunosensors, or immunological biosensors, the detection is direct, one of the immunoreagents is immobilized on the surface of the transducer, and a direct physical signal is produced when interaction occurs (Marty et al, 1998;Byfield et al, 1994; Montoya et al, 2008).In those techniques where labels are necessary, the actual www.intechopen.com