Bioreceptors are the key to specificity for biosensor technologies. They are responsible for binding the analyte of interest to the sensor for the measurement. These biorecep-tors can take many forms, and the different bioreceptors that have been used are as numerous as the different analytes that have been monitored using biosensors. However, bioreceptors can generally be classified into five different major categories: (1) antibody/antigen, (2) enzymes, (3) nucleic

acids/DNA, (4) cellular structures/cells, and (5) biomimetic. This chapter deals with biosensor systems using antibody probes, often called immunosensors, with optical detection.

The antigen-antibody (Ag-Ab) binding reaction, which is a key mechanism by which the immune system detects and eliminates foreign matter, provides the basis for specificity of immunoassays. Antibodies are complex biomolecules, made up of hundreds of individual amino acids arranged in a highly ordered sequence. Antibodies are produced by immune system cells when such cells are exposed to substances or molecules, which are called antigens. The antibodies appearing following antigen exposure have recognition/binding sites for specific molecular structures (or substructures) of the antigen. The way in which an antigen and an antigen-specific antibody interact is analogous to a lock and key fit, in which specific configurations of a unique key enable it to open a lock. In the same way, an antigen-specific antibody fits its unique antigen in a highly specific manner, so that the three-dimensional structures of antigen and antibody molecules are complementary. Due to this three-dimensional shape fitting, and the diversity inherent in individual antibody make up, it is possible to find an antibody that can recognize and bind to any one of a large variety of molecular shapes. This unique property of antibodies is the key to their usefulness in immunosensors; this ability to recognize molecular structures allows one to develop antibodies that bind specifically to chemicals, biomolecules, microorganism components, and so on. One can then use such antibodies as specific probes to recognize and bind to an analyte of interest that is present, even in extremely small amounts, within a large number of other chemical substances.

In the 1980s, advances in spectrochemical instrumentation, laser miniaturization, biotechnology, and fiber-optics research provided opportunities for novel approaches to the development of sensors for the detection of chemicals and biological materials of environmental and biomedical interest. Since the first development of a remote fiber-optics immunosensor for in-situ detection of the chemical carcinogen benzo[a]pyrene [1], antibodies have become common bioreceptors used in biosensors today.

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