Biosensors

1.2.1. Different Types of Bioreceptors

The key to specificity for biosensor technologies involves bioreceptors. They are responsible for binding the analyte of interest to the sensor for the measurement. These bioreceptors 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. These categories include: 1) antibody/antigen, 2) enzymes, 3) nucleic acids/DNA, 4) cellular structures/cells and 5) biomimetic. Figure 1.3 shows a schematic diagram of two types of bioreceptors: the structure of an immunoglobulin G (IgG) antibody molecule (Fig. 1.3A), and DNA and the principle of base pairing in hybridization (Fig. 1.3B).

1.2.1.1. Antibody Bioreceptors An antibody is a complex biomolecule, made up of hundreds of individual amino acids arranged in a highly ordered sequence. Antibodies are biological molecules that exhibit very specific binding capabilities for specific structures. For an immune response to be produced against a particular molecule, a certain molecular size and complexity are necessary: proteins with molecular weights greater then 5000 Da are generally immunogenic. The way in which an antigen and its antigen-specific antibody interact may be understood as analogous to a lock and key fit, by which specific geometrical configurations of a unique key enables it to open a lock. In the same way, an antigen-specific antibody "fits" its unique antigen in a highly specific manner. This unique property of antibodies is the key to their usefulness in immunosensors where only the specific analyte of interest, the antigen, fits into the antibody binding site.

Radioimmunoassay (RIA) utilizing radioactive labels have been applied to a number of fields including pharmacology, clinical chemistry, forensic science, environmental monitoring, molecular epidemiology and agricultural science. The usefulness of RIA, however, is limited by several shortcomings, including the cost of instrumentation, the limited shelf life of radioisotopes, and the potential deleterious biological effects inherent to radioactive materials. For these reasons, there are extensive research efforts aimed at developing simpler, more practical immunochemical techniques and instrumentation, which offer comparable sensitivity and selectivity to RIA. In the 1980s, advances in spectrochemical instrumentation, laser miniaturization, biotechnology and fiberoptic research have provided opportunities for novel approaches to the development of sensors

ANTIGEN (LOCK AND KEY FIT)

ANTIGEN (LOCK AND KEY FIT)

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