Biological change sensors

Chemical and biological sensors (qv) are important appHcations of LB films. In field-effect devices, the tunneling current is a function of the dielectric constant of the organic film (85—90). For example, NO2, an electron acceptor, has been detected by a phthalocyanine (or a porphyrin) LB film. The mechanism of the reaction is a partial oxidation that introduces charge carriers into the film, thus changing its band gap and as a result, its dc-conductivity. Field-effect devices are very sensitive, but not selective. 

Other sensor applications can be considered if some sensitive biological molecules (such as antibodies or receptors) are attached to the nanogranule. If, for example, an antibody molecule is attached to it, then the granule is placed between two electrodes, and single-electron current flows between them. The step value of the coulomb staircase depends on the capacity of the junctions. When the antibody molecule binds specific antigen, the capacity value will be changed, and, therefore, the step value of the VH characteristics will also change. 

As we have seen above, FRET is a technique that provides precise information about distances between 10 and 100 A which is in the range of the size of biological molecules and proteins. Researchers have taken advantage of this feature and developed different strategies to synthesize FRET sensors that are able to follow in real time and with high sensitivity very diverse processes such as enzymatic activity, conformational change, or molecule-molecule interaction. The design of these FRET sensors is described below. 

Sometimes people enter a room and notice at once that the air inside is stuffy , while most people inside the room have been getting tired, but were unaware of the poor air quality. This is because our biological air quality sensor, the nose, gets used to slow changes of surrounding smells. 

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