Cations as targets in biochemical sensing

The highest stability of the sensor-analyte complex is achieved when the substrate fits perfectly in the hole within the receptor. The analyte does not have to fit the receptor cavity perfectly efficient binding can be achieved by careful design of the receptor. In the case of metal ion sensors the receptor must contain a proper type and number of donor atoms angular orientation and directionality of lone electron pairs are also of crucial importance [5]. 

Metal ions play a quadruple role in fluorescent and chromogenic chemosensors they are the most common substrates some metal complexes are involved as receptors for anions and neutral molecules and some metal ions (of Ru, Re, Os, Eu, Tb) constitute fluorescent reporters. Furthermore, in some applications metal ions are structural elements of fluorescent chemosensors [7]. 

Colorimetric pH sensing has a long tradition and numerous chromogenic sensors such as phenolphthalein, bromothymol blue, methyl red, and many others, have been developed. These indicators are involved in protonation-deprotonation equilibria between two (or more) forms of different spectral properties. Much higher sensitivity can be obtained with fluorescent proton sensors. Furthermore, this technique can be widely used in bioanalytical chemistry, cellular biology, and medicine. Application of fluorescence imaging techniques provides spatial information on pH. 

Information on pH can be obtained from fluorimetric determination of proto-nated and deprotonated forms of proton chemosensor (equation 16.1), 

Depending on the photophysical properties the pH fluorescent sensors can be divided into three classes (1) chemosensors that undergo photoinduced proton transfer, (2) chemosensors that undergo photoinduced electron transfer, and (3) those sensors that undergo neither proton nor electron transfer. 

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