Sensing Based on Probe-Analyte Recognition

The photophysical properties (extinction coefficient, shifts in absorption and emission spectra, quantum yield, and lifetime) of a variety of probes are modified by pH changes, complexation by metal ions, or redox reactions. The resulting changes in photophysical parameters can be used to determine concentration of H+ and metal cations with suitably designed fluorophores. Most of these resulting sensors involve an equilibrium between the analyte, A, and the free probe (unprotonated or noncom-plexed by metal ion), Pf. If the stoichiometry of this reaction is 1 1, the reaction may be represented by 

The relative concentration of free, [Pf], and complexed (or bound), [Pb, forms are dependent on analyte concentration, [A], as follows 

There are a number indicators (most pH probes) which display changes in the absorption spectra on the complexation, but do not display useful fluorescence/51 In this case the change in absorbance is due to different extinction coefficients for the free and complexed forms. Because absorbance is proportional to the probe concentration, Eq. (10.11) can be expressed by 

A relation similar to Eq. (10.14) can be obtained for probes which display changes in the emission spectra on complexation. Since the intensity is proportional to the probe concentration, the analyte concentration can be obtained from 

Probe Absorption Emission tf(tb) (nsec) Dissociation constant (Kd) 

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