Nonlinear fluorescence-quenching model

Development of the Nonlinear Fluorescence Quenching Model. The main function of the nonlinear model is to relate aquatic equilibrium considerations based on conventional solution thermodynamics to observed fluorescence intensity changes occurring as metal ion is added at fixed pH. The fundamental relationship equating fluorescence quenching to complexation is present in equation 1. 

A second nonlinear fluorescence quenching data treatment method developed by Ventry and Ryan may also be used to extract conditional stability constants, and ligand concentrations from titrations of FA with Cu (23). The model designed is a modification of the original Stem-Volmer theory defined by equation 7 which accounts for either static or dynamic quenching of fluorescent species. 

The presence of at least two fluorophores, and possibly a third, associated with metal ion binding in fulvic acid strongly suggests the need for multiple binding site models. Existing linear and nonlinear models will be reviewed for both fluorescence quenching and enhancement. A new modified 1 1 Stem - Volmer model will be introduced as well as two site and multiple site models. Application of the models to Cu binding by fulvic acid and certain well defined model systems are discussed. 

Careful examination reveals that the modified Stem-Volmer equation is mathematically identical to the original nonlinear model developed by Ryan and Weber (22). Fluorescence quenching curves for Cu -FA and application of the modified Stem-Volmer data treatment to the experimental information are shown in figure 2. Since the nonlinear data treatment and the modified Stem-Volmer equations are algebraically identical, their ability to fit experimental data and provide meaningful parameters is the same. 

Lakowicz J R (1999) Principles of Fluorescence Spectroscopy, 2nd Ed, Kluwer Academic/Plenum Publishers. Demas J N, DeGraff BA, XuW(1995) Modeling of Luminescence Quenching-Based Sensors Comparison of Multisite and Nonlinear Gas Solubility Models. Anal Chem 67 1377-1380. 

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