Positive curvature in Stern-Volmer plot described by a generalized model for static quenching

The fluorescence quenching effect of unfolded-fullerene nanoparticles (UFNPs) in the presence of Cu2+ ions in water solution has been investigated for a broad range of fluorophore/quencher concentrations. The experimental Stern-Volmer plot exhibited a nonlinear behaviour with an initial upward curvature even though carbon nanoparticles are known to form stable complexes with metal ions and thus a dominant static quenching with a linear plot is expected according to the classical Stern-Volmer equation. A generalized model for static quenching, applicable to any value of fluorophore/quencher concentrations, provided a very good fit with the experimental data without the need to introduce any second-order effect, such as in the sphere of action model or diffusion-limited processes. Moreover, by changing the dilution of the UFNP solution, we could study how the quenching ratio depended on the fluorophore concentration, whereas it is predicted to be constant by the standard model. In addition to providing a significant test of the nonlinear model for static quenching, we believe this study could help find the best parameters for the optimization of sensitivity and signal-to-noise ratio in applications such as fluorescence sensing.