On the inadequacy of Stern-Volmer and FRET in describing quenching in binary donor-acceptor solutions

Quantitative fluorescence quenching is a common analytical approach to studying the mechanism of chemical reactions. The Stern-Volmer (S-V) equation is the most common expression used to analyze the quenching behavior and can be used to extract kinetics in complex environments. However, the approximations underlying the S-V equation are incompatible with Forster Resonance Energy Transfer (FRET) acting as the primary quenching mechanism. The nonlinear distance dependence of FRET leads to significant departures from "standard" S-V quenching curves, both by modulating the interaction range of donor species and by increasing the effect of component diffusion. We demonstrate this inadequacy by probing the fluorescence quenching of long-lifetime lead sulfide quantum dots mixed with plasmonic covellite copper sulfide nanodisks (NDs), which serve as perfect fluorescent quenchers. By applying kinetic Monte Carlo methods, which consider particle distributions and diffusion, we are able to quantitatively reproduce experimental data, which show significant quenching at very small concentrations of NDs. The distribution of interparticle distances and diffusion are concluded to play important roles in fluorescence quenching, particularly in the shortwave infrared, where photoluminescent lifetimes are often long relative to diffusion time scales.