Recovery of the Smoluchowski–Collins–Kimball kinetics parameters from fluorescence quenching decays

Abstract

Time-correlated single-photon counting (TCSPC) is a time-resolved fluorescence technique capable of monitoring transient diffusion-mediated kinetics. When the transients can be reliably quantified, TCSPC data can be used for extracting the underlying microscopic dynamics. In the present paper, we study the recovery of the Smoluchowski–Collins–Kimball model parameters from simulated fluorescence quenching decays. A Levenberg–Marquardt least-squares optimization routine was used for the estimation of the sum of the diffusion coefficients of the fluorophore and quencher, D=DF*+DQ, the sum of their radii, R=RF*+RQ, and the intrinsic quenching rate coefficient k. The accuracy and precision of parameter estimation were parameterized by the dimensionless quantities k/4πRD, τD/R2, and 4πR3[Q]/3, where τ is the fluorophore lifetime, and [Q] is the quencher concentration. The zero-time shift was an adjustable parameter. The best parameter estimates are obtained for long-lived fluorophores at high quencher concentrations. The estimated R and D are more accurate as the intrinsic quenching rate k becomes faster, but the estimation of k is optimal when k and the diffusion controlled rate 4πRD are comparable in value. The present study should be useful in planning and interpreting TCSPC experiments on nanosecond and picosecond time scales.