Analytic solution to the autocorrelation function for lateral diffusion and rare strong adsorption

An analytic expression is derived to describe the autocorrelation function for fluorophors interacting with heterogeneous chemical interfaces, where b oth lateral diffusion and reversible strong adsorption occur. The expressio n is accurate when the rate of strong adsorption is low compared to the rat es of both diffusion and desorption, enabling analysis by nonlinear regress ion. Simulations of single molecules are employed to investigate the applic ability of the analytic equation for interpretation of chemical equilibrium and kinetics of single fluorescent molecules undergoing both lateral diffu sion and varying amounts of specific adsorption at chemical interfaces. The simulations show that the analytic equation accurately describes the autoc orrelation decay, and the equation begins to deviate, as expected, when the adsorption rate becomes large. The results show that a smaller beam size e nhances the ability to extract sorption kinetics from the autocorrelation d ecay, and that a larger beam size enhances the ability to obtain informatio n about the diffusion coefficient with minimal interference from sorption p rocesses. The expression provides a tool for choosing between fluorescence correlation spectroscopy and single-molecule sorting in studies of heteroge neous surfaces.