Measuring reversible adsorption kinetics of small molecules at solid/liquid interfaces by total internal reflection fluorescence correlation spectroscopy

Reversible adsorption kinetics of a cationic dye molecule at a C-18 modified silica surface are measured in a fluorescence correlation spectroscopy experiment. An interfacial observation volume is defined by total internal reflection of a laser beam which excites fluorescence emission from Rhodamine 6G (R6G(+)) in adsorption equilibrium with the surface. Spontaneous fluctuations in the population of R6G(+) molecules within the interfacial volume are observed as excess low-frequency noise superimposed on a fluorescence transient. Autocorrelation of the transient allows the temporal characteristics of the fluctuations to be examined. The rate at which fluctuations occur depends on transport of molecules to the interface and their adsorption kinetics. Adsorption and desorption rates of R6G(+) in water/methanol solutions at a C-18 modified silica/solution interface were measured over a 3 order of magnitude range of the retention equilibrium; both rates were found to depend strongly on the composition of the overlying solution phase.