Probing the diffusion of a dilute dye solution in mesoporous glass with fluorescence correlation spectroscopy

The translational motion of rhodamine 6g dye molecules near the surface of a mesoporous glass with a disordered pore structure and a nominal pore diameter of 130 Å is measured by the application of fluorescence correlation spectroscopy (FCS) to fluorescence burst data as dye molecules diffuse through the laser excitation volume. FCS analysis indicates that the motion of the rhodamine 6g molecules in the mesoporous glass deviates from simple Brownian motion with a single diffusion constant. Two models, including two-component diffusion and diffusion plus adsorption, are utilized to fit the correlation data. The multicomponent diffusion model comprised of free diffusion (with a diffusion coefficient of 4.89 × 10^-7 cm²/s) and transient adsorption/desorption (desorption time of 67 ms) provides the best fit and the most plausible explanation of the correlation data. Possible explanation for the shift to multicomponent diffusion, such as restriction of the dye molecules within the pores of the glass, as well as increased viscosity and adsorption at the surface, are presented.