Fluorescence anisotropy in studies of solute interactions with covalently modified colloidal silica nanoparticles

Time-resolved anisotropy decays of a fluorescent cationic solute, rhodamine 6G (R6G), in Ludox sols were measured to characterize the extent of the ionic binding of the probe to silica particles after modification of the surface with neutral or cationic silane coupling agents. The anisotropy decays provided direct evidence for distribution of the dye between the aqueous solution (picosecond decay component) and silica particles (nanosecond decay component and residual anisotropy component, which were attributed to the wobbling motion of dye on the silica surface and to the ionically bound probe, respectively). The dye was strongly adsorbed to unmodified silica nanoparticles, to the extent that less than 1% of the dye was present in the surrounding aqueous solution. Significant decreases in the degree of probe adsorption were obtained upon covalent modification of the silica with neutral or cationic silanes, with up to 80% of the probe being present in the aqueous solution in cases where the surface was coated with (3-aminopropyl)triethoxysilane. The addition of such agents also altered the fractional distribution between the nanosecond decay component and the residual anisotropy component in favor of the nanosecond component, indicative of weaker interactions between the dye and the modified surface (i.e., more wobbling motion). The data clearly show the power of time-resolved fluorescence anisotropy decay measurements for probing the modification of silica surfaces and should prove useful in characterization of new chromatographic stationary phases.