Single-molecule studies of diffusion by oligomer-bound dyes in organically modified sol-gel-derived silicate films

Single-molecule fluorescence spectroscopy is used to study dye diffusion within organically modified silicate (ORMOSIL) films. ORMOSIL films are prepared from sols containing tetraethoxysilane and isobutyltrimethoxysilane in 2:1 and 1:9 molar ratios. Nile red and a new silanized form of nile red that can be covalently attached to the silicate matrix are used as fluorescent probe molecules. The number and rate of single molecules diffusing through these films increases dramatically with increasing film organic content. Autocorrelation of the fluorescence images yields a quantitative measure of the relative populations of fixed and diffusing species. Surprisingly, both "free" and silicate-bound nile red exhibit relatively facile translational motions. Single-molecule/single-point fluorescence correlation spectroscopy (FCS) is used to measure the dye diffusion coefficients in submicrometer-scale film regions. The most common diffusion coefficients for "free" and silicate-bound nile red molecules in the 1:9 films are 3.9 x 10(-10) and 1.6 x 10(-10) cm(2)/s, respectively. The unexpectedly rapid diffusion of silicate-bound nile red is attributed to the presence of liquidlike silicate oligomers in the films. A lower bound for the molecular weight of the oligomers is estimated at 2900. Bulk solution-phase FCS experiments performed on "free" and silicate-bound nile red species extracted into chloroform solutions provide valuable support for these conclusions. Comparison of the results derived from experimental and simulated time transients indicates film heterogeneity occurs on sub-100-nm-length scales and likely results from the presence of inorganic- and organic-rich domains.