Development of fluorescence methods for investigating diffusion in silica sol-gel films

Fluorescence methods are developed to investigate the diffusion of molecules in thin silica sol-gel films. Two experimental approaches are employed: total internal reflection fluorescence correlation spectroscopy (TIR-FCS) and single molecule tracking (SMT). In TIR-FCS, fluctuations in the fluorescence signal derive from molecular statistics due to the small number (1000) of molecules in the observation region. Autocorrelation of the fluctuating signal is fit to a model describing diffusion in the evanescent wave volume. The measured diffusivities ranged from one to two orders of magnitude slower than free diffusion and were found to be sensitive to the organization of particles in the film.; Single-molecule fluorescence tracking has been used to examine diffusion of small molecules in sol-gel films in order to identify spatial heterogeneity in the structure and molecular diffusivities for different regions of the film. Scatter plots and histograms of molecular step sizes indicate that the trajectories conform to the predictions of a two-dimensional random-walk. The diffusion coefficients measured for different molecules are subjected to F-test, which showed that the sol-gel film exhibits spatial variation in the diffusion coefficient on a micron length scale. The spatial variation in diffusivities is a measure of structural heterogeneity of these films.; Polyamidoamine (PAMAM) dendrimers were investigated for use as solution phase diffusion probes for silicate nanostructures. Because PAMAM dendrimers are positively charged in solution, having primary amines as end-groups, significant adsorption to the negatively charged silica surface was observed by TIR-FCS. The desorption rate decreases with larger dendrimer generations and the equilibrium constant for absorption increases with dendrimer generation. In order to alleviate this problem, the dendrimers were reacted with succinic anhydride, producing negatively charged carboxylic acid end-groups. These carboxylated dendrimers were shown to not appreciably adsorb to the silica surface. Carboxy-terminated PAMAM dendrimers were then used as diffusion probes in silica sol-gel films. Concentration jump experiments showed that larger dendrimers are excluded from the intraparticle pore volume more than small dendrimers. Smaller dendrimers experienced greater tortuosity in the film than larger dendrimers, because of size exclusion effects. The smaller dendrimers could explore smaller, more convoluted pores, while larger dendrimers could penetrate only large, open pore volumes. Pore size distribution analysis from a nitrogen adsorption isotherm was used to interpret these results.