Total internal reflection fluorescence-correlation spectroscopy study of molecular transport in thin sol-gel films

Total internal reflection fluorescence correlation spectroscopy is used to measure mass transport rates through thin sol-gel films. Fluctuations in the fluorescence signal derive from molecular statistics due to the small number (approximately 1000) of rhodamine 6G dye molecules in the observation region. Autocorrelation of the fluctuating signal is fit to a model describing diffusion in the evanescent wave excitation. Silica sol-gel films were prepared by dip-coating 27-nm porous silica particles, which were synthesized by a base-catalyzed sol-gel method, onto microscope slides. The measured diffusivities ranged from 1 to 2 orders of magnitude slower than free diffusion and decreased with increasing number of dips used to prepare the film. Scanning electron microscopy (SEM) was used to examine the film structure and showed that increasing the number of dips produced more uniform and well-ordered films. To determine what role the dip-coating process plays in inducing order, deposited films were further dipped into ethanol containing no particles. These films were annealed by this process and become more ordered, as determined by SEM, and show a corresponding reduction in the molecular diffusivity.