Single-Molecule Spectroscopic Imaging Studies of Polarity Gradients Prepared by Infusion-Withdrawal Dip-Coating

Sol–gel-derived thin films incorporating a one-dimensional polarity gradient are prepared by infusion-withdrawal dip-coating (IWDC) and characterized by single-molecule spectroscopic (SMS) imaging. Nonpolar phenyltrimethoxysilane (PTMOS) and polar tetramethoxysilane (TMOS) are used as precursor silanes during IWDC. Sessile drop water contact angle measurements demonstrate the presence of a wettability gradient in the resulting films, on macroscopic length scales. Raman maps of the phenyl C–H stretch at 3059 cm<sup>–1</sup> confirm gradient formation. Material polarity is assessed by SMS imaging, using Nile Red as a polarity-sensitive probe. Polarity data are obtained by recording fluorescence videos of single-molecule emission simultaneously in two spectral bands (590 ± 20 and 640 ± 20 nm). The pairs of fluorescence spots produced by each molecule are identified and analyzed using automated spot-tracking software. The local film polarity is quantified via the Clausius–Mossotti factors calculated from the single-molecule emission data. Histograms prepared from the SMS data confirm the presence of a macroscale polarity gradient. These same data reveal that the gradients are heterogeneous on nanometer to micrometer length scales. In some regions, the data are consistent with phase separation of the polar and nonpolar film components. These studies afford a better understanding of organosilane gradients prepared by sol–gel methods and will aid in the development of new materials for stationary-phase-gradient chemical separations, and for directed transport of liquids, macromolecules, vesicles, and cells.