posted on 2014-03-27, 00:00authored byDipak Giri, Chelsea
N. Hanks, Maryanne M. Collinson, Daniel A. Higgins
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–1 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.