Fluorescence Spectroscopy Studies of Silica Film Polarity Gradients Prepared by Infusion-Withdrawal Dip-Coating

The preparation of sol−gel-derived silica films incorporating macroscopic polarity gradients is reported for the first time. Such materials are likely to find broad applications in chemical separations, combinatorial catalysis, and biological sensing. Polarity gradients were prepared by an “infusion-withdrawal dip-coating” procedure that produces a sol of time-varying composition. Tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMOS) were employed as the precursor silanes. Gradient deposition was accomplished by first suspending a substrate in a TMOS-derived sol. Synchronized syringe pumps were then used to slowly infuse a MTMOS sol into the deposition reservoir while the mixed sol was simultaneously withdrawn at a higher rate. The difference in infusion and withdrawal rates caused the sol to slowly recede down the substrate surface, mimicking a dip-coating process. Film polarity was characterized by doping the films with Nile Red (NR), a fluorescent, polarity-sensitive probe. NR fluorescence spectra acquired as a function of position along the films provide strong evidence for the presence of macroscopic, unidirectional polarity gradients in these materials. As expected, more polar environments are found at the top of each film and less polar environments at the bottom. Water contact angle data provide supporting evidence, consistent with the presence of a polarity gradient.