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pumps were then used to slowly infuse a MTMOS sol into the deposition reservoir while the mixed sol was simultaneously withdrawn at a higher rate.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.
Investigations of single molecule diffusion and entrapment within sol-gel-derived silica thin film gradients are reported for the first time. Gradient films were prepared on silica-sublayer-coated substrates by infusionwithdrawal dip-coating [Chem. Mater. 2010, 22, 2970. This method employs a sol of time-varying composition obtained by slowly mixing two different sols in the deposition reservoir. Tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMOS) were employed as precursor silanes. Films exhibiting macroscopic, unidirectional gradients in methyl content were obtained. Fourier transform infrared (FTIR) microscopy data provide proof of gradient formation, depicting an increase along the gradient in Si-CH 3 absorption at 1275 cm -1 relative to Si-O-Si absorption at 1080 cm -1 . Widefield fluorescence videos recorded as a function of position for Nile Red-doped films depict spatially varying populations of immobile and mobile molecules. The immobile fraction is believed to be incorporated in the silica sublayer, while the mobile molecules reside in/on the gradient. Diffusion coefficients, D, measured from single molecule tracking data depict, on average, a gradual increase in molecular mobility between the TMOS and MTMOS ends of the gradient. The mobile fraction is observed to split into two separate populations toward the MTMOS end of the gradient. Simultaneously, the width of the D distribution for the most mobile population is found to increase. Comparison with simulated results suggests that the TMOS end of the film is relatively homogeneous, while the MTMOS end exhibits increased heterogeneity. It is concluded the MTMOS end may incorporate TMOS-and MTMOSrich "domains". Plots of mean square displacement in time provide no evidence for confined diffusion, suggesting the molecules move freely between these domains.
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