No abstract

No abstract

Single molecule spectroscopic methods are used to obtain detailed information on the polarity and rigidity of molecular-scale environments found in thin poly(vinyl alcohol) (PVA) and poly(methyl methacrylate) (PMMA) films. Nile Red is employed as a highly sensitive spectroscopic probe of environmental properties in these experiments. Fluorescence spectra are recorded for numerous single molecules and their peak positions and widths determined by fitting the spectra to Gaussian functions. The spectral data are analyzed using a new model for the dependence of the Nile Red charge-transfer transition on the properties of the surrounding medium. This model is based on previous work by Marcus (Marcus, R. A. J. Phys. Chem. 1990, 94, 4963). Additional information required for the analysis is obtained from extensive bulk solution-phase absorption and fluorescence studies. A broad inhomogeneous distribution of environments is found for PVA. The results are shown to depend significantly on PVA film water content, with the results for hydrated films indicating the presence of less rigid environments. In contrast to the PVA results, two distinct classes of environments are found in the PMMA films. On the basis of an analysis of the data using the aforementioned model, it is concluded that the two environments differ in rigidity but have nearly identical polarity.

Surface amine gradients that exhibit a wide variety of profiles, including those that incorporate spatially distinct regions having steep and gradual variations in chemical functionality, have been prepared by the sol-gel process using a controlled-rate infusion method. In this work, a substrate that incorporates dimethyl and Si-OH groups is temporally modified with an aminoalkoxysilane (NH(2)(CH(2))(3)Si(OC(2)H(5))(3)) to build a gradient film for which the amine content changes over a 10-20 mm distance. Both X-ray photoelectron spectroscopy (XPS) and contact angle measurements confirm the presence of a chemical gradient across the surface of the film. As expected, a greater density of amine functionalities and lower contact angle were found at the bottom of the gradient relative to the top. The local steepness of the gradient was systematically controlled by changing the rate of infusion. Fast rates of infusion created gradient surfaces where the amine content changed slowly along the surface and never reached saturation, whereas slow rates of infusion formed a surface exhibiting a steep rise in amine content followed by saturation. The steepness of the gradient was also changed at predefined positions along its length by programming the rate of infusion. Gradients prepared using six-step, three-step, and two-step programmed infusion rates are shown. The data fit nicely to a kinetic model that assumes first-order kinetics. The ability to manipulate the gradient profile is particularly vital for applications that rely on mass transport and/or those that require spatial control of gradient properties.

Single molecule spectroscopy is applied in studies of diffusion and surface adsorption in sol-gel-derived mesoporous silica thin films. Mesoporous films are obtained by spin casting surfactant-templated sols onto glass substrates. Small-angle X-ray diffraction results are consistent with hexagonally ordered mesophases in as-synthesized (i.e., surfactant-containing) films. Upon calcination, a 30% contraction and disordering of these structures occurs. Nile Red is used as a fluorescent probe of both the as-synthesized and calcined films. It is loaded into the samples at subnanomolar levels either prior to spin casting or after calcination. Fluorescence imaging and single-point fluorescence time transients show the dye molecules to be relatively mobile in the as-synthesized samples. In contrast, the molecules appear entrapped at fixed locations in dry calcined films. In calcined films rehydrated under high humidity conditions, the Nile Red molecules again become mobile. Time transients obtained from the as-synthesized and rehydrated samples provide clear evidence for frequent reversible adsorption of the dye to the silica surfaces. Autocorrelations of the time transients provide quantitative data on the mean diffusion coefficients (D = 2.4 x 10(-10) and 2.6 x 10(-10) cm2/s) and mean desorption times (1/k = 25 and 40 s) for the as-synthesized and rehydrated films, respectively. The results prove both water and surfactant play important roles in governing matrix interactions and mass transport.