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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.

Thin films of pseudoisocyanine (PIC) J-aggregates have been investigated by near-field scanning optical microscopy (NSOM), revealing the local molecular orientation, the molecular order, and the distance scale for exciton migration in these nanostructured films. Polarization-dependent images of J-aggregates grown in poly(vinyl sulfate) films were recorded by detecting the fluorescence from the excitonic state of the aggregates and are presented for three different excitation wavelengths corresponding to excitation to the lower (570 nm) and upper (514.5 and 488 nm) regions of the excitonic band. The images demonstrate that the fluorescence is highly polarized along the long axis of the aggregates. Aggregate absorption (i.e., fluorescence excitation) in the near field is highly polarized along the long axis of the aggregates at 570 nm, independent of polarization at 514.5 nm, and polarized perpendicular to the long axis at 488 nm. The polarization-dependent results are employed to determine the local excitonic transition dipole orientations and to determine the extent of orientational order within the aggregates. The local orientation of the PIC monomers in the aggregates is subsequently determined from the dipole orientations, and a herringbone-like structural model for the organization of the monomeric transition dipoles is proposed, based on these results. Finally, the fluorescence from the aggregates is observed to be highly polarized through curved and intersecting aggregate structures; the lack of fluorescence depolarization in these regions confirms our previous conclusion that exciton migration is limited to ≤50 nm.

The nanoscale properties of organically modified sol−gel-derived silicate thin films are studied in detail by single-molecule spectroscopic methods. For these studies, the solvent-sensitive probe Nile Red is doped into the films at nanomolar concentrations. Spectroscopic data are obtained for films prepared from sols containing different mole fractions of isobutyltrimethoxysilane and tetraethoxysilane. The data are analyzed using a model based on Marcus theory, providing important new information on static local film properties such as polarity and the extent of specific dopant−matrix interactions. Data on dynamic phenomena related to local matrix rigidity is also obtained. In general, throughout the range of samples studied, the most polar environments are also found to be the most rigid. With regard to their static properties, broad heterogeneous distributions are found in films of predominantly inorganic composition. In several instances, bimodal distributions are also observed, which result from specific chemical interactions and likely involve hydrogen bonding of the dye to the silicate matrix and/or to entrapped solvent. As the organic content of the film is increased, the film environments become less polar, less rigid, and more homogeneous. In addition, the effects of specific chemical interactions become dramatically less apparent. With respect to dynamic film properties, two distinct distributions are observed in films of intermediate organic/inorganic composition, reflecting the presence of environments differing in their rigidity. Studies of time-dependent single-molecule fluorescence fluctuations provide support for the conclusions derived from the spectroscopic data.

The quantitative assessment of single molecule diffusion trajectories by orthogonal regression analysis is reported. This analysis is broadly applicable to any single particle tracking experiments in which diffusion along one dimension (1D) is expected. It affords quantitative data on the (in plane) orientation of 1D trajectories, allowing for their absolute orientations to be determined. Histograms depicting the distribution of trajectory angles provide new physical insights into the degree of trajectory alignment, and by inference, materials order. Estimates of the errors in the trajectory angle and particle positioning along each trajectory are also obtained. The angle results are compared to those from single-step angle determinations. The advantages of the regression method include its simplicity and computational efficiency, and the ability to objectively differentiate between 1D and 2D/immobile trajectories. Its utility is demonstrated through analysis of single molecule diffusion trajectories in surfactant-templated mesoporous silica films as probed by wide-field fluorescence microscopy. The trajectory angle histograms obtained provide quantitative data on mean trajectory orientation and the degree of trajectory alignment in distinct populations and sample regions. Mesopore order was quantitatively assessed by implementation of an order parameter,

= 2-1, calculated from the individual trajectory angles in each of four representative sample regions. The results depict the presence of well-ordered domains (from microns to tens of microns in size), all having

≈ 0.9. The latter corresponds to an ≈14° average deviation of the individual trajectories from the mean trajectory (and mesopore) orientation in each domain.