The Higgins Research Laboratory
Research in the Higgins laboratory involves the implementation of novel optical microscopic techniques for characterization of mesostructured thin film materials. The techniques used include single molecule spectroscopy (SMS), single molecule tracking (SMT) and fluorescence correlation spectroscopy (FCS). We have also made use of near-field scanning optical microscopy (NSOM) and multiphoton-excited fluorescence microscopy (MPEFM) in earlier studies. We are presently using these methods to characterize mesoporous sol-gel derived silicate thin films and organically-modified silica film gradients. The main goal in all our research projects is to obtain a better understanding of the micron-to-nanometer-scale properties of these materials.
Single Molecule Polarity Studies
Using what is perhaps the most solvent sensitive fluorescent dye (Nile Red) known, we have developed and demonstrated unique SMS methods for characterizing the static and dynamic polarity properties of nanoscale environments in organic polymer films and in organically- modified silica films[36,40] prepared by the sol-gel process. We showed that the position and width of the emission spectrum for each molecule provide a measure of the static polarity of its local environment. The emission bandwidth itself provides a measure of the dynamic local polarity (i.e., the reorganization energy). We demonstrated these measurements by comparing results from polar and nonpolar organic polymer films and later made similar measurements in silica films containing covalently attached butyl and cyano functional groups.[36,40] These results showed that the nanoscale environments became distinctly less polar based on their static properties as organic groups were added into the silica matrix. Interestingly, the same results reveal an increase in dynamic polarity, i.e., increasing reorganization energies, across the same series of samples. These studies led us to develop and synthesize an unique form of Nile Red that could be polymerized into the silica matrix. The results of fluorescence correlation spectroscopy (FCS) studies performed with this dye showed that the organically modified silica films were actually comprised of viscous, liquid-like oligomers, rather than of solid silica frameworks. Since this early work, we have moved on to studies of silica film gradients. Aside from a single early report, sol-gel methods had not previously been employed to prepare gradient films, to our knowledge. We have since demonstrated a dip-coating method (termed infusion-withdrawal dip coating) for preparing organically-modified silica film gradients and later showed that these materials comprised mobility gradients, with Nile Red dye molecules exhibiting increased diffusivities along the gradient direction (i.e., with increasing film organic content). The silica work was performed in collaboration with Maryanne Collinson, with her group reporting XPS studies of materials composition along similar gradients.[76,81,88]
Single Molecule Tracking and Dichroism Measurements
In early studies from our group, we employed FCS methods to probe dye molecule motions and dye-surface interactions in surfactant-templated and calcined mesoporous silica materials.[58,63] These studies revealed distinct differences in the mass transport properties of these samples, with molecular mobility clearly depending on the dye charge, the level of solvent incorporated into the channels and the presence or absence of surfactant in the pores. Since this initial work, we have begun to directly record single molecule motions in the one-dimensional (1D) channels of nanostructured materials by single molecule tracking (SMT) methods. SMT methods afford unique advantages over FCS in that 1D molecular motions can be directly visualized. While such methods are well known, our unique contributions include their use for quantitative assessment of channel organization. Our first such studies were performed on spin-coated mesoporous silica. We reported the orthogonal regression methods used for data analysis in this first paper and also showed that the spin coated silica films employed were comprised of extremely well-ordered domains (having order parameters of ~ 0.9). The domains were found to be tens of micrometers in size. These results showed that materials disorder observed in X-ray studies was due to domaining (i.e., polycrystallinity) rather than short-range disorder in the individual domains. The same methods have been applied to flow-aligned silica monoliths prepared in microfluidic channels, revealing the presence of ~ mm sized, highly ordered monodomains. We have also used these methods to probe 1D diffusion in the hexagonal mesophase of Pluronic F127 gels. To our knowledge, this work was the first to show guided 1D diffusion of small dye molecules in structured surfactant gels by SMT methods. The results showed that the dye diffused within the micelle cores and that materials produced by flow alignment in microfluidic channels were extremely well ordered. In our most exciting work, we have since moved on to study single molecule orientational wobbling (i.e., confined orientational motions) within the surfactant-filled cylindrical pores of mesoporous silica. In these studies, we simultaneously record SMT data in two orthogonal polarizations. The results allow for single molecule emission dichroism (SMED) data to be obtained simultaneously with the trajectory data. Measurements of the 1D trajectory alignments allow us to determine the average orientation of individual dye molecules in the film plane. The dichroism data affords a highly quantitative measure of the degree to which the orientational motions of the molecules are confined within the pores. The results yield an in situ measure of the accessible cavity diameter with ~ 0.2 nm precision, much better than can presently be achieved by super-localization microscopy methods reported in the literature. In the next few weeks we will submit a second paper in which we show the wobbling angle dependence on probe molecule length. The results conclusively demonstrate our ability to detect the subtle, confined wobbling motions of single molecules within 1D nanomaterials. This work is performed in collaboration with Takashi Ito.
Field-Induced DNA Dynamics
In keeping with our interests in electric-field-induced dynamics (see PDLC studies), we recently undertook investigations of the field-induced reorientation dynamics of short, dye-labeled double stranded DNAs attached to glassy carbon electrode surfaces. This work was done in collaboration with Jun Lis group and his student Qin Li. Our work was built upon extensive studies of similar dynamics on gold electrodes performed by others. Over the long term, the goal is to investigate DNA dynamics at the ends of vertically-aligned carbon nanofibers produced by the Li Group. However, our initial studies showed that the DNA dynamics (detected by distance-dependent dipole-electrode fluorescence quenching of the dye) was more complicated than previously reported. We found that the overall response of the DNA was as expected: the DNA orients perpendicular to the surface at negative potentials and parallel to the surface at positive potentials. Interestingly, the DNA also appears to relax from its initial orientation on a ~ 1 sec timescale after initial potential switching. We hypothesize that this slow relaxation is due to direct participation of DNA in initial charging of the double layer, with diffusive exchange of ions from solution allowing for later, slow orientational relaxation. Electrical double layer dynamics in the presence of highly-charged, surface-bound polyelectrolytes remain largely unexplored. It is likely that none of the data reported in the literature were acquired under appropriate conditions to detect such relaxation phenomena.
Reference numbers refer to articles in the Higgins Group publication list