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.[77] 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.[89] We have also used these methods to probe 1D diffusion in the hexagonal mesophase of Pluronic F127 gels.[79] 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,[90] 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.

Reference numbers refer to articles in the Higgins Group publication list