ABSTRACT
Porous materials such as cellular cytosol, hydrogels, and block copolymers have nanoscale features that determine macroscale properties. Characterizing the structure of nanopores is difficult with current techniques due to imaging, sample preparation, and computational challenges. We produce a super-resolution optical image that simultaneously characterizes the nanometer dimensions of and diffusion dynamics within porous structures by correlating stochastic fluctuations from diffusing fluorescent probes in the pores of the sample, dubbed here as "fluorescence correlation spectroscopy super-resolution optical fluctuation imaging" or "fcsSOFI". Simulations demonstrate that structural features and diffusion properties can be accurately obtained at sub-diffraction-limited resolution. We apply our technique to image agarose hydrogels and aqueous lyotropic liquid crystal gels. The heterogeneous pore resolution is improved by up to a factor of 2, and diffusion coefficients are accurately obtained through our method compared to diffraction-limited fluorescence imaging and single-particle tracking. Moreover, fcsSOFI allows for rapid and high-throughput characterization of porous materials. fcsSOFI could be applied to soft porous environments such hydrogels, polymers, and membranes in addition to hard materials such as zeolites and mesoporous silica.
ABSTRACT
Single molecule tracking (SMT) and fluorescence correlation spectroscopy (FCS) are used to investigate probe molecule diffusion within the mesophase structures of Pluronic F127 gels. Mixtures are prepared in the hexagonal, lamellar, and cubic regions of the ternary F127/water/butanol phase diagram and are doped with nanomolar concentrations of a perylene diimide dye (DTPDI). Flow aligned F127 gels comprised of hexagonally arranged cylindrical micelles exhibit distinct one-dimensional (1D) DTPDI motion in wide-field videos, with diffusion occurring parallel to the flow alignment direction. The slow 1D dye motion observed is attributed to single molecule diffusion within the viscous, hydrophobic micelle cores. FCS data acquired from the same samples reveal a bimodal distribution of diffusion coefficients with the slower component assigned to 1D motion in the micelle core and the faster component to 3D diffusion in the interconnected micelle coronas. The rate of diffusion for both components increases with decreasing F127 concentration, reflecting a decrease in gel microviscosity. SMT data from the lamellar and cubic mesophases depict isotropic 2D and 3D diffusion, respectively, and provide supporting evidence for the role of the micelle core and corona in governing diffusion. Trajectory angle distributions from 1D diffusing species in the hexagonal mesophase provide quantitative information on the alignment of the cylindrical micelles. These results, and the rare observation of misaligned trajectories, indicate the hexagonal phase is highly ordered.
