Multiple Diffusion Pathways in Pluronic F127 Mesophases Revealed by Single Molecule Tracking and Fluorescence Correlation Spectroscopy

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.