Nanorheology of a Reconstituted Mucin Gel

I present results of diffusion nanospheres with a wide range of sizes (d) from 30 to 200 nm and nanorods of aspect ratio (≈3.5) within a model synthetic mucin gel possessing physiological and viscoelastic properties similar to lung mucus. The technique of two-photon fluorescence correlation spectroscopy was used, which provides high temporal resolution so that rotational dynamics of anisotropic particles occurring faster than milliseconds time scale can also be interrogated. The particle sizes in these experiments are smaller compared to mesh size (ax) in the gel, and thus, their motion is sensitive to nanoscale heterogeneity, segmental dynamics, and interstitial fluid properties. The particles of sizes 100 and 200 nm exhibited anomalous subdiffusion with an exponent ≈0.5, which is a manifestation of coupling of particle motion with local Rouse dynamics. The smaller spheres (30 and 50 nm) experience near normal diffusion but experience an effective viscosity that is proportional to the area of the particles. It can be explained by the presence of unreacted mucin within pores, which forms a semidilute solution with the associated correlation length, ξ < d. For nanorods of size 10 nm (d) × 38 nm (L), the rotational diffusion is slowed down more significantly compared to translational diffusion. The analysis indicated that translation is dominated by motion along the long axis of the rod when d < ξ, while the reduction of rotational motion is comparable to the slowdown of spheres with size equivalent to the length of the rod (L > ξ). The estimation of the first passage time distribution function for a model mucus layer of thickness 50 μm showed that subdiffusion can strongly affect the large length-scale transport of NPs.