Nanorheology\nof a Reconstituted Mucin Gel

I present results of diffusion nanospheres with a wide\nrange of\nsizes (<i>d</i>) from 30 to 200 nm and nanorods of aspect\nratio (≈3.5) within a model synthetic mucin gel possessing\nphysiological and viscoelastic properties similar to lung mucus. The\ntechnique of two-photon fluorescence correlation spectroscopy was\nused, which provides high temporal resolution so that rotational dynamics\nof anisotropic particles occurring faster than milliseconds time scale\ncan also be interrogated. The particle sizes in these experiments\nare smaller compared to mesh size (<i>a</i><sub><i>x</i></sub>) in the gel, and thus, their motion is sensitive\nto nanoscale heterogeneity, segmental dynamics, and interstitial fluid\nproperties. The particles of sizes 100 and 200 nm exhibited anomalous\nsubdiffusion with an exponent ≈0.5, which is a manifestation\nof coupling of particle motion with local Rouse dynamics. The smaller\nspheres (30 and 50 nm) experience near normal diffusion but experience\nan effective viscosity that is proportional to the area of the particles.\nIt can be explained by the presence of unreacted mucin within pores,\nwhich forms a semidilute solution with the associated correlation\nlength, ξ < <i>d</i>. For nanorods of size 10 nm\n(<i>d</i>) × 38 nm (<i>L</i>), the rotational\ndiffusion is slowed down more significantly compared to translational\ndiffusion. The analysis indicated that translation is dominated by\nmotion along the long axis of the rod when <i>d</i> <\nξ, while the reduction of rotational motion is comparable to\nthe slowdown of spheres with size equivalent to the length of the\nrod (<i>L</i> > ξ). The estimation of the first\npassage\ntime distribution function for a model mucus layer of thickness 50\nμm showed that subdiffusion can strongly affect the large length-scale\ntransport of NPs.