Unconventional Fast Nanorod Diffusion in Entangled Solutions of PEO Revealed by Mössbauer Spectroscopy

Macromolecular crowding plays an important role in the diffusion of nanoparticles (NPs) across various biological barriers. The transport of NPs in the confined environments created by macromolecular networks depends both on the characteristic length scales of such a network and on the morphology of the NPs themselves. Here, using 57Fe Mössbauer spectroscopy, we show that the diffusion of iron-based nanorods on a length scale determined by the entanglement relaxation time in poly(ethylene oxide) (PEO) network occurs 5 times faster than the diffusion of nanospheres with a similar diameter and up to 20 times faster than predicted from the continuum Stokes–Einstein model. Furthermore, the diameters of nanospheres and nanorods are comparable to the tube diameters of the PEO entanglement network. While the diffusion behavior of nanospheres is governed by the macroscopic bulk viscosity of PEO solutions, nanorods experience reduced local friction and experience nanoviscosity. This result demonstrates the important role of the shape anisotropy of NPs in determining their mobility in confined environments on the scale of characteristic network lengths.