Unconventional\nFast Nanorod Diffusion in Entangled\nSolutions of PEO Revealed by Mossbauer Spectroscopy

Macromolecular crowding plays an important role in the\ndiffusion\nof nanoparticles (NPs) across various biological barriers. The transport\nof NPs in the confined environments created by macromolecular networks\ndepends both on the characteristic length scales of such a network\nand on the morphology of the NPs themselves. Here, using <sup>57</sup>Fe Mössbauer spectroscopy, we show that the diffusion of iron-based\nnanorods on a length scale determined by the entanglement relaxation\ntime in poly(ethylene oxide) (PEO) network occurs 5 times faster than\nthe diffusion of nanospheres with a similar diameter and up to 20\ntimes faster than predicted from the continuum Stokes–Einstein\nmodel. Furthermore, the diameters of nanospheres and nanorods are\ncomparable to the tube diameters of the PEO entanglement network.\nWhile the diffusion behavior of nanospheres is governed by the macroscopic\nbulk viscosity of PEO solutions, nanorods experience reduced local\nfriction and experience nanoviscosity. This result demonstrates the\nimportant role of the shape anisotropy of NPs in determining their\nmobility in confined environments on the scale of characteristic network\nlengths.