(27e) The Hindered Translational and Rotational Dynamics of Anisotropic Nanoparticles Diffusing Near a Solid-Liquid Interface Measured in Aqueous Solution

We discuss a new method for measuring the three-dimensional translational and rotational dynamics of various nanoparticles (e.g. ZnO, gold and clay nanorods) diffusing in aqueous solution above a solid-liquid interface. The utility of conventional optical tools for probing these systems is limited by the proximity of an interface and the presence of particle anisotropy; yet it is the influence of these factors that makes such systems interesting, introducing asymmetric interfacial forces and separation-dependent hydrodynamic hindrance in each of the spatial modes of diffusion. We simultaneously record the spatially correlated scattering of multiple evanescent light sources by isolated anisotropic particles, and use this data to reconstruct instantaneous positions and orientations at millisecond time-intervals. By observing diffusion in each spatial mode over time we are able to quantify each translational and rotational diffusion coefficient as a function of interfacial separation, and show an asymptotic approach to the expected constant value in the bulk. These same data can also be used to measure equilibrium particle-interface interaction potentials with orientation and position effects optionally decoupled, completing a rich set of tools for probing anisotropic interactions. Aside from fundamental applications, this approach will be particularly useful for understanding and tuning the self-assembly of films and other structures incorporating anisotropic nanoparticles.