Resolving Translational and Rotational Diffusion of Gold Nanorods by Confocal Scattering Correlation Spectroscopy

Abstract Anisotropic gold nanorods (GNRs) are widely recognized for their tunable plasmonic properties and their utility in sensing, photothermal therapy, and metamaterials. Accurate characterization of their size, shape, and hydrodynamic behavior is critical, yet conventional techniques such as scanning electron microscopy provide limited statistical insight and are labor-intensive. Accessing the coupled translational and rotational dynamics of anisotropic nanoparticles in solution remains experimentally challenging due to the weak and orientation-dependent nature of their scattering signals. Here, we demonstrate that confocal scattering correlation spectroscopy (SCS) enables the label-free and simultaneous quantification of both translational and rotational diffusion of gold nanorods within a femtoliter detection volume. Using commercially available GNRs spanning aspect ratios from 1.89 to 6.44, we demonstrate that SCS can disentangle rotational and translational dynamics on micro- and millisecond time scales, respectively, providing high-resolution hydrodynamic characterization. Experimental diffusion coefficients are compared with classical hydrodynamic models by Broersma and Tirado-Garcia de la Torre, revealing aspect-ratio-dependent agreement and highlighting the influence of end effects and nanoscale surface interactions. These results establish SCS as a powerful nanometrology tool for anisotropic colloids, enabling rapid determination of nanoparticle morphology and dynamics in solution while enabling the temporal resolution of rotational and translational dynamics within a highly localized femtoliter observation volume.