We record dark-field scattering bursts of individual
gold nanorods,
52 × 15 nm2in average size, freely diffusing in water
suspension. We deduce their Brownian rotational diffusion constant
from autocorrelation functions on a single-event basis. Due to spectral
selection by the plasmonic resonance with the excitation laser, the
distribution of rotational diffusion constants is much narrower than
expected from the size distribution measured by TEM. As rotational
diffusion depends on particle hydrodynamic volume, viscosity, and
temperature, it can sense those parameters at the single-particle
level. We demonstrate measurements of hot Brownian rotational diffusion
of nanorods in temperature and viscosity gradients caused by plasmonic
heating. Further, we monitor hydrodynamic volumes of gold nanorods
upon addition of very low concentrations of the water-soluble polymer
PVA, which binds to the particles, leading to measurable changes in
their diffusion constant corresponding to binding of one to a few
polymer coils. We propose this analysis technique for very low concentrations
of biomolecules in solution.