  The optical trapping and manipulation of small particles is an important tool
for probing fluid properties at the microscale. In particular, microrheology
exploits the manipulation and rotation of micron-scale particles to probe local
viscosity, especially where these properties may be perturbed as a function of
their local environment, for example in the vicinity of cells. To this end,
birefringent particles are useful as they can be readily controlled using
optically induced forces and torques, and thereby used to probe their local
environment. However the magnitude of optical torques that can be induced in
birefringent particles is small, and a function of the particle diameter,
meaning that rotational flow cannot readily be probed on length scales much
small than the micron level. Here we show modelling that demonstrates that
eccentric spherical core-shell nanoparticles can be used to generate
considerable optical torques. The eccentricity is a result of the displacement
of the centre of the core from the shell. Our results show that, for particles
ranging from 90~nm to 180~nm in diameter, we may achieve rotation rates
exceeding 800~Hz. This fills a missing size gap in the rotation of
microparticles with optical forces. The diameter of particle we may rotate is
almost an order of magnitude smaller than the smallest birefringent particles
that have been successfully rotated to date. The rotation of eccentric
core-shell nanoparticles therefore makes an important contribution to
biophotonics and creates new opportunities for rheology in nanoscale
environments.
