  Controlling the motion of nano and microscale objects in a fluid environment
is a key factor in designing optimized tiny machines that perform mechanical
tasks such as transport of drugs or genetic material in cells, fluid mixing to
accelerate chemical reactions, and cargo transport in microfluidic chips.
Directed motion is made possible by the coupled translational and rotational
motion of asymmetric particles. A current challenge in achieving directed and
controlled motion at the nanoscale lies in overcoming random Brownian motion
due to thermal fluctuations in the fluid. We use a hybrid lattice-Boltzmann
Molecular Dynamics method with full hydrodynamic interactions and thermal
fluctuations to demonstrate that controlled propulsion of individual
nanohelices in an aqueous environment is possible. We optimize the propulsion
velocity and the efficiency of externally driven nanohelices. We quantify the
importance of the thermal effects on the directed motion by calculating the
P\'eclet number for various shapes, number of turns and pitch lengths of the
helices. Consistent with the experimental microscale separation of chiral
objects, our results indicate that in the presence of thermal fluctuations at
P\'eclet numbers $>10$, chiral particles follow the direction of propagation
according to its handedness and the direction of the applied torque making
separation of chiral particles possible at the nanoscale. Our results provide
criteria for the design and control of helical machines at the nanoscale.
