“Our nanomotor rotates faster than a jet engine,” says team member and lead author of the study Lei Shao of Chalmers University of Technology. “What is more, the scattering mechanism that drives the rotation prevents the gold nanorods from overheating, making the motors suitable for biological applications.”

The researchers, led by Mikael Käll, also at Chalmers, made their motor by trapping gold nanorods in optical tweezers, which work by confining the nanorods near the focus of a laser beam. They rotated the rods by circularly polarizing the laser light and observed how fast the structures spun round by measuring the light scattered off them.

“The recorded signal provides us with information about the rotation, as well as the local temperature of the nanorods,” Shao tells nanotechweb.org. “We confirmed the scattering mechanism by theoretical calculations and rotational dynamical analysis.”

Stir bars and drills

The fast rotating nanomotors could be used in a variety of optomechanical sensing and actuation applications, he adds. “One possibility is to use them as stir bars to rapidly mix solutions in very small volumes, and perhaps thus control chemical reactions – for example, in micro or nanofluidic chips. It might also be possible to exploit the rotation to probe the local viscosity and temperature of a sample, as well as molecular surface reactions – as we indeed show by proof-of-principle demonstrations in our study. The mechanical rotary motion of the nanorods might also be used to ‘drill’ tiny holes in thin membranes or surfaces.”

The team, reporting its work in ACS Nano DOI: 10.1021/acsnano.5b06311, is now busy looking at how the rotation depends on nanoparticle shape, size and composition. “We are also trying to better understand the movement of nanoparticles trapped in focused light fields and want to develop the idea of using these particles as molecular sensors – for example, by functionalizing them so that they can capture specific molecules from solution.”

We also hope to employ spectroscopic probes, such as surface plasmon resonance and Raman analysis, to obtain more information about what is happening on the surfaces of rotating nanoparticles – for instance, when they are exposed to molecules.