Rolling on water

Rolling nanorods on the surface of polar liquids using a coulombically induced torque.

Credit: © 2009 APS

The transportation of nanoscale packages regularly occurs in biological systems, carried out, for example, by proteins such as kinesin or myosin. Equivalent synthetic processes are sought for a range of nanotechnologies, and methods for manipulating species in solution and on solid surfaces have been developed. Lela Vuković and Petr Král of the University of Illinois at Chicago have now explored the possibility of rolling nanorods on the surface of water¹.

The researchers designed an electrically driven 'roller' made up of a metal nanorod covered with photoactive hydrophobic surfactant molecules. The surfactant molecules had three sections: an inner unit to bind to the metallic rod; a middle unit composed of photoactive and partially orientated chromophores; and an outer unit of alkane chains, which prevent the rods from submerging in water. Light excitation of the chromophores could provide an asymmetric charging of the nanorod surface and, as a result, an asymmetric coulombic attraction to the highly polar molecules of water. This transient coupling should then lead to a unidirectional rolling of the nanorods across the water surface.

Using molecular dynamics simulations, Vuković and Král examined a simplified model of the nanorod and found that they could roll on water with translational velocities of 1–5 nm ns⁻¹. In addition, they also explored the idea of using the roller to pull other objects.