The control of single molecules is of considerable interest in areas of molecular assembly such as the control of individual chemical reactions and interactions, single molecular sensing, advanced therapeutics where specific targeting is envisaged, and molecular computers.

Geoghegan and colleagues note long-standing efforts to produce directed, controlled movement of individual molecules in the nano world, where objects are about 1/50,000th the width of a human hair. The main solutions so far have involved use of expensive, complex machines to move the molecules and they have been only partially successful, the scientists say.

Mark Geoghegan and colleagues [ACS Nano, (2009) DOI: 10.1021/nn900991r] demonstrate the diffusion of single poly(ethylene glycol) molecules on surfaces which change from hydrophilic to hydrophobic over a few micrometers. These gradients in surface energy are shown to drive the molecular diffusion in the direction of the hydrophilic component. The polymer diffusion coefficients on these surfaces are measured by fluorescence correlation spectroscopy and are shown to be elevated by more than an order of magnitude compared to surfaces without the surface energy gradient. Along the gradient, the diffusion is asymmetric, with diffusion coefficients 100 times greater in the direction of the gradient than orthogonal to it. This diffusion can be explained by a Stokes−Einstein treatment of the surface-adsorbed polymer.

The applications are great and range from the possibility of coaxing cells to move and grow in specific directions to treat diseases. It also could speed development of some long-awaited nanotech innovations. They include self-healing structures that naturally repair tears in their surface and devices that deliver medication to diseased while sparing healthy tissue.