Finding new ways to assemble nano-objects is important because the properties of these materials depend not only on their size and shape but also on how they are arranged in space, and the degree of ordering among the individual building blocks. Although ring-like assemblies are fairly common and have been seen for various polymers, small organic molecules and spherical inorganic nanoparticles, they have never been observed for rod-shaped nanocrystals before. Nearly all experiments on nanorods have found that they assemble parallel to each other. Moreover, theoretical models also predict that these materials should pack together side by side.

Now, Bishnu Khanal and Eugene Zubarev have shown that rings of gold nanorods can form within seconds when water droplets condense onto the surface of a solution of the rods in a non-polar solvent. The Rice University duo began by coating gold nanorods with polystyrene. The tiny polymer chains in the coating stick out from the rod-like bristles and this makes the rods soluble in organic solvents but insoluble in water -- something that is important for the self-assembly process. Next, the researchers dissolve the rods in dichloromethane. A carbon-coated grid is then dipped into this solution.

When the grid is pulled out, a thin layer of solution clings to it. The highly volatile dichloromethane quickly evaporates, which strongly cools the surface of the liquid film. As a consequence, humidity from the air condenses onto the surface of the film and, because the water and dichloromethane are immiscible, the condensation forms tiny droplets of water.

When the dichloromethane has almost completely evaporated, the last remnants of the solution form rings around the water droplets. And once the solvent has evaporated altogether, the substrate warms back up to room temperature so that the water droplets also evaporate. This leaves behind ring-shaped structures made of nanorods. The diameter of the rings varies from 300 nm to a few microns, and they are typically about 50 nm wide (see figure).

Electron microscope images show that the nanorods in the rings are randomly oriented when their concentration in the original solution is high. However, at lower concentrations, the nanorods are oriented in a head-to-tail sequence along the edge of the ring -- a phenomenon the researchers describe as "truly amazing".

Zubarev told that the ring-like assemblies of nanorods can act as highly sensitive optical nanodevices that can detect biological molecules, such as DNA and enzymes at exceedingly low concentrations, which in turn could have broad applications in medicine and in diagnosing cancer.

The researchers reported their work in Angewandte Chemie International Edition.