Nanodevice, Build Thyself

By B. J. Almond

Rice University chemists have discovered that tiny building blocks known as gold nanorods spontaneously assemble themselves into ring-like superstructures.

The finding, which was published in the chemistry journal Angewandte Chemie International Edition, could potentially lead to the development of novel nanodevices like highly sensitive optical sensors, superlenses and even “invisible” objects for use in the military.

“Finding new ways to assemble nano-objects into superstructures is an important task because, at the nanoscale, the properties of those objects depend on the arrangement of individual building blocks,” said principal investigator Eugene Zubarev, the Norman Hackerman-Welch Young Investigator and assistant professor of chemistry.

Although ringlike assemblies have been observed in spherical nanoparticles and other symmetrical molecules, until now such structures had not been documented with rod-shaped nanostructures.
Like many nanoscale objects, gold nanorods are several billionths of a meter in size. The nanorods Zubarev used have a central core of inorganic crystal, and attached to their surfaces are thousands of flexible, chainlike organic polymer molecules. The combination of inorganic and organic features results in a hybrid structure that proved to be critical to the research.

Working with Rice graduate student Bishnu Khanal, Zubarev placed the nanorods in a solution of chloroform, which is an organic solvent. As the chloroform evaporated, its surface temperature dropped low enough to cause condensation of water droplets from the air, much like what happens when dew forms. As thousands and thousands of microdroplets of water condensed on the surface of the chloroform, the nanorods that had been suspended in the solution started to press up against the droplets and form rings around them. The polymer coating prevented the rods from being absorbed into the droplets because it is insoluble in water. After the droplets evaporated, the nanorods remained in their ring formation.

Thousands of well-defined rings can be produced in a matter of seconds using this method. “It is surprisingly simple and can be used for organizing nanocrystals of various shapes, size and chemical compositions into circular arrays,” Zubarev said. “When nanorods are organized into a ring, significant changes occur in their optical and electromagnetic properties.”