Self-assembling gold rings

Chemists from Rice University in Houston, US, have discovered that gold nanorods can self-assemble into ring structures that may one day form the basis of highly sensitive optical detectors and superlenses.

Bishnu Khanal and Eugene Zubarev created their nanorings by first mixing polymer-coated gold nanorods with chloroform. “When a volatile solvent evaporates, its surface temperature cools below the dew point, and causes the condensation of micro-droplets of water from the atmosphere,” says Khanal. “Those water droplets act as templates for the formation of rings.”

Self-asssembling gold nanorod ring

After the droplets evaporate, the rods retain their ring structure, and nearly all the rods that were in solution end up being part of one ring or another. Unlike with previous attempts to create metallic nanorings, there are virtually no nanoparticles left either between or inside the rings.

As for potential applications, optical properties depend not only on the type of metal, but also the shape of the structures. “The big advantage of short gold nanorods is that they have two plasmons – transverse and longitudinal – and both are in the visible range,” says Zubarev. “This is in stark contract to spherical particles which have only one plasmon.”

“We are working really hard to measure the optical properties of the rings,” says Zubarev. “The technical problem we are facing is how to collect an extinction spectrum from a ring which is only one micron in diameter. Right now, we have just demonstrated that rings of rods can be made, and if somebody someday could show that they are useful for something, then our method would allow people to make such superstructures.”

Catherine Murphy, the University of South Carolina chemist who originally developed the synthesis of gold nanorods, comments: “This is a really nice illustration of how simple physical effects (such as evaporation) can be used to enable self-assembly of nanostructured objects. The authors did the proper control experiments to pinpoint the underlying explanation of their observations.”


Further reading

Rings of Nanorods, Bishnu P Khanal & Eugene R Zubarev, Angewandte Chemie Int. Ed. 46, 2124 (2007) (subscription required)

Figure

Transmission electron microscope image of a circular superstructure of hybrid nanorods templated by a water micro-droplet.

Image: Bishnu Khanal, Rice University

Article first published in Nanomaterials News.