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Gold Clusters Are Optically Active

Photonics Spectra
Aug 2000
Daniel S. Burgess

Metals, as a rule, do not display optical activity. So researchers at the Georgia Institute of Technology were surprised to find that their gold nanocrystals did so intensely and across the spectrum. Gold nanoparticles already have found a niche as biomolecular markers, but with the addition of these optical effects to their repertoire, they may find added work as high-contrast colorimetric probes and sensors.

A material is optically active if it preferentially absorbs or affects the polarization of light passing through it. In the case of the gold structures, the researchers suspect it is due to chirality, or handedness, which are mirror-image variations in the atomic geometries of molecules.

Chirality has been observed in the semimetallic carbon fullerenes and nanotubes, and metallic nanostructures such as nanowires and clusters of nickel had been theorized to display optical activity. However, Robert L. Whetten of Georgia Tech, co-author of a report describing the work in the March 30 Journal of Physical Chemistry, said that the researchers did not expect to confirm those predictions with their investigations into gold nanoclusters.

Whetten and T. Gregory Schaaff, now at Oak Ridge National Laboratory in Tennessee, prepared the clusters by mixing HAuCl4 and the tripeptide glutathione to form a gold:glutathione polymer. Chemical decomposition of the polymer produced the crystals, and the researchers used gel electrophoresis to isolate the species with only 20 to 80 metal atoms. They used spectrophotometers from PerkinElmer Analytical Instruments of Norwalk, Conn., and Jasco Corp. of Tokyo to determine the optical absorption spectra and circular dichroism of the clusters, respectively. Those with 40 or fewer atoms displayed circular dichroism from the ultraviolet to the infrared: more than 300 ppm in the yellow-green in one species, and 1100 ppm in the red and near-IR in another.

Researchers at the Georgia Institute of Technology unexpectedly discovered intense optical activity in gold nanoclusters. The clusters may find applications as bioconjugate probes. Courtesy of T. Gregory Schaaff, Oak Ridge National Laboratory.

"These effects have been important both for what they reveal about the geometrical and electronic structure of matter, including various spectroscopic diagnostics, and also in optics for the control of the polarization of light," Whetten said. The team believes that the phenomenon is the result of a helical structure at the cores of the clusters.

While a primary effect of the work is a reiteration of the importance of using separation procedures in the study of nanoclusters, the potential commercial applications of the gold clusters, such as bioconjugate probes, are sparking interest as well.

"We've had an unusual number of inquiries from other researchers who are replicating our preparation and separation procedures," Whetten said. "They each have their own reasons, usually unspecified, for pursuing them." He said that a high-throughput automated separation process must be devised to produce the clusters commercially.

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