A report published in the March 30 issue of the Journal of Physical Chemistry presents the first experimental evidence that tiny nanoclusters of metallic gold -- assemblies containing between 20 and 40 gold atoms encapsulated by a common biomolecule -- can display distinctly chiral properties.
The chiral nature of the clusters, which means they exist in distinct right-handed and left-handed variations, dramatically affects the way in which they absorb polarized light. This optical effect had been predicted theoretically to occur in metal nanostructures, but Georgia Institute of Technology researchers were the first to measure it in a special class of clusters they formulated.
"When clusters are prepared in this way, we see that the conduction electrons in the gold circulate in such a way as to have the unique optical effect of preferring one direction of circularly-polarized light over the other direction," explained Dr. Robert L. Whetten, a professor in Georgia Tech's School of Physics and School of Chemistry and Biochemistry. "The effect was enormous, which was unexpected."
The gold nanoclusters are believed to be the smallest ever prepared. Dr. T. Gregory Schaaff, a former graduate student in Whetten's lab and now a staff scientist at the Oak Ridge National Laboratory, first attached the molecule glutathione, a common sulfur-containing tripeptide, to individual gold atoms to form a gold-glutathione polymer in which the gold atoms make no direct contact with one another. The decomposition of this polymer yields the gold clusters, which have glutathione molecules adsorbed to their surface so as to physically limit the number of metal atoms that could join together in each cluster.
While measuring the properties of the cluster, Schaaff noted dramatic differences in the way the smallest clusters absorbed polarized light in the visible and near-infrared spectra. In one cluster, this circular dichroism effect exceeded 300 ppm in the yellow-green region, while in another cluster, the effect exceeded 1,000 ppm in the red and near-infrared.
These optical measurements suggest that the clusters have a helical structure that Whetten compared to the stripes on a candy cane or a barbershop pole.
"We had to double-check our instruments and repeat the measurements a number of times because the effect was enormous," he said. "This effect is comparable to what is seen in naturally-helical structures. Such effects had not previously been measured in metal-cluster compounds and it's kind of a shock that small metals might prefer to have a helical structure."
Using gel electrophoresis to separate the clusters by weight, Schaaff found that certain cluster sizes dominated, with 28-atom assemblies -- slightly less than one nanometer across -- being the most common. The chiral properties varied by the size of the cluster, and therefore were only observed clearly when the clusters were separated by weight. Only clusters with 40 or fewer atoms displayed the intense optical properties.
The optical effect changed direction as the researchers moved from one cluster size to the next, suggesting a direct correlation to the energies of the conduction electrons in the metal's outer shell.
"Even though the optical absorption increases more or less monotonically here, the preferences for right- versus left-handed light changes direction from one band to another," Whetten noted. "The optical spectra are not smeared out. They each have their own distinct character, plus or minus, corresponding to the energy level."
He believes the effect is related to the high level of confinement created in the conduction electrons by formation of the small clusters, though research has not yet confirmed that. A helical geometrical pattern or "tiling" of the glutathione adsorption sites (gold-sulfur bonds) could also affect the circulation of the conduction electrons. The implications and potential uses for the effect also remain to be determined.
"Having this kind of a structure is a big deal in terms of the way they interact with light, and maybe the way they interact with other things that are chiral," Whetten said. "From the point of view of metallic bonding, its actually a subtle difference physically. But it makes an enormous difference in how they interact with light, and perhaps under certain circumstances, what they can do chemically."
The next step in the research is to characterize the clusters to determine what causes this effect. Whetten and Schaaff want to study the internal structure of the gold clusters in addition to their interactions with the tripeptides.
"We need to rigorously determine the arrangement of the atoms, to find out what aspects of the internal structure give rise to this effect," Whetten continued. "We want to understand the reasons why, out of this whole range of sizes from about 20 atoms to about 80 atoms, there are only a handful of distinct sizes that are dominating."
Whetten would also like to make the new gold-glutathione clusters available to other researchers for study -- and potential development of new applications. "We can make these very easily in large enough quantities to share with others interested in working with them," he said.
For several years, Whetten's group has been collaborating with researchers at the University of North Carolina-Chapel Hill to study the unique electrical properties of gold nanoclusters.
The new gold-glutathione clusters are of interest not only because of potential electronic applications, but because they can be used as markers in nanoprobes -- their presence indicated by their unique light absorption.
Glutathione is a non-protein combination of amino acids synthesized by cells to help maintain proper reduction-oxidation levels. [i.e. To prevent ordinary oxygen and free radicals from 'burning up' the fragile biomolecules in the living cell.] Its importance to this work stems from its unique molecular structure that tends to favor the formation of small gold nanoclusters.
The research was sponsored by the U.S. National Science Foundation and the Georgia Tech Foundation.
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Visuals: Images of gold bands separated by weight, molecular diagrams.
Writer: John Toon
Journal
The Journal of Physical Chemistry