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The Next Big Thing:
Amazing Metal Nanoclusters and Nanoparticles

More efficient catalysts. Improved sensors. A new optical data storage technique. Fluorescent labels for biological studies.

photo by Gary Meek Thin films of silver nanoclusters could provide a new means for storing data optically. Researchers Robert Dickson, Lynn Peyser and Amy Vinson expose a silver nanocluster sample to green laser light and study the resulting fluorescence. Typical responses are shown on the computer monitor behind them. (300-dpi JPEG version - 827k)

These are just a few of the applications being developed by Georgia Tech researchers to take advantage of the unique properties found in nanometer-scale metals.

In the Laser Dynamics Laboratory at the School of Chemistry and Biochemistry, Mostafa El-Sayed and colleagues have studied catalytic properties of nanoparticles since the mid-1990s. The El-Sayed group reported in Science that because nanoparticles are so small, it was possible to synthesize them with different shapes. Now they are examining the different catalytic properties the various shapes might have for industrial and environmental uses.

Catalytic reactions take place only on the surface of these materials. Atoms not exposed to the reaction cannot participate, so increasing the percentage of atoms with surface exposure maximizes use of the costly platinum or palladium catalysts. Up to 70 percent of atoms in nanoscale catalysts can be involved in reactions, compared to a very small percentage of atoms in bulk catalysts.

"Most of the atoms then are helping you in the reaction, so if you are using a very expensive material, you don't need as much," El-Sayed explains. "The efficiency per gram of material can be orders of magnitude higher." Furthermore, nanoscale catalysts are more efficient than bulk catalysts and thus can catalyze reactions at lower temperatures to save energy.

But the nanoparticles tend to aggregate, forming larger and less efficient structures unless "capped" by a polymer material. That capping, however, reduces the surface available for catalysis. And researchers must devise a structure to hold the tiny particles without the capping material.

"We have to weigh these things against each other," El-Sayed notes. "It is always a compromise."

Laser Dynamics Laboratory researchers also study the unique optical properties of gold nanoparticles and nanorods. At the nanometer scale, these metal particles absorb light in unique ways, producing characteristically bright red colors that once found use in stained glass windows. The phenomena may find more modern use making sensors more efficient. The luminescent ability of gold nanorods may also find applications in imaging.

Robert Whetten also studies the unique optical properties of gold at the very smallest of scales. Examining nanoclusters containing between 20 and 40 gold atoms encapsulated by an ordinary biomolecule, he and Gregory Schaaff found distinctly chiral properties in the way the nanoclusters absorb light.

Theory had suggested such right-handed and left-handed absorption properties, but Whetten and Schaaff reported the first experimental evidence in a Journal of Physical Chemistry paper.

"When clusters are prepared 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," explains Whetten, a professor in the School of Physics and the School of Chemistry and Biochemistry.

The study measured nanoclusters created by attaching glutathione - a common sulfur-containing tripeptide - to individual gold atoms. In the resulting gold-glutathione polymer, gold atoms make no direct contact with one another. Decomposing the polymer produces the gold nanoclusters, which have glutathione molecules adsorbed to their surface so as to physically limit the number of metal atoms that could join each cluster.

At the nanoscale, metal particles behave differently than they do at bulk quantities. Gold nanoparticles and nanorods absorb light in unique ways, producing characteristic colors. (Larger JPEG version - 304k)

The phenomena could be used for labeling biological molecules during experimentation.

Silver nanoclusters composed of 2 to 8 atoms also produce interesting optical properties, demonstrating fluorescence that could lead to a new type of optical data storage. Writing in the journal Science, Georgia Tech researchers described binary optical storage based on writing and reading simple images recorded on thin films of silver oxide nanoparticles.

"These nanomaterials have a remarkable new property: When you shine blue light with a wavelength of less than 520 nanometers onto them, you switch on their ability to fluoresce," explains Robert M. Dickson, an assistant professor in the School of Chemistry and Biochemistry. "You can then read the fluorescence nondestructively by illuminating the clusters with longer-wavelength light."

The researchers begin by producing extremely thin films (less than 20 nanometers thick) of silver oxide nanoparticles on a glass slide. They then selectively expose portions of the film to light in the blue spectrum. The light chemically reduces particles near the surface of the film, partially converting them to clusters of silver atoms. When researchers expose these photo-activated silver clusters to longer wavelength green light, the clusters fluoresce strongly, emitting red light. Silver oxide particles not photo-activated by exposure to blue light don't fluoresce.

Studied under a microscope, the individual silver particles show an additional property that may be useful for increasing the density of optical data storage.

"If you look at an individual particle through the microscope, you see green emission, then red emission, then yellow emission all from the same particle," Dickson says. "Not only are you generating fluorescence, but you presumably are also changing the size and/or geometry of the cluster, which causes it to emit different wavelengths."

By using the right distribution of particle sizes, these multi-color emissions could allow storage of more than one bit of information in each data point. Distributed in a three-dimensional matrix, the particles could provide a dense storage medium that could be written and read in parallel.

Simple images stored on the silver oxide film can be read nondestructively by green light for at least two days, the longest time the researchers studied them. Though they have demonstrated an ability to optically write and read information, the researchers do not yet know if their film can be optically erased and rewritten.

For more information, contact Mostafa El-Sayed, School of Chemistry and Biochemistry, Georgia Tech, Atlanta, GA 30332-0400. (Telephone: 404-894-0292) (E-mail: mostafa.el-sayed@chemistry.gatech.edu);    or Robert Whetten, School of Chemistry and Biochemistry, Georgia Tech, Atlanta, GA 30332-0400. (Telephone: 404-894-8255) (E-mail: robert.whetten@chemistry.gatech.edu);   or Robert Dickson, School of Chemistry and Biochemistry, Georgia Tech, Atlanta, GA 30332-0400. (Telephone: 404-894-4007) (E-mail: robert.dickson@chemistry.gatech.edu)