On the Other Hand
Researchers find first experimental evidence of optical chirality in tiny nanoclusters of gold.
Tiny nanoclusters of metallic gold — assemblies containing between 20 and 40 gold atoms encapsulated by a common biomolecule — can display distinctly chiral properties.
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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. Their research was published in the March 30 issue of the Journal of Physical Chemistry.
"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," explains Dr. Robert L. Whetten, a professor in the 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 Oak Ridge National Laboratory, attached 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 clusters, 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 parts per million (ppm) in the yellow-green region, while in another, 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 says.
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 notes. "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.
— John Toon
The full-text version of this article is posted at www.gtri.gatech.edu/res-news/CHIRALGOLD.html. For more information, you may contact Dr. Robert Whetten, School of Chemistry and Biochemistry, Georgia Tech, Atlanta, GA 30332-0400. (Telephone: 404-894-8255) (E-mail: robert.whetten@physics.gatech.edu)
Feeding the World by Cleaning the Air
Study ties heavy regional haze to reductions in China's crop production.
A recent Georgia Institute of Technology study suggests that cleaning up the air may help feed the world.
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The study found that heavy regional haze in China's most important agricultural areas may be cutting food production there by as much as one-third. The study was published in the Proceedings of the National Academy of Sciences last fall.
Covering one million square kilometers or more, the haze scatters and absorbs solar radiation, reducing the amount of sunlight reaching key rice and winter wheat crops. That decreases plant growth and food production.
"For crops that are irrigated and fertilized, there is often a direct correlation between how much is grown and how much sunlight reaches those crops," says Dr. William L. Chameides, Smithgall Chair and Regents Professor in the School of Earth & Atmospheric Sciences. "In China, there is a significant amount of haze that reduces the sunlight reaching the surface by at least 5 percent, and perhaps as much as 30 percent. The optimal yields of crops in China are likely reduced by the same percentage."
Chameides says the NASA-funded study provides China — and other nations with similar issues — another option in the struggle to feed their growing populations. It is believed to be the first work to quantitatively assess the direct impact of regional haze on the yields of these crops.
"China is already losing 10, 20 or even 30 percent of its crop production to haze," he says. "Controlling the sources of the haze represents a potential way to increase crop production because the technology exists to control air pollution."
The estimates of crop production losses are based on detailed long-term measurements at Nanjing, 200 miles southwest of Shanghai, but are extrapolated to other areas of China. They consider only the direct effects of haze on sunlight, and do not include the indirect effects on sunlight potentially caused by haze interacting with clouds or the toxic effects of air pollutants that also reduce crop growth.
Extensive studies by agricultural researchers have documented the relationship between crop production and the sunlight received.
The haze affecting China is made up of aerosols composed of solid and liquid particles of varying sizes. The aerosols likely result from the burning of coal, biomass and other fuels, though scientists lack detailed information on their origins.
Large-scale regional hazes exist in other developing countries, suggesting food production may be similarly reduced in India and African nations that are also struggling to feed their people.
"Any economically developing or developed country will have these large regional hazes associated with burning," Chameides explains. "Burning fossil fuels, burning wood and burning biomass for clearing fields causes production of a significant amount of haze that leads to a reduction in the solar radiation reaching the earth's surface."
The same effect has been measured on the East Coast of the United States, though China's haze levels are roughly twice as bad. Records suggest that China's haze problem has worsened over the past 20 years, a time of massive industrialization.
The study, for which Chameides is the lead investigator, found that the regional haze affects approximately 70 percent of crops grown in China. The haze tends to be worst in the eastern part of the country that includes the most productive and heavily cultivated areas. It can be measured year around.
The study produced two different estimates of sunlight reduction, one based on direct measurements and one based on a model of China's atmosphere. Data based on direct measurements suggest an even larger effect than the 5 to 30 percent crop reduction calculated by the model.
— John Toon
The full-text version of this article is posted at www.gtri.gatech.edu/res-news/CHINA-AIR.html. For more information, you may contact Dr. William Chameides, School of Earth and Atmospheric Sciences, Georgia Tech, Atlanta, GA, 30332-0340. (Telephone: 404-894-1749) (E-mail: william.chameides@eas.gatech.edu)
Finding the Right Recipe
Researchers control chemistry to tailor magnetic nanoparticles for medical treatment and diagnosis.
Nanoparticles that possess magnetic properties offer exciting new opportunities for delivering drugs to targeted areas in the body, replacing radioactive tracer materials, improving the quality of noninvasive medical imaging and producing ever-smaller data storage devices. But before these magnetic nanoparticles gain widespread use, scientists must learn to consistently control their key properties.
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Using only variations in chemistry and process conditions, researchers at the Georgia Institute of Technology have learned to precisely control the size and magnetic properties of one class of magnetic nanoparticles. Their goal is a "recipe book" other researchers could use to produce nanoparticles with exactly the right properties for different applications.
"If you are going to produce these nanoparticles for large-scale use, you cannot guess at the conditions or rely on intuition," says Dr. John Zhang, Georgia Tech assistant professor of chemistry and biochemistry. "We are understanding the fundamental ways to control the properties of these particles, chemically manipulating the magnetic interactions at the atomic level. We want to control these properties through chemical means."
Zhang presented his research team's latest findings earlier this spring at the American Chemical Society's 219th national meeting. The team includes Adam J. Rondinone, Anna C.S. Samia, Chao Liu, and Richard Anderson.
Because each potential application for the magnetic nanoparticles requires different properties, the work is essential to their future use as carriers of drugs, tracers and MRI contrast enhancement agents. Also, it will provide insights to some key technical issues in high-density information storage.
For instance, each particle possesses certain magnetic orientations just as the north or south pole in a tiny magnet. Magnetic digital data bits in a computer hard disk have magnetic states similar to the nanoparticles. When the bits get smaller as the storage density increases, the magnetic state could become unstable. To avoid data loss caused by magnetic state change from simple temperature fluctuations, computer makers need to install a high enough magnetic energy barrier to stabilize the magnetic states.
But for magnetic nanoparticles to be used in the body, physicians need particles with a low-energy barrier to allow magnetic state to change constantly. Because magnetic opposites attract one another, the magnetic particles could potentially clump together, clogging blood flow. Rapidly changing the magnetic direction, therefore, would be essential to prevent the particles from aggregating.
"We know that the energy barriers in these magnetic nanoparticles are due to atomic-level magnetic interactions," Zhang explains. "We want to make the connection between these atomic-level interactions and the macroscopic behavior that we want in these materials."
The energy barrier between magnetic states — which Zhang likens to a hill that requires a certain amount of energy to climb over — is proportional to the size of the particle as well as magnetic interactions. Zhang and his team have learned to control this energy barrier through chemical means. Another critical property is the size. Magnetic nanoparticles for in-vivo biomedical use must be small enough to avoid detection by the immune system, yet large enough to remain in the body long enough to be circulated through the blood stream.
And because magnetic properties vary by size, the particles must all be about the same diameter to ensure consistent properties. Zhang and his team have developed a statistical model to predict and control the size of the nanoparticles from synthesis process variables. They produce nanoparticles with size variations of less than 15 percent, but hope to reduce that further.
— John Toon
The full-text version of this article is posted at www.gtri.gatech.edu/res-news/NANOPART2.html. For more information, you may contact Dr. John Zhang, School of Chemistry and Biochemistry, Georgia Tech, Atlanta, GA 30332-0400. (Telephone: 404-894-6368) (E-mail: john.zhang@chemistry.gatech.edu)
Faculty Awards and Honors
Dr. Krishan Ahuja of the Georgia Tech Research Institute was named AIAA Engineer of the Year, which is a national award. Ahuja conducts research in the Aerospace, Transportation and Advanced Systems Laboratory. He is also a Regents researcher and professor in the School of Aerospace Engineering.
Dr. Erian Armanios of the School of Aerospace Engineering received a Regents' Teaching Excellence Award for 2000. The awards are designed to honor impressive work being done within the University System of Georgia.
Oxford University Press recently published Strategic Corporate Management for Engineering by Dr. Paul Chinowsky of the School of Civil and Environmental Engineering. Chinowsky has been studying the management practices of engineering and construction organizations throughout the United States since 1995. This book is based on the largest collection of research data on these organizations in the country.
Dr. Imme Ebert-Uphoff in the School of Mechanical Engineering received a 2000 CAREER Award from the National Science Foundation. Her project is titled "New Research Directions for Parallel Manipulators-Investigation of Redundant Actuation, Redundant Sensing and Static Balancing" and is funded for a four-year period at about $200,000.
Dr. Charles Eckert of the School of Chemical Engineering received a Regents' Research in Undergraduate Education Award for 2000. The awards are designed to honor impressive work being done within the University System of Georgia.
Dr. Augustine O. Esogbue of the School of Industrial and Systems Engineering was elected to the 2000 Class of Fellows of IEEE, the largest engineering professional society in the world. Esogbue was cited for "his contributions to theoretical and computational dynamic programming and applications."
Dr. Rigoberto Hernandez in the School of Chemistry and Biochemistry was named a 2000 Sloan Research Fellow. Awarded by the Alfred P. Sloan Foundation, the two-year fellowship is intended to enhance the careers of the best young faculty members in the nation, especially those who have demonstrated independent creativity in their work. Hernandez's group develops models to better understand the dynamics of thermosetting polymers.
Dr. Michael D. Meyer, professor in and chair of the School of Civil and Environmental Engineering, received the Theodore M. Matson Award for outstanding contributions in the field of transportation engineering. Meyer was recognized not only for his writings and teaching, but also for his leadership in the profession.
The Institute of Electrical and Electronics Engineers honored 21 Georgia Tech faculty members recently with Third Millennium Medals recognizing outstanding contributions in their areas of research expertise. Those recognized were Drs. Donald E. Clark, David P. Millard, Edward K. Reedy, Mark A. Richards, Bob Trebits and James C. Wiltse, all of GTRI; and Drs. Tom Barnwell, John A. Buck, Nikil Jayant, Nan Jokerst, John Limb, Jim McClellan, Jim Meindl, Russ Mersereau, Andrew F. Peterson, Teddy Puttgen, Pete Rodrigue, Bill Sayle, Ron Schafer, Rao Tummala and Roger P. Webb, all of the School of Electrical and Computer Engineering.
Also see Research Notes news stories.
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