Both shape and size matter in tiny world of nanoparticles

November 30, 2001

EVANSTON, Ill. -- Scientists at Northwestern University have created a nanoparticle with a new shape that could be a useful tool in the race to detect biological threats. The nanoprism, which resembles a tiny Dorito, exhibits unusual optical properties that could be used to improve biodetectors, allowing them to test for a far greater number of biological warfare agents or diseases at one time.

The simple method used to produce triangular nanoprisms in large quantities, using commercially available silver nanospheres, soap and visible light, is reported in the Nov. 30 issue of the journal Science.

"Many detection systems are based on small particles and their individual properties," said Chad A. Mirkin, director of Northwestern's Institute for Nanotechnology, who led the experimental portion of the study. "As we get more building blocks, the types of detectors one can build and the agents one can detect grow. With its intriguing optical properties, the nanoprism is a new and important building block for detection science. It's a major new addition to our set of nanoscale Tinker Toys."

"The nanoprisms are a major fundamental advance because they have previously unidentified optical properties that derive from their unusual shape," said George C. Schatz, professor of chemistry, who led the theory part of the work. He and his team of researchers have used calculations to successfully model these properties and are excited about the prospects for using theory to guide the design of these novel nanomaterials for applications not only in detection but also in optics and electronics.

In nanotechnology, researchers are not proficient at making controlled shapes of materials; thus far, most of the bulk preparatory methods are for spheres and rods, added Mirkin. (A nanometer is one-billionth of a meter.)

The nanoprisms, made up of silver atoms, gives off a rich red color when exposed to light. The nanoparticles could be used as new diagnostic labels, lighting up when target DNA -- of anthrax or HIV, for example -- is present. By developing nanoprisms made of different materials and with varying shapes and sizes, a large number of multicolor diagnostic labels could become available.

Once the technology is optimized, biodetectors incorporating nanoprisms could be used to quickly, easily and accurately detect biological weapons such as anthrax, smallpox and tuberculosis, as well as a wide range of genetic and pathogenic diseases, from genetic markers for cancer and neurodegenerative diseases to HIV and sexually transmitted diseases.

In their experiments, the researchers found that when they placed common nanospheres made of silver in a solution containing soap molecules and irradiated the mixture with room light for three days, the spheres were converted into triangular prisms a mere 15 nanometers thick.

The light induced the nanospheres to break up into silver atoms, which then fed the growth of the nanoprisms, a process called ripening. The conversion process could be arrested at any point by stopping the light exposure, thus giving the researchers control over the shape and size of the nanoparticles.

"When you go from a sphere to a prism you get new properties -- new optical properties, electrical properties, chemical properties and catalytic properties," said Mirkin, also George B. Rathmann Professor of Chemistry. "It's the same material, but a different shape and size. And that makes all the difference."

In addition to biological diagnostics, development of the nanoprism in a wide variety of materials holds promise for electronics, materials, optics and catalysis.
Other authors on the paper are Rongchao Jin (lead author), YunWei Cao, K. L. Kelly and Gand J. G. Zheng, all of Northwestern. The research was supported by the National Science Foundation, Army Research Office and the Air Force Office of Scientific Research.

Northwestern University

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