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First Carbon-Based Quantum Dots Created

Photonics.com
May 2006
CLEMSON, N.C., May 26, 2006 -- A new type of quantum dot, the first to be made from carbon, is showing promise for a broad range of applications, including biosensors, medical imaging devices and tiny LEDs, and could be more environmentally friendly than its metal-based counterpart, the dots' creators said.

The nano-sized (billionths of a meter) "carbon dots" glow brightly when exposed to light, said the chemists at Clemson University who developed them. The carbon dots show less potential for toxicity and environmental harm and have the potential to be less expensive than metal-based quantum dots, they said. Cheap disposable sensors that can detect hidden explosives and biological warfare agents such as anthrax also are among the possibilities envisioned by the researchers.

“Carbon is hardly considered to be a semiconductor, so luminescent carbon nanoparticles are very interesting both fundamentally and practically,” said study leader and chemist Ya-Ping Sun. “It represents a new platform for the development of luminescent nanomaterials for a wide range of applications.”

Quantum dots have generated much interest in recent years, especially for potential applications in biology and medicine. These tiny particles -- thousands of times smaller than the width of a human hair -- have been developed from compounds composed of lead, cadmium and, more recently, silicon. But these materials have raised concerns over potential toxicity and environmental harm. As a result, scientists have begun to look for more benign compounds for making quantum dots, Sun said.

Researchers have known for some time that carbon nanoparticles, due partly to their enormous surface area, have unusual chemical and physical properties quite different from their bulk form. Using nanoparticles produced from graphite, Sun and his associates demonstrated that when these carbon nanoparticles are covered with special polymers, they glow brightly when exposed to light, behaving as tiny light bulbs. The dots glow continuously as long as a light source is present, they said.

The scientists said they believe that this photoluminescence may be due to the presence of “pockets” or holes on the surface of the carbon dots that trap energy. The polymer coating acts as a “molecular band-aid,” enabling light emission from the inside of the polymer casing, they said. The current thinking among scientists, they said, is that metal-based quantum dots emit light by a somewhat different mechanism.

The two-sided polymer coating allows researchers to attach antibodies or other labeling materials to the carbon dot, said Sun. This could lead to improved dyes for medical imaging and also the development of sensors that light up in the presence of a target, such as anthrax or even food-borne pathogens. In lab studies, the researchers successfully labeled anthrax-like spores with luminescent carbon dots, resulting in glowing spores that were easily viewed under a microscope.

The research, funded by Clemson and the federal government, will appear in the June 7 edition of the Journal of the American Chemical Society. For more information, visit: www.clemson.edu



GLOSSARY
polymer
A material whose molecular structure consists of long chains made up by the repetition of many (usually thousands) of similar groups of atoms.
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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