Nano noses


By Mark Schrope in San Francisco AFTER nano-fountain pens and nano-tweezers comes a new, more down-to-earth application for the wunderkind of chemistry, the carbon nanotube. Researchers in California have discovered that the cylindrical versions of buckyballs can be used to detect the toxic gases nitrogen dioxide and ammonia. The scientists hope that their discovery will lead to a new generation of tiny environmental sensors. Both nitrogen dioxide and ammonia contribute to greenhouse warming and acid rain, so levels in the atmosphere must be constantly monitored. Engineers also need to measure concentrations of these gases at, for instance, coal-burning plants to check the efficiency of scrubbing systems that remove pollutants. Existing measurement techniques are plagued by high cost, a lack of portability or high-temperature requirements. The prototype nanotube sensors, which have been developed by a team of researchers at Stanford University led by Hongjie Dai, may solve those problems. The team has found that the flow of electric current through carbon nanotubes increases when they are exposed to nitrogen dioxide, and decreases in response to ammonia. The researchers are not yet sure what triggers this change, but Dai says a likely explanation is that the gas molecules are donating or removing electrons, thus changing the resistance of the nanotubes. The team’s prototype sensors, which the researchers hope to describe in a forthcoming issue of Science, consist of nanotubes with metallic leads connected at either end. Unlike current sensors, they operate at room temperature and are cheap to produce. They are also tiny enough—just 3 micrometres long—to be used in a “lab-on-a-chip”, a microchip-based tool for performing chemical analyses. “That would be extremely handy,” says Iris Anderson, a chemist at the Virginia Institute of Marine Science in Gloucester Point. “Ammonia is a real pain to measure, and it’s a growing problem.” And William Linak, a chemical engineer with the US’s Environmental Protection Agency in North Carolina, says that nitrogen dioxide measurements are valuable but difficult to make. “A new system would be wonderful.” Dai says his team has just filed patents on the work and is looking for a commercial partner. The prototype sensors have some drawbacks, such as a slow recovery time—it takes about 12 hours after exposure to a sample before they can be reused. But the team is already exploring ways to alter the nanotubes physically and chemically to make them respond faster. In addition, Dai says his group is making alterations to the nanotubes with other applications in mind. They have already had some success in detecting carbon monoxide,
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