Silicon Wires Sniff Out Bombs

Silicon polymer nanowires may cheaply and effectively detect traces of TNT and picric acid in both water and air, researchers say. The tiny wires could thus be used to help locate explosives in terrorist bombs and land mines.

Nearly 2,000 times thinner than a human hair, the nanowire consists of a long string of silicon atoms surrounded by organic molecules. The wire normally conducts electricity and glows under ultraviolet light. But by tweaking the polymer, chemists at the University of California at San Diego were able to make it cease conducting electricity and glowing whenever it came into contact with TNT or picric acid. "The chief advantage of this polymer is that it's stable in air and water, as well as extremely sensitive to explosive residues," team member William Trogler says. "With relatively crude engineering, we were able to detect the presence of TNT down to about one part in a billion in air and some 50 parts per billion in seawater."

To demonstrate the technology, the researchers put 0.1 gram of TNT on a latex glove. They then wiped off the glove and made a handprint on a piece of paper containing the glowing polymer. A similar hand print without the TNT was placed next to it as a control. Whereas the TNT handprint showed up as a darkened silhouette, the non-TNT handprint had no visible effect (see image). "The TNT turns off the green luminescence of the polymer or, in chemical terms, quenches the excited state," Trogler explains.

While this may be a handy application for airport security, for example, the researchers see other applications as well. "These polymers, or nanowires, can be dissolved in solvents and painted on surfaces just as you would spray-paint a house," Michael Sailor notes. When sprayed on filters, they could detect TNT in the air or in the water. "There are millions of unexploded land mines from past wars all over the world," Sailor remarks. "And people are very interested in getting rid of them for humanitarian purposes."