TNT-sensing plastic exposes land mines.

Placed in the heart of a lightweight, portable device, a new polymer that sniffs out trinitrotoluene, or TNT, could help in the search for millions of unexploded land mines buried around the world.

Designed by Timothy M. Swager and his colleagues at the Massachusetts Institute of Technology, the polymer can detect TNT concentrations of less than a few parts per billion, making it one of the most sensitive materials of its kind. The polymer glows when exposed to light and indicates the presence of TNT through a decrease in this fluorescence.

An estimated 100 million land mines lie buried in fields, forests, and villages worldwide, presenting a deadly hazard for those who live and work in those areas (SN: 3/28/98, p. 202). Most land mines use TNT as their explosive and leak small amounts into the soil and the air above.

A prototype device can detect these TNT traces. It shines a light onto the sensitive polymer and indicates on a display when TNT molecules, diffusing into the polymer, make its fluorescence dim.

The polymer is extremely sensitive because it soaks up TNT vapor "like a sponge" and its intrinsic electronic properties amplify the signal that TNT triggers, Swager says.

The material absorbs photons of light, which produce bundles of energy, called excitons, that travel throughout the polymer. In the absence of TNT, the excitons eventually release their energy and emit photons, leading to the observed fluorescence. TNT molecules deactivate the excitons, releasing the excess energy as heat instead of fluorescence. Just a few TNT molecules can deactivate many excitons, so small concentrations of TNT cause a large decrease in the intensity of emitted light.

Swager and his coworker Jye-Shane Yang reported their latest findings on Nov. 11 in the online version of the Journal of the American Chemical Society.

David R. Walt of Tufts University in Medford, Mass., has been developing sensors for finding old mines and has tested the polymer. "It's better than any other materials we've examined for [TNT-like] compounds," he says.

The prototype device, made by Nomadics of Stillwater, Okla., "looks like an oversized TV remote," Swager says.

Preliminary tests in a simulated mine-field showed that the device and polymer both need improvement before they can be used in a real de-mining situation, he adds. The researchers missed some mines in their test, and an overlooked mine can be disastrous. "You cannot miss," Swager says. "It's OK to have false positives, but you cannot miss."