Sniffing Polymers: A Soldier’s Best Friend?

At the beginning of september, MIT chemist Timothy Swager and several of his students were set to travel to the United States Army’s Fort Leonard Wood in Missouri to try their skill at detecting land mines. It’s only a preliminary test and the mines were deactivated. But the scientists were armed with a portable device that incorporates a novel conducting polymer-and a clever molecular detection scheme-that could trigger a breakthrough in chemical sensing.

Land mines continue to be a menacing presence around the world, and methods for quickly and safely spotting them remain woefully inadequate. Indeed, dogs are still one of a soldier’s most reliable friends in sniffing out the deadly devices. Metal detectors often turn up false alarms. More sophisticated and expensive analytical instruments tend to be bulky and can have trouble picking up the “smell” of land mines.

What the U.S. military “really wants is the equivalent of the tricorder they have in Star Trek,” says Swager. Short of that, he says that the Defense Advanced Research Projects Agency, which is helping to fund his work, would like a very sensitive detector that is easy to use. “Something basically like a TV remote,” says Swager.

And that’s exactly what Swager is hoping to make, using conducting polymers to do the sensing. The MIT chemist has synthesized polymers that are, in essence, molecular wires with receptors strung along the polymer backbone. The polymers fluoresce, but when the right molecule, say, nitrogen-containing TNT (2,4,6-trinitrotoluene), binds with one of the sites, it reduces the fluorescence, indicating the compound’s presence.
But here’s the trick. A single molecule binding to one of many sites in a polymer might be expected to produce only a weak signal. In the conducting polymers, however, energy races through the macromolecules prior to fluorescence; a molecule that binds with even one of the sites deactivates this energy flow, greatly increasing the reduction in fluorescence and amplifying the signal.

Swager has made thin films of a fluorescent conducting polymer that are stable and capable of detecting a whiff of TNT at levels below one part per billion. Swager’s ambitions, however, go far beyond TNT; the group is modifying the polymers with different types of sites capable of detecting other molecules, as well as increasing the sensitivity by switching to an electrical signal, rather than fluorescence. Targeted applications include portable, real-time sensors for biological and medical applications.

That will take time. For now, Swager is intent on mimicking a canine’s talents. “A dog smelling a buried land mine twenty yards away will trot and stop on a dime 2 feet away from it,” points out Swager. A man-made device with that kind of sensitivity is still a ways off, he acknowledges. “It’s a big challenge in any vapor detection to do it as well as a dog can.”