Laser sniffs explosives

Improving sensors means finding ways to detect smaller concentrations of target chemicals.

Researchers from the Massachusetts Institute of Technology have built a device that detects when molecules of the explosives TNT and DNT stick to a thin film of polymer, or plastic. Their prototype sensor shows that it is possible to detect vapors emitted by TNT at concentrations of five parts per billion and DNT at 100 parts per billion.

The method could eventually be used to make ultrasensitive biosensors, breath sensors and gas sensors, according to the researchers.

The sensor is similar to traditional fluorescence sensors, which indicate the presence of target molecules by dimming when the molecules disrupt the florescent material's electrical properties. Instead of functioning in a spontaneous emission mode -- emitting photons one at a time -- the researchers' device works as a laser.

Lasers emit intense streams of photons at a single wavelength after the atoms of the laser's active material are excited en mass. This makes the device more sensitive because the material's lasing state is readily disrupted by the target molecules. The material is 30 times more sensitive at detecting the molecules through lasing than through spontaneous emission, according to the researchers.

The method could be used to increase the sensitivity of explosives detectors in two to five years, according to the researchers. The work appeared in the April 14, 2005 issue of Nature (Sensitivity Gains in Chemosensing by Lasing Action in organic Polymers).