Detecting plastic explosives in air

At the parts-per-trillion level has been achieved. Explosive compounds such as PETN and RDX are easy to mold, remain stable until detonated, and can inflict significant damage even in small amounts. Now, researchers at Oak Ridge National Laboratory and the University of Tennessee, Knoxville, have reported using commercially available atomic force microscope cantilevers for detecting PETN and RDX with great sensitivity. One surface of the cantilever was coated with a monolayer of 4-mercapto-benzoic acid, which can bind to both PETN and RDX. As the binding occurs, the cantilever bends significantly due to differential stress. The researchers estimate that a sensor based on their technique could detect the explosives at a level of 14 parts per trillion after only 20 seconds of operation. Such a sensor is also potentially cheap and easy to mass-produce. (L. A. Pinnaduwage et al, Appl. Phys. Lett. 83, 1471, 2003  https://doi.org/10.1063/1.1602156 .)