Detecting Plastic Explosives in Air

Detecting plastic explosives in air at the parts-per-trillion level has been achieved by researchers at Oak Ridge National Laboratory and the University of Tennessee (Thomas Thundat, 865-574-6201), potentially leading to a fast, portable, and ultrasensitive plastic-bomb "sniffer." Plastic explosives such as pentaerythritol tetranitrate (PETN) and hexahydro-1,3,5-triazine (RDX) pose serious threats because (1) they are easily to mold into desired shapes, (2) they remain highly stable until detonated, and (3) they can inflict significant damage even in small amounts. In fact, the infamous shoe bomber had PETN in his footwear. Most current plastic-bomb sensors are bulky and expensive. In contrast, the new sensor is a microelectromechanical system (MEMS), or a tiny mechanical device with microscopic dimensions. Potentially cheap and easy to mass-produce, the bomb-sniffing MEMS device is a microcantilever, a 180-by-25-micron slab of silicon attached to a spring-loaded wire. Similar in structure to a diving board attached to a pool, the microcantilever is coated on one side with gold. On one end of the gold-coated surface is a single layer of 4-MBA (4-mercaptobenzoic acid), to which PETN and RDX both attach. Like hair that curls up on a humid day as water molecules adsorb to it, this specially coated cantilever will bend by significant amounts when PETN and RDX molecules attach to it. A laser-microscope system can detect the degree of bending to nanometer precision. Placed in a vacuum-tight glass cell, the cantilever was exposed to a stream of ambient air with tiny traces of plastic explosive. Using a modified atomic force microscope to measure the deflections of the cantilever, the researchers determined that their MEMS device could detect the explosives at a level of 14 parts per trillion, after only 20 seconds of operation. By another measure, the device becomes sensitive to plastic explosives even if only a few femtograms (1 fg=10^-15 g) impinges upon it. A future step is to take the device out of the laboratory and develop it into a portable sensor. While much activity has centered on the development of sensors for detecting vapors from all kinds of explosives, this is, to the authors' knowledge, only the third device of its kind that uses MEMs. (Pinnaduwage et al., Applied Physics Letters, 18 August 2003)