In the trenchesBetter explosives detection for soldiers, first responders in the field
From a chemical viewpoint, developing a detector for nitroaromatic compounds such as TNT is difficult because such compounds have a low vapor pressure, meaning their concentration in air at room temperature is around six parts per billion; MIT researchers develop cantilever sensors which use functional coatings to transduce detection of chemicals into a signal; the coating, usually a polymer, swells up when it reacts with the target analyte and deflects the cantilever
Explosives detection is a field that has long been the purview of the animal kingdom: most people are familiar with the idea of sniffer dogs (though perhaps less familiar with the idea of sniffer bees; also see “Bee Alert Technology Offers New Explosives Detection System,” 9 March 2007 HSNW). Lisa Wylie writes in Materials Views that more recently, technological substitutes have started to come into use, and techniques such as gas chromatography, ion mobility spectrometry, and X-ray are now standard equipment in airport security suites.
For explosives detection in the field, however, such techniques are not really practical. Land-mine cleanup (an issue affecting around seventy-six countries worldwide) needs a reliable, portable detection method, as does the clearing of improvised explosive devices (IEDs) in military conflicts and counterterror operations. This is easier said than done, however. Wylie notes that from a chemical viewpoint, developing a detector for nitroaromatic compounds such as TNT is difficult because such compounds have a low vapor pressure, meaning their concentration in air at room temperature is around six parts per billion (and lower than that when the material is encapsulated in a device). “Technologically speaking, the machines in use at airports are not really scalable,” she writes, “and although progress has been made in producing small, incredibly sensitive microcantilever-based systems, the need for high-precision positioning of components and a laser source (and subsequent large power source) for optical detection limits their potential for fieldwork.”
Microcantilever systems for nitroaromatic detection have nonetheless presented an interesting angle for Professor Karen Gleason and her team at MIT’s Institute of Soldier Nanotechnologies to pursue. Cantilever sensors use functional coatings to transduce detection of chemicals into a signal. The coating, usually a polymer, swells up when it reacts with the target analyte and deflects the cantilever. What the MIT team have done is combine this polymer coating (poly[4-vinylpyridine]) with a vapor deposition technique and standard nanofabrication processes, permitting miniaturization of the sensor system.
Here is how it works: nanoscale trenches of about 600nm width are etched into a silicon wafer using microfabrication techniques. The polymer coating is then deposited using initiated CVD (iCVD), a low-pressure CVD process pioneered by the Gleason lab, where the monomer and initiator are vaporized by passing them over hot filaments in the reactor. In so doing, the initiator breaks down into free radicals and starts the polymerization process. iCVD allows fine control of deposition thickness and conformality, meaning the team could coat the inside of the trenches completely and evenly with the detector polymer, in a layer around 120 nm thick. Then, a thin film of gold was sputtered over the top edge of the trench and connected up to a power source to form an open circuit. When exposed to a nitroaromatic (nitrobenzene in the initial tests), the polymer coating swells, expanding outwards to fill the trench. The gold films on either side are brought into contact, closing the circuit and generating a detectable signal.
Modeling based on the initial device performance suggests that on optimized set-up (thicker coatings, narrower trenches, orientation of the coating) might be able to detect as little as 0.95 parts per billion nitroaromatics concentration.
This elegant device setup has real potential for developing reliable, practical explosives detection systems for the field. As Professor Gleason explains, “Miniaturization enabled by standard fabrication combined with CVD polymerization results in resistive-base sensors of extremely small footprint and low unit cost.” Not only can the sensing elements themselves be miniaturized, making small devices and mass-production feasible, the system only draws on its power source when it actually senses its target, so the need for a large power supply is also greatly reduced. And it’s all been brought about by the world’s smallest trench coat.
