Dogs, pigs – even dolphins – have been trained to sniff out different agents, from illicit drugs and rare truffles to explosives. The exquisite sensitivity of their olfactory glands allows these mammals to distinguish unique aromas associated with the targeted agent. Many bombs use nitrated compounds – such as TNT or dynamite, which contain volatile components – as explosives. These compounds emit scent molecules that may be detected by trained animals. The challenge of developing an electronic nose system for explosive detection lies at the heart of a newly funded project that will involve a multidisciplinary team of primarily UConn researchers.
With nearly $800,000 in funding awarded by the National Science Foundation, over the course of three years, assistant professor Yu Lei and his colleagues hope to develop real-time, ultra-sensitive sensor arrays capable of sniffing out even trace quantities of explosives. Dr. Lei, who joined the Chemical, Materials & Biomolecular Engineering Department in 2006, will lead the investigation. He is joined by University of Connecticut (UConn) colleagues Christian Brückner, an associate professor of Chemistry, and Ali Gokirmak, an assistant professor of Electrical & Computer Engineering, along with University of California – Riverside professor Yushan Yan. UConn’s Krishna Pattipati, professor of Electrical & Computer Engineering, will assist the team on pattern recognition facets of the research.
The team will focus on the development of the science behind a miniaturized sensing device capable of detecting potential explosives with greater speed, selectivity and accuracy than ever before using simple instrumentation. Thus, Dr. Lei and his colleagues plan to lay the groundwork for a hand-held unit that inspectors could use, say, to inspect luggage of passengers boarding a plane. Dr. Lei explained that, as the team envisions it, the unit will combine a number of features: the ability to capture and concentrate airborne explosive molecules, and the real-time capacity to distinguish and identify compounds commonly found in explosives.
To understand the challenge of building an accurate, real-time electronic sensor, it’s helpful to review how animal noses – including our human noses – detect and process scents. Animal noses do not contain one sensor for, say, pineapple scent, another for mint, others for oranges or Chanel No. 5. Rather, animal noses contain an array of sensors that respond to all gaseous components of a scent, though each sensor type responds to a differing degree. The brain then collects the output from these sensors and memorizes the pattern of responses for roses, cinnamon, or carrion, for example. The next time the animal brain detects this pattern, the animal recognizes the scent as corresponding to a rose or apple – even if the apple aroma comes mingled with all the other scents of a supermarket, or even if it is another type of apple than you smelled before. The complexity of the animal scent detection/recognition “system” illustrates the difficulty of designing an electronic sensor.
With the electronic sensor, a security employee might wave the device near a package or piece of luggage. In an open space such as a luggage handling room, volatile explosive vapors are found at such low concentrations – in the range of parts per billion or even parts per trillion – that detection is hampered. To overcome this problem, the unit will employ an ultra-thin molecular sieving membrane that will sample ambient air and concentrate any explosives vapors encountered. Concentration is possible because the membrane’s pores are about half the size of a single nanometer (a typical human hair is about 100,000 nanometers wide), through which small molecules of nitrogen and oxygen found in air pass easily but which are too small for the passage of larger explosive molecules. The unit is, thus, expected to concentrate these explosive molecules by many orders of magnitude within a short period of time.
Having concentrated the molecules on the membrane surface, the unit’s next task is detection. Dr. Lei said the device will incorporate an array of single-walled carbon nanotube (SWNT)-porphyrin conjugates as sensors. Planted onto microelectronic circuitry, these are capable of signaling the presence of explosives (or many other volatile compounds) by a change of their conductivity. Using a variety of different porphyrins, large organic molecules that are particularly suited to interactions with nitroaromatic compounds, different sensor elements will respond differently to particular explosives vapors. This generates a distinct electric response pattern that, properly processed using pattern recognition software, will identify the explosive. Once this electronic nose ‘smells’ an explosive, the software can trigger an alarm, alerting the user to the presence of explosives vapors.
Preliminary proof of concept data have been most encouraging. The team now will focus on building a solid-state 32-sensor array to generate the signature for common explosives such as TNT. They will expand the device’s recognition capacity to include other explosives over time. The sensor device then will be combined with the molecular sieving membrane to complete the unit.
The project is of significance in helping the nation attain a greater level of security in various venues, from airports and bus terminals to post offices.
