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Research team developing electronic nose to detect explosives

by Nan Cooper - October 27, 2008

A team of researchers headed by a UConn engineering professor is working to develop an electronic nose system to detect explosives.

With a three-year grant of almost $800,000 from the National Science Foundation, a team headed by Yu Lei, an assistant professor of chemical, materials, and biomolecular engineering, hopes to develop real-time arrays of ultra-sensitive sensors that can sniff out even trace quantities of explosives.

The team includes UConn colleagues Christian Brückner, an associate professor of chemistry, and Ali Gokirmak, an assistant professor of electrical and computer engineering, and University of California-Riverside professor Yushan Yan.

UConn’s Krishna Pattipati, a professor of electrical and computer engineering, will assist the team with aspects of the research that involve pattern recognition.

The team is focusing on developing the science behind a miniaturized sensing device capable of detecting potential explosives with greater speed, selectivity, and accuracy than ever before, using simple instrumentation.

Lei and his colleagues hope this will lay the groundwork for a hand-held unit that could be used by officials to inspect the luggage of passengers boarding a plane, for example, or mounted on a small robotic vehicle to ‘sniff out’ land mines.

The team envisions a unit that will combine the ability to capture and concentrate airborne explosive molecules, and the real-time capacity to distinguish and identify compounds commonly found in explosives.

A real-time system requires that the operation be performed not only correctly but also within a specified time.

Lei says the complexity of the animal scent detection/recognition “system” illustrates the challenge of designing an electronic sensor.

Animal noses – including those of humans – detect and process scents with an array of sensors that respond to all gaseous components of a scent, although each sensor type responds to a differing degree. The brain collects the output from these sensors and memorizes the pattern of responses for roses, apples, or carrion, for example.

The next time the animal brain detects this pattern, the animal recognizes the scent as corresponding to a rose or an apple – even if the apple aroma comes mingled with all the other scents of a supermarket or the apple is of a type that’s different from those smelled previously.

Many bombs use nitrated compounds – such as TNT or dynamite – with volatile components that emit scent molecules which can be detected by trained animals, such as dogs or rats.

From left, researchers Ali Gokirmak, Christian Brückner, and Yu Lei in their lab.
From left, researchers Ali Gokirmak, Christian Brückner, and Yu Lei in their lab. Photo by Christopher LaRosa

An electronic sensor for detecting explosives would likely be used in an open space, such as a luggage handling room, where volatile explosive vapors are found at such low concentrations – in the range of parts per billion or even parts per trillion – that detection is difficult.

To overcome this problem, the unit being developed 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). The small molecules of nitrogen and oxygen found in air can pass easily through these pores, but larger explosives molecules cannot. The unit will concentrate the explosives molecules by many orders of magnitude within a short period of time.

Having concentrated the molecules on the membrane’s surface, the unit’s next task is detection. The device will incorporate an array of single-walled carbon nanotube-porphyrin conjugates as sensors. Planted onto microelectronic circuitry, these are capable of signaling the presence of explosives (and many other volatile compounds) by a change in their conductivity.

Using a variety of different porphyrins – large organic molecules that are particularly suited to interacting with nitroaromatic compounds – different sensor elements will respond differently to particular explosives vapors.

This will generate 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 will trigger an alarm, alerting the user to the presence of explosives vapors.

Preliminary proof-of-concept data have been encouraging. The team will now focus on building a solid-state 32-sensor array to generate the signature for common explosives such as TNT. They will expand the recognition capacity of the device to include other explosives over time. The sensor device will then be combined with the molecular sieving membrane to complete the unit.

The researchers hope the project will help the nation attain a greater level of security in a variety of venues, including airports, bus terminals, and post offices.

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