Wanted: A Bomb Detector as Sensitive as a Dog's Nose

If a suicide bomber lurks in the public with an explosive device, bomb-sniffing dogs can often detect the explosive chemicals from the tiniest whiff-these canine superheroes can sense the presence of the explosive triacetone triperoxide (TATP) if just a few molecules are present, on the scale of parts per trillion.

Researchers at the University of Rhode Island are striving to make a comparable device for detecting TATP in its vapor form. Their new detection system, which pairs a conductance sensor with a traditional thermodynamic sensor, confirms the presence of TATP at the level of parts per billion (ppb). Their work is described in a study published on 2 October in IEEE Sensors Letters.

TATP is a common choice of ingredient for terrorists who make explosives. It was used in the 2015 Paris attacks, the 2016 Brussels airport bombings, the 2017 concert bombing in Manchester, and most recently the 2018 bombings in Surabaya, Indonesia.

"All of the IEDs used in these attacks relied on TATP as the detonator and often times the energetic material itself," explains Otto Gregory, a researcher involved in the study. But, he notes, "No electronic trace detection system currently exists that is capable of continuously monitoring TATP or its precursors that can compete with a dog's nose."

His team, which has received funding for TATP sensors from the U.S. Department of Homeland Security for the last 10 years, hopes to change that. The researchers first developed a thermodynamic sensor that detects TATP at 78 ppb, and can detect 2,4-DNT (a decomposition product of the explosive substance TNT) at 2 ppb.

Images: University of Rhode Island/IEEE A photograph (left) and top view (middle) of an orthogonal sensor. A cutaway view (right) shows the various layers that comprise the sensor, from top: a metal oxide (catalyst) layer, nickel interdigitated electrodes, alumina coatings, nickel microheater, and alumina substrate.

The thermodynamic sensor uses two microheaters: one coated with a metal oxide catalyst, and one without a catalyst coating. When explosive substances like TATP or 2,4-DNT come in contact with the microheaters, the device analyzes the thermodynamic difference between the catalyst and non-catalyst reactions-which reveals the nature of the substance.

To make the device more accurate at confirming the presence of these explosives, the researchers added a conductance sensor. The conductance sensor analyzes the same catalyst reactions as the thermodynamic sensor, but instead measures the resistivity changes that occur.

"Combining a thermodynamic platform with a conductometric platform provides a built-in redundancy that can mitigate false positives and negatives. Both platforms operate as independent systems looking for a unique 'fingerprint' or response to same explosive molecule," explains Peter Ricci, a co-author and chemical engineer at the University of Rhode Island.

The researchers tested mircoheaters made from different catalyst metals, including tin oxide, zinc oxide, and copper oxide. In terms of measuring conductance, the copper oxide was particularly sensitive to detecting 2,4-DNT. Copper oxide was more than three times as sensitive as the other materials in measuring the heat effect and 13 times as sensitive for conductance.

Gregory notes that implementing this device in a real-world setting depends on how well they can downsize the system and make it portable. "Our current detection system employs both a thermodynamic and conductometric platform that fits into a small toolbox," he says. "The final version of our device would have a much smaller footprint, in the form of a handheld or even a wearable [device]."

The catalyst coatings are currently layered on ultrathin alumina ceramic substrates, which are responsible for the systems extraordinary sensitivity. But these substrates are still thicker than desired.

"Sensors fabricated on much thinner substrates would drastically reduce operating temperature and would also lower the power requirements of the system, thus enabling a smaller portable detection system. We are working towards that goal everyday," says Ricci.

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