Disposable sensor detects “shoe bomb” explosive

Margaret W. Bushee, margaret.bushee@photonics.com

A new disposable postage stamp-size colorimetric sensor array can detect a high explosive in its vapor phase, even at very low concentrations. The scientists at the University of Illinois at Urbana-Champaign who created it also have designed a prototype handheld scanner that can read the results.

The explosive, triacetone triperoxide, or TATP, was made infamous by al-Qaida member Richard Reid, better known as the “shoe bomber.” On Dec. 22, 2001, while airborne on American Airlines Flight 63 from Paris to Miami, he tried to detonate the explosives hidden in his shoes.

TATP vapor concentrations are indicated by the patterns and shades of color on these postage stamp-size disposable sensors. Images courtesy of the University of Illinois at Urbana-Champaign.

First synthesized in 1895, TATP has been used in at least three major terrorist acts over the past decade. Not only are its chemical ingredients readily available, but also it is difficult to detect. It does not fluoresce or absorb UV light and, unlike other nitro explosives, is not traceable by ion mobility spectrometry. Although there are commercial methods that can identify TATP, they cannot detect it in its vapor form and are expensive and nonportable.

The new sensor is a by-product of the researchers’ long-term goal of creating a personal safety device for personnel who are exposed to volatile chemicals in the workplace. To that end, they have been developing sensors comprising arrays of tiny dots, each of which is a unique chemical substance that changes color when oxidized – similar to litmus paper, which changes color in an acid or base.

“Our goal is to create the chemist’s equivalent of the physicist’s radiation badge,” explained Kenneth S. Suslick, the university’s Marvin T. Schmidt professor of chemistry. A radiation badge, worn at critical chest height, detects a worker’s exposure level.

“We turned our attention to TATP because we realized that there was no current technology capable of rapid and sensitive detection of peroxide explosives and because we believed that our colorimetric approach could be easily extended to this analyte,” Suslick said.

Enabling the breakthrough was the discovery that the solid-acid catalyst Amberlyst-15 converts the relatively unreactive vapor form of TATP to the much more easily detected hydrogen peroxide, plus acetone – acids that register readily on a colorimetric sensor.

With the benefit of this advance, the researchers produced a 4 x 4 array of chemically responsive dot-size colorants on a sensor measuring approximately 1 x 1 cm. With an ordinary flatbed scanner, images were made of the dots before and after exposure to various concentrations of TATP vapor.

Each concentration level, they determined, has its own distinct pattern. “The pattern of the color change is a unique molecular fingerprint for TATP at any given concentration, and we can identify it in a matter of seconds,” Suslick said.

By measuring the red, green and blue values of each dot, they created color difference maps that enabled them to detect TATP vapor concentration levels lower than 2 parts per billion. Data was verified with in-line analysis in real time using a Fourier transform IR multigas analyzer.

As for the handheld scanner developed by the team, it consists of a CMOS camera – similar to what is found in a cell phone – illuminated by a white LED. Despite its simplicity, it has a signal-to-noise ratio that is three times better than that of the flatbed scanner used in the study.

Potential applications of this handheld scanner include airport security and biomedicine.

The handheld scanner is being commercialized by iSense, a company co-founded by Suslick. The university has granted iSense an exclusive license to commercialize the technology, which is still in the R&D phase. Although airport security is one potential application, the company’s main focus is on biomedical uses, such as rapid identification of bacteria.

The study, titled “A Colorimetric Sensor Array for Detection of Triacetone Triperoxide Vapor,” was published in the Nov. 10, 2010, issue of the Journal of the American Chemical Society. Its co-author was postdoctoral researcher Hengwei Lin.