Triacetone triperoxide (TATP) is an explosive that's been used in past shoe-bombing attempts. TATP is a difficult chemical to detect, but researchers have developed a colorimetric sensor array that can expose minute amounts of TATP vapor in the air.
The array, which was developed by Kenneth Suslick and Hengwei Lin of the University of Illinois, consists of 16 tiny pigmented dots that change color based on the amount of TATP vapor present. Each pigment changes colors depending on the concentration of TATP in the air, and the array is then imaged using a scanner or camera. Previous TATP detection strategies required expensive equipment or large amounts of TATP in the vicinity, but this array can detect levels of 2 parts per billion. From The University of Illinois:
"Imagine a polka-dotted postage stamp sensor that can sniff out the shoe-bomber explosive simply by using a digital camera to measure the changing colors of the sensor's spots," Suslick said. "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."
The array is uniquely sensitive to TATP. Unlike many other chemical sensors, Suslick and Lin's array is unaffected by changes in humidity or exposure to other chemicals, such as personal hygiene products or laundry detergents. It also has a long shelf life, so airport security and other users can keep a supply on hand.
In addition to demonstrating their sensing technique with an ordinary flatbed scanner, the researchers also developed a functional prototype hand-held device. The portable instrument, designed to easily screen luggage or shoes, uses inexpensive white LED illumination and an ordinary digital camera similar to a cell-phone camera.