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Image: A handprint containing trace amounts of TNT—produced by placing a tenth of a gram of TNT on a latex glove, then wiping the glove clean—shows up as a darkened silhouette of the hand. A similar handprint without the TNT has no effect on the flat, greenish glowing paper.
The chief goal of the research was to develop new selective solid state sensors for explosives based on redox or energy transfer quenching of the luminescence from polysilole "nanowires." Highly luminescent polysiloles that are chain-terminated with anion binding regions are also being explored as chromate and arsenate sensors. Chemical modification to vary the redox potential of the polysilole excited state is used as a way to impart chemical selectivity. Published in Sohn, H.; Calhoun, R. M.; Sailor, M. J.; Trogler, W. C. Angew. Chem. Int. Ed. 2001, 40, 2104-5.

FUNDING: This work was originally supported through DARPA's Tactical Sensors Program via a Space and Naval Warfare Systems Center Contract (N66001-98-C-8514). The technical point of contact for this DARPA program is Dr. Edward Carapezza.

Technology: A silicon polymer "nanowire," some 2,000 times smaller than the diameter of a human hair, is synthesized via a Wurtz coupling reaction. The polymer is essentially a long string of silicon atoms surrounded by phenyl rings that conducts electricity and glows under ultraviolet light due to a property known as photoluminescence. We found that luminescence from the polymer is "shorted out" whenever it comes into contact with molecules of TNT or picric acid. That happens chemically because the TNT and picric acid—electron-deficient molecules—accept electrons from the excited silicon polymer. We found that this material is capable of detecting trace amounts of TNT and picric acid, an explosive commonly used in terrorist bombs. The polysilole nanowires may provide a sensitive new tool to combat terrorist attacks and locate unexploded mines and bombs on land as well as in the ocean. The chief advantage of this polymer is that it’s stable in air and water. With relatively crude engineering, we were able to detect the presence of TNT down to one part in a billion in air and a few parts in a billion in seawater. The TNT turns off the green luminescence of the polymer or, in chemical terms, quenches the excited state. In addition to detecting residues of explosives on hands or clothing, we see numerous potential applications for their development for anyone needing an inexpensive, highly sensitive explosives detector. The polymers can be dissolved in solvents and painted on surfaces just as you would spray paint a house. When the polymers are sprayed on pieces of paper they can easily detect trace amounts of explosives.
Image: Chemical model of the silole oligomer showing the Si atoms in purple, carbon atoms in black, and hydrogen atoms in blue.
Image: Closeup view of the thumbprint from the ticket on the right.
Image (above): Polysilole nanowires used to image a TNT-contaminated thumbprint on a transit ticket from the San Francisco BART line. The ticket on the left is the control. Both tickets were treated with the nanowires to develop the latent image of the thumbprint after it had been held in a contaminated hand.

(1) Detection of Nitroaromatic Explosives Based on Photoluminescent Polymers Containing Metalloles. Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc. 2003, 125, 3821-3830.

(2) Detection of TNT and Picric Acid on Surfaces and in Seawater Using Photoluminescent Polysiloles, Sohn, H.; Calhoun, R. M.; Sailor, M. J.; Trogler, W. C., Angew. Chem. Int. Ed., 2001, 40, 2104-5.

(3) Sailor, M. J.; Trogler, W. C.; Létant, S. E.; Sohn, H.; Content, S.; Schmedake, T. A.; Gao, J.; Zmolek, P. B.; Link, J. R.; Fainman, Y.; Xu, F.; Shames, P. Low-Power Microsenors for Explosives and Nerve Warfare Agents Using Silicon Nanodots and Nanowires, SPIE Meeting on Unattended Ground Sensor Technologies and Applications, Orlando, FL, 2001.