UCSD Chemists Develop Tiny Silicon Wires To Detect Trace Residues Of Explosives

The achievement, detailed in the June 1 issue of the German chemistry journal Angewandte Chemie, provides 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, as well as extremely sensitive to explosive residues," says William C. Trogler, a professor of chemistry and biochemistry at UCSD. "With relatively crude engineering, we were able to detect the presence of TNT down to about one part in a billion in air and some 50 parts per billion in seawater."

Honglae Sohn, a postdoctoral student in Trogler’s laboratory, produced the silicon polymer and discovered its explosive detection potential with the assistance of Trogler; Michael J. Sailor, a professor of chemistry and biochemistry; and Rebecca M. Calhoun, a graduate student in Sailor’s laboratory.

The polymer is essentially a long string of silicon atoms surrounded by organic molecules that conducts electricity and glows under ultraviolet light, much like the heated filament in a light bulb, due to a property known as photoluminescence. The UCSD chemists modified the polymer to behave like a "shorted out" electrical circuit whenever it comes into contact with molecules of TNT or picric acid. That happens chemically because the TNT and picric acid—electron-deficient molecules—grab electrons from the silicon polymer whenever it is excited by ultraviolet light. This prevents the silicon nanowires from glowing.

A piece of paper containing the polymer, which normally glows under a black light, demonstrates the effect visually. 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 remains a flat, greenish glowing slate (see photographs above).

"The TNT turns off the green luminescence of the polymer or, in chemical terms, quenches the excited state," says Trogler.

In addition to detecting residues of explosives on hands or clothing, the UCSD scientists see numerous potential applications for their development for anyone needing an inexpensive, highly sensitive explosives detector.

"These polymers or nanowires can be dissolved in solvents and painted on surfaces just as you would spray paint a house," says Sailor. When the polymers are sprayed on pieces of filter paper, he adds, they can easily detect trace amounts of TNT in air or water containing trace amounts of TNT or picric acid.

The UCSD scientists were able to demonstrate this experimentally in the laboratory by spiking seawater with trace amounts of TNT and picric acid, then passing the liquid through thin films of the polymer. Their films were able to detect TNT in seawater at a level of 50 parts per billion and picric acid at a level of 6 parts per billion. The scientists were also able to detect TNT vapor in air with thin films of the polymer down to a level of 4 parts per billion.

Using similar methods, Sailor believes films of polymers could be used to reveal the presence of land mines, unexploded bombs in shallow reefs and explosive residues passing through airport screening systems.

"There are millions of unexploded land mines from past wars all over the world and people are very interested in getting rid of them for humanitarian purposes," he says. "You might imagine using these silicon nanowire detectors to look for TNT leaking out of land mines."

Similarly, the polymers might be used to locate TNT leaking from old, unexploded bombs lying in seawater. Such bombs now litter the reefs surrounding Kahoolawe—the smallest of the eight major Hawaiian Islands, located six miles southwest of Maui—which the U.S. Navy used as a target range for 50 years since World War II and is being turned into a state reserve.

"If you could take these TNT indicators, you could find the bays and inlets where some of the bombs are located before they explode and hurt someone," says Sailor.

The UCSD development was supported by grants from the National Science Foundation and the Defense Advanced Research Projects Agency, or DARPA.