Detecting explosives on a nanowire

Chemists at the University of California have developed a silicon polymer nanowire that is capable of detecting trace amounts of TNT and picric acid.

Chemists at the University of California, San Diego have developed a silicon polymer nanowire that is capable of detecting trace amounts of TNT and picric acid, an explosive commonly used in terrorist bombs.

The achievement is said to provide a sensitive new tool to combat terrorist attacks and locate unexploded mines and bombs on land as well as at sea.

‘The chief advantage of this polymer is that it’s stable in air and water, as well as extremely sensitive to explosive residues,’ said 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.

The polymer is a long string of silicon atoms surrounded by organic molecules that conduct electricity and glow under ultraviolet light.

The UCSD chemists modified the polymer to behave like a short-circuited electrical circuit whenever it comes into contact with molecules of TNT or picric acid.

This occurs because the TNT and picric acid take electrons from the silicon polymer whenever it is excited by ultraviolet light. This is said to prevent 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 shows up as a darkened silhouette of the hand. A similar handprint without the TNT remains a flat, greenish glowing slate.

‘The TNT turns off the green luminescence of the polymer or, in chemical terms, quenches the excited state,’ said Trogler.

In addition to detecting residues of explosives on hands or clothing, the UCSD scientists foresee numerous applications for their development for those 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,’ said Sailor. When the polymers are sprayed on pieces of filter paper they can easily detect trace amounts of TNT in air or water containing trace amounts of TNT or picric acid, he added.

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 vapour 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.