Detecting TNT made easy

Agencies handling the security of sensitive installations like airports can now relax. Advances in materials science and chemistry have now brought to them two innovative tools to contain the damage caused by explosives. A group effort of European Union scientists has designed a special bag which could absorb the shock created by an explosion and limit the damage. Elsewhere, Chinese chemists have devised a special material which could make even the tiniest of specks of explosives visible. 

That airport security agencies prevent carrying even innocuous objects like mineral water into aircraft might be annoying for the travellers.

Nevertheless, it is a necessary caution. Given the height that an aircraft travels, even a tiny explosion sufficient to make a small hole in its pressurised body could tear the aircraft apart. Hence, the extra caution.

No wonder, security agencies suffer the perennial fear of an explosive device entering the aircraft right in spite of their hawk-like supervision. The infamous Kanishka tragedy in 1985 that killed all 175 people on board an Air India flight happened because of a bomb planted in its cargo. Such incidents prompted a group of chemists and technologists to devise a novel device that contains even the fearsome of explosions. Called fly-bag, it is an innovative clothing for air cargo which prevents the disastrous consequences of an undetected bomb.

How does a fly-bag work?

Fly-bag is a result of putting together some novel chemistry into play. It uses a cleverly designed ‘shear thickening fluid’ (STF) along with Kevlar, the toughest of the fibres in use today. Kevlar, touted to be stronger than steel, is the fabric of bullet-proof vests and its fibres are very hard to stretch. In other words, Kevlar fibres bend very little even when impacted with the hardest of forces.

The property helps a Kevlar fabric to hold itself together even after a bullet hits at a very close range and hence is very popular among the military. The Fly-bag, according to James Warren of University of Sheffield, UK and one of the members of the international team, is made from a special fabric woven with Kevlar yarn that is coated with STF.

Suspension of solid particles

The STF is a colloid. It is a suspension of hard solid particles in a liquid which under excess stress hardens much like a solution of corn flour which hardens like rock when stamped on it.

The hardening of STF adds to the inherent strength of Kevlar enabling the fly-bag to take the impact of an explosion besides helping to contain the sudden release of gases during explosion. The international team which has kept the composition of the STF a secret, tested the new fabric successfully a few months ago and expects it to be in commercial use in a couple of years from now. The colloid nature makes the bag a light-weight protective shield, appropriate for air cargo, used for luggage to be zipped in. 

While the fly-bag is a preventive measure, the Chinese effort aims at easy detection of tri-nitrotoluene (TNT) residues, the essential component of gelatine explosives. Professor Zhongping Zhang and his team from the Institute of Intelligent Machines, Anhui, China have devised quantum dot crystals that make even the minutest speck of TNT visible. The invention would be of help in “instant on-site identification of trace TNT,” in mail sorting centres, airports and luggage and other situations where an explosive threat is possible.

Quantum dots to trace TNT

It is well known that however carefully handled, explosives such as TNT contaminate clothing and packaging surfaces. Unfortunately, detection of such contaminants is not easy and requires sampling of the clothing or the package and analysis in a distant laboratory with sophisticated instruments. This delay could sometimes be costly. Sniffer dogs may also miss such faint clues. Zhang’s team has come up with the idea of using flickering Quantum dots that attach specifically to TNT molecules. Such dots can be sprinkled on the suspect surface and shone with a torch. Any TNT dust on the suspect surface would show up as shining dots. 

Quantum dots are crystals of special materials with properties that change with their shape or size. For TNT detecting quantum dots, Zhang’s team has used two differently-sized quantum dots made from Cadmium-Tellurium crystals. The two dots shine with different colours under ultraviolet light. These are then encased in silica, a transparent material, such that the inner quantum dot shines red while the outer one shines green. Zhang’s team then attached these ‘hybrid’ two-colour emitting dots to an organic compound which specifically binds to TNT. 

The interaction between the organic compound and TNT demands some energy. This results in a dimming of light produced by the quantum dot. Zhang’s team has devised the quantum dots in such a way that only the green light is dimmed in intensity and not the red.

When the quantum dots attach to TNT, the green shine of the enveloping quantum dot is dimmed, resulting in a stronger red light, which can be easily detected.

On a surface containing TNT, these hybrid quantum dots show the TNT molecules as red dots against a yellow background. The brightness of the red speck increases with the amount of TNT present. In a paper published in the Journal of American Chemical Society recently, Zhang’s team demonstrates that their quantum dots can detect 5-50 nanograms (nanogram is a billionth of a gram) on a square millimeter, on such different surfaces like fabric bags, rubber stamps and paper. The idea can also be adapted for detection of other explosive materials.