Conventional bomb detection; Partha Sarathi Mukherjee (below)

Chemist Partha Sarathi Mukherjee loves making cages. This ‘carpenter’ of sorts, however, doesn’t use timber or metal to build the enclosures. Instead, he uses chemical molecules. The boxes that he crafts are so tiny that they can’t be seen with the naked eye. The largest of the cages — created in his lab at the Indian Institute of Science (IISc), Bangalore — is nearly 300 times smaller than a dust particle.

Mukherjee, who did his doctorate in the magnetic properties of copper-containing polymers from the Indian Association for the Cultivation of Science, Calcutta, just six years ago, isn’t creating these molecular boxes, or nanocages, just for fun. Though Mukherjee is very reluctant to admit it, these cages,synthesised by putting together just the right combination of chemicals, may find application in myriad fields — from biomedicine to the chemical industry to explosive detection.

The one that currently occupies his time and mind is sure to make security experts sit up and listen. For these tiny prisms have the ability to sniff out many explosives that go into making bombs. “I don’t want to claim anything, it is really in a very preliminary stage,” says Mukherjee in his characteristic unassuming manner.

Whether he admits it or not, the paper that detailed the novel mechanism to detect chemical signatures of explosive materials like TNT (tri-nitro-toluene) and DNT (di-nitro-toluene) was one of the 10 most accessed chemistry papers in the first quarter of the year. The paper by Mukherjee and his student Sushobhan Ghosh was published in Organometallics, the journal of chemistry.

Mukherjee’s team is focused on the cutting edge of materials science research, pursuing molecular self-assembly — a peculiar process in which some chemical entities come together to create compounds of complex but uniform architecture.

If materials scientists carefully choose the starting materials and know well their chemical and physical properties, they can have a fine control over the formation process. Normally, such molecules are uniform in shape and size. Many of them are hollow inside, offering endless possibilities. For instance, Mukherjee elaborates, one can pack a drug molecule inside the self-assembled material for drug delivery or put in a sensor to detect pollutants in the air or water.

The nano-sized molecular prisms designed by the IISc scientists by combining a platinum compound and a “clip-shaped” organic compound, emit a fluorescent light once formed. However, when they come in contact with nitroaromatic compounds such as TNT, this fluorescence dies out. “We tried this out on three different nitroaromatic compounds that have explosive potential — TNT, DNT and picric acid. We got the same encouraging results in all the cases,” Mukherjee told KnowHow.

Although he thinks it is too early to talk about its application, he feels it has a potential that needs to be explored further. As the next step, Mukherjee plans to make a thin film of the material containing a large number of the tiny molecular prisms. This film, in turn, can be pasted on a metal plate to make it a handy explosive detector. “We presume that if it is a good detector, when taken into a room where a suspected explosive is kept the glow will go out. We expect it to be sensitive enough to pick up vapours emanating from the nitroaromatic compounds to quench the fluorescence.”

A represetation of how the molecular prisms are made in the lab

Vijayamohanan Pillai, head of materials chemistry at the National Chemical laboratory, Pune, thinks that it is an exciting piece of work. He says that remote, visual detection of explosives is surely the way forward. Even experts abroad have had only limited success in developing such a fluorescence-based detection system. But, Pillai cautions that specificity — the ability to detect only the explosives — is very important. “We can’t give out false alarms,” he points out.

Mukherjee says they have yet to go into that stage. “There is a lot of fine tuning to be done.” As of now, the IISc team is trying to knock out platinum from the final material. Removing a metal like platinum can actually improve the sensitivity of the sensor.

Mukherjee’s lab has also ‘self-assembled’ several other materials. In a very recent issue of the journal Angewandte Chemie, his team announced the design and synthesis of a hexagonal molecular box which is open on both ends. “It is fairly big by molecular standards; we can easily pack a fullerene — a carbon molecule of 60 atoms — inside it,” says Mukherjee. A wonder molecule, which won its inventors Robert Curl, Harold Kroto and Richard Smalley the Nobel Prize for chemistry in 1985, fullerene has since opened up a new branch in materials science.