Published in the Hindu on August 28, 2012

Identifying TNT explosives present at a very low concentration—nine molecules—has become possible with a nanostructure-sized sensor developed by Prof. T. Pradeep of the Department of Chemistry, IIT Madras.

There is a complete change in luminescence from red to green when TNT is added to the sensor.

All that is required to detect the change in luminescence is a fluorescence microscope.

The results can be double-checked by Raman spectroscopy. Raman spectrum, specific to TNT, gets enhanced and hence identifying the signature becomes easy.

“So false alarm can be avoided by using Raman spectroscopy,” said Prof. Pradeep. “Multiple confirmations improve accuracy and are essential in sensor-like applications.”

The sensor has two main components—a unique flower-shaped gold particle (mesoflower). “The unique shape of the flower is important in identifying the genuineness of the sensor,” he said.

Silver particle

Sitting on top of the gold mesoflower is a less than one nanometre-sized silver particle that is embedded in a protein (bovine serum albumin).

The gold mesoflower, together with the protein-protected silver cluster, is then coated with a dye.

When irradiated with a right wavelength of light, the silver cluster produces a red luminescence.

But adding a very dilute solution containing TNT at concentrations of one ppb (parts per billion) can quench the red luminescence.

Explaining the principle behind the sensor, he said: “the amine groups of the protein form a complex with TNT.

This complex quenches the red luminescence of the silver cluster; the red light therefore goes missing. But absence of luminescence alone is not sufficient as the object would appear dark.”

This is where the dye coating comes into play. The dye by itself produces green luminescence and is unaffected by TNT.

Dye’s luminescence

When the protein forms a complex with TNT and the red luminescence goes missing, the dye’s green luminescence becomes visible.

The change in luminescence can be observed with a fluorescence microscope. “Similar sensors using the same approach can be developed for other molecules,” he said.

According to Prof. Pradeep, it is possible to make one billion sensors with one gram of gold. “So it is not worth reusing the sensors after cleaning them,” he said.

A device based on this principle is under development. Work on the mesoflower started in 2008 with P. R. Sajanlal, who is currently doing his postdoc.

In 2010, Ammu Mathew, a doctoral student, began the work of developing the sensor.

“We could get the sensor in place in just one-and-half years,” he said. The results were published on Monday in the journal Angewandte Chemi.