April 28 : Researchers at the Northwestern University, US, claim to have developed a simple ‘litmus test’ for mercury that could be used for on-the-spot environmental monitoring of water bodies like rivers, streams, lakes and oceans to evaluate their safety as food and drinking water sources.

The method takes advantage of gold’s intense colour when the metal is measured on the scale of atoms.

According to Chad A. Mirkin, professor of medicine and professor of materials science and engineering at the University, who led the study, the method is highly sensitive, capable of detecting mercuric ions at the 100-nanomolar level.

“To the best of my knowledge, we have set a record for the most sensitive colorimetric sensor. A glucose meter, for example, operates at a high micromolar scale, with glucose being 100,000 times more concentrated than the mercury we are detecting,” said Prof. Mirkin.

Prof. Mirkin and his team started with gold nanoparticles, each just 15 nanometers in diameter, held together by complementary strands of DNA.

Since they are held together within a certain critical distance, the gold nanoparticles — and the solution they are in — are blue. When the solution is heated, the DNA breaks apart, and the gold nanoparticles, no longer in close proximity to each other, turn bright red.

The key, as Prof. Mirkin said, is that the colour changes from blue to red at 46 degrees Celsius if the solution has no mercury, and it occurs at a higher temperature if mercury is present.

“When mercury binds to the T-T mismatch site it is like adding some superglue — the gold nanoparticles are now held together even more tightly. The mercury creates a stronger bond that requires a higher temperature to break apart the DNA strands,” said Prof. Mirkin.

Also the temperature it takes to break apart the strands, when the colour changes from blue to red, indicates how much mercury is present — the higher the temperature, the more mercury or “super glue” is present.

Prof. Mirkin said the next step is to increase the sensitivity of the colorimetric screening technology, as well as expand the scope of environmental targets.

“Using similar principles, we have started developing a colorimetric screening method for cadmium and lead. This is a simple method that we can tailor easily for other metals as well,” he said.

The findings appear online in the journal Angewandte Chemie. (ANI)