Gold Nanoparticles Used to Detect a Toxic Metal - Mercury

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Throughout history, mercury has widely been used by alchemists that were trying to produce gold. Now, the roles have changed, and gold is what helps scientists find mercury.

In

the litmus test, litmus is a water-soluble mixture of different dyes extracted from certain lichens, especially Roccella tinctoria, often absorbed on to filter paper. The resulting piece of paper or solution with water becomes a pH indicator (one of the oldest), used to test materials for acidity. Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic (alkaline) conditions, the color change occurring over the pH range 4.5-8.3 (at 25 degrees Celsius). Neutral litmus paper is purple in color.

Scientists at Northwestern University have recently developed a simple "litmus test" for mercury that could be used for environmental monitoring of bodies of water, such as rivers, streams, lakes and oceans, to evaluate their safety as food and drinking water sources.

The colorimetric screening technology and its success in detecting mercury rely on using gold nanoparticles and DNA.

Produced directly and indirectly as part of several industrial processes such as the manufacture of acetaldehyde, methyl mercury is a neurotoxin that is particularly dangerous to young children and pregnant women and the form of mercury people ingest when they eat contaminated fish and shellfish.

Mercury is released into the air through industrial pollution, falling into bodies of water and polluting the waters in which fish and shellfish live. Bacteria in the aquatic environment then convert water-soluble mercuric ion (Hg2+) into methyl mercury, which accumulates in varying amounts in fish and shellfish.

Chad A. Mirkin, George B. Rathmann Professor of Chemistry, Professor of Medicine and Professor of Materials Science and Engineering, who led the study, said that "It is critical to detect mercury quickly, accurately and at its source. Most existing detection methods require expensive complicated equipment forcing tests to take place in a lab. Our method is simpler, faster and more convenient than conventional methods, and results can be read with the naked eye at the point of use."

Being highly sensitive, they are 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," said Mirkin. "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."

Future applications using similar principles will include developing a colorimetric screening method for cadmium and lead.