Detecting Mercury with Gold

Hank Hogan

As evidenced (albeit comically) by Lewis Carroll’s madcap hatter, mercury exposure can have serious health consequences, such as tremors, memory loss and personality changes. Free mercury can arise from volcanic emissions, the mining of precious metals, the incineration of waste or the burning of coal. Airborne vapors can deposit the heavy metal in streams and in other bodies of water.

Tracking the toxin is important, but traditional monitoring techniques often require expensive equipment, complicated sample preparation or both. Fluorescent chemosensors are promising, but the current ones are not very soluble and exhibit either poor specificity or weak fluorescence.

Now scientists from National Taiwan University in Taipei have devised a technique — based on interactions of gold nanoparticles, rhodamine B and mercury — that could overcome these problems while delivering high selectivity and a detection limit of two parts per billion of mercury in water.

The fluorescence intensity of gold nanoparticles to which rhodamine B has been adsorbed (AuNP-RB) increases by 400 times in the presence of mercury. The insets show the absorbance spectra (A) and transmission electron microscopy images (B) of AuNP-RB in the presence (1) and absence (2) of mercury. Image courtesy of Huan-Tsung Chang, National Taiwan University.

“Our method should be useful for monitoring mercury(II) in biological and environmental samples such as drinking water, meats, and water in rivers and lakes,” said researcher Huan-Tsung Chang, a chemistry professor at the university.

The technique exploits the fact that the fluorescence of rhodamine is quenched when it is adsorbed onto gold nanoparticles. That fluorescence, however, is restored in the presence of metal ions such as mercury(II), which release the rhodamine from the particle surface.

In a demonstration of their technique, the researchers fabricated gold nanoparticles with an average size of 13.3 nm. They determined the nanoparticle concentration in solution using a double-beam UV-VIS spectrophotometer from GBC Scientific Equipment Pty Ltd. of Dandenong, Victoria, Australia.

Rhodamine adsorption

They then adsorbed rhodamine onto the gold nanoparticles, followed by mercaptopropionic acid, mercaptosuccinic acid or homocystine. The latter three thiol ligands, according to Chang, made the method selective to mercury because they formed stable complexes with the gold-rhodamine compounds.

The investigators measured the fluorescence of each nanoparticle solution after adsorption using a Hitachi spectrophotometer with excitation at 510 nm. That showed almost complete quenching. They exposed their probes to battery samples (a standard for detecting mercury) and pond water. In the presence of mercury, the fluorescence increased 400 times, yielding a detection limit of 10 nM concentration, or about two parts per billion.

Chang said that the researchers plan to improve the sensitivity of the method because the present detection limit is at the maximum mercury level allowed in drinking water by the US Environmental Protection Agency. They also intend to test various metallic nanoparticles and fluorophores and hope to enhance the method and move it out of the lab.

“Our major goal is to construct a portable fluorometer for detection of mercury(II) by using the nanosensor,” he said.

Analytical Chemistry, Dec. 15, 2006, pp. 8332-8338.