Sensitive, Selective Mercury Sensor

A new colorimetric method provides a simple and sensitive way to detect mercury in aqueous samples. The advance may form the basis of an instrument-free procedure for monitoring mercury levels in lakes and rivers and may lead to similar types of detection methods for other metals.

A variety of harmful effects in humans, from brain damage to DNA alteration, are attributed to mercury exposure. In addition to direct exposure to the metal's poisonous vapors, indirect exposure caused by eating mercury-tainted fish and other water-derived foods has also been fingered as a common route to the metal's toxic effects. Accordingly, a number of procedures have been developed to monitor the heavy metal and its compounds in various settings.

Colorimetric methods are especially convenient because analytes can be detected with the naked eye by observing color changes in reagent solutions. Some colorimetric methods for detecting mercury are available. but they are limited to micromolar concentration levels. They are also nonselective and don't work in aqueous environments.

The new colorimetric method, demonstrated by researchers at Northwestern University, detects Hg²⁺ ions at concentrations as low as 100 nM (20 ppb) in aqueous samples. Based on DNA-functionalized gold nanoparticles, the procedure is highly selective and, unlike other Hg²⁺ detection methods, does not require organic solvents or cosolvents (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200700269).

The basis of the method, which was developed by chemistry professor Chad A. Mirkin, graduate student Jae-Seung Lee, and postdoc Min Su Han, is thymidine—Hg²⁺–thymidine coordination chemistry. To turn that coordination reaction into a chemical sensing method, the team functionalized gold nanoparticles with one of two types of thiolated DNA sequences, A and B. The sequences are complementary except for a single thymidine-thymidine mismatch.

In the absence of Hg²⁺ ions, A- and B-functionalized nanoparticles aggregate and form duplexes—despite the single mismatch—that melt (dissociate) at 46 ^??C. The researchers note that the melting gives rise to a dramatic purple-to-red color change. The presence of Hg²⁺ ions, however, strengthens the complexes and raises their melting temperature and hence the temperature at which the color of the solution changes. These effects are mediated by Hg²⁺ coordinating the pair of mismatched thymidine groups.

The team showed that the melting temperature and color change depend linearly on the Hg²⁺concentration from the high nanomolar to low micromolar range. They also showed that the method is selective for Hg²⁺ and insensitive to Mg²⁺, Pb²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and other metal ions.

The researchers note that, in principle, the new method could be used to detect other metal ions by substituting thymidine with artificial nucleobases that selectively bind other metal ions.

Yi Lu, a chemistry professor at the University of Illinois, Urbana-Champaign, remarks that compared with other mercury detection techniques, the Mirkin group's "impressive" method stands out due to its high sensitivity, compatibility with aqueous media, and high selectivity.