Sensitive, Selective Mercury Sensor
Nanoparticle-based colorimetric method detects part-per-billion levels of mercury
Mitch Jacoby
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.
SAMPLING Northwestern’s Lee collects water samples from Lake Michigan to test a new mercury-detection method.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.
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COLOR COORDINATED Gold nanoparticles (orange and purple spheres) functionalized with one of two complementary DNA sequences (black and green curved lines, bottom) do not form stable complexes in the absence of mercury ions (all vials except Hg2+, black arrow) due to a single thymidine group mismatch (red T). Mercury ions bind the two types of DNA sequences at the point of the T-T mismatch and cause the otherwise red nanoparticle solutions to appear purple (red arrow).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.
Chemical & Engineering News
ISSN 0009-2347
Copyright © 2007 American Chemical Society
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