Mercury is toxic, showing up in water and the fish we eat to the point where pregnant women are advised against eating certain top-level fish, namely shark, swordfish, king mackerel and tilefish.
Researchers at Northwestern University and the Ecole Polytechnique Fédérale de Lausanne, led by Eun Sun Cho and Bartosz Grzybowski, have developed a nanoparticle system sensitive enough to pick up even tiny levels of heavy metals.
The nanoparticles are laid on a strip of glass between two gold electrodes. The nanoparticles are also gold, but are coated with mixtures of organic molecules -- n-hexanethiol and alkanethiols that end with ethylene glycol groups, for aficionados. The organic molecules have a "hairy" structure that traps positively charged ions.
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When a current is sent across the nanoparticles, the voltage indicates the concentration of whatever chemical one is trying to measure, because the positive ions conduct electricity. The technique is very sensitive since ions will have differing resistances depending on what elements they are.
In this case, the team tested the device on methylmercury (CH3Hg+), cadmium and zinc. In the paper describing the experiments, they say they could pick up concentrations as small as one in one billion billion.
The sensitivity is an important point. Most common detection methods can't see mercury in concentrations less than about one part per trillion.
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That may sound like a tiny bit, but even small amounts of methylmercury can add up over time if people keep ingesting it. Mercury stays in the body for long periods, and is concentrated in the bodies of fish as it moves up the food chain. Mercury also gets into the water from industrial runoff and burning coal (one of the biggest sources).
Grzybowski and his team say the nanoparticle film costs only about $10 to make. The devices for measuring the current flow across it are more expensive, but if this can be widely disseminated, then the one-time cost matters less.
The nanoparticle experiments appeared in the Sept. 9 issue of Nature Materials.