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A CHEMICAL sensor has been developed that is so sensitive it can detect just one molecule of an environmental toxin or warfare agent.

The detector is modelled on the tiny openings that let ions in and out of the cells in our body. These openings, called ion channels, span the walls of human cells. A thin protein flap guards each opening, and in nerve cells the flap can be controlled by neurotransmitters. When a neurotransmitter binds to a site in the channel it pushes open the flap, letting ions flow into or out of the cell.

Hagan Bayley of Texas A&M University in College Station wondered if a chemical sensor could be based on a similar principle. Many of the nerve agents used in chemical warfare, such as sarin, block these channels, which is why they are so toxic.

Bayley started with an existing experimental set-up in which solutions of ions are separated by a barrier containing a single protein pore. Electrodes on each side of the barrier measure the current that flows when ions move through the pore. But certain nerve agents stop the pore working by permanently opening it. So a chemical sensor based on this idea would be useless as soon as it came into contact with a toxin. That means it would be impossible to know the identity or the concentration of the offending compound.

Bayley’s answer is to genetically engineer a modified version of the protein pore, so that the binding site for the particular toxin he’s interested in doesn’t fit quite as well as normal. For example, he made a binding site containing some but not all of the chemical bonds necessary for it to bind to a toxic arsenic compound. Molecules of the compound that made their way into the site could bind only transiently, so the pore opened but snapped shut again a fraction of a second later.

The frequency at which this happens indicates how concentrated the neurotoxin is. The more molecules around, the more often one finds its way in to block the pore. Even better, says Bayley, the site can be designed so that a variety of chemicals bind to it more or less tightly. They sit there for different lengths of time before popping out, and can even push the flap open by varying amounts, allowing more or fewer ions through. The length of the current pulse and the rate at which it flows provide a kind of electrical signature that can be used to identify the compound, he says.

Bayley’s ability to detect and identify single molecules is unique, says Tim Swager of the Massachusetts Institute of Technology in Cambridge, who also works on developing ultrasensitive chemical sensors. But a device based on this technology will have to be extremely selective, to avoid false alarms.