Super-Sensitive and Small: MIT Detector Senses Deadly Gases

As bioterrorist attacks increase, improved detection plays an important role. An MIT team found a way to detect microscopic threats in a portable form factor, holding life-saving potential.

Using carbon nanotubes, MIT chemical engineers have built the most sensitive electronic detector yet for sensing deadly gases, such as the nerve agent sarin.

The technology, which could also detect mustard gas, ammonia and VX nerve agents, has potential to be used as a low-cost, low-energy device that could be carried in a pocket or deployed inside a building to monitor hazardous chemicals.

"We think this could be applied to a variety of environmental and security applications," said Michael Strano, Associate Professor of Chemical Engineering and senior author of a paper describing the work published in the June edition of Angewandte Chemie.

Formula for Success

Strano's sensor has exhibited record sensitivity to molecules mimicking nerve toxins such as sarin: It can detect minute quantities as low as 1 femtomole (1 billion molecules), roughly equivalent to a concentration of 25 parts per trillion. "There's nothing that even comes close," he said.

Sarin, which killed 12 people in a 1995 terrorist attack on the Tokyo subway, can kill at very low concentrations (parts per million) after 10 minutes, so highly sensitive detection is imperative to save lives. The new detector is far more sensitive than needed to detect lethal doses. To build their super-sensitive detector, Strano and his team used an array of carbon nanotubes aligned across microelectrodes. Each tube consists of a single-layer lattice of carbon atoms, rolled into a long cylinder with a diameter about 1/50,000 of the width of a human hair.

The nanotube sensors require very little power about 0.0003 watts. One sensor could run essentially forever on a regular battery.

When a particular gas molecule binds to the carbon nanotube,the tube's electrical conductivity changes. Each gas affects conductivity differently, so gases are identified by measuring the conductivity change after binding.

The researchers achieved new levels of sensitivity by coupling the nanotubes with a miniature gas-chromatography column etched onto a silicon chip smaller than a penny. The column separates different gases before feeding them into the nanotubes.

Lab Work
Using a chemistry process outlined in a separate paper published in January's Journal of the American Chemical Society, Strano and coworkers showed that this can be done by coating the nanotubes with amine-type molecules, which donate an extra pair of electrons to the nanotubes.

The coating allows gas molecules to bind to nanotubes but detach a few milliseconds later, allowing another molecule from the column to move in. With a network of these reversible sensors, a gas could be tracked as it spreads through a large area.

The work was funded by the Department of Homeland Security under contract to the Federal Aviation Administration and MITˇs Institute of Soldier Nanotechnology. Characterization facilities used for this work were supported by the Department of Energy. Microcolumn and detector development was funded in part by the Defense Advanced Research Projects Agency.

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