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TERRORISM is most commonly associated with the bomb and the bullet, but ever since an incident on the Tokyo subway system in 1995, the security services have also had to worry about poison gas.
That attack, which used a nerve-gas called sarin, killed 12 people and severely injured another 50. Sarin is to be feared because it is invisible, odourless and 500 times more deadly than cyanide. However, other gases (not least cyanide itself) could be used instead. What is needed is a cheap way of detecting such gases and, having raised the alarm, of identifying which gas is involved so that anyone who has succumbed can be treated.
And that is what a team of chemical engineers at the Massachusetts Institute of Technology, led by Michael Strano (pictured on the left), think they have created. Not only can their new sensor tell between chemical agents, it can detect them at previously unattainable concentrations—as low as 25 parts in a trillion.
The core of Dr Strano's invention, which he describes in Angewandte Chemie, is an array of treated carbon nanotubes. Each is, in essence, a layer of carbon atoms that has been coated with nitrogen-containing molecules called amines and rolled into a cylinder with the amines on the outside. Individual tubes, which are about 1/50,000 of the width of a human hair in diameter, are arranged so that they run between pairs of tiny electrodes. When the device is switched on the nanotubes carry an electric current with a power of about 300 microwatts.
The gases to be analysed reach the nanotubes through a miniature column etched onto a silicon chip. As they pass along this column they tend to stick to its sides. Some gases stick more than others, and hence travel more slowly down the column. In this way the components of the sample separate from one another. Each component is puffed onto the nanotubes, where it sticks to the carbon atoms. This, in turn, causes the conductivity of the nanotubes to change—how much is a characteristic of each gas.
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