MIT pencil 'draws' gas sensors onto paper

MIT sensor pencil The researchers create a sensor by drawing with the pencil onto paper imprinted with gold

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Chemists have created a pencil-shaped device which can draw tiny sensors onto a sheet of paper that detect harmful gases.

The "pencil lead" is made out of sheets of carbon rolled into tubes 50,000 times smaller than a human hair.

The researchers said it was a safer alternative to an existing process that used a toxic substance which could damage humans' nervous systems.

The development was funded by the US Army - suggesting a military use.

The work was carried out at the Massachusetts Institute of Technology (MIT) and published in the German journal Angewandte Chemie.

Lead and gold

The MIT chemists said that they had used their technique to make sensors capable of detecting ammonia - a colourless gas used by farms and industry, which has also been identified by the US authorities as a poison that might be used by terrorists.

The researchers said that the nanotubes could also be adapted to detect other types of dangerous gases.

"These sensors might ultimately find applications in the food industry, healthcare and homeland security," said team member Dr Katherine Mirica.

"Our approach for overcoming the challenges of working with carbon nanotubes relies on a solvent-free method that is essentially similar to drawing with pencil on paper."

To produce their "pencil lead" the team compressed a powder made up of the carbon nanotubes until it formed a solid substance.

US soldiers in Afghanistan The US Army already uses paper-based sensors to detect the presence of harmful gases

This was then placed in a pencil and used to scribble on paper imprinted with broken gold stripes.

The technique takes advantage of the fact that ammonia binds to the nanotubes when it passes through them. Other gases can be targeted by adding metal atoms to the tubes' walls or wrapping other materials around them.

The gold stripes on the paper act as electrodes. By running electricity through them the team can measure how much current flows through the gaps coated with the nanotube scribble.

If a gas has bound to the the nanotubes it will impede the electron flow, altering the current.

Easy-to-carry tech

One analyst told the BBC the military might ultimately wish to equip soldiers with a range of these pencils to provide a portable test kit for a range of harmful gases.

"The reason the army would be funding this is the fear of chemical attacks," Jennifer Cole, research fellow in emergency management at the British defence think tank Rusi, told the BBC.

"They already use different strips of paper that are stuck onto suits and change colour when exposed to a specific substance.

"This would be a more advanced type of this technology which might help provide information about a wider range of substances and perhaps give more detail about how high the risk is."

The US government had previous funded MIT to do research into how carbon nanotubes could be used to detect nerve toxins including Sarin following a terrorist attack on the Tokyo subway which killed 12 people in 1995.

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