By Liat Clark, Wired UK

A team of MIT chemists has published a paper revealing how it developed a method for drawing gas sensors onto paper using a tailor-made, super-conductive carbon nanontube pencil.

Electrically charged tiny carbon nanontubes – about 50,000 times thinner than a human hair – make great sensors because when a foreign gas molecule disturbs their surface it binds to the nanotubes, immediately altering the current flow. This kind of alert system can be used to detect chemical changes in the air, and so would be useful in developing things like biosensors for national security reasons. It even has uses in the food industry, as MIT demonstrated earlier this year when it developed a carbon nanotube sensor that monitors ethylene levels excreted by ripening fruits.

According to MIT, building these sensors – though inexpensive – is a hazardous process that involves dissolving the nanotubes in dangerous solvents such as dichlorobenzene, and therefore mass production has never been possible. The team, led by professor Timothy Swager, has come up with a novel, solvent-free solution to overcome this. Postdoctoral student Katherine Mirica realised that pencils, which share properties with carbon nanotubes, could provide an easy method for transferring the conductive coating on to all kinds of surfaces.

"Carbon nanotubes are chemically related to the main component in the lead of a pencil, which is graphite," she said. The team used commercial carbon nanotube powder and compressed it into the shape of a pencil's lead. This renders it stable and therefore incredibly reliable, says Swager: "You can't imagine a more stable formulation. The molecules are immobilised". They then printed gold electrodes onto standard paper (the smoother the paper, the better) and drew a line of carbon nanotubes onto this using the new pencil. All that's left is to pass an electric current through the nanotubes and monitor it for changes. This particular sensor can measure ammonia in the atmosphere, but all different types can be made using the same process.

"We think we can make sensors for almost anything that's volatile," says Swager. By bonding different metal atoms to the carbon atoms that make up the nanotubes, different gases can be targeted.

Not only is the process safer than the usual method, it means nearly anyone can apply the coating to a surface – experiments showed that irregularities in the drawn on sensor strips did not affect their reliability, so you can have a shaky hand and still be an expert carbon nanontube sensor-maker.

Video: MIT

Source: Wired.co.uk