MIT Pencil Draws Sensors Directly onto Paper

A few scribbled lines on paper can instantly create a sensor for detecting dangerous gases. Massachusetts Institute of Technology chemists pulled off that neat trick by using a pencil in which the graphite "lead" was replaced by a special material called carbon nanotubes.

Lines of carbon nanotubes are drawn on specially treated paper. When an electrical current is then run through the paper, a change in resistance is a sign that gas molecules have bound to the nanotubes.

MIT's test sensor successfully detected small traces of ammonia gas. But the same idea could work for almost any type of gas, said Timothy Swager, a chemist at MIT.

Best of all, the special pencil lead is fairly cheap and easy to use, according to the MIT researchers. The new method of compressing powdered nanotubes into pencil lead is also safer than the usual sensor-manufacturing method of dissolving the nanotubes in hazardous chemicals.

"You can't imagine a more stable formulation," Swager said. "The molecules are immobilized."

The carbon nanotubes are tiny cylinders of rolled-up carbon sheets — thousands of times thinner than a human hair.

Simplification of such sensor manufacturing has huge implications ranging from battlefield scenarios to modern medicine. For that reason, MIT's research received funding from both the Army Research Office and a National Institutes of Health fellowship.

The MIT team hopes to create "drawn" sensors that can detect ethylene levels, for monitoring the ripeness of fruits being shipped or stored, and sulfur compounds, which could warn of natural gas leaks in homes or businesses.

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