Scientists develop carbon nanotube sensors etched with pencils

Scientists develop carbon nanotube sensors etched with pencils

7:15 AM, 10th October 2012
Scientists develop carbon nanotube sensors etched with pencils
MIT chemists designed a new type of pencil lead consisting of carbon nanotubes, allowing them to draw carbon nanotube sensors onto sheets of paper.

CAMBRIDGE, US: Carbon nanotubes offer a powerful new way to detect harmful gases in the environment. However, the methods typically used to build carbon nanotube sensors are hazardous and not suited for large-scale production. A new fabrication method created by MIT chemists, as simple as drawing a line on a sheet of paper, may overcome that obstacle. Katherine Mirica, postdoc, MIT has designed a new type of pencil lead in which graphite is replaced with a compressed powder of carbon nanotubes. The lead, which can be used with a regular mechanical pencil, can inscribe sensors on any paper surface.

The sensor detects minute amounts of ammonia gas, an industrial hazard. According to Timothy Swager and John D MacArthur, Professor, and leader of the research team, the sensors could be adapted to detect nearly any type of gas. “The beauty of this is we can start doing all sorts of chemically specific functionalized materials. We think we can make sensors for almost anything that’s volatile,” said Swager.

Carbon nanotubes are sheets of carbon atoms rolled into cylinders that allow electrons to flow without hindrance. Such materials have been shown to be effective sensors for many gases, which bind to the nanotubes and impede electron flow. However, creating these sensors requires dissolving nanotubes in a solvent such as dichlorobenzene, using a process that can be hazardous and unreliable.

Researchers are set out to create a solvent-free fabrication method based on paper. Inspired by pencils on her desk, Mirica had the idea to compress carbon nanotubes into a graphite-like material that could substitute for pencil lead. To create sensors using their pencil, the researchers draw a line of carbon nanotubes on a sheet of paper imprinted with small electrodes made of gold. They then apply an electrical current and measure the current as it flows through the carbon nanotube strip, which acts as a resistor. If the current is altered, it means gas has bound to the carbon nanotubes.

Two major advantages of the technique are that it is inexpensive and the ‘pencil lead’ is extremely stable. You can’t imagine a more stable formulation. The molecules are immobilized,” said Swager.

“I can already think of many ways this technique can be extended to build carbon nanotube devices. Compared to other typical techniques, such as spin coating, dip coating or inkjet printing, I am impressed with the good reproducibility of sensing response they were able to get,” said Zhenan Bao, Associate Professor, Stanford University.

In this study, the researchers focused on pure carbon nanotubes, but they are now working on tailoring the sensors to detect a wide range of gases. Selectivity can be altered by adding metal atoms to the nanotube walls, or by wrapping polymers or other materials around the tubes. One gas the researchers are particularly interested in is ethylene, which would be useful for monitoring the ripeness of fruit as it is shipped and stored. The team is also pursuing sensors for sulfur compounds, which might prove helpful for detecting natural gas leaks.

© Massachusetts Institute of Technology News

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