redOrbit Staff & Wire Reports – Your Universe Online

Chemists at MIT have created a customized, carbon nanotube lead that can be used to draw freehand electronic circuits using a conventional, mechanical pencil.

A typical pencil lead is fashioned out of graphite — a form of carbon made up of layers of graphene — and a clay binder. Whenever a person writes with a graphite pencil, a mixture of tiny graphene flakes and clay are deposited on the paper, leaving behind a mark.

In the current study, the MIT researchers developed a new type of pencil lead, in which the graphite is replaced with a compressed powder of carbon nanotubes, allowing the pencil to inscribe sensors onto any paper surface.

Electrically charged carbon nanotubes, which are 50,000 times smaller than a human hair, make ideal sensors. This is because anytime a foreign gas molecule disturbs their surface, it binds to the nanotubes and immediately changes the current flow.

These properties mean the sensor could be used, for instance, to detect chemical changes in the air. Such a biosensor could have important national security applications.

Other potential applications exist in the area of food safety, where the sensors could be used to monitor changing ethylene levels that indicate fruit ripeness.

The MIT team built their sensor to detect tiny amounts of ammonia gas, an industrial hazard. It could be modified to detect nearly any type of gas.

“The beauty of this is we can start doing all sorts of chemically specific functionalized materials,” said MIT Chemistry Professor Timothy Swager, who led the research team.

“We think we can make sensors for almost anything that’s volatile.”

The MIT team built their sensor by drawing a line of carbon nanotubes on a sheet of paper imprinted with small electrodes made of gold. An electric current was then applied and measured as it flowed through the carbon nanotube strip, which acted as a resistor. Whenever the current changed, it meant that gas had bound to the carbon nanotubes.

The researchers tested their device on several different types of paper, and found that the best response came with sensors drawn on smoother papers. They also found that the sensors gave consistent results regardless of whether or not the marks were uniform.

Swager said the new technique offers two key advantages: low cost and stability.

“You can’t imagine a more stable formulation. The molecules are immobilized,” he said.

In the current study, researchers focused on pure carbon nanotubes, but are now working on customizing 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.

The researchers said they are particularly interested in 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.

The sensor could also prove useful in a variety of other applications.

“I can already think of many ways this technique can be extended to build carbon nanotube devices,” said Zhenan Bao, an associate professor of chemical engineering at Stanford University, who was not involved in the research.

“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.”

The research was published online October 4 in the journal Angewandte Chemie.