Carbon nanotubes can provide a very useful function as a new way to detect harmful gases that are present in the air. But, currently, the methods that are used to build carbon nanotube sensors involve hazardous materials and aren’t really suited for production on a large-scale.
But now, a newly created fabrication method that is done by simply drawing a line on a sheet of paper with a special lead may fix those problems. “MIT postdoc Katherine Mirica 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,” an MIT news release states.
The new sensor is able to detect very small amounts of ammonia gas, which is a major industrial hazard. The lead researcher, Timothy Swager, the John D. MacArthur Professor of Chemistry, says that the sensors can be easily adapted to be used in the detection of almost any type of gas.
“The beauty of this is we can start doing all sorts of chemically specific functionalized materials,” Swager says. “We think we can make sensors for almost anything that’s volatile.”
MIT adds: “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.”
“Swager and Mirica 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.”
To test their new device, the researchers tried it out on a variety of different types of paper, finding that “the best response came with sensors drawn on smoother papers. They also found that the sensors give consistent results even when the marks aren’t uniform.”
The main advantages of this new technique is its inexpensive nature, and that the “pencil lead” is extremely stable, Swager says. “You can’t imagine a more stable formulation. The molecules are immobilized.”
According to Zhenen Bao, an associate professor of chemical engineering at Stanford University, the new sensor has many potential uses. “I can already think of many ways this technique can be extended to build carbon nanotube devices,” says Bao, who was not part of the research team. “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 MIT news release continues:
“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.”
The new research was just published in the journal Angewandte Chemie.
Source: Massachusetts Institute of Technology
Image Credits: Jan Schnorr; Courtesy of Mark Hersam, Northwestern University