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Simple, Hand-Drawn Carbon Nanotube Sensor System Devised

Drawing with nanotube pencil to make gas sensor (Jam Schnorr, MIT)

Drawing with nanotube pencil to make gas sensor (Jam Schnorr, MIT)

Chemistry researchers at Massachusetts Institute of Technology in Cambridge developed a safer method of building hazardous gas sensors from carbon nanotubes that is literally as easy as drawing by hand. The system designed by MIT postdoctoral fellow Katherine Mirica and colleagues is described online in the journal Angewandte Chemie (paid subscription required).

Harmful gases in the air can be detected with carbon nanotubes, but methods used to build carbon nanotube sensors can be hazardous themselves and thus not readily applicable for large-scale production. Mirica’s team found a way around this obstacle by putting carbon nanotube powder in a pencil-like dispenser that lets the sensor material flow easily onto a substrate.

Carbon nanotubes are sheets of carbon atoms rolled into nanoscale cylinders — 1 nanometer equals 1 billionth of a meter — that allow electrons to flow without hindrance. Many gases have been shown to bind to the nanotubes and impede electron flow, which makes nanotubes useful for detecting these gases. To create these sensors using current methods, however, first requires dissolving nanotubes in a solvent such as dichlorobenzene, a chemical that causes irritation to the skin, throat, and eyes and classified by the Environmental Protection Agency as a possible human carcinogen.

Mirica, working in the lab of MIT chemistry professor Timothy Swagger, devised a solvent-free sensor by first compressing carbon nanotubes into a material resembling graphite so it could substitute for pencil lead. The carbon nanotubes could then be dispensed by hand by drawing on a conductive substrate, in this case paper imprinted with small electrodes made of gold (pictured at top).

An electrical current can then be sent through the paper, with the carbon nanotube strip acting as a resistor. If a gas is present and is exposed to the the nanotube strip, the gas binds to the nanotubes and alters electrical current.

The team tested the sensor on ammonia gas and found it able to detect the gas in low (sub-parts-per-million) concentrations. The tests also showed the hand-applied nanotube strips worked better on smoother paper substrates, and did not need uniformly consistent application.

The MIT chemists now plan to customize the sensors by adding atoms from other materials to the nanotubes to detect a wider range of gases, including ethylene, a gaseous hormone emitted in the ripening process of fruit. The team is also looking into sensors for sulfur compounds, which could help detect natural gas leaks.

The following video, Mirica tells more about building sensors from carbon nanotubes.

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