Chemists at the Massachusetts Institute of Technology have new tools for creating carbon-nanotube sensors: pencils. But the graphite in these pencils has been replaced with a compressed powder of carbon nanotubes. This makes creating sensors as easy as drawing a line on a sheet of paper.
Carbon nanotubes are carbon atoms linked together to form a tube. The carbon provides little opposition to the flow of electrons, creating a low-resistance path compared to normal carbon. Gases bind to the nanotubes, impeding electron flow and raising the tube’s electrical resistance, which is key to making the simple sensors.
The first sensor made with the new technology detected ammonia. Researchers drew lines of nanotubes between gold electrodes imprinted on a piece of paper and applied an electrical current. As the nanotubes attracted ammonia gas, the resistance changed. So resistance measurements indicated the amount of ammonia present.
Different kinds of paper provide different responses, with sensors drawn on smooth paper yielding the best reactions. Surprisingly, test results have remained consistent even when marks aren’t uniform.
Researchers hope that with adjustments, sensors could detect nearly any gas, including nerve gas. Soldiers could then carry pencils to quickly build sensors to detect chemical weapons.
Other gases being investigated include ethylene, emitted by ripening fruit. Ethylene sensors might show whether fruit being shipped or stored is too ripe. Likewise, nanotubes sensitive to sulfur could lead to sensors that detect natural gas leaks.