Carbon nanotube pencil leads used for drawing sensors onto paper
By Ben Coxworth
October 10, 2012
We’ve already seen a pen with silver-based ink, that lets its user draw electrical circuits on ordinary paper. Now, scientists from MIT have brought similar “hands on” technology to the humble pencil – they’ve compressed carbon nanotubes together to form a pencil lead substitute, that has been used to draw gas sensors onto regular paper imprinted with gold electrodes.
Carbon nanotubes have already found use in various chemical sensors. Typically, molecules of a specific gas will bind with the electrically-conductive microscopic tubes, impeding the flow of electrons through them. When the sensing device subsequently attempts to run an electrical current through the nanotubes, the amount of resistance created by the gas molecules will be measured, indicating how much (if any) of the gas is present in the environment.
Unfortunately, in order to create such sensors, nanotubes must be dissolved in a toxic solvent such as dichlorobenzene. The process is potentially hazardous, and it doesn’t always work.
MIT Professor of Chemistry Timothy Swager and postdoctoral fellow Katherine Mirica decided to find a safer way to create such sensors, that didn’t require solvent. Mirica came up with the idea of compressing a carbon nanotube powder into a graphite-like solid, then using it in place of the lead in a mechanical pencil.
To create sensors, the pencil is used to draw a bar on a sheet of the gold-imprinted paper, that bar bridging a gap within one of the electrodes. When an electrical current is then run through the electrode and measured, any resistance that it encounters when passing through the bar will indicate the presence of ammonia gas molecules.
Smoother grades of paper appear to work best for creating the sensors, and the marks themselves don’t need to be particularly uniform. Not only is the process of creating the sensors safer than using solvents, but it’s also less expensive, and the “leads” themselves are very stable.
So far, they have been made from nanotubes composed of pure carbon. However, the researchers believe that by altering the nanotubes – through adding metal atoms or wrapping them in polymers, for instance – they could be tailored to detect gases other than ammonia. Swager and Mirica are particularly interested in developing the capability to detect ethylene, which indicates the ripeness of fruit, and sulfur compounds, which would be present in the event of a natural gas leak.
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