A team of MIT chemists have created a carbon nanotube “lead” that can be used to draw freehand electronic circuits using a standard, mechanical pencil.
In a normal pencil, the lead is usually fashioned out of graphite and a clay binder. Graphite, as you may already know, is a form of carbon that is made up of layer after layer of the wonder material graphene. When you write or draw with a graphite pencil, a mixture of tiny graphene flakes and clay are deposited on the paper, creating a mark. (Incidentally, pencil leads never contained lead; it’s just that when graphite was first used in the 1500s, they thought it was lead ore, and the name stuck).
With MIT’s carbon nanotube pencil, the lead is formed by compressing single-walled carbon nanotubes (SWCNT), until you have a substance that looks and behaves very similarly to graphite. The difference, though, is that drawing with MIT’s pencil actually deposits whole carbon nanotubes on paper — and carbon nanotubes have some rather exciting properties.
In this case, MIT is utilizing the fact that SWCNTs are very electrically conductive — and that this conductivity can be massively altered by the introduction of just a few other atoms, namely ammonia.
In the picture above, electricity is applied to the gold electrodes (which are imprinted in the paper). The carbon nanotube pencil is used to fill in the gaps, and effectively acts as a resistor. When ammonia gas is present, the conductivity of the nanotubes decreases, and thus resistance increases — which can be easily measured. Carbon nanotubes are so sensitive that MIT’s hand-drawn sensor can detect concentrations of ammonia as low as 0.5 parts per million (ppm).
There are two main takeaways here. The first is that MIT has found a form of carbon nanotubes that is stable, safe, and cheap to produce. Second, carbon nanotubes have been used in sensors before, but usually the process involves dissolving SWCNTs in solvents, which can be dangerous. Here, creating a carbon nanotube sensor is as simple as drawing on a piece of paper — either by a human, or an automated process.
The team will now work on other carbon nanotube leads that can be used to detect other gases, such as ethylene (produced by fruit as it ripens) and sulfur (for detecting natural gas leaks). It’s also worth noting that the research was partly funded by the US Army/MIT Institute for Soldier Nanotechnologies — so it wouldn’t be surprising if military personnel are eventually outfitted with these sensors… or perhaps their very own carbon nanotube pencil, for MacGyver-like sensor fabrication in the field.