Paper and pencils used to create chemical vapour detection system


Three students from a university in Illinois, USA, have published a paper about a new way to turn everyday pencils and paper into a system that can detect chemical vapours.

In 2011 Cheng-Wei Lin, Zhibo Zhao and Jaemyung Kim sat in on a lecture about conducting polymers at Northwestern University’s McCormick campus. Jiaxing Huang, their professor, explained in the lecture that sheets of graphene would be shed from a pencil when drawing on a piece of paper. Graphene, of course, is the miracle material that’s made of a one-atom-thick layer of graphite. And the lead in pencils isn’t lead, but actually graphite.

Using this knowledge, one of the students ask the obvious question: can that graphene be used for something?

Armed with a selection of pencils and some paper the group set about experimenting. First they draw on paper using the pencils and measured the conductivity of the graphite path they’d created. After that, they bent the paper to measure the conductivity when the layer of graphite was compressed or expanded. Bending the paper inwards, compressing the layer of graphite, the conductivity increased. And doing the opposite yielded a decrease in conductivity.

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Then the team used a different kind of pencil. Conventional wooden pencils have their lead core made up of graphite and clay, but novelty pencils that are flexible use a polymer – a synthetic chemical compound – to retain flexibility while still keeping the graphite particles together. As science would have it, the team’s experiments were repeated with these pencils and the results were repeated.

However, with a polymer embedded in the graphite there was another benefit. Any chemical vapours in the area would be detected by this rudimentary circuit thanks to the absorbency of the polymer in the graphite. Basically, the glue in the graphite would absorb chemical gases, which caused the polymer to expand – and decrease the conductivity of the graphite circuit. The result is something that’s known as a chemiresistor – a circuit that changes resistance based on chemical input. These are usually used in many industrial machines for detecting chemical leaks or even bombs.

Fun fact: When pencils were first manufactured the graphite used in them was originally thought to be lead ore, and the name’s stuck ever since. In some languages the word pencil literally means “lead pen”. The only lead ever to be used in pencils was in the paint on pencils in the 50s and 60s.

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