Pencils and regular office paper can work to create functional devices that can measure strain and detect hazardous chemical vapors. Safety at the tip of a pencil.
It all started in the middle of an Introduction to Conducting Polymers course during a discussion about the conductive properties of graphene, a one-atom thick layer of carbon that can come from a regular pencil.
“When you draw a line on a piece of paper, the graphite may shed numerous graphene sheets,” said Jiaxing Huang, associate professor of materials science and engineering at Northwestern University’s McCormick School of Engineering. “A student asked, ‘Can we use that graphene for something?’ That started an exploration of what pencil traces can do.”
One team of students — including lead authors Cheng-Wei Lin and Zhibo Zhao — started by measuring the conductivity of a pencil trace on paper, then used the traces to create a rudimentary electrode. They learned that curling the paper in one direction increased the trace’s conductivity by compressing the conductive graphene particles. Curling the paper in the other direction loosened the graphene network and decreased conductivity.
The students then turned to the traces of a bendable toy pencil. (These novelty pencils are flexible because the graphite mixes not with clay, but with a polymer binder.) Again, conductivity could increase and decrease by manipulating the paper, but the students found it also felt the presence of volatile chemical vapors, such as those from toxic industrial solvents.
When the chemical is present, the polymer binder absorbs the vapors and expands, pushing the graphene network apart and decreasing conductivity. The conductivity decreased the most in the presence of vapors more readily absorbed by the polymer binder.
These types of chemical sensor — also called “chemiresistors” — are key elements in “electronic noses” for detecting toxic chemical vapors. In creating chemiresistors, researchers often use more expensive materials, such as networks of carbon nanotubes or metal nanoparticles, and need to disperse them in polymer matrix to form a network.
“Now our students showed that this can be done simply with a pencil and paper — and it works,” Huang said. “This is a great example showing how curiosity leads to innovative work.”
Click here to view a paper on the subject.