Curiosity is often a driving force to innovative work. A team of students lead by Prof. Jiaxing Huang, at Northwestern University, show in an article just published in Scientific Reports that chemical sensors capable of detecting toxic vapors can be drawn with a toy pencil on paper.
The “lead” in ordinary pencils is made of graphite fine powders bound together with clay. In a flexible toy pencil, the graphite powders are glued together with a soft polymer binder such as polyvinyl chloride (PVC), which makes the “lead” bendable and unbreakable. Drawing with such flexible pencils leaves a trace that is essentially a thin film of polymer/graphite composite. When the graphite fine powders, or “few-layer graphene flakes” (as they are sometimes delightfully rebranded after the discovery of graphene), are interconnected to form a continuous network, a conductive path is established. Upon exposure to volatile chemical vapors such as toluene, hexane and tetrahydrofuran, the polymer binder swells and loosens up the graphite network, causing a decrease in electrical current. This effectively creates a chemiresistor type of sensor. Among the vapors tested, the toy pencil drawn sensor showed greatest response to toluene, which is consistent with the high solubility of PVC in toluene.
Chemiresistors based on a network of conductive materials embedded in polymer have been demonstrated before with carbon blacks, carbon nanotubes, graphene or metal nanoparticles. Other than being low cost, disposable and environmental friendly, the toy pencil drawn devices reported by Huang and his students are a great reminder that “Science can be fun and inspirations can be drawn from daily life objects around you,” Huang said.