A highly sensitive, wearable gas sensor like an 'electronic nose' for environmental and human health monitoring may soon become commercially available, say researchers.

According to the study, published in the Journal of Materials Chemistry, the sensor device is an improvement on existing wearable sensors because it uses a self-heating mechanism that enhances sensitivity. It allows for quick recovery and reuse of the device. Other devises of this type require an external heater. In addition, other wearable sensors require an expensive and time-consuming lithography process under cleanroom conditions, the study said. "People like to use nanomaterials for sensing because their large surface-to-volume ratio makes them highly sensitive," said study researcher Huanyu Cheng, Assistant Professor at Penn State University in the US.

"The problem is the nanomaterial is not something we can easily hook up to with wires to receive the signal, necessitating the need for something called interdigitated electrodes, which are like the digits on your hand," Cheng added. For the results, the research team use a laser to pattern a highly porous single line of nanomaterial similar to graphene for sensors that detect gas, biomolecules, and in the future, chemicals. In the non-sensing portion of the device platform, the team creates a series of serpentine lines that they coat with silver.

When they apply an electrical current to the silver, the gas sensing region will locally heat up due to significantly larger electrical resistance, eliminating the need for a separate heater, according to the study. The serpentine lines allow the device to stretch, like springs, to adjust to the flexing of the body for wearable sensors, the research said. The nanomaterials used in this work are reduced graphene oxide and molybdenum disulfide, or a combination of the two; or a metal oxide composite consisting of a core of zinc oxide and a shell of copper oxide, representing the two classes of widely used gas sensor materials -- low-dimensional and metal oxide nanomaterials.

"We showed that we could detect nitrogen dioxide, which is produced by vehicle emissions. We can also detect sulfur dioxide, which, together with nitrogen dioxide, causes acid rain. All these gases can be an issue in industrial safety," said study co- author Ning Yi. "Using a CO2 laser, often found in machine shops, we can easily make multiple sensors on our platform," Cheng said. "We plan to have tens to a hundred sensors, each selective to a different molecule, like an electronic nose, to decode multiple components in a complex mixture," Cheng added.

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