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Cleaning products, candles, cribs, and cosmetics are just a few of the common household items that emit formaldehyde, a colorless, odorless chemical that, when present in the air at levels higher than 0.1 parts per million, has been found to be a risk to human health.

To make indoor air quality monitoring more accessible, researchers at Carnegie Mellon University have developed a low cost, long-lasting, indoor formaldehyde sensor. A unique polymer coating on the MXene-based sensor not only extends its half life by 200%, but also enables it to regenerate when performance begins to degrade.

MXene is a class of compounds that has shown promise in energy storage and gas sensing because of its superior electrical properties and versatile surface chemistries. Despite these advantages, MXenes are known to be highly susceptible to oxidation, particularly when exposed to air and/or humidity, posing a major challenge for MXene-based air quality monitors.

New research this week in Science Advances, overcomes this challenge by encapsulating the MXene in a polymer coating. Using Chemical Vapor Deposition, a materials processing technique that is fundamental to electronics manufacturing, the research team led by Reeja Jayan, Mechanical Engineering Professor, pumps vaporized precursor materials into a vacuum chamber housing the MXene sensor. The hot gases polymerize and form a nano-coating on the cold sensor in a way similar to how condensation coats the outside of an ice-cold drinking glass on a hot day.

Without the polymer coating, the MXene sensor lasted for a little over two months, but when the polymer layer was applied, the sensor ran for over five months.

Shwetha Sunil Kumar, the first author of the research paper and Ph.D. candidate in Mechanical Engineering, noticed something unexpected during the observation period.

"We found that our polymer layer was also enabling a chemical reaction that allowed the sensor to detect lower levels of formaldehyde in the air. Because a new molecule was forming to make the sensor more sensitive, we began to wonder if enabling the creation of more of those molecules when the sensor performance begins to degrade, could help the sensor regenerate," explained Kumar.

Sure enough, the team found that by introducing humidity to the sensor at the end of its life it regained about 90% of its sensing ability.

"The polymer layers we synthesized are dielectric and highly insulating, acting as effective structural barriers," explained Jayan. "That makes our sensors both stable and sustainable."

Simulations run by Jerry Wang, Assistant Professor of Civil and Environmental Engineering, further proved the effectiveness of the sensors. By testing how quickly large amounts of different molecules could move through the layer, the team confirmed that the polymer layer swiftly slows oxygen and moisture permeation.

Jayan is confident that these materials could be deployed to other devices to enhance lifetime and performance. She is currently developing to extend the life and safety of batteries.

Albert Presto, director of the Center for Atmospheric Particle Studies at CMU and co-author of the paper, believes that this technology can make indoor air quality sensors more versatile. The sensor is easily interfaced to cell phones or smart home devices, and with continued development, he believes sensors could someday be painted onto our walls or sewn into our clothing for consistent monitoring.

"Indoor air quality is often overlooked," he said. "We are living in a plastic heavy world and that has implications. We want to better educate people on indoor pollutants, so that they can make informed decisions. Only then can we improve indoor air quality and eliminate the inherent health risks."