Sensors made from ‘frozen smoke’ can detect toxic formaldehyde in homes and offices

Researchers have developed a sensor made from ‘frozen smoke’ that uses artificial intelligence techniques to detect formaldehyde in real time at concentrations as low as eight parts per billion, far beyond the sensitivity of most indoor air quality sensors.

Researchers at the University of Cambridge have developed sensors made from highly porous materials called aerogels. By precisely engineering the shape of the holes in the aerogels, the sensors were able to detect the fingerprint of formaldehyde, a common indoor air pollutant, at room temperature.

The proof-of-concept sensors, which require minimal energy, could be adapted to detect a wide range of hazardous gases, and could also be miniaturized for wearable and healthcare applications. The results are reported in the journal Scientists progress.

Volatile organic compounds (VOCs) are a major source of indoor air pollution, causing watery eyes, burning eyes and throat, and difficulty breathing at high levels. High concentrations can trigger attacks in people with asthma and prolonged exposure can cause certain cancers.

Formaldehyde is a common VOC and is emitted from household items, including pressed wood products (such as MDF), wallpaper and paints, and some synthetic fabrics. For the most part, the levels of formaldehyde emitted by these items are low, but levels can build up over time, especially in garages where paints and other formaldehyde-emitting products are more likely to be stored.

According to a 2019 report from campaign group Clean Air Day, a fifth of UK households had notable concentrations of formaldehyde, with 13% of residences exceeding the recommended limit set by the World Health Organization (WHO).

“VOCs such as formaldehyde can cause serious health problems with prolonged exposure, even at low concentrations, but current sensors do not have the sensitivity or selectivity to distinguish between VOCs that have different impacts on health,” said Professor Tawfique Hasan of Cambridge Graphene. Center, who led the research.

“We wanted to develop a sensor that was small and low-power, but could selectively detect formaldehyde at low concentrations,” said Zhuo Chen, the first author of the paper.

The researchers based their sensors on aerogels: ultra-light materials sometimes called “liquid smoke” because they are made up of more than 99% air by volume. The open structure of aerogels allows gases to enter and exit easily. By precisely engineering the shape or morphology of holes, aerogels can act as highly effective sensors.

Working with colleagues at the University of Warwick, the Cambridge researchers optimized the composition and structure of aerogels to increase their sensitivity to formaldehyde, transforming them into filaments around three times the width of a human hair.

The researchers 3D printed lines of a paste made from graphene, a two-dimensional form of carbon, then freeze-dried the graphene paste to form the holes in the final airgel structure. Aerogels also incorporate tiny semiconductors called quantum dots.

The sensors they developed were able to detect formaldehyde at concentrations as low as eight parts per billion, or 0.4% of the level deemed safe in UK workplaces. The sensors also operate at room temperature and consume very little power.

“Traditional gas sensors need to be heated, but because of the way we designed the materials, our sensors work incredibly well at room temperature, so they use between 10 and 100 times less energy than other sensors,” he said. Chen said.

To improve selectivity, the researchers then integrated machine learning algorithms into the sensors. The algorithms were trained to detect the “fingerprints” of different gases, so the sensor can distinguish the formaldehyde fingerprint from other VOCs.

“Existing VOC detectors are blunt instruments: You only get one number for the overall concentration in the air,” Hasan said. “By building a sensor that can detect specific VOCs at very low concentrations in real time, it can give home and business owners a more accurate picture of air quality and any potential health risks .”

The researchers say the same technique could be used to develop sensors to detect other VOCs. In theory, a device the size of a standard household carbon monoxide detector could integrate several different sensors, providing real-time information on a range of different dangerous gases.

The Warwick team is developing a low-cost, multi-sensor platform that will integrate these new airgel materials and, coupled with AI algorithms, detect different VOCs.

“By using highly porous materials as a sensing element, we are opening up entirely new ways to detect hazardous materials in our environment,” Chen said.