Sensing a silent killer

Tawfique Hasan advises paying attention to more than just particulate matter when it comes to indoor air pollution. The University of Cambridge Professor emphasises that we shouldn't overlook other harmful gases like formaldehyde. According to the Dyson Global Connected Air Quality Data published in January, India ranks 7th in indoor air pollution due to volatile organic compounds (VOCs).

Describing VOCs, such as formaldehyde, as 'silent killers', Hasan says that because we spend most of our time indoors – in cars, offices and homes, the effect of the indoor air pollution is quite severe. The U.S. Environment Protection Agency notes damage to liver, kidneys and central nervous system, and even cancers, as health effects of VOCs.

While there are sensors available that can measure the total VOC content of air, they lack selectivity – not every VOC is equally harmful. Hasan has invented a smart sensor (bit.ly/AerogelSensor) that can detect formaldehyde, a cancer-causing substance, at room temperature and with low power usage (130 microwatts). He 3D-prints a special 'ink' made by mixing small particles of tin dioxide known as quantum dots (QDs) with graphene, a two-dimensional material made of a single layer of carbon atoms, onto a printed circuit board. The QDs are the main sensing elements and are incorporated with metal catalysts.

When 3D-printed, this ink forms a highly porous structure similar to a sponge, called an aerogel. Due to their unique structure and properties, aerogels have many applications like thermal insulation and cancer diagnosis (see Nature's wonder material). Hasan drew out a synergy between the highly porous structure of graphene aerogel and the sensing capability of QDs. The porosity of the aerogel helps fast-moving gases to spread out and interact with the QDs easily. This interaction changes the QD's conductivity, allowing it to sense formaldehyde. Since there are several VOCs that may be present in the air, Hasan and his colleagues went a step ahead and incorporated a clever algorithm with the sensor to distinguish and recognise formaldehyde selectively in real time.

They can sense one part of formaldehyde in a million parts of air and can go to an ultralow limit of detection of around 8 parts of formaldehyde in a billion parts of air. "This combination of accuracy, stability and gas recognition capability is a game-changer for prolonged, real-world gas-sensing applications," says Zhuo Chen, research student at Cambridge and first author of the study. The ultimate goal is to commercialise the sensor. However, first the researchers plan on bringing together similar sensors but with different QDs to enable the selective sensing of more than one VOC.