Graphene-based NO₂ detectors could be a breath of fresh air for polluted urban centres

Graphene Flagship partners at the National Physical Laboratory, UK, and Chalmers University of Technology, Sweden, alongside colleagues at the Advanced Institute of Technology, UK, Royal Holloway University, UK, and Linköping University, Sweden, have created a low-cost, low-energy consuming NO₂ sensor that measures NO₂ levels in real-time – and it could help to visualize pollution in urban areas.

NO₂ gas is produced by burning fossil fuels, and it can cause airway inflammation, leading to breathing problems and even asthma attacks. Recent research also links NO₂ exposure to childhood obesity and dementia. The European Union and UK Parliament have introduced legislation to regulate the amount of NO₂ in the air – but significant portions of the population are exposed to NO₂ levels above this limit, so experts are calling for new ways to monitor pollutant levels. The two usual methods to monitor air pollution are optical laser techniques, such as chemiluminescence, but this requires large and expensive lab equipment – and metal oxide detectors, which are small but lack sensitivity. Neither of these are good enough for large scale, continuous NO₂ monitoring, so Graphene Flagship researchers are working on new, innovative sensing devices.

Christos Melios, from the National Physical Laboratory and the Advanced Institute of Technology, UK, and colleagues, have now developed a graphene-based NO₂ detector that reports pollutant levels based on changes in its electrical resistance. They grew graphene on silicon carbide, etched it into an appropriate shape and fused it to a detector chip using metal wires. When NO₂ from the air is physically absorbed by the graphene layer, the resistance of the graphene changes, which produces a recordable signal. "When a molecule like NO₂ is absorbed on a graphene layer, it withdraws electrons from graphene, which increases its resistance significantly," Melios says. "You only need very simple electronics for the signal read-out."

Graphene has a large surface-to-volume ratio, with most of its free electrons on the surface. This means that when an NO₂ molecule becomes attached to the surface, significant charge transfer takes place, so even very low concentrations of NO₂ can be detected. "We achieved 10 ppb sensitivity for NO₂, which is extremely low and really desirable for environmental monitoring," Melios continues. "We also measured it in different environments – we tried to mimic the ambience, so we varied the humidity and temperatures, and also mixed different gases – so we could confirm that the sensor works in a real environment."

The simplicity of the device means that small, commercially available sensors can be easily adapted for accurate NO₂ detection. These cheap, adaptable sensors could then be set-up across a city – such as on lampposts and street signs – to form a robust network of sensors and create a 'heat map' of air quality, collecting real-time pollution data all day and night. Alternatively, mobile sensors could be carried by pedestrians and cyclists for personal environmental data monitoring. Co-author Olga Kazakova, also from the National Physical Laboratory, UK, comments: "Understanding the problem is the first step to solving the problem. If you only monitor some junctions or roads for NO₂ pollution, you do not get an accurate picture of the environment. In order to do this, a network must be set up to show the dynamically changing level of pollution through different times of day and year, so you can get to know the real level of critical exposure."

With this data, people could use an app to check how much NO₂ pollution they might be exposed to on their planned route, and city councils could use this information to restrict and divert cars near schools and hospitals. According to Melios, this would enable governing bodies to adopt restrictive measures in specific areas recognised as being highly pollutive. "If you can measure NO₂ pollution accurately, you can produce explicit, temporary laws for specific areas, instead of covering entire cities," he says.

Sanna Arpiainen, Deputy Work Package Leader of the Graphene Flagship's Sensors division, said that "evaluating the sensor performance in conditions resembling the real operation environment is an essential step in the industrialisation process. The cross sensitivity, stability and drift in typical analysis conditions, response and recovery times and possible limitations of each sensor concept need to be clarified, not to forget the energy consumption, fabrication cost and scalability issues. This work is done in the Graphene Flagship in many fronts."

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel adds: "NO₂ sensing was one of the first applications of graphene demonstrated. Flagship partners have now developed this into practical devices for everyday use. This is yet another example of innovation fuelled by the collaborative nature of our project"

'Detection of Ultralow Concentration NO2 in Complex Environment Using Epitaxial Graphene Sensors', ACS Sensors, 2018, 3, 9, 1666-1674. Christos Melios, Vishal Panchal, Kieran Edmonds, Arseniy Lartsev, Rositsa Yakimova and Olga Kazakova.