The National Physical Laboratory has, as part of an international research collaboration, discovered a novel technique to monitor extremely low concentrations of NO2 in complex environments, using epitaxial sensors containing the “wonder material” graphene.
The research, as published in ACS Sensors, was led by an international collaboration of scientists from Linköping University, Chalmers University of Technology, Royal Holloway, University of London and the University of Surrey.
The findings demonstrate why single-layer graphene should be used in sensing applications and opens doors to new technology for use in environmental pollution monitoring, new portable monitors and automotive and mobile sensors for a global real-time monitoring network.
As part of the research, graphene-based sensors were tested in conditions resembling the real environment we live in and monitored for their performance. The measurements included, combining NO2, synthetic air, water vapor and traces of other contaminants, all in variable temperatures, to fully replicate the environmental conditions of a working sensor.
Key findings from the research showed that, although the graphene-based sensors can be affected by co-adsorption of NO2 and water on the surface, at about room temperature, their sensitivity to NO2 increased significantly when operated at elevated temperatures, 150 C. This shows graphene sensitivity to different gases can be tuned by performing measurements at different temperatures.
Testing also revealed a single-layer graphene exhibits two times higher carrier concentration response upon exposure to NO2 than bilayer graphene—demonstrating single-layer graphene as a desirable material for sensing applications.
Christos Melios, a lead scientist on the project from NPL, said: “Evaluating the sensor performance in conditions resembling the real environment is an essential step in the industrialization process for this technology.
“We need to be able to clarify everything from cross-sensitivity, drift in analysis conditions and recovery times, to potential limitations and energy consumption, if we are to provide confidence and consider usability in industry.”
By developing these very small sensors and placing them in key pollution hotspots, there is a potential to create a next-generation pollution map—which will be able to pinpoint the source of pollution earlier, in unprecedented detail, outlining the chemical breakdown of data in high resolution in a wide variety of climates.
Christos continued: “The use of graphene into these types of gas sensors, when compared to the standard sensors used for air emissions monitoring, allows us to perform measurements of ultra-low sensitivity while employing low cost and low energy consumption sensors. This will be desirable for future technologies to be directly integrated into the Internet of Things.”
NO2 typically enters the environment through the burning of fuel, vehicle emissions, power plants, and off-road equipment. Extreme exposure to NO2 can increase the chances of respiratory infections and asthma. Long-term exposure can cause chronic lung disease and is linked to pollution related death across the world.
Figures from the European Environment Agency also links NO2 pollution to premature deaths in the UK, with the UK being ranked as having the second highest number of annual deaths in Europe. In 2014, 14,050 deaths in the UK were recorded as being NO2 pollution related, 5,900 of which were recorded in London alone.
When interacted with water and other chemicals, NO2 can also form into acid rain, which severely damages sensitive ecosystems, such as lakes and forests.
Existing legislation from the European Commission suggests hourly exposure to NO2 concentration should not be exceeded by more than 200 micrograms per cubic metre (µg/m3) or ~106 parts per billion (ppb), and no more than 18 times annually. This translates to an annual mean of 40 mg m3 (~21 ppb) NO2 concentration.
In central London, for example, the average NO2 concentration for 2017 showed concentration levels of NO2 ranged from 34.2 to 44.1 ppb per month, a huge leap from the yearly average.
These figures show there is an urgent need for a low-cost solution to mitigate the impact of NO2 in the air around us. This work could provide the answer to early detection and prevention of these types of pollutants, in line with the government’s Clean Air Strategy.
Further experimentation in this area could see the graphene-based sensors introduced into industry within the next 2 to 5 years, providing an unprecedented level of understanding of the presence of NO2 in our air.
