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A breath of fresh air

Researchers at Australia National University have developed a miniaturised gas sensor to continuously monitor air quality in our vehicles.

Well known for its clean air, Australia ranks among the top 10 countries globally in terms of overall air quality. However, even at low levels, air pollution can negatively impact our health. Although particulate matter from dust storms and bushfires causes extreme air pollution events regionally, motor vehicles and road traffic are the main generators of pollution in urban areas.

Sitting in traffic can literally be a threat to your health. Gasoline-powered vehicles produce the pollutant nitrogen dioxide (NO2) and are the highest contributors to NO2 emissions in Australia. Exposure to this gas can increase a person’s vulnerability to, and the severity of, asthma and respiratory infections and can irreversibly damage the human respiratory tract. Long-term exposure will reduce a person’s sense of smell and can cause chronic lung disease.

Even drivers of electric vehicles (EVs) can be exposed to high levels of this toxic gas. While sitting in traffic, exhaust from other cars can enter and become trapped within a vehicle. In daily traffic, NO2 levels inside a car can be 10 times higher than the air outside.

The levels of the major air pollutants, such as carbon monoxide, sulphur dioxide, NO2, ozone and particulate matter, are monitored by the Australian government This information is reported on weather and air quality apps and websites and in daily news. However, the numbers you see are not reporting the air quality in your vicinity. The equipment for measuring air pollutants is stored in air quality monitoring stations (AQMS), so the air quality report is for the station’s location. In addition to poor geographical resolution, our current air monitoring systems rely on specialised large-footprint instruments that require trained personnel to operate. Air sampling is not continuous, and instrument processing causes a delay in getting the analysis. Real-time, local measurement of NO2 pollution is not available in a crucial location – our vehicles.

Chemiresistive sensing is one of the most promising technologies for portable and miniaturised chemical sensing. Chemiresistive materials experience changes in electrical resistance in response to changes in the nearby chemical environment. Recently, there have been growing efforts to develop microchip-based chemical sensors operating at room temperature with high sensitivity, selectivity, spatial and temporal resolution, long-term stability, and cost-effectiveness.

Shiyu Wei, a PhD student of Prof. Lan Fu in The Australian National University’s Department of Electronic Materials Engineering, has taken on this challenge, developing a highly performing miniaturised gas sensors containing chemiresistive indium phosphide (InP) vertical nanowire arrays. Chemical adsorption of NO2 onto the InP surface increases the wire’s resistance, resulting in measurable change in the electrical current. Nanostructuring of the InP creates a high surface-to-volume ratio that allows more efficient gas adsorption and resistive change for greater sensing performance.

Working hand in hand with Dr Kaushal Vora and the staff of ANFF-ACT, Ms Wei optimized various parameters of her design, including array geometry and nanowire diameter and length. Under Dr Vora’s guidance, she mastered the art of fine tuning her fabrication method and troubleshooting her design to successfully produce one of the world’s best chemical sensors that selectively detects NO2.

The Fu research group’s proof-of-concept device is a game changer for air pollution monitoring, providing local data in real-time with outstanding selectivity. Current instruments have limited sensitivity at parts per million. Their chemiresistive InP gas sensor can detect amounts as little as 3.1 parts per billion, which is necessary to accurately measure repeated exposures to this chemical at the relevant concentration levels.

At less than 5 mm x 5 mm in size, this miniaturised sensor has the potential for integration into the on-board electronics in our vehicles. Providing real-time data on NO2 levels in a car’s interior will provide drivers with critical information to limit their exposure to this toxic gas with simple actions such as recirculating the air and closing the windows.