As kids, most of us were introduced to various air pollutants and their harmful effects in our schoolbooks. Over the past few decades, the number of these pollutants and the industries that spew them have increased manifold, to the extent that people walking around in anti-pollution masks has become a common sight in most Indian cities these days. Some of these toxic pollutants are known to cause asthma, lung cancer and even death.
Now, researchers from the Indian Institute of Science (IISc), Bengaluru, have designed a novel gas sensor for monitoring air pollution by quantifying various gaseous pollutants. The gas sensor, the researchers say, can accurately measure carbon monoxide (CO), carbon dioxide (CO2), nitrogen dioxide (NO2) and sulphur dioxide (SO2). It is also cost-effective. They recently published their results in the Journal of Microelectromechanical Systems from IEEE Xplore. The Times of Indiareported recently on the CO detection capabilities of the sensor.
The researchers have assembled an array of gas sensors — a microelectromechanical system — on a single chip. Microelectromechanical systems (MEMS) are miniaturised mechanical and electromechanical devices ranging from well below one micron (a billion times smaller than 1 metre) to several millimeters (1,000 times smaller than 1 metre) in dimension. “Since they consume just a few milliwatts of power for operation, they fulfil the current demands for integration in battery-operated (portable) devices. This kind of small footprint is not possible to achieve using macro components,” says professor Navankanta Bhat from the Centre of Nano Science and Engineering (CeNSE), IISc, who led this research effort at the institute.
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This sensor array contains four micro-heaters and four sensor elements, each of them customised to sense a specific gas: zinc oxide (ZnO) for CO, barium titanate-cupric oxide (BaTiO3-CuO) doped with 1% silver (Ag) for CO2, tungsten trioxide (WO3) for NO2 and vanadium pentoxide (V2O5) for SO2.
This sensor array contains four micro-heaters and four sensor elements, each of them customised to sense a specific gas: zinc oxide (ZnO) for CO, barium titanate-cupric oxide (BaTiO3-CuO) doped with 1% silver (Ag) for CO2, tungsten trioxide (WO3) for NO2 and vanadium pentoxide (V2O5) for SO2
Gas sensors usually suffer from insufficient sensitivity to gases, lack of reproducibility, temporal drift, and instability. However, the newly developed sensor array acts as an “electronic nose” due to its enhanced selectivity and sensitivity. This is because of its different sensing pattern for each gas in a mixture. The data collected from the different array elements can be utilised to distinguish constituent gases of the mixture, thus making it capable of onsite monitoring of air pollution.
The researchers have designed the new sensor such that it consumes very low power (∼10 mW for the micro-heaters to achieve 300°C), is very compact, and allows for the temperature of each sensing unit to be controlled separately, thus being highly selective. Such a feature was made possible by using a stress-engineered silicon dioxide (SiO2) diaphragm, made using plasma-enhanced chemical vapour deposition. A diaphragm is a platform that hosts all the array elements. It is a chemically inert material that makes it possible to join together with a wide variety of media, thus helping reduce both the size and the cost of the sensors.
One of the investigations that was carried out for optimising the system was the synthesis process of the diaphragm (SiO2). Taking into account the efficiency of various synthesis routes, the plasma-enhanced chemical vapour deposition demonstrated higher fabrication yield compared to thermally grown SiO2. The investigators carried out simulations of both the diaphragms under thermal, bending and tensile stresses, and the yield was found to be higher in the former route.
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The researchers also ensured that the new array was not “cross-sensing” interfering gases, a drawback of most metal oxide-based sensors. By recording the response of interfering gases, they chose the optimal sensing material, thus eliminating cross-sensing. “We are already evaluating the interference from ambient temperature and humidity and appropriate algorithms can be developed to achieve robust gas-sensing performance against all interfering agents,” says Dr Chandra Shekhar Prajapati, a postdoctoral fellow who worked on the project, talking about future plans to improve the sensor.
“This sensor array technology was developed as a prototype device for air quality monitoring, and is currently being used for selective detection of NO2 at Satish Dhawan Space Centre SHAR, Isro. The future possibilities include food quality monitoring and breath analysis for disease diagnosis”
— Professor Navankanta Bhat, CeNSE, IISc
The newly developed micro-sensor array will find a range of applications in various fields that include sensing and diagnosis. “This sensor array technology was developed as a prototype device for air quality monitoring, and is currently being used for selective detection of NO2 at Satish Dhawan Space Centre SHAR, Isro. The future possibilities include food quality monitoring and breath analysis for disease diagnosis. The miniature size of the micro-sensors can also enable the integration of such devices in cellphones in the near future,” says Bhat.
There is a global need for reliable and low-cost sensors. With air pollutants rising over time, sensors help address issues related to public health by augmenting data regarding their quantity, source and distribution, as well as develop methods to subside them. In the long run, they could possibly aid in environmental public policy. Also read other Science Language articles
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