Towards a polymer CO2 sensor

Concentrations of carbon dioxide in greenhouses may be monitored by a cheap and small solid-state nano device. A special polymer coating makes it sensitive to the greenhouse gas.

Market gardeners are among the few people who welcome elevated CO2-levels. It makes their plants grow faster, but the CO2 levels in the greenhouses must be continuously monitored since it forms a health hazard for people working there. However, existing technologies based on metal oxide sensors or on spectroscopy are cumbersome and expensive.

During his PhD research, Dr. Xiangping Chen set out to develop a new type of solid-state CO2-sensor that would be fast, sensitive and affordable. He based his design on the work of Zhong Lin Wang (Georgia Institute of Technology, US) who presented a gas sensor that works with a single zinc oxide nanowire. Nature explains: ‘When a gas molecule sticks to the surface of the nanowire, it changes the wire’s ability to conduct electrons, and this change can be used to sense the presence of gasses such as carbon monoxide or oxygen.”

The snag is that the device is most sensitive at about 275 Celsius, which makes it hard to turn it into a cheap, low-energy and continuous gas sensing device since it requires heating to function.

Dr. Chen initially proposes to increase the sensitivity of the device by covering the nanowire with a specially designed polymer such as polyaniline (also known as PANI) and later does away with the nanowire altogether, thereby simplifying the construction of the device to two electrodes on a substrate with a strip of polymer in between.

Polymers are more often used to add chemical functionality to electronic devices, potentially turning them into electronic noses, taste buds or indeed gas detectors. Chen explains the function of the polymer is twofold: it increases the sensitivity and makes the sensor more specific for CO2 detection.

In the moist environment of a greenhouse, CO2 reacts with water vapour to form H+ and a HCO3 group. This means that a polymer sensitive to proton-doping may be used for moist CO2-detection. Chen proposes the polymer emeraldine base PANI (or EB-PANI) that changes its electrical resistance in response to protons, or acid (H+), which makes it a suitable candidate for a solid-sate CO2-sensor.

By modifying the EB-PANI with NaSO3 groups along the polymer, Chen shows that the sensitivity can be increased by a factor of hundred, making the polymer sensitive to concentrations between 1 and 100 ppm of CO2 (ppm stands for parts per million; the atmospheric concentration nowadays is a little under 400 ppm). That’s about the right range for the greenhouse range.

Prof. Paddy French, who was in Chen’s doctoral committee, says the polymer technology is promising, but that more work is needed to proof that the proposed gas sensor is specifically sensitive to CO2 and not to whichever other gases may pass. Another point of interest for a first prototype would be the robustness and the lifespan of the device. 

Xianping Chen, Molecular Modelling in Design of Polyaniline for Polymer-based Carbon Dioxide Sensor, PhD supervisor Prof. G.Q. Zhang, 02 March 2013


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