Oxygen plasma treated carbon nanotubes for the wireless monitoring of nitrogen dioxide levels

There is a growing interest in the development of smart global systems of wireless, distributed sensors that can be deployed and then operated with no human intervention. However, there are still important technological barriers related to sensor performance and power consumption that limit the implementation of such systems. This paper shows that nanotechnology could help overcome these barriers. An oxygen plasma treatment has been used to functionalise, in a controlled way, the outer walls of multiwalled carbon nanotubes. This treatment respects the integrity of carbon nanotubes but gives rise to carbonyl groups grafted at their sidewalls, which greatly enhance the room-temperature sensitivity and selectivity of the nanomaterial to nitrogen dioxide. The detection mechanism of nitrogen dioxide in the presence of ambient moisture is discussed in light of computational modelling, compositional characterisation and gas sensing tests. The optimised nanomaterial has been used to develop a semi-passive radio frequency identification tag with the capability of sensing nitrogen dioxide in the environment. The tag, which is read by a low-cost commercially available ultra-wideband radar, is normally in sleep mode and includes a wake-up circuit, so power consumption is minimised. This wireless sensor is able to reproducibly detect nitrogen dioxide concentrations from 10 ppm to 100 ppm, with a mean relative error of 0.29%.

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