Since the environmental aspects are one of the biggest concerns nowadays, new ways to detect and monitor ambient air pollutants and toxic gases became critical. One of the most dangerous species amongst them is nitrogen dioxide (NO₂), which has a major negative effect on the environment. NO₂ is accountable for acidic rains, ozone formation and is a major environmental cause of morbidity and mortality worldwide, even in low concentrations, if there is a repetitive or long-term exposure.

Most of the current NO₂ SC sensors work at high temperature (typically between 200 and 400^◦C) because the performances at room temperature are generally very poor. Moreover, heating the sensitive layer leads to higher power consumption.

Electrochemical sensors are an alternative to SC devices for NO₂ detection. The key advantages of EC detection include low level linear output with high resolution, good selectivity and repeatability, ppm level detection with high accuracy and low costs compared to other techniques.

Surface acoustic wave sensors are another available sensing technology, which offers a fast and highly sensitive way for NO₂ detection. Deployment of SAW device in the sensors' domain is an emerging area of research due to its miniaturization, high sensitivity, extremely low power consumption, and fast response time along with inherent passive nature, wireless and portable. In addition, the fabrication process of SAW device is compatible to microelectronics technology, which offers high throughput, possibility of integration with MEMS for smart sensors development, mass production and scalability.

SAW operation is based on both direct and reverse conversions between electrical and mechanical energies, using the piezoelectric effect and specific interdigital transducers (IDTs). Three stages are involved in this process: (i) excitation of the acoustic wave in a piezoelectric material, (ii) modulation of both amplitude and phase characteristics, as an effect of the sensing process, of the acoustic wave along the propagation path, and (iii) re-conversion of the acoustic wave into an electrical signal for successful detection.

The present Postdoc project will introduce for the first time the integration of the CVD graphene sensing nanomaterial with a surface acoustic wave resonator (SAWR) for detection at room-temperature of very low NO₂ concentrations.

The novel sensor will consist of two quartz-based Rayleigh SAWRs arranged in a dual oscillator configuration, where one resonator will be coated with gas-sensitive graphene, while the other is left uncoated, in order to act as a reference. Thus, the variation of the measured signal parameter, i.e., the difference between the frequencies of these oscillators, will depend only on the changes in NO₂ concentration. Furthermore, graphene can be non-covalently functionalized to improve its specificity for selective sensing of the NO₂.