Microwave flexible gas sensor based on polymer multi wall carbon nanotubes sensitive layer

This study presents the feasibility and the first real time results of microwave flexible gas sensor based on poly (3,4-ethylenedioxythiophene) polystyrene sulfonate - multi wall carbon nanotubes (PEDOT:PSS-MWCNTs) as sensitive material, deposited by inkjet printing technology. The sensor is suitable for wireless applications, it consists of two stub resonators on kapton in order to provide a differential detection. The final aim of this work is to develop a low cost communicating sensor which can be integrated into real time multi sensing platform dedicated to the applications requiring low power consumption and adaptable for the Internet of Things (IoT), in order to do the detection of harmful gases such as Volatile Organic Compounds (VOCs). Preliminary results have shown a large influence of ethanol concentration on the electrical properties of the passive resonators at radio-frequency range. These vapors have induced additional insertion losses and frequency shifts on the first resonant frequency mode around 0.65 GHz. The sensor sensitivity to ethanol vapors exposition has been estimated to −642.9 Hz/ppm and −7 μdB/ppm for resonant frequency and insertion losses variations in differential mode, respectively, according to the values at 4 min of exposure to 500, 1000 and 2000 ppm. To deepen the study of the sensor, we have focused on the influence of ethanol on the conductivity of the sensitive layer, in terms of repeatability and sensitivity. We proposed a way of real-time reconstruction of the response by representing the difference of the insertion losses as well as the difference of frequencies calculated on the basis of the phase average value within a specified frequency range near the resonance. This lead to estimate a sensitivity of −9 μdB/ppm and 648.1 Hz/ppm, respectively, for ethanol concentrations ranging from 500 ppm to 2000 ppm at 10 min of exposure.