This work reports the design of a resistive gas sensor based on 2D mats of multi-walled carbon nanotubes (MWCNT). The sensor sensitivity was optimised using chlorine by tuning both MWCNT network morphology and MWCNT electronic properties. Raw CNT were compared with annealed CNT. Besides, with the aim to enhance the sensor sensitivity and selectivity for detection of several gases, MWCNT were functionalized with poly (phenylene)-like or vinyl polymers using a process based on the diazonium chemistry. In this paper, we will mention the preparation of such sensors and we will demonstrate that the optimized devices are operating at room temperature, for the detection of pollutants such as chlorine, hydrogen chloride and ammonia. Such sensors are able to detect down to 30 ppb of pollutant, in particular for chlorine.