This paper presents a flexible and reliable chemiresistor-type NO2 gas sensor based on single-walled carbon nanotubes (SWNTs) on polytetrafluoroethylene (PTFE) membrane filter substrates. The sensor is realized by using a cost-effective spray coating in the preparation of SWNTs thin film, followed by the fabrication of metal contacts using a shadow mask and polyethyleneimine (PEI) noncovalent functionalization of the SWNTs. This showed a high sensitivity to NO2 gas at room temperature in dry air; 21.58% to 167.7% for concentrations of 0.75 ppm to 5 ppm, and was almost nonsensitive to ammonia. Gas sensing characterization results, obtained for various substrate bending/wrapping over different cylinders with diameters of 75 mm, 12.5 mm, and 6 mm showed that bending does not significantly affect sensitivity for NO2 concentrations of 0.75 ppm to 2 ppm, while in the case of 3 ppm to 5 ppm NO2, the bent samples indicate enhanced sensitivity. This is probably because of the porous nature of PTFE substrates; these sensors were 1.5 to 2.7 times more sensitive than those fabricated over silicon substrate for 1 ppm and 5 ppm, respectively. Moreover, the relative humidity of 10% and 30% significantly reduced the sensitivity of the sensors. The presented results could be useful for the future development of flexible electronics/sensors for monitoring outdoor air quality and for the detection of volatile organic compounds. © 2018 IOP Publishing Ltd.