SWCNT-based CO sensors. (a, b) Sensing scheme of binding event of CO by cobalt complex showing the freeing of amine ligand upon binding of CO. The averaged change in conductance of cobalt complex/SWCNT chemiresistor in response to several gases and gas mixtures. Reproduced with permission from Ref. 311. Copyright 2016, American Chemical Society. (c) Schematic depiction of CO sensor comprising aniline-functionalized SWCNTs and iron porphyrin (Fe(tpp)ClO4). The covalent pyridyl ligands facilitate the in situ reduction of iron porphyrin to modulate the sensitivity of the sensor. (d) Sensing responses of iron porphyrin/aniline-functionalized SWCNTs towards 200 ppm CO with applied gat voltage of −3, 0, and +3V. Reproduced with permission from Ref. 127. Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA.