The electrostatically functionalized self-assembled MWCNT (f-MWCNTs) were employed to develop a sensing device to selectively sense most toxic and suffocating gases such as CO and NOx with high sensitivity and repeatability to as low as sub ppm levels. The resistive gas sensor’s operation is primarily based on changes in the electrical resistance of the f-MWCNT network as a result of its selective interaction with the specific target gas in two-pole format. The degree to which the electrical resistance of the sensing film increases or decreases is determined by the concentration of the target gas to which it is exposed. As a result, the target gas can be detected both qualitatively and quantitatively. The sensitivity of 100 ppb and 300 ppb with sensor response time of ~30 s and ~50 s for NOx and CO respectively were recorded using our gas sensor.