Display omitted •The active sites on the MWNTs’ sidewall were increased by a two-step purification process.•The interaction of the adsorbate molecules mainly occurred at the defect sites of MWNTs’ sidewall.•Highly sensitive mass-type SO2 gas sensor was fabricated by using QCM coated with the functional MWNTs.•The increase in the active sites increases the adsorption of the SO2 molecules on surface of the MWNTs. In this work, a room temperature gas sensor based on functional multi-wall carbon nanotubes (MWNTs) coated quartz crystal microbalance (QCM) was developed. The two-step purification process was used to remove amorphous carbon and inactivated catalyst particles, and added functional groups on the sidewall of tubes. The functional MWNTs were then dispersed in deionized water to deposit on the gold electrode of the QCM. The operative MWNTs-coated QCM sensor was tested with toxic hazardous gases such as hydrogen sulfide, hydrogen, nitrogen dioxide, carbon monoxide, and sulfur dioxide. The sensing results expose that the fabricated sensor exhibits good sensitivity with sulfur dioxide gas at room temperature. The sensitivity of the functional MWNTs based sensor presented to be superior to that of the as-grown MWNTs based sensor.