Detection of formic acid and acetic acid gases by a QCM sensor coated with an acidified multi-walled carbon nanotube membrane

In this paper, the acidified multi-walled carbon nanotubes (MWCNTs) film was coated on the quartz crystal microbalance (QCM) to prepare a high-performance sensor for the real-time detection of organic acid gases. The material characteristics of the thin films were analysed by field emission scanning electro microscopy (FESEM), Raman spectra and X-ray photoelectron spectroscopy (XPS). The organic acid vapours' sensing results indicated that acidized-MWCNTs thin film exhibited good frequency response, repeatability, reversibility and stability. There is a clear linear relationship between the frequency offset and the organic acid vapours with concentration below 5.0 ppm, and the detection limit of 0.77 and 0.73 ppm for formic and acetic acid vapours, respectively. The sensor shows the highest response to formic acid vapour than acetic acid vapour which may be ascribed to molecular polarity. Furthermore, a sensing mechanism model was introduced to understand the adsorption reaction between organic acid molecules and acidized-MWCNTs. This paper proves that acidized-MWCNTs is a potential and suitable material for organic acid vapour detection when combined with a QCM sensor.