Thin-Film Sensors for Detection of Formaldehyde: A Review

This paper reviewed the films used in formaldehyde gas sensing recently. They can be divided into three groups: 1) metal oxide semiconductor films; 2) polymer films; and 3) carbon nanotubes (CNTs) films. Detection limits of these three groups were 1 ppb, 1 ppm, and 20 ppb, respectively. In metal-oxide-semiconductor films, the sensitivity of ZnO conductimetric type sensor was down to 1 ppb with the detectable response of 7.4. Polyethyleneimine/TiO₂ was the most sensitive film with quartz crystal microbalance sensor in polymer films, which could detect 1-ppm formaldehyde with the response (Δf ) of 0.8 Hz. Either multi-wall CNTs (MWCNTs) or single-walled CNTs, forming a CNTs film, had the higher sensitivity so far, and the MWCNTs-NH₂ interdigital electrodes sensor exhibited 1.73% relative resistance change to 20 ppb of formaldehyde. The further research will be, however, needed to deal with the situation of real scenario influences, such as temperature, humidity, and interferents.