Nano-assembled thin film gas sensors. IV. Mass-sensitive monitoring of humidity using quartz crystal microbalance (QCM) electrodes

Nano-assembled thin films prepared by a layer-by-layer approach on QCM resonators were used as sensitive elements for monitoring relative humidity. Different types of films, polymer-based (PDDA/PSS)_n film and porphyrin-based (PDDA/TSPP)_n and (PDDA/MnTSPP)_n films (where n = 5, 10, and 15), showed linear responses to relative humidity changes in the range of 4-94%. Increase of the number of bilayers deposited on the QCM electrode enhanced the sensitivity to relative humidity. The porphyrin-based films showed ca. 4.5 times higher sensitivity towards relative humidity than the polymer-based films, owing to presence of the higher number of free sulfonic acid groups. Incorporation of the metal ion into the porphyrin pyrrole ring was not significant in sensitivity. However, all films showed a good reversibility to the stepwise changes of relative humidity and the response and recovery times (t₉₀) were very fast within 15 s.