Environmental and bio-medical applications using quartz crystal microbalance (QCM) humidity sensors modified with nano-assembled thin films

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%. An 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 the 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 sec. Additional examples of the QCM humidity sensors for environmental and medical sensing are presented. In particular, a respiratory monitoring system based on QCM frequency oscillations was designed and investigated. The developed system, in which the sensor response reflects lung movements, was able to track human breathing parameters such as respiratory rate, tidal volume, and inspiratory/expiratory flow, which were tested on healthy volunteers.