Humidity sensing properties of calix[4]arene and functionalized calix[4]arene measured using a quartz-crystal microbalance

Humidity sensors were made by coating 25,26,27-tribenzoyloxy-28-hydroxycalix[4]arene (TBHC) and 5-(4′-nitrophenyl)azo-25,26,27-tribenzoyloxy-28-hydroxycalix[4]arene (NTBHC) thin films on a gold electrode of a quartz crystal microbalance (QCM). The thin films were analyzed by atomic force microscopy (AFM). The humidity sensing properties, including sensitivity, sensing linearity, hysteresis, cross-sensitivity effects and long-term stability were also investigated. Water vapor molecules that were adsorbed onto the NTBHC thin film exhibited a larger frequency shift than those adsorbed onto TBHC thin films. Adsorption dynamic analysis and a molecular mechanical calculation (of the association constant) were made to elucidate the way in which a substituent on the upper and lower rims of calix[4]arene increases the sensitivity to humidity.