Vacuum-Evaporated Cavitand Sensors: Dissecting Specific from Nonspecific Interactions in Ethanol Detection

High vacuum evaporation (VE) is used for the first time to grow thin films of novel tetraphosphonate, Tiiii[H, CH3, Ph], and tetrathiophosphonate, TSiiii[H, CH3, Ph], cavitands for gas sensing applications. The sublimation rate of the compounds was monitored during the depositions and related to the final physical properties of the samples. The properties of deposited films were investigated by various techniques. FT-IR and ESI-MS indicate that the samples consist of pristine cavitand molecules and demonstrate the high purity of the VE films. AFM images show the Tiiii and TSiiii films to possess similar thickness and globular morphology. These physical analyses indicate the uniformity and homogeneity of the final samples as well as the high reproducibility of the VE technique. The sensing capabilities of the samples were investigated by exposing Tiiii- and TSiiii-coated QCMs to ethyl alcohol in very low concentrations. The sensitivity, the speed, and the detection limit of the samples were determined, indicating highly competitive sensing capabilities. Elovich kinetics and Langmuir-Henry isotherms were used to analyze the sorption process occurring onto the different samples and showed that, whereas TSiiii samples consist mainly Of unspecific adsorption sites, Tiiii films show specific active sites where analyte molecules can be trapped and detected.