A study on the discrimination of xylene isomers vapors by quartz crystal microbalance sensors

A series of quartz crystal microbalance (QCM) gas sensors were developed for discrimination of the vapor of xylene isomers (ortho-xylene, meta-xylene and para-xylene). The investigated QCM sensors included an Au-bare sensor and those modified by self-assemble monolayer (SAM) formation by 1-butanethiol and 3-methyl-1-butanethiol in the presence of xylene isomers, as reaction solvent. The presence of the investigated thiols made a slight difference on the polarity of the sensors surface, estimated by measuring the contact angles. Although the modified sensors with 1-butanethiol were selective toward ortho-xylene vapors as that presented by Au-bare sensor, the sensitivity of the 1-butanethiol-modified sensors was significantly improved with respect to that demonstrated by the unmodified sensor. The sensors modified with 3-methyl-1-butanethiol presented different demeanor for the xylene isomers vapor. This attitude varied with the solvent used through SAM formation process. It was suggested that the observed attitude of the sensors is a reflection of the solvent effect on the structure and decoration of the formed SAM on the sensor surface and the polarity of the sensors’ surface. The results allowed suggesting the fabrication of sensitive sensor arrays for the discrimination of xylene isomers vapors.