Organic vapor detection by quartz crystal microbalance modified with mixed multilayer Langmuir-Blodgett films

Abstract

Mixed chloroform solution of Polysiloxane PS-264 and stearic acid was made into multilayer Langmuir-Blodgett (LB) films on the surface of quartz crystal microbalance (QCM). The condition for forming mixed multilayer LB films is discussed in this paper. In addition, PS-264, stearic acid and their mixture (molar ratio 1:1) were coated on the surface of QCM sensors by a spin coating method. The above modified crystals were exposed to different concentrations of various organic vapors, including related chlorinated hydrocarbons and alcohol vapors. The results of frequency changes corresponding to concentration change of the above vapors were compared. The linear regression coefficients of the QCM with LB film had better values (0.9881 < R < 0.9981) than QCM with spin-coated films. The sensitivity between analytes/coating was interpreted in terms of solubility interactions. For the more polar organic vapors investigated, even at the same concentration, frequency changes of the QCM with LB films were somewhat larger, especially for chloroform and dichloromethane. However, the sensitivity of many organic vapors investigated on the mixed multilayer LB films is similar to the spin-coated films of mixture and PS-264, except that chloroform and dichloromethane only were sensed with much higher sensitivity on the mixed monolayer. The sensitivity of the QCM spin-coated with the mixture of PS-264 and stearic acid was between mixed LB films and PS-264 for many organic vapors investigated. In this study, the surface morphologies of the uncoated and coated QCM were characterized by atom force microscopy (AFM).