Cyclodextrin functionalized mesoporous silica films on quartz crystal microbalance for enhanced gas sensing

We have attempted to enhance the detection sensitivity of frequency domain sensors such as quartz crystal microbalance (QCM) by depositing a mesoporous silica film on their flat electrode surface. The mesoporous silica film is prepared by using a sol-gel process in combination with plasma calcination. To demonstrate the effect of the mesoporous silica film on the sensitivity of QCM, beta-cyclodextrin (beta-CD) is trapped in the mesoporous network to detect benzene and ethanol vapors. To ensure a covalent immobilization, the beta-CD molecules are modified through alkenylation of the hydroxyl groups and then immobilized on a thiolated silica film by using the anti-Markovnikov reaction. A field emission scanning electron microscopy (FESEM) and nitrogen adsorption analysis are used to characterize the mesoporous structure of the silica film. NMR and time-of-flight secondary ion mass spectroscopy (ToFSIMS) are used to confirm the alkenylation of beta-CD and the presence of beta-CD in the silica network, respectively. The sensing performance of the beta-CD/silica hybrid coatings are tested to benzene and ethanol vapors. The results show that the responses of the QCM to benzene and ethanol vapors are enhanced by the introduction of the mesoporous sensing film on the electrodes. Through sufficient control experiments, i.e. sensing response of non-beta-CD modified silica-QCM and beta-CD modified planar QCM without a silica film to the target chemical vapors, we confirm that the total surface area of the silica controls the sensitivity and the chemical property of the beta-CD controls the selectivity of the sensing system. (c) 2005 Elsevier B.V. All rights reserved.