Preparation and identification of β-cyclodextrin polymer thin film for quartz crystal microbalance sensing of benzene, toluene, and p-xylene

β-Cyclodextrin (β-CD) and maleic anhydride (MA) were used as the monomer and the crosslinker, respectively, from which maleic anhydride-bridged poly(β-CD-co-MA) (PCDMA) was thus prepared. The BET (Brunauer-Emmett-Teller) surface area of β-CD polymer was 2.1 m²/g. Thermal properties of this polymer were studied using a thermogravimetric analyzer (TGA) and a differential scanning calorimeter (DSC). Fourier transform infrared (FTIR) spectroscopy was also used to confirm the difference of the functional structures between β-CD monomer and its polymer. The polymer showed absorption bands at 1719 and 1660 cm^-1 in addition to the β-CD functional groups, which represented the C{double bond, long}O and C{double bond, long}C stretches, respectively. ¹H NMR spectroscopy was primarily used to identify the mono- and di-ester fractions of the β-CD polymer. However, most of the skeleton proton signals were located in an extremely small range of 3.5-5.0 ppm. Hence, ¹³C NMR assignments for the PCDMA were further examined. This acid-crosslinked β-CD polymer was then used to be a sensing material of a quartz crystal microbalance (QCM) sensor. The polymer was also fabricated into a thin film on a gold-covered quartz crystal chip. The measured film thickness was compared with the calculated values and most deviations were under 7%. Detection of benzene, toluene, and p-xylene of low concentrations was studied. The film surfaces adsorbed with the above-mentioned compounds were inspected and compared using a scanning probe microscope (SPM). Performance of the QCM sensor coated with the PCDMA film was also evaluated. Linear calibrations were achieved for these three compounds with linear ranges of 400, 300, 150 ppm for benzene, toluene and p-xylene, respectively. Among them, p-xylene showed the highest detection sensitivity, which was 1.236 ± 1.994 Hz/ppm in average.