A Benzimidazole-Containing Covalent Organic Framework-Based QCM Sensor for Exceptional Detection of CEES

Mustard gas poses a substantial concern for homeland security, and the development of sensors for facile, rapid, highly sensitive, and selective detection of a mustard gas simulant is much desirable. Herein, a new benzimidazole-containing covalent organic framework (COF), termed BABE-TFPy COF, was synthesized by the Schiff base condensation of 1-(4,7-bis(4-aminophenyl)-1H-benzoimidazole-2-yl)ethan-1-ol (BABE) with 1,3,6,8-tetrakis(4-formylphenyl)pyrene (TFPy). By virtue of its crystallinity, high porosity, excellent chemical stability, and abundant accessible benzimidazole sites, an unprecedented COF coated quartz crystal microbalance (QCM) sensor was judiciously fabricated for the detection of various volatile organic compounds (VOCs). Remarkably, the resultant BABE-TFPy COF coated QCM sensor exhibited significantly higher sensitivity to 2-chloroethyl ethyl sulfide (CEES), a typical hazardous mustard gas simulant, than those of the other common VOCs. Moreover, the frequency changes of the COF coated QCM sensor were linearly related to the concentrations of CEES vapor within the range of 5.6–19.7 ppm, and derived a strikingly low detection limit of 0.96 ppm (signal-to-noise ratio = 3), rendering it an extraordinary QCM sensor for the detection of CEES. The enthalpy change indicated by the temperature-varying microgravimetric experiment, along with the Gaussian theoretical calculation, highly suggests a dual-hydrogen bonding formed between the BABE-TFPy COF and the CEES molecule accounts for such an exceptional recognition of CEES.