Capacitive humidity sensing using a metal–organic framework nanoporous thin film fabricated through electrochemical in situ growth

The preparation of nanoporous thin film of metal–organic framework (MOF), Cu–BTC [1,3,5-benzenetricarboxylate or trimesate (BTC)], on the copper plate electrode as dielectric layer of the capacitive sensor was achieved by electrochemical in situ synthesis and film growth. An ionic liquid (IL), 1-methyl-3-octylimidazolium chloride as conducting salt, was used and aid synthesis in the electrochemical synthesis procedure. The structure and morphology of MOF film were properly characterized using scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction analysis and BET techniques. The fabricated sensor was used for the humidity measurement at ppm level using the parallel plates’ capacitive sensor structure. Capacitance variations in the presence of humidity at concentration range of 20–100 ppm was investigated using the fabricated sensor. The performances of the sensor have been examined by measuring the capacitance changes using a LCR meter [inductance (L), capacitance (C), and resistance (R)]. Variations of capacitance versus concentration were linear in the range of humidity concentrations which was used here. Sensitivity of the fabricated sensor was 1.13 pF/ppm. Limit of detection (LOD) of the fabricated sensor calculated as low as 5.45 ppm. n-Hexane and toluene vapors as nonpolar analytes were used to investigate the selectivity of the sensor.