Investigation of the Multiple Doping of Citric Acid and Chitosan in Nanofiber for Enhancement of a Quartz Crystal Microbalance-Based Ammonia Sensor

Herein, the use of multiple doping of citric acid (CA) and chitosan (CS) in polyacrylonitrile (PAN) nanofibers over a quartz crystal microbalance is investigated as a method for enhancing the performance of an ammonia sensor at room temperature. It was found that the PAN/CA/CS sensor has superior sensitivity and better selectivity. The PAN/CA/CS sensor demonstrated sensitivity of (0.629 ± 0.005) Hz ppm−1, which increased by 2.75 times compared to the PAN/CA sensor and 39 times compared to the PAN sensor. Chitosan doping also resulted in better selectivity, as shown by the decreased response of the PAN/CA/CS sensor compared to the PAN/CA sensor to other analytes including formaldehyde (−147%), acetic acid (−22%), ethanol (−19%), methanol (−15%), and acetone (−1%). The viscoelastic properties of chitosan might be responsible for the anti-Sauerbrey phenomena behind the enhanced selectivity. The detection and adsorption mechanisms of the fabricated sensors towards ammonia were studied using adsorption kinetics and isotherms. The adsorption kinetics varied and exhibited the limits of the high-concentration region of each sensor. Moreover, all the fabricated sensors followed the Freundlich adsorption isotherm with different adsorption processes, which were confirmed by scanning electron microscopy, x-ray diffraction, and Fourier transform infrared spectroscopy concerning the morphology, crystal structure, and active groups after the loading of citric acid and chitosan. Thus, the use of multiple doping can improve the sensor abilities, as well as causing changes in the detection and adsorption mechanisms.