Quartz crystal microbalance humidity sensors integrated with hydrophilic polyethyleneimine-grafted polyacrylonitrile nanofibers

In this work, we devised hybrid humidity sensors by integrating quartz crystal microbalance (QCM) platforms with polyethyleneimine (PEI)-grafted polyacrylonitrile (PAN) nanofibrous mats (i.e., PAN/PEI nanofibers) as active layers. The produced devices were characterized using a scanning electron microscope (SEM), an atomic force microscope (AFM), and a Fourier transform infrared (FTIR) spectrometer to investigate their morphologies, roughness, and molecular compositions, respectively. To understand the humidity sensing mechanism, an exponential kinetic model was developed for the adsorption and desorption processes of water molecules on nanofibers. The viscoelasticity of the active layer started to increase when the sensor was exposed to relative humidity (RH) of > 60 %rH. The PAN/PEI nanofiber-functionalized QCM sensors exhibited a sensitivity of up to 164 Hz/%rH with the response and recovery times of 13 and 7 s, respectively, during humidity exposure assessments performed at room temperature of (30 ± 1) °C. Moreover, they also provided high selectivity, long-term stability, and excellent repeatability with low error rates. Among the other existing QCM-based humidity sensors that were coated with various materials (e.g., metal oxides, carbon, and polymers), our unique electrospun PAN/PEI nanofibrous mat-functionalized QCM sensing devices have demonstrated the most superior performances in terms of their sensitivity and responsivity, which could then offer an alternative for high-sensing-performance yet low-cost humidity sensors.