Ultrafast 10 ppb Acetonitrile THz Gas Sensor Based on Flexible Low-Loss Ag/PP Hollow Waveguide

Acetonitrile is widely used in pharmaceutical and industrial processes, but it is a volatile, flammable, and toxic gas. The rapid detection of trace acetonitrile gas is crucial for industrial production, environmental monitoring, and public safety. This study presents a novel high-performance acetonitrile terahertz (THz) gas sensing system based on a flexible, low-loss Ag/PP THz hollow waveguide (HWG). The mathematical model of the established sensing system calculates that the designed sensor exhibits a high effective path length rate (0.9942), which can achieve high gas sensing performance. Based on the molecular rotational transition theory, the frequency positions of typical absorption peaks of acetonitrile were calculated, which were consistent with THz-TDS measurements. According to the strong selective absorption of acetonitrile at 275.6 GHz and the Lambert–Beer law, the concentration of acetonitrile was measured by the HWG-based THz gas sensor, where HWG serves as both the gas chamber and the transmission channel of THz waves. The effects of the length and bending angle of HWG on sensitivity, response/recovery time, and LOD were investigated systematically. Both the 30 cm-long straight HWG and 75 cm-long curved (90°) HWG sensor could detect 10 ppb acetonitrile gas samples in 10 kinds of mixed gases, with response/recovery times of 2.6/2.7 and 3.0/3.1 s, respectively. Meanwhile, the developed sensor demonstrated excellent reversibility and reasonable reliability (RSD < 0.5%) in six test cycles. The as-built acetonitrile sensor exhibits high selectivity, rapid response/recovery times, low LOD, and easy operability, opening up new avenues for the development of high-performance THz gas sensors.