Abstract
Long-term, quantitative monitoring of ammonia (NH₃) concentrations across wastewater treatment plants (WWTPs) is essential for characterizing fugitive emission patterns, identifying critical source zones, and guiding targeted process-unit retrofits. However, chemiresistive sensors capable of simultaneously delivering wide dynamic range, humidity tolerance, and multi-month operational stability in complex field environments remain critically lacking. Herein, we report a chemiresistive NH₃ sensor based on a Ga₂O₃/polyaniline (PANI) nanocomposite. β-Ga₂O₃ nanorods synthesized via a precipitation-calcination route serve as a high-affinity adsorption scaffold, and PANI deposited through in situ oxidative polymerization forms a p-n heterojunction interface with the oxide. Combined experimental characterization and density functional theory (DFT) calculations reveal a distinctive sensing mechanism wherein the incorporation of PANI does not substantially enhance the thermodynamic adsorption energy of NH₃ but instead functions as an electronic-state modulator that amplifies the charge-transfer efficiency per adsorption event through heterojunction-induced depletion-layer modulation. The optimized sensor delivers a response of 51.5% toward 4 ppm NH₃ across a dynamic range of 0–7 ppm, rapid response/recovery kinetics of 19 s / 8.3 s, and excellent selectivity against common interfering gases. A fixed monitoring instrument with on-board temperature and humidity compensation was deployed continuously for over 180 days in an operating WWTP. It achieved a daily drift rate below 1% and close agreement with a commercial reference analyzer. This validates a practical and reliable solution for long-term NH₃ monitoring in complex field environments.