Highly sensitive and chemically stable NH3 sensors based on an organic acid-sensitized cross-linked hydrogel for exhaled breath analysis

Due to interference by the high moisture content and complicated compositions of human exhaled breath, the trace-level detection of ammonia (NH3) with desirable selectivity and stability is a large challenge for exhaled breath analysis. Carboxyl-sensitized hydrogels can be activated by moisture to exhibit a significant response and excellent selectivity to NH3. However, the high activity of carboxyl groups in hydrogels is a double-edged sword, resulting in poor chemical stability during NH3 detection. Herein, organic acids were embedded into a cross-linked poly(ethylene glycol) diacrylate (PEGDA) hydrogel via thiol−ene photochemistry to form stable hydrogels for NH3 detection in a humid atmosphere. As a result, under high humidity conditions (80% RH), the optimal sensors exhibited superior selectivity to NH3 among various interfering gas species, a remarkably high NH3 response (Za/Zg=6.20) towards 20 ppm NH3, and an extremely low actual detection limit (50 ppb) at room temperature. Moreover, the sensors exhibited excellent chemical stability due to the moderate equilibrium water content of the hydrogel composites and acid dissociation constant of the acid groups. The moisture-activated NH3 sensing mechanism was thoroughly investigated by complex impedance spectroscopy (CIS), quartz crystal microbalance (QCM) measurements, Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). To explore the application prospects of cross-linked hydrogel sensors for detecting NH3 in exhaled breath, a simulated exhaled breath test was also performed. •The strategy for improving the repeatability of hydrogel NH3 sensors was approached.•The CA/PEGDA sensor exhibited a remarkable NH3 response (Za/Zg=6.20, 20 ppm) at 80% RH.•The extremely low detection limit (50 ppb) to ammonia at 80% RH was achieved.•The novel sensing mechanism gives a potential way to realize the exhaled breath ammonia detection.