High Humidity-Enhanced Stability Fabry-Pérot Microcavity Sensor With ppb-Level Ammonia Detection Limit for Room-Temperature Breath Analysis

Reliable trace-level detection of ammonia (NH3) in exhaled breath remains challenging because of its low concentration, high relative humidity (RH), complex gas composition, and temperature variations. In this work, a Fabry–Pérot (FP) microcavity sensor with a poly(diallyldimethylammonium chloride)/poly(acrylic acid)/reduced graphene oxide (PDDA/PAA/rGO) composite coating was developed for NH3sensing under high-RH conditions. The PDDA/PAA network provided accessible carboxyl groups and a water-retaining environment for NH3capture, while rGO provided abundant adsorption sites and limited excessive water uptake and swelling of the polymer network. A cascaded fiber Bragg grating was further employed for temperature monitoring and compensation. At room temperature and 90% RH, the FP sensor exhibited a linear response over 1–50 ppm NH3with a sensitivity of approximately −0.02 nm/ppm and a detection limit of 740 ppb. The RH and NH3responses remained nearly stable over 85–95% RH, and the response and recovery times were approximately 35 and 95 s, respectively. Furthermore, the sensor produced a distinguishable additional response to healthy exhaled breath spiked with 1 ppm NH3, supporting its potential for noninvasive breath NH3analysis.