Compact Differential Photoacoustic Exhaled Gas Sensor for Online ETCO2 and ETO2 Monitoring.

Online exhaled breath analysis holds great promise for noninvasive medical diagnostics, health monitoring, and environmental exposure assessment. However, the complex nature of the breath matrix and strong interference from water vapor and carbon dioxide make high-precision, real-time sensing challenging, often requiring large and costly systems such as mass spectrometers. In this work, we present a compact differential photoacoustic gas sensor for end-tidal carbon dioxide (ETCO2) and end-tidal oxygen (ETO2) monitoring. The sensor employs a dual-resonator photoacoustic cell with resonant frequencies of 4110 and 13,115 Hz for selective ETCO2 and ETO2 detection, respectively. A small sample gas volume of 2.6 mL enables a rapid response time of <0.5 s, enabling rapid tracking of changes in exhaled gas concentration. The optimized system achieves detection limits of 12.6 ppm for CO2 and 18.4 ppm for O2, with corresponding normalized noise equivalent absorption values of 2.9 × 10-8 and 1.6 × 10-7 cm-1⋅W⋅Hz-1/2. Real-time monitoring during human respiration demonstrates clear tracking of physiological O2 depletion and CO2 enrichment, consistent with respiratory dynamics. The developed sensor combines high sensitivity, fast response, compact size, and low cost, showing strong potential for continuous clinical monitoring, perioperative care, and metabolic studies.