Compact Differential Photoacoustic Exhaled Gas Sensor for Online ETCO ₂ and ETO ₂ 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.