CuO/PVA-based ultra-low-volume oral H₂S detection system to reveal tongue coating effects

Hydrogen sulfide (H₂S), a key volatile biomarker from the oral cavity, is closely associated with oral health conditions. While numerous semiconductor-based H₂S sensors exist, the development of an H₂S sensing system with high selectivity, room-temperature operation, and low volume (5 mL) detection ability to focus on oral emission gas remains limited. This study introduces a compact and portable sensing platform integrating a double-layer CuO nanoparticle/polyvinyl alcohol (CuO/PVA) sensor with a CaCl₂-based drying tube for selective detection of H₂S in the 5–400 ppb range at room temperature (24 ± 1 °C) and 25 % relative humidity (RH). A novel modular double-tube detection system was designed to allow the separately control of the analyte injection (5 mL in 10 s) and the fixed relative humidity (RH = 25 %) with 500 mL/min background airflow. Using the extractor to obtain oral emissive gas without breathing, a preliminary clinical test was conducted on 20 healthy young volunteers to measure H₂S levels at three stages: early morning before brushing (M), after tooth brushing (MB), and after both tooth and tongue brushing (MTB). The MTB stage showed a significant decrease in H₂S levels compared to M (p < 0.01), while the difference between M and MB was not statistically significant. These results highlight the tongue coating as the primary source of oral H₂S emissions in healthy individuals. This work presents a novel integration of selective sensing, controlled sampling, and a user-friendly interface, offering a promising approach for non-invasive oral gas analysis and potential clinical applications.