Highly Sensitive and Selective NiO/WO₃ Composite Nanoparticles in Detecting H₂S Biomarker of Halitosis

Indirectly monitoring halitosis via the detection of hydrogen sulfide (H₂S) biomarkers using gas sensors is a newly emerging technique. However, such H₂S sensors are required with critically high selectivity and sensitivity, as well as a ppb-level detection limit, which remains technologically challenging. To address such issues, here, we have developed highly sensitive and selective H₂S sensors with NiO/WO₃ nanoparticles (NPs), which have been synthesized by firstly hydrolyzing WO₃ NPs and subsequently decorating with NiO NPs in a hydrothermal process. Theoretically, the NiO/WO₃ NPs assist in forming a thicker electron depletion layer, adsorbing more oxygen species O₂^- to oxidize H₂S and finally release more electrons. Beneficially, 2.1 wt % NiO/WO₃ NPs show high sensitivity to H₂S (R_a/R_g = 15031 ± 1370 @ 10 ppm, 100 °C), which is 42.6-fold higher than that of the pristine WO₃ NPs (R_a/R_g = 353 ± 5.6 @ 10 ppm, 100 °C). Further, the H₂S sensor shows ppb-level detection limit (R_a/R_g = 4.95 ± 2.9 @ 0.05 ppm, 100 °C) and high selectivity. Practically, NiO/WO₃ NP sensor prototype has been employed to detect the simulated exhaled halitosis compared with that of gas chromatography, revealing a close concentration of H₂S. Our investigation offers an experimental base in future intelligent medical applications.