Hollow porous opal structured sensors for ultrasensitive and fast-response NO₂ fluorescence detection via fiber optic spectroscopy

Thin-film fluorescent sensors (TFFSs) suffer from low NO₂ detection sensitivity and long response time due to the poor NO₂ adsorption capability. Herein, a Rh6G/SnO₂–SnS₂/H–SiO₂ TFFS with a photonic crystal structure was fabricated for NO₂ sensing: Hollow SiO₂ (H–SiO₂) nanospheres, prepared via a sacrificial template method, formed a microporous structure and increased the specific surface area (SSA), facilitating efficient NO₂ adsorption. Additionally, by modifying the H–SiO₂ nanospheres with SnO₂–SnS₂ heterojunctions, the generated electron accumulation layer can trap escaping NO₂ molecules, further improving the adsorption performance. When vertically self-assembled into a photonic crystal film (PCF), the SnO₂–SnS₂/H–SiO₂ nanospheres could specifically amplify fluorescent signals, thereby enhancing the sensor's sensitivity. Rhodamine 6G (Rh6G) was selected as the probe molecule, and the specific nitrosation reaction was employed to optimize the selectivity of the TFFS. Dynamic fluorescent detection of NO₂ gas concentrations was realized using a fiber optic spectroscopy test system. The as prepared Rh6G/SnO₂–SnS₂/H–SiO₂ TFFS could achieve high-sensitivity room-temperature detection of NO₂ in the range of 1–1000 ppm within 24 s, with a limit of detection (LOD) as low as 100 ppb. Coupled with its advantages of low cost, portability, and visual detection capability, the sensor exhibits considerable commercial potential.