Nitrophenol isomers, characterized by similar chemical structures, high toxicity, and low saturated vapor pressures, pose significant challenges for highly sensitive and discriminative detection in the gaseous phase, which is critical for environmental monitoring and safety screening. In this study, a benzothiadiazole derivative containing dialdehyde groups (denoted as BTD) was designed and synthesized, and subsequently utilized to fabricate a BTD/agar composite fluorescent hydrogel. We investigated the effect of BTD molecules on the hydrogel using rheological techniques, and the results demonstrated that the incorporation of BTD molecules contributes to enhancing the stability of the agar hydrogel. Meanwhile, compared with solid BTD, the BTD composite gel exhibited a significant blue shift in fluorescence wavelength, indicating that the BTD molecules are dispersed within the gel network structure, and the π-π stacking between molecules is effectively suppressed. Through freeze-drying, a porous fluorescent xerogel with favorable flexibility was obtained. The morphology and composition of the composite xerogel were characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. The fluorescent gel exhibited excellent performance in distinguishing and detecting gaseous ortho-, meta-, and para-nitrophenol isomers. The unique porous network structure of the xerogel not only facilitates efficient adsorption and enrichment of analyte vapors but also provides a favorable microenvironment for fluorescence sensing. Leveraging the fluorescent gel materials, a portable gas-phase sensing device was constructed using a quartz glass tube connected to a mini pump. The sensor demonstrated rapid response, completing detection within one minute, along with high sensitivity and selectivity. The detection limit for ortho-nitrophenol reached 3.70 μg•L⁻¹, and common volatile organic compounds did not interfere with the measurements. Based on variations in fluorescence intensity, the sensor enabled clear discrimination among nitrophenol isomers. Furthermore, using this portable device equipped with automatic air sampling and detection functions, rapid and reliable detection of ortho-nitrophenol leakage was successfully achieved. This work provides a novel, efficient, and low-cost strategy for monitoring nitrophenol pollutants in the air and offers insights for the development of high-performance gas-phase sensing materials.

Key words: fluorescence, gel, nitrophenol, sensor, gaseous detection