Dual-emission [email protected]: A platform for ultra-low LOD ammonia detection and on-site visual recognition

Abstract

Luminescent metal-organic frameworks (LMOFs) have emerged as promising materials for optical sensing, owing to their tunable luminescent properties. However, most existing LMOF-based sensors rely on single-wavelength fluorescence intensity changes, which suffer from poor resistance to environmental fluctuations due to the absence of auxiliary reference signals, limiting their accuracy, especially for trace ammonia(NH₃) below the human olfactory threshold, which evades sensory perception yet poses long-term risks. To address this limitation, UIO-67-x where x denotes the molar ratio of terephthalic acid (H₂BDC) to biphenyl-4,4′-dicarboxylic acid (H₂BPDC) was designed. By optimizing the ligand ratio, UIO-67-0.2 was synthesized with well-defined crystallinity, large specific surface area and high fluorescence emission peak intensity. Its regular structure enables efficient encapsulation of rhodamine B (RhB) forming the dual-emission [email protected] platform. This platform has two adjustable emission peaks, both of which respond sensitively and synchronously to ammonia, thereby strengthening signal reliability and anti-interference capability through dual-channel synergistic verification. Notably [email protected] achieves an ultra-low NH₃ limit of detection (LOD) of 0.2 ppm, far below the olfactory threshold, and maintains remarkable stability after 5 cycles with stable reusability. Combining experiments and Density functional theory (DFT) calculations the quenching mechanism is clarified as the synergistic effect of static quenching and charge transfer. A [email protected] drop-cast film exhibits naked-eye distinguishable color change at 2.5 ppm NH₃ under 365 nm Ultraviolet(UV) light. It remains unchanged with other volatile organic compounds (VOCs) and shows concentration-dependent color gradation for semi-quantitative assessment. Furthermore, Digital conversion of fluorescence color signals via ΔE analysis enabled truly quantitative ammonia detection with a linear calibration curve (R² = 0.968), eliminating the subjectivity of naked-eye judgment. This work develops a dual-mode LMOF for sub-olfactory-threshold NH₃ detection and informs anti-interference dual-emission platform design enabling on-site low-cost monitoring.