Upconversion Nanorods@Covalent Organic Framework with Enhanced Red Emission: A Ratiometric Fluorescent Probe for Uranyl Ions Detection.

Uranium-contaminated aqueous systems pose severe threats to life and health due to their combined chemical toxicity and radioactivity, particularly through bioaccumulation in the food chain. In this study, a novel near-infrared (NIR)-driven ratiometric fluorescent probe (UCNRs@COF) was fabricated by surface-growing a covalent organic framework (COF) onto upconversion nanorods (UCNRs). The synergistic effect between the COF and UCNRs significantly enhanced the upconverted red emission, enabling the composite to emit dual fluorescence peaks at 480 nm and 650 nm under 980 nm excitation. The intensity ratio (I650/I480) exhibited a linear relationship with UO22+ concentration in the range of 1-45 μM, achieving a detection limit of 66 nM. Mechanistic studies confirmed that UO22+ quenches the fluorescence of UCNRs@COF via a synergistic process of intramolecular charge transfer (ICT) and static quenching (SQE). The developed sensor demonstrates high sensitivity and selectivity for detecting UO22+ in water, urine, and food samples. Furthermore, a portable smartphone-based sensing platform was developed for the visual detection of UO22+, enabling visible color changes under 980 nm excitation. This method offers accurate and selective sensing of UO22+, highlighting its potential for practical applications across various fields.