EY@UiO-67-CDC-(CH₃)₂ as an ultra-sensitive ratiometric fluorescence sensor for visible and selective detecting of nitrite

Nitrite is widely used in production and daily life, and detecting this substance has always been crucial for human health or environment monitoring. Here, a dual-emitting dye@MOF composite, EY@UiO-67-CDC-(CH₃)₂, has been constructed and demonstrated that this material could detect nitrite ions with highly selective and anti-interference properties. By incorporating an organic anionic dye (Eosin Y) within a cationized EY@UiO-67-CDC-(CH₃)₂ framework through the post-synthetic method, the composite presents an effective combination due to electrostatic attraction. It features a dual emission effect, which belongs to the blue emission of MOF at 428 nm and yellow emission of EY at 559 nm, respectively. After adding NO₂⁻, the fluorescence emission peaks of EY and MOF showed opposite changes due to the inner filter effect of NO₂⁻ and MOF, as well as effective excitation of the dye's fluorescence and slight dye leakage. Therefore, I_dye/I_MOF is used as the detection signal value instead of absolute fluorescence intensity, effectively reducing the detection limit to 0.224 μM and improving its detection accuracy. In addition, during the detection process, the fluorescence sensor showed noticeable color changes, so a smartphone-based analytical device without additional instruments was developed for the quantitative, visual analysis of nitrite by identifying the RGB values of the fluorescence sample solution, with a detection limit of 6.74 μM. The application of the sensor visualizes nitrite detection and reduces the cost of detection, providing a feasible method for on-site detection of nitrite in water samples.