Crystal Growth and Ratiometric Fluorescence Tuning of the Eu-Metal Oxide Framework for Formaldehyde Sensing via Photoinduced Electron Transfer

Combining the potential advantages of metal–organic frameworks (MOFs) with the distinctive luminescent behavior of lanthanide metal ions, a fluorescence probe (Eu-MOF, Eu-BDC-NH₂) with dual fluorescence emissions of ligand (λ = 431 nm) and Eu³⁺ (λ = 617 nm) is synthesized using a straightforward room-temperature crystal growth process. The obtained Eu-BDC-NH₂ crystal possesses an abundance of uncoordinated amino groups, which serve as effective active sites for the selective recognition of formaldehyde (FA). Owing to the presence of photoinduced electron transfer (PET) between the amino group and the adjacent ligand, the fluorescence of the ligand in the Eu-BDC-NH₂ is turned off. Upon the introduction of FA, the amino group interacts with FA, which inhibits the PET process and simultaneously diminishes the “antenna effect” of sensitizing Eu³⁺. Consequently, the ligand fluorescence is turned on, and the Eu³⁺ fluorescence is weakened, enabling the highly sensitive ratiometric fluorescence detection of FA. Compared with the single emission fluorescence spectrum, Eu-BDC-NH₂ can achieve internal self-calibration by measuring the ratio of fluorescence intensity of the two wavelengths in the system to overcome signal fluctuations and provide more accurate and reliable information. Furthermore, by integrating smartphones, an intelligent sensing system has been developed to enhance the visualization of FA detection, where the fluorescence color visible to the naked eye shifts from red to blue. Specifically, this work utilizes a straightforward dripping method performed at room temperature, which not only simplifies the experimental synthesis process but also adheres to the principles of green chemistry, thereby offering a novel perspective and methodology for the development of MOF-based fluorescence probes.