Lanthanide-bimetallic organic frameworks: from mechanism and sensors design to ratiometric fluorescent applications

Metal-organic frameworks (MOFs) exhibit strong sensing performance due to their porous structure, large specific surface area, and ease of functionalization. Notably, MOF sensors utilizing lanthanide ions (Ln³⁺) with the unique 4f-4f transitions luminescent properties have attracted great attention in fluorescence sensing. In the last decade, Lanthanide bimetallic organic frameworks (Ln-BMOFs) with bimetallic centers have become a research hotspot in the field of fluorescence sensing due to unique energy transfer of bimetals, tunable ratiometric fluorescence signal, high stability, and high luminescence efficiency. This review is organized to highlight the current progress of Ln-BMOFs in the four fluorescence sensing mechanisms, including energy transfer between ligands and metals (such as LMET, MLET), metal-metal energy transfer (MMET), metal-analyte interaction and ligand-analyte interaction. Additionally, the types of Ln-BMOFs sensor-based designs are also highlighted including Ln-BMOFs self-sensors, Ln-BMOFs@biorecognition elements and Ln-BMOFs@molecular imprinting polymers. Based on these fluorescence sensing mechanism/design, the topical developments in the application of Ln-BMOFs ratiometric fluorescent sensors for environmental monitoring, food safety, and biomedical sensing are also summarised. Furthermore, the discussion has been extended to describe the current challenges and prospects of Ln-BMOFs ratiometric fluorescence sensors, and to facilitate the development of Ln-BMOFs sensor.