Ratiometric fluorescence sensing of metal-organic frameworks: Tactics and perspectives

Metal-organic frameworks (MOFs) are an important class of inorganic-organic hybrid crystals with applications in various fields. In last ten years, MOFs-based fluorescence sensing has gained much attention. MOFs can exhibit luminescence by metal nodes, ligands and the inserted or absorbed guests, which offer an excellent fluorescence response in analyzing. However, MOFs-based monochromatic fluorescence probes are limited by concentration, the effect of ambient and excitation light intensity, resulting in low detection accuracy. To overcome this defect, MOFs-based ratiometric fluorescence (RF) sensors have been proposed and rapidly developed. This review focuses on the design tactics of MOFs-based RF sensing and describes the detection mechanism of different RF sensors. Based on different strategies of synthesis, the MOFs-based RF sensors are categorized into three classes: MOFs’ intrinsic dual-emission, single-emissive MOFs with a chromophore and non-emissive MOFs with two chromophores. In each categorization, the design approaches and relative merits were discussed separately. Finally, the applications of MOFs-based RF sensors are summarized and prospected.