A MOF-on-MOF composite encapsulating sensitized Tb(III) as a built-in self-calibrating fluorescent platform for selective sensing of F ions

The establishment of simple and sensitive detection methods for fluoride ion (F-) is of great importance for its effective prevention and control, and metal-organic framework (MOF) has attracted much attention for sensing applications due to its high surface areas and tunable structures. Herein, we successfully synthesized a fluorescent probe for ratiometric sensing of F- by encapsulating sensitized Tb3+ in a MOF-on-MOF material (UIO66/MOF801, with the formula of C48H28O32Zr6 and C24H2O32Zr6, respectively). We found that Tb3+@UIO66/MOF801 can be used as a built-in fluorescent probe for fluorescence-enhanced sensing of F-. Interestingly, the two fluorescence emission peaks of Tb3+@UIO66/MOF801 at 375 nm and 544 nm exhibit different fluorescence responses to F- under excitation at 300 nm. The 544 nm peak is sensitive to F-, while the 375 nm peak is insensitive to it. Photophysical analysis indicated that the photosensitive substance was formed, which promotes the absorption of 300 nm excitation light by the system. Self-calibrating fluorescent detection of F- was achieved due to the unequal energy transfer toward the two different emission centers. The detection limit of Tb3+@UIO66/MOF801 for F- was 4.029 μM, which is far lower than the WHO guideline for drinking water. Moreover, the ratiometric fluorescence strategy showed a high concentration tolerance of interference, because of its inner-reference effect. This work highlights the high potential of lanthanide ion encapsulated MOF-on-MOF as environmental sensors, and offers a scalable way for construction of the ratiometric fluorescence sensing systems.