Visual Detection of Fluoride Anions Using Mixed Lanthanide Metal-Organic Frameworks with a Smartphone

Fluoride ion (F^-) is one of the most harmful elements in drinking water. Over-intake of F^- can result in dental fluorosis, kidney failure, or DNA damage. Therefore, developing affordable, equipment-free, and reliable methods for F^- detection is an important goal. In this work, a series of mixed lanthanide metal-organic frameworks were synthesized using a triazine-based planar ligand, 4,4',4″-s-triazine-2,4,6-triyltribenzoate (TATB) and mixed lanthanide ions (Tb³⁺ and Eu³⁺). The luminescent color of the Tb/Eu(TATB) can be finely modulated by changing the Tb³⁺/Eu³⁺ ratio in the synthesis procedure. Benefiting from the unique host-guest interaction (e.g., Lewis acid-base interaction), between F^- and MOF host, a highly selective, sensitive, and reliable fluoride sensor was then developed. Moreover, visual detection of F^- was achieved with a smartphone by identifying the RGB value. Drinking water samples were analyzed for F^-, and the results obtained by our ratiometric luminescent method were consistent with those by ion chromatographic strategy. This easy-to-use sensor provides reliable detection of F^- in everyday applications for nonexpert users, especially in remote rural areas.