Ratiometric fluorescence and a smartphone platform for the visual detection of fluoride ions using [Ru(bpy)₃]²⁺-encapsulated Zr-based metal–organic frameworks

Fluoride anion plays a vital role in the human health, but over exposure to fluoride can lead to severe diseases. Thus, it is of continuing interest to establish facile and effective methods for the quantitative analysis of fluoride ions. Herein, a dual-emission [Ru(bpy)₃]²⁺-encapsulated Zr-MOF composite material (Ru(bpy)₃@UiO-67-NH₂) was prepared by confining [Ru(bpy)₃]²⁺ within the cavities of Zr-MOF (UiO-67-NH₂) <i>via</i> a solvothermal method. A drastic 478 nm blue fluorescence enhancement of UiO-67-NH₂, induced by fluoride ions was demonstrated as a response signal, while the 600 nm orange-red emission of [Ru(bpy)₃]²⁺ was utilized as an internal reference, enabling fluoride-specific recognition through signal ratio variation. The response mechanism of Ru(bpy)₃@UiO-67-NH₂ towards fluoride was due to an N-H…F hydrogen bond structure formation, hindering the ligand-to-metal charge transfer (LMCT) and causing the restoration of blue fluorescence of H₂BPDC-NH₂. Thus, the ratiometric fluorescence measurement of fluoride ions was easily achieved with a detection limit of 0.2 μM. Furthermore, Ru(bpy)₃@UiO-67-NH₂ was loaded onto a paper substrate, and a chromaticity-based test strip integrated with a smartphone enabled the visual detection with a detection limit of 4.1 μM, providing a new option for the on-site analysis of fluoride ions in real samples.