Facile synthesis of binary two-dimensional lanthanide metal-organic framework nanosheets for ratiometric fluorescence detection of mercury ions

Three boric acid-functionalized two-dimensional metal-organic frameworks (2D MOFs) nanosheets, namely, 2D Tb-bop, Eu-bop and Tb/Eu-bop nanosheets, were synthesized through simple mixing of a reaction mixture of Tb/Eu salts and 5-boronoisophthalic acid (5-bop) in the presence of triethylamine (TEA) at room temperature. The product had excellent mercury ions (Hg2+) sensing properties. The highly exposed boric acid sites on the surface of the 2D structure can undergo a transmetalation reaction with Hg2+ to promote the energy transfer between the ligand and the lanthanide ions, thus enhancing the emission of 2D Tb-bop and Eu-bop nanosheets and resulting in high sensitivity for Hg2+ sensing. Significantly, the Hg2+-induced transmetalation reaction in 2D binary Tb/Eu-bop nanosheets also enhanced the energy transfer between Tb3+ and Eu3+ nodes, showing a ratiometric fluorescence response toward Hg2+, and further improving the sensitivity. This novel 2D ratiometric fluorescence probe showed good linearity from 0.1 to 40 μM for Hg2+ with a detection limit of 4.83 nM. This work not only provided a simple and efficient strategy for synthesizing 2D MOFs but also yielded new insights for designing fluorescence probes with excellent sensing properties.