A Boric Acid-Functionalized Lanthanide Metal–Organic Framework as a Fluorescence “Turn-on” Probe for Selective Monitoring of Hg2+ and CH3Hg+

Mercury detection remains an important task because of its high toxicity. Herein a new dual-signal probe based on a boric acid (BA)-functionalized lanthanide metal-organic framework (BA-Eu-MOF) was developed for the detection of Hg²⁺ and CH₃Hg⁺ ions for the first time. The BA-Eu-MOF was synthesized by coordination of Eu³⁺ with 5-boronobezene-1, 3-dicarboxylic acid (5-bop) through a one-pot method. The 5-bop ligand not only acted as the "antenna" to sensitize the luminescence of Eu³⁺ but also provided reaction sites for Hg²⁺ and CH₃Hg⁺. Owing to the electron-withdrawing effect of the BA group, the "antenna" effect of the ligand was passivating and the BA-Eu-MOF showed weak red emission in water. Upon addition of Hg²⁺ or CH₃Hg⁺ into the system, a transmetalation reaction took place, i.e., BA groups were replaced by Hg²⁺ or CH₃Hg⁺; therefore, the "antenna" effect of the ligand was triggered, leading to the enhancement of red emission. As Hg²⁺ or CH₃Hg⁺ concentration increased, the red emission was gradually enhanced, and the color change was also observed with the naked eye under 365 nm ultraviolet light. Owing to the porous characteristics and the surface effect of the MOF, as well as the unique transmetalation reaction between the BA group and Hg²⁺ or CH₃Hg⁺, the developed nanoprobe showed excellent characteristics for simultaneous detection of Hg²⁺ and CH₃Hg⁺, such as simple preparation, convenient operation, "turn-on" signal output, high sensitivity, and selectivity. The unique features of the BA-Eu-MOF make it an attractive probe for monitoring Hg²⁺ and CH₃Hg⁺.