A Tb3+-anchored Zr(iv)-bipyridine MOF to promote photo-induced electron transfer and simultaneously enhance photoluminescence ability and photocatalytic reduction efficiency towards Cr2O72−

Abstract

Effective detection and removal of Cr(VI) from various waters still remains a severe challenge. Given the aggregation-induced emission (AIE) and photo-induced electron transfer (PET) effect, a few MOFs perform dual-functional fluorescence sensing and photochemical reduction towards Cr(VI). Nevertheless, how to enhance photoluminescence ability and photocatalytic reduction efficiency simultaneously has been rarely studied and reported. Here, a novel dual-emissive platform of Tb@Zr-MOF with high water stability was ingeniously designed by anchoring lanthanide Tb³⁺ ions in a Zr(IV)-bipyridine framework. Owing to the greatly promoted PET effect, Tb@Zr-MOF exhibits a unique double-enhancement feature compared to the pristine Zr-MOF. After Tb³⁺ ions were immobilized by carboxyl O atoms, Tb@Zr-MOF emits a typical bright green fluorescence signal of lanthanide metal Tb³⁺ through the “antenna effect”. Notably, Tb@Zr-MOF can be deployed as a self-calibrating photochemical sensor for high-precision fluorescence quenching detection of Cr₂O₇²⁻ ions, showing a rather high K_sv value of 8.093 × 10⁴ M⁻¹ and ultra-low detection limit of 4.03 ppb (17.6 ppb for pristine Zr-MOF). Moreover, the highly efficient PET process combined with an appropriate energy band structure, i.e., the rather negative CB potential confers Tb@Zr-MOF excellent ability to photochemically reduce high-oxidation-state Cr(VI) ions under UV light, with rather a high reduction efficiency of up to 97.5% (1.46 times of pristine Zr-MOF) just within 60 min. Prominently, the highly efficient PET effect from organic modules of Zr-MOF to anchored Tb³⁺ ions was confirmed by modeling the ground state and excited states of Tb@Zr-MOF using DFT and TD-DFT. This study innovatively combines porous MOFs and the lanthanide metal ion anchoring technique to provide a feasible strategy for improving the existing dual-functional MOF-based fluorescence sensing and photocatalytic platforms.