Dual-Color Visual Ratiometric Fluorescence Sensing for H2O and D2O Mixtures Using a Hexanuclear Ln(III) Cluster-Based Metal–Organic Framework

High-purity heavy water (D2O) is a strategic material owing to its important application in the fields of nuclear energy and scientific research. D2O always tends to get contaminated by H2O owing to its strong hygroscopicity. Herein, a bimetallic hexanuclear Ln(III) cluster-based metal–organic framework (Eu0.5Tb0.5-TZB-MOF) has been synthesized for fluorescence sensing of the D2O–H2O binary mixtures. Eu0.5Tb0.5-TZB-MOF can be used to immediately differentiate D2O or H2O via fluorescent color responses that are obvious to the naked eye and allow for quantitative ratiometric analysis using simple spectrophotometry. Fluorescence titration experiments demonstrate that both trace H2O in D2O and trace D2O in H2O can be quantitatively detected. Mechanistic studies demonstrate that the weaker vibrational quenching of the O–D oscillator compared to the O–H oscillator, in addition to the terbium-to-europium energy transfer, triggered the fluorescence signal response.