Coordination-Site-Engineered Hydrophilic Ln-MOFs for Ultrasensitive Ratiometric Sensing of Trace H ₂ O in D ₂ O

Trace H 2 O detection in D 2 O is important for isotope-related technologies but remains challenging owing to subtle isotopologue differences. Herein, we propose a coordination-site engineering strategy based on hydrophilic lanthanide MOFs. Tuning Ln 3+ coordination strengthens framework–isotopologue interactions and amplifies isotope-dependent nonradiative quenching. Eu(BTC)(H 2 O) and Eu(BTC)(H 2 O) 6 reveal that coordinated water governs hydrophilicity, vibrational coupling, and sensing performance. Activation exposes accessible lanthanide sites, while Tb 3+ enables a self-calibrated I 545 /I 618 ratiometric signal. Optimized Eu 0.15 Tb 0.85 (BTC) achieves linear detection with a 0.023 vol % H 2 O limit and supports smartphone-assisted fluorescence/photocurrent dual-mode sensing.