Ligand-Engineered Eu-MOF for Ratiometric/Visual Urease Biosensing with Encrypted Optical Readout

Programming energy-transfer pathways in lanthanide metal-organic frameworks (Ln-MOFs) through rational ligand design is essential for modulating the antenna effect and developing high-performance ratiometric sensors. However, the effective synthetic regulation of the coordination microenvironment remains a key challenge. Herein, we demonstrate that the ligand-to-Eu3+ energy transfer efficiency can be precisely regulated simply by tuning the molar ratio of pyromellitic acid to 2-aminoterephthalic acid during MOF assembly. This compositional regulation strategy not only enables fine modulation of the antenna effect to optimize Eu3+ emission but also directly controls the density of amino sites for postsynthetic conjugation with fluorescein isothiocyanate. The resulting EuMOF@FITC probe integrates multiple emission centers and exhibits a distinct red-to-green fluorescence response toward NH3 generated from urease-catalyzed urea hydrolysis. The ratiometric signal improves analytical reliability and signal-to-noise ratio through minimizing the influence of excitation fluctuation, probe concentration variation, and complex environmental interference, enabling accurate quantification of urease activity and efficient screening of urease inhibitors. This synthetic strategy can systematically modulate the energy-transfer pathway through simple ligand ratio regulation without changing the overall synthetic conditions, providing a direct and generalized route for the rational design of Ln-MOF ratiometric sensors. Furthermore, we embed the probe into a hydrogel matrix to achieve portable, visual, and real-time urease monitoring. The excitation-dependent and reversible luminescence characteristics of the composite hydrogel further enable dual-key optical information encryption. This study highlights the critical role of precisely controlling the coordination microenvironment during MOF assembly to improve the sensing reliability and expand the multifunctional optical applications of Ln-MOFs.