Microenvironment-engineered Brønsted acidity in MOF-808 for efficient organophosphate hydrolysis and sensing

Organophosphate esters (OPEs) are representative phosphate ester substrates, whose efficient hydrolysis critically depends on cooperative Lewis and Brønsted acid catalysis as observed in natural hydrolases. However, most artificial catalysts primarily emphasize Lewis acidic metal centers while the regulatory role of Brønsted acidity in hydrolysis nanozymes has rarely been systematically investigated or mechanistically validated. Here, we report a hydroxyl-modified metal-organic framework nanozyme (MOF-808-OH), in which the catalytic microenvironment is engineered by introducing hydroxyl groups into the organic linkers of MOF-808 to enhance Brønsted acidity. As a result, MOF-808-OH exhibits a 1.9-fold increase in phosphate ester hydrolysis activity compared with pristine MOF-808. Combined experimental investigations and density functional theory calculations reveal that strengthened Brønsted acid sites facilitate substrate activation and transition-state stabilization during P-O bond cleavage. Benefiting from this enhanced catalytic efficiency, MOF-808-OH enables reliable colorimetric quantification of p-nitrophenyl phosphate (p-NPP) with a linear range of 5-1000 μg mL^-1 and a limit of detection of 2.5 μg mL^-1. Furthermore, the hydrolyzed phosphate can be subsequently captured via Ca²⁺-induced precipitation, offering a feasible route for downstream phosphorus utilization. This work demonstrates the key role of Brønsted acidity in hydrolysis nanozymes, which differs from traditional studies that only focus on Lewis acid sites. The establishment of structure-activity relationship provides a new and transferable design principle for high-performance MOF-based hydrolytic nanozymes.