Eu³⁺-doped CeO₂ with regulated oxygen vacancies for highly sensitive organophosphorus detection

Eu³⁺-doped CeO₂ nanoparticles with regulated oxygen vacancies (V_o^••) have been developed as phosphatase-like nanozymes for highly sensitive colorimetric detection of p-nitrophenyl phosphate (p-NPP), a model phosphate ester substrate. Eu³⁺ serves simultaneously as a defect regulator that modulates V_o^•• formation and as a luminescent probe for tracking the local environment associated with V_o^••. Spectroscopic analyses revealed that moderate Eu³⁺ doping promotes the preferential formation of Ce-adjacent V_o^••, generating highly active Ce⁴⁺-V_o^••-Ce³⁺ configurations that enhance substrate adsorption and facilitate P-O bond cleavage. CeO₂: 2%Eu exhibits the highest hydrolysis activity, enabling highly sensitive detection of p-NPP with a detection limit of 54 nM over a linear range of 1.2-200 µM, approximately 12-fold lower than that of undoped CeO₂. Coupled with a smartphone-assisted paper-based platform, the nanozyme enables portable, visual and quantitative colorimetric detection. This work provides new insights into defect-engineered nanozyme design and offers a promising strategy for the development of highly sensitive OPC sensors.