Active Site Engineering of Mn‐Doped CeO ₂ Nanozymes: Enhanced Reactivity and Specificity for Colorimetric Detection of Perfluorooctanoic Acid in Waters

ABSTRACT The on‐site monitoring of perfluorooctanoic acid (PFOA) in aquatic ecosystems is critical for safeguarding environmental and public health. However, conventional colorimetric sensing platforms are often constrained by a fundamental trade‐off between signal sensitivity and target selectivity. To overcome this limitation, we developed manganese‐doped ceria nanozymes (Mn‐CeO ₂ NSs) with precisely engineered active sites to enhance both catalytic reactivity and molecular recognition capability. Combined experimental and theoretical analyses reveal that Mn doping not only increases catalytic site density and elevates the Ce ^{4 +} /Ce ^{3 +} redox ratio—enhancing peroxidase‐like activity—but also modulates the charge density at metal centers. This electronic regulation strengthens PFOA binding through synergistic electrostatic attraction, Ce─F coordination, and hydrophobic interactions. The colorimetric system was integrated with smartphone‐based grayscale analysis, enabling rapid on‐site detection of PFOA across diverse real water samples and food matrices, with high recovery rates (93.10–106.75%) and reliable self‐calibration. This work provides a facile and robust strategy for mapping PFOA distribution in natural waters.