Photochemical Anchoring of Ultrahigh‐Loading Single‐Atom Catalysts in MOFs for Enhanced Oxidase‐Mimicking Activity

Achieving high metal loadings in metal–organic frameworks (MOFs)‐based single‐atom catalysts (SACs) remains a major challenge due to the degradation of anchoring sites during high‐temperature synthesis. Here, a low‐temperature photochemical reduction strategy that preserves the structural integrity of MOF and maximizes the density of unsaturated pyridinic nitrogen sites for efficient metal atom anchoring is reported. This pyrolysis‐free approach enables the synthesis of SACs with record‐high metal loadings, up to 20.5 wt.% for Pt, 16.9 wt.% for Ru, 15.4 wt.% for Os, 12.9 wt.% for Fe, and 9.6 wt.% for Cu, surpassing previous MOF‐derived SACs by one order of magnitude. Density functional theory (DFT) calculations reveal that the unique Pt‐N 2 Cl 2 coordination significantly enhances oxidase‐like activity compared to conventional Pt‐N 3 configurations. Furthermore, the high metal loading increases the density of catalytically active sites, thereby improving overall catalytic efficiency. As a proof of concept, a Pt‐SACs@MOF‐based immunosensor achieves ultrasensitive detection of α‐fetoprotein (AFP) with a detection limit as low as 3 fg mL −1 . This work offers a general and scalable strategy for synthesizing high‐density SACs, addressing the long‐standing trade‐off between metal loading and structural stability in MOF‐based catalysts.