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-N₂Cl₂ coordination significantly enhances oxidase-like activity compared to conventional Pt-N₃ 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.