Axial\nChlorination Engineering of Single-Atom Nanozyme:\nFe‑N<sub>4</sub>Cl Catalytic Sites for Efficient Peroxidase-Mimicking

Developing\naxial coordination engineering of single-atom nanozymes\n(SAzymes), directly regulating the axial coordination environment\nof the catalytic site, and optimizing the axial adsorption are meaningful\nand challenging for boosting the enzyme-like activities. Herein, the\naxial chlorination engineering of SAzyme with the Fe-N<sub>4</sub>Cl catalytic site (Fe-N<sub>4</sub>Cl/CNCl) was first proposed, exhibiting\nsuperior peroxidase-like activity compared to the traditional Fe-N<sub>4</sub>/CN SAzyme with Fe-N<sub>4</sub> site. The maximal reaction\nvelocity (4.73 × 10<sup>–5</sup> M min<sup>–1</sup>), the catalytic constant (246.4 min<sup>–1</sup>), and the\nspecific activity (81 U/mg) catalyzed by the Fe-N<sub>4</sub>Cl/CNCl\nSAzyme were 4.9 times, 3.9 times, and 2.7 times those of the Fe-N<sub>4</sub>/CN SAzyme, revealing the enormous advantages of axial chlorination\nengineering of SAzymes for remarkably improving enzyme-like activities.\nMoreover, the Fe-N<sub>4</sub>Cl/CNCl SAzyme also exhibited an enhanced\ninhibition effect of tumor cell growth in vitro and in vivo. The density\nfunctional theory calculation revealed that the Fe-N<sub>4</sub>Cl\nsite was more favorable for releasing <sup>•</sup>OH radical,\nlowering the energy barrier of rate-determining step, and accelerating\nthe reaction rate compared to the Fe-N<sub>4</sub> site. This work\ndemonstrated the outstanding potential of axial chlorination engineering\nof SAzymes for improving enzyme-like activities and practical application\nin tumor therapy.