Reducing Valence States of Co Active Sites in a Single‐Atom Nanozyme for Boosted Tumor Therapy

Abstract The construction of biocompatible and trackable‐imaging single‐atom nanozymes (SAzymes) with efficient catalytic activities is particularly desirable. Here, cobalt/titanium oxide (Co/TiO 2 ) SAzymes are presented with cobalt atomically dispersed on nanoporous hollow TiO 2 using a cation‐exchange strategy. Significantly, by varying the calcination conditions, the enzyme‐like activity can be enhanced tenfold. It is determined that different calcination treatments result in valence state shifts of the Co active site due to changes in the amounts of defects, which affects the catalytic kinetics. Moreover, Co/TiO 2 SAzymes exhibit good intrinsic biocompatibility and excellent tolerance toward the biological medium, while the hollow structure facilitates the loading of drugs and imaging agents for image‐guided chemo‐chemodynamic therapy via intravenous injection. This study not only provides a paradigm shift for the preparation of biocompatible SAzymes but also presents new insights for modulating the catalytic activity of SAzymes.