Since Fe 3 O 4 was reported to mimic horseradish peroxidase (HRP) with comparable activity (2007), countless peroxidase nanozymes have been developed for a wide range of applications from biological detection assays to disease diagnosis and biomedicine development. However, researchers have recently argued that Fe 3 O 4 has no peroxidase activity because surface Fe(III) cannot oxidize tetramethylbenzidine (TMB) in the absence of H 2 O 2 (cf. HRP). This motivated us to investigate the origin of transition metal oxides as peroxidase mimetics. The redox between their surface M n+ (oxidation) and H 2 O 2 (reduction) was found to be the key step generating OH radicals, which oxidize not only TMB for color change but other H 2 O 2 to produce HO 2 radicals for M n+ regeneration. This mechanism involving free OH and HO 2 radicals is distinct from that of HRP with a radical retained on the Fe-porphyrin ring. Most importantly, it also explains the origin of their catalase-like activity (i.e., the decomposition of H 2 O 2 into H 2 O and O 2 ). Because the production of OH radicals is the ratelimiting step, the poor activity of Fe 3 O 4 can be attributed to the slow redox of Fe(II) with H 2 O 2 , which is two orders of magnitude slower than the most active Cu(I) among common transition metal oxides. We further tested glutathione (GSH) detection on the basis of its peroxidase-like activity to highlight the importance of understanding the mechanism when selecting materials with high performance.

Although Fe3O4 nanoparticles were early reported to outperform horseradish peroxidase (HRP), recent studies suggested that this material bears a very poor activity instead. Here, we resolve this disagreement by reviewing the definition of descriptors used and provide an atomic view into the origin of Fe3O4 nanoparticles as peroxidase mimetics. The redox between H2O2 and Fe­(II) sites on the Fe3O4 surface was identified as the key step to producing OH radicals for the oxidation of colorimetric substrates. This mechanism involving free radicals is distinct from that of HRP oxidizing substrates with a radical retained on its Fe-porphyrin ring. Surprisingly, the distribution and chemical state of Fe species were found to be very different on single- and polycrystalline Fe3O4 nanoparticles with the latter bearing not only a higher Fe­(II)/Fe­(III) ratio but also a more reactive Fe­(II) species at surface grain boundaries. This accounts for the unexpected gap in the catalytic constant (k cat) observed for this material in the literature.

Nanozymes are promising alternatives to natural enzymes, but their use remains limited owing to poor specificity. For example, CeO 2 activates H 2 O 2 and displays peroxidase (POD)-like, catalase (CAT)-like, and haloperoxidase (HPO)-like activities. Since they unavoidably compete for H 2 O 2 , affecting its utilization in the target application, the precise manipulation of reaction specificity is thus imperative. Herein, we showed that one can simply achieve this by manipulating the H 2 O 2 activation pathway on pristine CeO 2 in well-defined shapes. This is because the coordination and electronic structures of Ce sites vary with CeO 2 surfaces, wherein the (100) and (111) surfaces display nearly 100% specificity toward POD-/CAT-like and HPO-like activities, respectively. The antibacterial results suggest that the latter surface can well-utilize H 2 O 2 to kill bacteria (cf., the former), which is promising for anti-biofouling applications. This work provides atomic insights into the synthesis of nanozymes with improved activity, reaction specificity, and H 2 O 2 utilization. KEYWORDS: nanozymes, CeO 2 , coordination/electronic structure, H 2 O 2 activation pathway, H 2 O 2 -associated enzymatic reactions

Urea pollution is a growing environmental concern and its removal via catalytic hydrolysis is challenging due to the resonance stabilized amid bonds. In nature, this reaction is catalyzed by ureases...

A cross-coupling reaction via the dehydrogenative route over heterogeneous solid atomic catalysts offers practical solutions toward an economical and sustainable elaboration of simple organic substrates. The current utilization of this technology is, however, hampered by limited molecular definition of many solid catalysts. Here, we report the development of Cu−M dual-atom catalysts (where M = Co, Ni, Cu, and Zn) supported on a hierarchical USY zeolite to mediate efficient dehydrogenative cross-coupling of unprotected phenols with amine partners. Over 80% isolated yields have been attained over Cu−Co−USY, which shows much superior reactivity when compared with our Cu 1 and other Cu−M analogues. This amination reaction has hence involved simple and non-forceful reaction condition requirements. The superior reactivity can be attributed to (1) the specifically designed bimetallic Cu−Co active sites within the micropore for "co-adsorption−co-activation" of the reaction substrates and (2) the facile intracrystalline (meso/micropore) diffusion of the heterocyclic organic substrates. This study offers critical insights into the engineering of next-generation solid atomic catalysts with complex reaction steps.