Enzymes have catalytic turnovers. The field of nanozyme endeavors to engineer nanomaterials as enzyme mimics. However, a discrepancy in the definition of "nanozyme concentration" has led to an unrealistic calculation of nanozyme catalytic turnovers. To date, most of the reported works have considered either the atomic concentration or nanoparticle (NP) concentration as nanozyme concentration. These assumptions can lead to a significant under-or overestimation of the catalytic activity of nanozymes. In this article, we review some classic nanozymes including Fe 3 O 4 , CeO 2 , and gold nanoparticles (AuNPs) with a focus on the reported catalytic activities. We argue that only the surface atoms should be considered as nanozyme active sites, and then the turnover numbers and rates were recalculated based on the surface atoms. According to the calculations, the catalytic turnover of peroxidase Fe 3 O 4 NPs is validated. AuNPs are self-limited when performing glucose-oxidase like activity, but they are also true catalysts. For CeO 2 NPs, a self-limited behavior is observed for both oxidase-and phosphatase-like activities due to the adsorption of reaction products. Moreover, the catalytic activity of single-atom nanozymes is discussed. Finally, a few suggestions for future research are proposed.

Abstract“Nanozyme” is used to describe various catalysts from immobilized inorganic metal complexes, immobilized enzymes to inorganic nanoparticles. Here, the history of nanozymes is dvescribed in detail, and they can be largely separated into two types. Type 1 nanozymes refer to immobilized catalysts or enzymes on nanomaterials, which were dominant in the first decade since 2004. Type 2 nanozymes, which rely on the surface catalytic properties of inorganic nanomaterials, are the dominating type in the past decade. The definition of nanozymes is evolving, and a definition based on the same substrates and products as enzymes are able to cover most currently claimed nanozymes, although they may have different mechanisms compared to their enzyme counterparts. A broader definition can inspire application‐based research to replace enzymes with nanomaterials for analytical, environmental, and biomedical applications. Comparison with enzymes also requires a clear definition of a nanozyme unit. Four ways of defining a nanozyme unit are described, with iron oxide and horseradish peroxidase activity comparison as examples in each definition. Growing work is devoted to understanding the catalytic mechanism of nanozymes, which provides a basis for further rational engineering of active sites. Finally, future perspective of the nanozyme field is discussed.

The field of nanozyme aims to use nanomaterials to replace protein-based enzymes. Nanozymes have attracted extensive interest because of their stability, cost-effectiveness, and versatility. While the focus of the nanozyme field has mainly been the discovery of new nanozyme materials and the exploration of their analytical, biomedical, and environmental applications, the number of fundamental studies is growing. Nanozymes are related to two important fields: enzymology and heterogeneous catalysis. Although fitting nanozyme kinetic data to the Michaelis–Menten kinetics is a very common practice, using the surface science methods of heterogeneous catalysis can provide insights about their catalytic mechanisms. The definition of a nanozyme unit is critical to understanding and comparing nanozyme activities. In this perspective, we articulate the use of a surface science approach to study nanozymes and discuss the various application scenarios of using different nanozyme units.

Polydopamine (PDA) is a widely used universal coating for a broad range of materials. Interfacing PDA with various biomolecules, such as DNA, is critical for applications such as sensing, intracellular delivery, and material fabrication. Because of the negative surface charge of PDA at neutral pH, electrostatic repulsion exists between PDA and DNA. In previous studies, modified DNA or low pH was used to overcome this repulsion for DNA adsorption. More recently, divalent Ca2+ was found to bridge DNA and PDA. Herein, we studied four transition metals (Mn2+, Co2+, Zn2+, and Ni2+) and compared their efficiencies with Ca2+ for promoting DNA adsorption. These transition metals induced a more efficient and tighter DNA binding compared to Ca2+. In all these cases, the DNA phosphate backbone played a dominant role in adsorption, although DNA bases might also interact with strong binding metals such as Ni2+. Moreover, when the adsorption affinity was stronger, sensing was more selective to complementary DNA. Finally, aging of PDA appeared to be detrimental for DNA adsorption, which could be due to further oxidation of PDA. We showed that using Zn2+ or Ni2+ could considerably relieve the aging effect, while storing PDA at 4 °C could slow down aging.

Removal and degradation of microplastics are often carried out separately. In this work, hydrophilic bare Fe3O4 nanoaggregates allowed efficient removal of the most common microplastics including high‐density polyethylene, polypropylene, polyvinyl chloride, polystyrene, and polyethylene terephthalate. Full extraction was achieved using Fe3O4 at 1 % of the mass of microplastics. Hydrogen bonding is the main force for the adsorption of Fe3O4. Unlike the more commonly used hydrophobically modified Fe3O4 nanoparticles, the bare Fe3O4 benefitted from the peroxidase‐like activity of its exposed surface, enabling further catalytic degradation of microplastics with nearly 100 % efficiency and easy recovery of the Fe3O4.