Atomic engineering of single-atom nanozymes for enzyme-like catalysis

Enzyme mimics, especially nanozymes, play a crucial role in replacing natural enzymes for diverse applications related to bioanalysis, therapeutics and other enzyme-like catalysis. Nanozymes are catalytic nanomaterials with enzyme-like properties, which currently face formidable challenges with respect to their intricate structure, properties and mechanism in comparison with enzymes. The latest emergence of single-atom nanozymes (SAzymes) undoubtedly promoted the nanozyme technologies to the atomic level and provided new opportunities to break through their inherent limitations. In this perspective, we discuss key aspects of SAzymes, including the advantages of the single-site structure, and the derived synergetic enhancements of enzyme-like activity, catalytic selectivity and the mechanism, as well as the superiority in biological and catalytic applications, and then highlight challenges that SAzymes face and provide relevant guidelines from our point of view for the rational design and extensive applications of SAzymes, so that SAzyme may achieve its full potential as the next-generation nanozyme.

ZIF-67 as a Template Generating and Tuning "Raisin Pudding"-Type Nanozymes with Multiple Enzyme-like Activities: Toward Online Electrochemical Detection of 3,4-Dihydroxyphenylacetic Acid in Living Brains

Due to its unique structure and high porosity, metal-organic frameworks (MOFs) can act not only as nanozyme materials but also as carriers to encapsulate natural enzymes and thus have received extensive attention in recent years. However, a few research studies have been conducted to investigate MOF as a template to generate and tune nanozymes in the structure and performance. In this work, the "raisin pudding"-type ZIF-67/Cu_0.76Co_2.24O₄ nanospheres (ZIF-67/Cu_0.76Co_2.24O₄ NSs) were obtained by rationally regulating the weight ratio of ZIF-67 and Cu(NO₃)₂ in the synthesis process. Here, ZIF-67 not only acts as a template but also provides a cobalt source for the synthesis of cobalt copper oxide on the surface of ZIF-67/Cu_0.76Co_2.24O₄ NSs with multiple enzyme-like activities. The ZIF-67/Cu_0.76Co_2.24O₄ NSs can mimic four kinds of enzymes with peroxidase-like, glutathione peroxidase-like, superoxide dismutase-like, and laccase-like activities. Based on its laccase-like activity, an online electrochemical system for continuous monitoring of 3,4-dihydroxyphenylacetic acid with good linearity in the range of 0.5-20 μM and a detection limit of 0.15 μM was established. Furthermore, the alteration of DOPAC in the brain microdialysate before and after ischemia of the rats' brain was also successfully recorded. This work not only raises a new idea for the synthesis of nanozyme materials with multiple enzyme activities but also provides a new solution for the detection of neurotransmitters in living brains.

Embedding Functional Biomacromolecules within Peptide-Directed Metal-Organic Framework (MOF) Nanoarchitectures Enables Activity Enhancement

Rationally tailoring a robust artificial coating can enhance the life-time of fragile biomacromolecules. However, the coating also can restrain the activity of the guest because of the decreased substrate accessibility. Herein, we report a peptide-directed strategy that enables in situ tailoring of the MOF-shrouded biohybrids into controllable nanoarchitectures. The MOF biohybrid can be shaped from different 3D microporous architectures into a 2D mesoporous layer by a peptide modulator. Using this mild strategy, we show that the nanoarchitectures of the MOF coatings significantly affect the biological functions of the contained biomacromolecules. The biomacromolecules entrapped within the novel 2D mesoporous spindle-shaped MOFs (2D MSMOFs) have significantly increased bioactivity compared to when encased within the hitherto explored 3D microporous MOFs. The improvement results from the shortened diffusion path and enlarged pore channel in 2D MSMOFs. Meanwhile, the thin 2D MSMOF layer also can provide excellent protection of the hosted biomacromolecules or protein-scaffolded biominerals through structural confinement.

Revealing the Intrinsic Peroxidase-Like Catalytic Mechanism of Heterogeneous Single-Atom Co-MoS2

The single-atom nanozyme is a new concept and has tremendous prospects to become a next-generation nanozyme. However, few studies have been carried out to elucidate the intrinsic mechanisms for both the single atoms and the supports in single-atom nanozymes. Herein, the heterogeneous single-atom Co-MoS₂ (SA Co-MoS₂) is demonstrated to have excellent potential as a high-performance peroxidase mimic. Because of the well-defined structure of SA Co-MoS₂, its peroxidase-like mechanism is extensively interpreted through experimental and theoretical studies. Due to the different adsorption energies of substrates on different parts of SA Co-MoS₂ in the peroxidase-like reaction, SA Co favors electron transfer mechanisms, while MoS₂ relies on Fenton-like reactions. The different catalytic pathways provide an intrinsic understanding of the remarkable performance of SA Co-MoS₂. The present study not only develops a new kind of single-atom catalyst (SAC) as an elegant platform for understanding the enzyme-like activities of heterogeneous nanomaterials but also facilitates the novel application of SACs in biocatalysis.