Nanozymes: Design, Synthesis, and Applications

The discovery of highly stable and low-cost nanozymes, nanomaterials with enzyme-like activity, opens a new portal for breaking through the application limitations of delicate natural enzymes. Iron oxide nanozymes, as one of the most representative nanozymes, were first discovered to exhibit enzyme-like activity, and were systematically and in-depth studied. Researches have found some structural factors that have an important impact on the catalytic activity. In this chapter, we introduce the enzyme-like activities of iron oxide nanozymes and systematically review the elucidated structure-activity relationships to gain insight into their enzyme-like catalytic mechanisms.

Owing to their facile synthesis, low cost, excellent stability, and tunable activity, nanomaterials with enzyme-mimicking activities (nanozymes) are becoming robust alternatives to natural enzymes. However, the poor activity and relatively low selectivity of nanozymes have seriously limited their rapid development. Therefore, an in-depth understanding of experimental phenomena and catalytic mechanisms is urgently needed to move away from the traditional trial-and-error approach and develop more efficient nanozymes through rational design. So far, numerous studies have shown that the catalytic performance of nanozymes is significantly correlated with many factors, such as their size, exposed crystalline surface, element composition, and surface defects. This chapter aims to elucidate the structure-activity relationships of nanozymes, elucidate the key factors influencing their catalytic behaviors, and provide insights and ideas for designing and developing highly active nanozymes.

Nanozyme is one of the research hotspots in the field of nanocatalysis. More than 300 different nanomaterials have been reported possessing intrinsic enzyme-mimic activities to convert the substrates of oxidoreductase, hydrolase, lyase and isomerase. Among them, the number of redox nanozymes accounts for the majority, including peroxidases, catalase, oxidase and superoxide dismutase. Redox nanozymes can regulate reactive oxygen species in living cells and thus have great application potential in tumor chemodynamic therapy, improvement of tumor hypoxia, biological antioxidant and bacteriostasis. Recently, carbon-based nanozymes have received extensive attention, such as graphene oxide, carbon nanotubes, carbon dots and carbon-based single-atom enzymes etc. The carbon-based structure is similar to the molecular elements of enzyme proteins and thus has good biocompatibility and may be used as an ideal substitute for natural enzymes. How to prospectively select nanomaterials meeting specific application needs is one of the key scientific issues in nanozyme as an emerging and interdisciplinary field. The first principle calculation can deeply describe the dynamic changes in the geometric and electronic structures of materials during chemical reactions at the molecular level and has become the most effective and irreplaceable theoretical tool for studying chemical reaction mechanisms. In this chapter, theoretical investigations on the four oxidoreductase-mimicking carbon-based nanozymes are summarized and future perspectives are proposed.

With the attractive merits of good stability, easy large-scale preparation, low cost, and tailorable catalytic performance, nanozymes have experienced a rapid development period in the past few years. Especially, nanomaterials with not only the enzyme-like catalytic feature but also other optical, electrical and magnetic characteristics have found great promise in the biochemical sensing community. These multifunctional nanozymes can play versatile roles to advance the analytical chemistry field. In this chapter, currently explored nanozymes with optical, electrical and magnetic properties or acting as potential carriers are summarized in detail. Their typical applications in the field of biochemical analysis are emphatically introduced. Future prospects and challenges of multifunctional nanozymes for biochemical sensing are also discussed.

Cupric oxide (CuO) nanomaterials have been attractive to researchers worldwide because of their desired properties for technological applications in sensing, diagnosis, cancer therapy, antibacterial activity, etc. With the increasing research, enzyme-like CuO nanomaterials (CuO nanozymes) have brought about a significant contribution to the nanozyme’s field. In the following chapter, we have elaborated on details about the enzyme-like activities of CuO nanozymes and their biomedical applications. A fundamental understanding of the catalytic mechanisms and types of enzyme-like actions has been discussed, which sheds significant influence on the biomedical applications of CuO nanozymes. Overall, this chapter endeavors to sum up the research progress of CuO nanozymes, which reminds us that there are valuable discoveries that make attempts at CuO nanozymes worthwhile. The outlook of CuO nanozymes in biomedical applications has also been presented.

Oxidases are highly efficient biocatalysts in enzymes-involved cellular metabolism of all living systems, which have been widely used in various fields including biosensing, therapeutics, environmental protection and other industries. With the enormous development of nanozymes, oxidase-mimicking nanomaterials have been recognized as promising alternatives for natural oxidases due to the tunable catalytic activity and high stability. Recently, increasing researchers are focusing their attention on the oxidase-mimicking nanomaterials with specific substrates of various natural oxidases, such as glucose, cysteine, and cytochrome c. To highlight the attractive development of oxidase-mimicking nanomaterials, this chapter introduces the current progress of various oxidase-mimicking nanozymes based on the acting group of specific substrates, such as hydroxyl groups, sulfur group, and metal ions. In addition, their biomedical applications in biosensing and disease therapeutics are outlined in detail. We anticipate this review will provide useful guidance for the innovation and development of oxidase-mimicking nanomaterials.

Nanozymes refer to the nanomaterials with enzyme-like characteristics. Currently, the introduction of light is considered as an effective means to regulate the catalytic activity of nanozymes. In this chapter, we focus on the progress of photoresponsive nanozymes. The interconnection between the photoactivity and enzyme-like activity of nanozymes is discussed. In addition, the types of photoresponsive nanozymes based on various nanostructures, including metal-based, metal compound-based, carbon-based, and MOF-based, are presented. Finally, the biomedical applications of photoresponsive nanozymes are highlighted.

Cell therapy, including stem cell therapy and cancer immunotherapy, as a third generation of emerging therapeutic strategy after drug and surgical treatment, has opened up a new horizon for intractable diseases. However, the adverse microenvironment of the lesion sites reduced the efficacy of cell therapy in vivo. Reportedly, nanozymes play prominent roles in modulating microenvironment to improve cell-based therapy. Therefore, in this chapter, we introduce and exemplify the development of nanozymes-based modulation-enhanced strategies for cell therapy, ranging from antioxidant nanozymes improving stem cell therapy to pro- and anti-oxidant nanozymes that activate immune responses. It will provide valuable guidance for the synergism of cell therapy and nanocatalytic therapy.

Owing to multiple enzyme-like activities and functionality, nanozymes have shown great potentials in biomedicine, such as antibacterial, antiviral, antitumor, and antioxidant. Importantly, nanozymes can be integrated as key and stable material in medical devices including hydrogels, bandages, coatings, nanorobots, etc. In this chapter, we summarized the recent progress of nanozyme-based medical devices for biomedical applications, in particular with therapeutic purpose. We hope that this will help readers learn the perspective of nanozyme-based medical devices, and boost nanozymes to be translated to improve human health as soon as possible.