Over the past few decades, researchers have established artificial enzymes as highly stable and low-cost alternatives to natural enzymes in a wide range of applications. A variety of materials including cyclodextrins, metal complexes, porphyrins, polymers, dendrimers and biomolecules have been extensively explored to mimic the structures and functions of naturally occurring enzymes. Recently, some nanomaterials have been found to exhibit unexpected enzyme-like activities, and great advances have been made in this area due to the tremendous progress in nano-research and the unique characteristics of nanomaterials. To highlight the progress in the field of nanomaterial-based artificial enzymes (nanozymes), this review discusses various nanomaterials that have been explored to mimic different kinds of enzymes. We cover their kinetics, mechanisms and applications in numerous fields, from biosensing and immunoassays, to stem cell growth and pollutant removal. We also summarize several approaches to tune the activities of nanozymes. Finally, we make comparisons between nanozymes and other catalytic materials (other artificial enzymes, natural enzymes, organic catalysts and nanomaterial-based catalysts) and address the current challenges and future directions (302 references).

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes

Metal–organic frameworks (MOFs), also known as porous coordination polymers (PCPs), synthesized by assembling metal ions with organic ligands have recently emerged as a new class of crystalline porous materials. The amenability to design as well as fine-tunable and uniform pore structures makes them promising materials for a variety of applications. Controllable integration of MOFs and functional materials is leading to the creation of new multifunctional composites/hybrids, which exhibit new properties that are superior to those of the individual components through the collective behavior of the functional units. This is a rapidly developing interdisciplinary research area. This review provides an overview of the significant advances in the development of diverse MOF composites reported till now with special emphases on the synergistic effects and applications of the composites. The most widely used and successful strategies for composite synthesis are also presented.

Metal–organic framework composites

Recently sulfate radical-based advanced oxidation processes (SR-AOPs) attract increasing attention due to their capability and adaptability in decontamination. The couple of cobalt and peroxymonosulfate (PMS) is an efficient way to produce reactive sulfate radicals. This article reviews the state-of-the-art progress on various heterogeneous cobalt-based catalysts for PMS activation, including cobalt oxides, cobalt-ferrite and supported cobalt by diverse substrates. We summarize the intrinsic properties of these catalysts and their fundamental behaviors in PMS activation, as well as synthetic approaches. In addition, influencing factors and synergistic techniques of Co/PMS systems in organic degradation and possible environmental applications are also discussed. Finally, we propose perspectives on challenges related to cobalt-based catalysts, heterogeneous Co/PMS systems and their potential applications in practical environmental cleanup.

Cobalt-catalyzed sulfate radical-based advanced oxidation: A review on heterogeneous catalysts and applications

Because of the high catalytic activities and substrate specificity, natural enzymes have been widely used in industrial, medical, and biological fields, etc. Although promising, they often suffer from intrinsic shortcomings such as high cost, low operational stability, and difficulties of recycling. To overcome these shortcomings, researchers have been devoted to the exploration of artificial enzyme mimics for a long time. Since the discovery of ferromagnetic nanoparticles with intrinsic horseradish peroxidase-like activity in 2007, a large amount of studies on nanozymes have been constantly emerging in the next decade. Nanozymes are one kind of nanomaterials with enzymatic catalytic properties. Compared with natural enzymes, nanozymes have the advantages such as low cost, high stability and durability, which have been widely used in industrial, medical, and biological fields. A thorough understanding of the possible catalytic mechanisms will contribute to the development of novel and high-efficient nanozymes, and the rational regulations of the activities of nanozymes are of great significance. In this review, we systematically introduce the classification, catalytic mechanism, activity regulation as well as recent research progress of nanozymes in the field of biosensing, environmental protection, and disease treatments, etc. in the past years. We also propose the current challenges of nanozymes as well as their future research focus. We anticipate this review may be of significance for the field to understand the properties of nanozymes and the development of novel nanomaterials with enzyme mimicking activities.

Nanozymes: Classification, Catalytic Mechanisms, Activity Regulation, and Applications

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

Hemin@metal–organic framework with peroxidase-like activity and its application to glucose detection

Adsorptive removal of bisphenol A (BPA) from aqueous solution has been studied over two highly porous metal-organic frameworks (MIL-101(Cr) and MIL-100(Fe)) in view of the adsorption kinetics, adsorption isotherm, effect of initial pH and effect of ionic strength. The adsorption kinetics fit pseudo-second-order kinetic model well and the adsorption isotherms follow the Langmuir model. The adsorption kinetics and capacity of BPA over MIL-101(Cr) generally depend on the average pore size and specific surface area (or pore volume), respectively. The adsorption mechanism may be explained with π–π interaction and hydrogen bonding between BPA and MIL-101(Cr). Finally, it can be suggested that metal-organic frameworks possessing high porosity and large pore size can be used as potential adsorbents to remove harmful endocrine disrupting chemicals in contaminated water.

Adsorptive removal of bisphenol A from aqueous solution using metal-organic frameworks

Efficient removal of nitrobenzene by Fenton-like process with Co-Fe layered double hydroxide

Preparation of CdIn2S4 microspheres and application for photocatalytic reduction of carbon dioxide

SO4 − and OH radicals are important oxidative species that play important roles in the oxidation of organic contaminants by peroxomonosulfate (PMS)/persulfate (PS) and Co3O4 under acidic conditions. However, the facet effect of Co3O4 on the catalytic oxidation of phenol by PS under alkaline conditions has not been investigated yet. In this study, we synthesized three Co3O4 nanoparticles with different facets [rod (110), cube (100) and octahedra (111)]. The Co3O4 rods show the highest catalytic reactivity in the oxidation of phenol by PS, followed by Co3O4 cubes and Co3O4 octahedra. Quenching experiments and EPR analysis rule out contributions of SO4 −, OH, O2 − and 1O2 to phenol oxidation and suggest that H-abstraction from phenol to phenoxyl radicals is an important pathway in all Co3O4-PS systems. The analysis of total organic carbon (TOC) and solid characterizations reveal that more than 96.6 % phenol has been transformed into polyphenols, instead of being mineralized into smaller organic molecules or CO2 in the rod-PS-phenol system. The DFT (density functional theory) calculations further confirm that PS is more readily activated via a non-radical pathway on the Co3O4(110) plane with appropriate Co(II)-Co(II) distances, and the activated O O bonds then abstract H atoms from phenol molecules with the production of phenoxyl radicals, as well as SO4 − and OH radicals. The produced phenoxyl radicals then link together to form dimers, trimers, oligomers and eventually insoluble polymers. This study not only enriches our understanding on the facet effect of different Co3O4 in the oxidation of phenol, but also gives new insight into turning phenolic wastes into polyphenol "treasures".

Yong Liu

Papers

Hemin@metal–organic framework with peroxidase-like activity and its application to glucose detection

Adsorptive removal of bisphenol A from aqueous solution using metal-organic frameworks

Efficient removal of nitrobenzene by Fenton-like process with Co-Fe layered double hydroxide

Preparation of CdIn2S4 microspheres and application for photocatalytic reduction of carbon dioxide

Catalytic reactivity of Co3O4 with different facets in the hydrogen abstraction of phenol by persulfate