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

A review on g-C3N4-based photocatalysts

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

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

Ag2O/g-C3N4 composites synthesized in this study were applied in the photocatalytic degradation of phenol under UV- and visible-light irradiation. X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy analysis demonstrated that Ag2O nanoparticles were well distributed on the surface of g-C3N4, and the heterostructure of Ag2O/g-C3N4 was formed. Compared with pure g-C3N4 and Ag2O, the Ag2O/g-C3N4 composite (8:1) displayed much higher photocatalytic activities in phenol degradation under UV- and visible-light irradiation. The degradation rate constant of 8:1 was 0.069 min–1 under visible light, which was almost 230 and 2.1 times more than that of pure g-C3N4 and Ag2O, respectively. Moreover, the formation of a certain amount of Ag0 on the surface of Ag2O under illumination contributed to the high stability of Ag2O/g-C3N4 photocatalysts. It was also found that photogenerated holes during the photocatalytic process played the predomin...

Improved Photochemical Reactivities of Ag2O/g-C3N4 in Phenol Degradation under UV and Visible Light

Studies of Fe3O4-chitosan nanoparticles prepared by co-precipitation under the magnetic field for lipase immobilization

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

Ag 2 O/P25 and Ag/P25 catalysts were synthesized and applied in the photocatalytic reduction of nitrate with formic acid as a hole scavenger. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM) analysis demonstrate that Ag 2 O or Ag nanoparticles were well distributed on the surface of P25. Under UV irradiation, 5% Ag 2 O/P25 showed a fast rate (0.95 h −1 , 6.3 times of P25), a high conversion (97.2%, 3.63 times of P25) and a high N 2 selectivity (83.1%, 1.15 times of P25) for nitrate reduction. The improved separation of electron–hole pairs on Ag 2 O/P25 results in the high conversion of nitrate. Moreover, compared with the Ag/P25 catalyst, Ag 2 O/P25 exhibited a good reusability in nitrate reduction. The formed Ag–Ag 2 O structure [Ag(0) present on the surface of Ag 2 O] over P25 by partial photoreduction of deposited Ag 2 O contributed to the better stability of Ag 2 O/P25 in nitrate reduction. The present study provides a new sight into the use of Ag 2 O/P25 catalyst in the reduction of nitrate.

Shao-Yi Jia

Papers

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

Improved Photochemical Reactivities of Ag2O/g-C3N4 in Phenol Degradation under UV and Visible Light

Studies of Fe3O4-chitosan nanoparticles prepared by co-precipitation under the magnetic field for lipase immobilization

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

A facile preparation of Ag2O/P25 photocatalyst for selective reduction of nitrate