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

Nanozymes are nanomaterial-based artificial enzymes. By effectively mimicking catalytic sites of natural enzymes or harboring multivalent elements for reactions, nanozyme systems have successfully served as direct surrogates of traditional enzymes for catalysis. With the rapid development and ever-deepening understanding of nanotechnology, nanozymes offer higher catalytic stability, ease of modification and lower manufacturing cost than protein enzymes. Additionally, nanozymes possess inherent nanomaterial properties, providing not only a simple substitute of enzymes but also a multimodal platform interfacing complex biologic environments. Recent extensive research has focused on designing various nanozyme systems that are responsive to one or multiple substrates by tailored means. Catalytic activities of nanozymes can be regulated by pH, H2O2 and glutathione concentrations and levels of oxygenation in different microenvironments. Moreover, nanozymes can be remotely-controlled via different stimuli, including a magnetic field, light, ultrasound, and heat. Collectively, these factors can be adjusted to maximize the diagnostic and therapeutic efficacies of different diseases in biomedical settings. Therefore, by integrating the catalytic property and inherent nanomaterial nature of nanozyme systems, we anticipate that stimuli-responsive nanozymes will open up new horizons for diagnosis, treatment, and theranostics.

/pdf/nanozyme-new-horizons-for-responsive-biomedical-applications-3ii8qmytjy.pdf

Nanozyme: new horizons for responsive biomedical applications.

Structure and activity of nanozymes: Inspirations for de novo design of nanozymes

It is difficult and significant to realize the aim of “one-pot” and “nonenzyme” for traditional colorimetric detection of blood glucose. The synthesis of nanomaterials with 2D morphology is also a challenge for the bovine serum albumin (BSA)-directed method. Here, the BSA-directed synthesis avenue for metal oxide with 2D nanomorphology is developed. MnO2 nanoflakes (NFs) with controllable morphology can be obtained by changing the synthesis conditions. Fortunately, not only is the glucose oxidase (GOx)-like nanozyme (MnO2 NFs) discovered, but MnO2 NFs also show dual enzyme activities (GOx-like activity and peroxidase-like activity) in similar pH range. That is to say, a “tandem nanozyme” (nanomaterial with tandem enzyme-like characteristics) is presented here. Further, the one-pot nonenzymatic strategy is proposed for the colorimetric detection of glucose, where the oxidation of glucose and the colorimetric detection of H2O2 are simultaneously conducted under the catalysis of the single nanozyme (MnO2 NFs). The method shows high sensitivity, low limit of detection, and short detection time, due to the proximity effect and in situ reaction. The as-synthesized 2D tandem nanozyme expands the species of nanozymes, and the proposed strategy breaks traditional colorimetric detection process, accomplishing the purposes of “one-pot” and “nonenzyme” in the true sense.

Protein-Directed Metal Oxide Nanoflakes with Tandem Enzyme-Like Characteristics: Colorimetric Glucose Sensing Based on One-Pot Enzyme-Free Cascade Catalysis

Nanomaterials with intrinsic enzyme-like activities, namely "nanozymes," are showing increasing potential as a new type of broad-spectrum antibiotics. However, their feasibility is still far from satisfactory, due to their low catalytic activity, poor bacterial capturing capacity, and complicated material design. Herein, a facile synthesis of a defect-rich adhesive molybdenum disulfide (MoS2 )/rGO vertical heterostructure (VHS) through a one-step microwave-assisted hydrothermal method is reported. This simple, convenient but effective method for rapid material synthesis enables extremely uniform and well-dispersed MoS2 /rGO VHS with abundant S and Mo vacancies and rough surface, for a performance approaching the requirements of practical application. It is demonstrated experimentally and theoretically that the as-prepared MoS2 /rGO VHS possesses defect and irradiation dual-enhanced triple enzyme-like activities (oxidase, peroxidase, and catalase) for promoting free-radical generation, owing to much more active edge sites exposure. Meanwhile, the VHS-achieved rough surface exhibits excellent capacity for bacterial capture, with elevated reactive oxygen species (ROS) destruction through local topological interactions. As a result, optimized efficacy against drug-resistant Gram-negative and Gram-positive bacteria can be explored by such defect-rich adhesive nanozymes, demonstrating a simple but powerful way to engineered nanozymes for alternative antibiotics.

Defect-Rich Adhesive Molybdenum Disulfide/rGO Vertical Heterostructures with Enhanced Nanozyme Activity for Smart Bacterial Killing Application.

The activity of a gold nanoparticle-based catalyst can be reversibly up- and down-regulated by light. Light is used to switch a small molecule between cis- and trans-isomers, which inhibits the catalytic activity of the nanoparticles to different extent. The system is functional in aqueous buffer, which paves the way for integrating the system in biological networks.

/pdf/photoswitchable-catalysis-by-a-nanozyme-mediated-by-a-light-4y3dynvvj1.pdf

Photoswitchable Catalysis by a Nanozyme Mediated by a Light-Sensitive Cofactor.

The development of non-equilibrium synthetic systems provides access to innovative materials with life-like properties. Non-equilibrium systems require a continuous input of energy to retain their functional state, which makes for a fundamental difference to systems that operate at thermodynamic equilibrium. Kinetic asymmetry in the energy consumption pathway is required to drive systems out of equilibrium. This understanding has permitted chemists to design dissipative synthetic molecular machines and high-energy materials. Here we show that kinetic asymmetry also emerges at the macroscopic level by demonstrating that local energy delivery in the form of light to a hydrogel containing gold nanoparticles installs a non-equilibrium steady state. The instalment and maintenance of the macroscopic non-equilibrium state is facilitated by the gel matrix in which motion is governed by diffusion rather than convection. The non-equilibrium state is characterized by a persistent gradient in the surface composition of the nanoparticles embedded in the gel, which affects the fluorescent and catalytic properties of the system. We show that the overall catalytic performance of the system is enhanced under these non-equilibrium conditions. In perspective it will be possible to develop out-of-equilibrium matrices in which functional properties emerge as a result of spatially controlled energy delivery and spatially controlled chemistries. A diffusion-reaction system in a gel matrix shows that concentration gradients can lead to an overall enhanced catalytic activity.

Enhanced catalytic activity under non-equilibrium conditions

Transient self-assembly of molecular nanostructures driven by chemical fuels.

Herein, we present a modular indicator-displacement assay able to selectively recognize small molecules with biological relevance under competitive conditions. The assay relies on the change in affinity of macrocyclic receptors, such as cavitands, cyclodextrins or calixarenes, for monolayer-protected gold nanoparticles upon complexation of the respective target analyte. This change affects the equilibrium between the nanoparticles and a fluorescent reporter leading to a change in intensity of the fluorescent output signal. The recognition modules can be changed in order to tune the selectivity of the assay without affecting the nature of the output signal. The combined use of recognition modules results in an assay able to detect multiple analytes simultaneously with high selectivity.

Orthogonal Sensing of Small Molecules Using a Modular Nanoparticle-Based Assay

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Photoswitchable Catalysis by a Nanozyme Mediated by a Light-Sensitive Cofactor.

Enhanced catalytic activity under non-equilibrium conditions

Transient self-assembly of molecular nanostructures driven by chemical fuels.

Orthogonal Sensing of Small Molecules Using a Modular Nanoparticle-Based Assay