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

Carbon dots: synthesis, formation mechanism, fluorescence origin and sensing applications

There is great need for stand-alone luminescence-based chemosensors that exemplify selectivity, sensitivity, and applicability and that overcome the challenges that arise from complex, real-world media. Discussed herein are recent developments toward these goals in the field of supramolecular luminescent chemosensors, including macrocycles, polymers, and nanomaterials. Specific focus is placed on the development of new macrocycle hosts since 2010, coupled with considerations of the underlying principles of supramolecular chemistry as well as analytes of interest and common luminophores. State-of-the-art developments in the fields of polymer and nanomaterial sensors are also examined, and some remaining unsolved challenges in the area of chemosensors are discussed.

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Supramolecular Luminescent Sensors

This review summarizes recent progress in the design and applications of cadmium-free quantum dots (Cd-free QDs), with an emphasis on their role in biophotonics and nanomedicine. We first present the features of Cd-free QDs and describe the physics and emergent optical properties of various types of Cd-free QDs whose applications are discussed in subsequent sections. Selected specific QD systems are introduced, followed by the preparation of these Cd-free QDs in a form useful for biological applications, including recent advances in achieving high photoluminescence quantum yield (PL QY) and tunability of emission color. Next, we summarize biophotonic applications of Cd-free QDs in optical imaging, photoacoustic imaging, sensing, optical tracking, and photothermal therapy. Research advances in the use of Cd-free QDs for nanomedicine applications are discussed, including drug/gene delivery, protein/peptide delivery, image-guided surgery, diagnostics, and medical devices. The review then considers the pharma...

New Generation Cadmium-Free Quantum Dots for Biophotonics and Nanomedicine

Photodynamic therapy is arising as a noninvasive treatment modality for cancer and other diseases. One of the key factors to determine the therapeutic function is the efficiency of photosensitizers (PSs). Opposed to traditional PSs, which show quenched fluorescence and reduced singlet oxygen production in the aggregate state, PSs with aggregation-induced emission (AIE) exhibit enhanced fluorescence and strong photosensitization ability in nanoparticles. Here, the design principles of AIE PSs and their biomedical applications are discussed in detail, starting with a summary of traditional PSs, followed by a comparison between traditional and AIE PSs to highlight the various design strategies and unique features of the latter. Subsequently, the applications of AIE PSs in photodynamic cancer cell ablation, bacteria killing, and image-guided therapy are discussed using charged AIE PSs, AIE PS molecular probes, and AIE PS nanoparticles as examples. These studies have demonstrated the great potential of AIE PSs as effective theranostic agents to treat tumor or bacterial infection. This review hopefully will spur more research interest in AIE PSs for future translational research.

Photosensitizers with Aggregation-Induced Emission: Materials and Biomedical Applications

Materials for photosensitized oxygen activation are extremely important for a suite of photodynamic applications in biomedical, analytical, and energy sectors. Carbon-based photosensitizers are attractive for their low cost and high stability, but most of them such as fullerene and graphene quantum dots suffer from low efficiency, and the rational design of carbon-based photosensitizers remains a challenge. Given the similar chemical origin of phosphorescence and photosensitization, we herein synthesized a series of nitrogen-doped carbon dots (C-dots) and confirmed that their photo-oxidation activity correlated with their phosphorescence quantum yields, providing a direction for the rational designing of such materials. Compared to other carbon nanomaterials and molecular photosensitizers, these C-dots have the highest activity, and they can finish oxidation reactions in a few seconds. The excellent photosensitized oxygen activation makes these water-soluble C-dots a promising oxidase-mimicking nanozyme f...

Phosphorescent Carbon Dots for Highly Efficient Oxygen Photosensitization and as Photo-oxidative Nanozymes.

A long-standing challenge in nanozyme catalysis is low activity at physiological pH, especially for oxidase- and peroxidase-mimicking nanozymes. We herein communicate that Mn(II) can promote catalysis at neutral pH for carbon dots (C-dots) as a photo-oxidase nanozyme. The C-dots produce singlet oxygen upon light irradiation to oxidize Mn(II) to Mn(III), which is confirmed by a suite of spectroscopic evidence. The in situ produced Mn(III) acts as a mediator, analogous to mediators in electrochemistry to enhance electron transfer. None of the other divalent metal ions show such an effect, allowing the selective detection of Mn(II) down to 5 nM. EDTA further enhances the activity by stabilizing the highly active Mn(III), producing an intense blue color by oxidizing 3,3',5,5'-tetramethylbenzidine (TMB) in just 10 s. Finally, this reaction was used to evaluate antioxidants. With this method, more analytical and biomedical applications of nanozymes can be exploited at neutral pH, and it may inspire other strategies to overcome the pH limitation in nanozyme catalysis.

Manganese as a Catalytic Mediator for Photo-oxidation and Breaking the pH Limitation of Nanozymes.

Among various photophysical processes for fluorescence modulation, inner filter effect (IFE) is well-known for its simplicity and no requirement of the distances between the fluorophores and the corresponding modulators. Theoretically, the key to maximize the sensitivity of IFE-based fluorescent assays is to enlarge the overlap between the absorption of the absorber and the excitation/emission of the fluorophores. However, it is difficult to achieve perfect spectral overlap for IFE in conventional organic fluorophores with constant excitation/emission. The emergence of various optically-active nanocrystals greatly revolutionize the IFE-based fluorescent sensing. Therefore in this critical review, we first made an introduction to IFE from the viewpoint of its photophysical process. Particularly, the similarities and differences of IFE and fluorescence resonance energy transfer for better understanding of IFE are discussed, and the general rules for verification of IFE are proposed. The use of various optically-active nanocrystals for enhanced IFE-based sensing is especially concerned in this review.

Optically-active nanocrystals for inner filter effect-based fluorescence sensing: Achieving better spectral overlap

Recently, nanozymes have attracted enormous interest for their high stability, low cost and various enzyme-like activities. In nature, many biochemical reactions require light. Recently, introducing light to nanozymes has also been reported, especially for photosensitized oxygen activation. Compared to normal nanozymes, light-activated nanozymes possess several advantages including light-regulated activity, using molecular oxygen as a green oxidant, and often higher activity can be achieved. Herein, we summarize light-activated nanozymes, starting from their photophysical processes and identification of reactive oxygen species (ROS). Although the types of light-activated nanozymes are still quite limited and cannot yet mimic the same reactions as natural photo-related enzymes, they have widened the range of nanozymes. A few specific applications are highlighted, including sensing, chemical synthesis, degradation of organic pollutants, and cleavage and repair of DNA. Finally, a few future research opportunities are discussed.

Light-activated nanozymes: catalytic mechanisms and applications

Singlet oxygen (1O2) plays a central role in photochemical and photobiological research. Although many photosensitizers for efficient 1O2 generation were reported, further improving its yield and oxidation power is still highly desirable. Instead of developing new ones, current photosensitizers might be boosted by mediators to facilitate energy transfer. Taking advantage of the long triplet state lifetime of lanthanide ions (Ln3+), we herein demonstrate their roles as potent oxidation mediators. Using oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) at neutral pH as a difficult model reaction, based on the fluorescence, phosphorescence, and metal-binding properties, various dyes and nanomaterials were classified into four types. The 1O2 emission of carbon dots and rose bengal was enhanced 4 times in the presence of Ce3+. Some nonphosphorescent, but strongly fluorescent dyes that are not known as photosensitizers can still be mediated by Ln3+ to produce 1O2, but metal-chelating calcein was not enhanced. Finally, nonemissive dyes failed to show activity. As mediators, the excited Ln3+ can migrate a long distance and transfer energy to O2, resulting in high 1O2 yield. Since redox-active Ce3+ and Eu3+ had the highest activity, participation of oxidation involving excited lanthanides might be possible too. In addition, Ln3+ also enhanced the activity of graphene quantum dots, graphene oxide, and g-C3N4. Rapid degradation of organic dyes was demonstrated, further supporting a high photocatalytic activity of the Ln3+-mediated system.

Jinyi Zhang

Papers

Phosphorescent Carbon Dots for Highly Efficient Oxygen Photosensitization and as Photo-oxidative Nanozymes.

Manganese as a Catalytic Mediator for Photo-oxidation and Breaking the pH Limitation of Nanozymes.

Optically-active nanocrystals for inner filter effect-based fluorescence sensing: Achieving better spectral overlap

Light-activated nanozymes: catalytic mechanisms and applications

Lanthanide-Boosted Singlet Oxygen from Diverse Photosensitizers along with Potent Photocatalytic Oxidation