Iron oxide nanozyme catalyzed synthesis of fluorescent polydopamine for light-up Zn(2+) detection

Co-based Nanozymatic Profiling: Advances Spanning Chemistry, Biomedical, and Environmental Sciences

Nanozymes, next-generation enzyme-mimicking nanomaterials, have entered an era of rational design; among them, Co-based nanozymes have emerged as captivating players over times. Co-based nanozymes have been developed and have garnered significant attention over the past five years. Their extraordinary properties, including regulatable enzymatic activity, stability, and multifunctionality stemming from magnetic properties, photothermal conversion effects, cavitation effects, and relaxation efficiency, have made Co-based nanozymes a rising star. This review presents the first comprehensive profiling of the Co-based nanozymes in the chemistry, biology, and environmental sciences. The review begins by scrutinizing the various synthetic methods employed for Co-based nanozyme fabrication, such as template and sol-gel methods, highlighting their distinctive merits from a chemical standpoint. Furthermore, a detailed exploration of their wide-ranging applications in biosensing and biomedical therapeutics, as well as their contributions to environmental monitoring and remediation is provided. Notably, drawing inspiration from state-of-the-art techniques such as omics, a comprehensive analysis of Co-based nanozymes is undertaken, employing analogous statistical methodologies to provide valuable guidance. To conclude, a comprehensive outlook on the challenges and prospects for Co-based nanozymes is presented, spanning from microscopic physicochemical mechanisms to macroscopic clinical translational applications.

Polydopamine-Based Nanoprobes Application in Optical Biosensing

Polydopamine (PDA), the synthetic counterpart of melanin, is a widely investigated bio-inspired material for its chemical and photophysical properties, and in the last few years, bio-application of PDA and PDA-based materials have had a dramatic increase. In this review, we described PDA application in optical biosensing, exploring its multiple roles as a nanomaterial. In optical sensing, PDA can not only be used for its intrinsic fluorescent and photoacoustic properties as a probe: in some cases, a sample optical signal can be derived by melanin generation in situ or it can be enhanced in another material thanks to PDA modification. The various possibilities of PDA use coupled with its biocompatibility will indeed widen even more its application in optical bioimaging.

Secondary metal doped cuprous-cyanoimidazole frameworks for triple-mode detection of dopamine

BACKGROUNDS

Metal-organic framework-based nanozymes enable several opportunities for designing novel analysis methods for the detection of pesticides, heavy metal ions, and biomolecules; however, practical applications are still limited by a complicated synthesis route, lower catalytic activity, and single detection mode. Dopamine (DA) is a crucial catecholamine substance in the human body that acts as a neurotransmitter regulating a variety of physiological functions of the central nervous system. Therefore, it is highly significant to explore simple nanozymes synthesis methods for constructing a multiple analysis system to detection DA.

RESULTS

Herein, we elaborately selected cobalt ions as the secondary metal doping in cuprous-cyanoimidazole frameworks (CuCo-CIFs) with a mass-production strategy. CuCo-CIFs possess intrinsic peroxidase-like activity that can convert hydrogen peroxide into various reactive oxygen species (i.e., 1O2, OH·, O2·-) and thereby oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) and DA to blue oxTMB and orange polydopamine (PDA), respectively. The absorption of the detection system increases at 460 nm while decreases at 652 nm as the concentration of DA increases under near-neutral pH (6.1), resulting in a color transition from blue to orange. Consequently, an unprecedented triple-mode analysis system of DA monitored by naked eyes, ratiometric-absorption, and scanometric was constructed. The limit of detection for the ratiometric-absorption and scanometric mode can reach 20 nM and 28 nM, respectively. CuCo-CIFs were successfully used for the rapid and accurate detection of DA in practical samples.

SIGNIFICANCE

As a simple, low-cost, multi-mode colorimetric platform, this kind of nanozyme detection with peroxidase-like activity exhibits significant potential for the detection of DA. Our work not only expands the applications of MOFs in analytical fields but also addresses the general challenges faced by nanozyme-based colorimetric detection systems of DA. This work provides valuable insights for the rational application of nanozyme and the design of new analysis systems.

Research Progress in Iron-Based Nanozymes: Catalytic Mechanisms, Classification, and Biomedical Applications

Natural enzymes are crucial in biological systems and widely used in biology and medicine, but their disadvantages, such as insufficient stability and high-cost, have limited their wide application. Since Fe₃O₄ nanoparticles were found to show peroxidase-like activity, researchers have designed and developed a growing number of nanozymes that mimic the activity of natural enzymes. Nanozymes can compensate for the defects of natural enzymes and show higher stability with lower cost. Iron, a nontoxic and low-cost transition metal, has been used to synthesize a variety of iron-based nanozymes with unique structural and physicochemical properties to obtain different enzymes mimicking catalytic properties. In this perspective, catalytic mechanisms, activity modulation, and their recent research progress in sensing, tumor therapy, and antibacterial and anti-inflammatory applications are systematically presented. The challenges and perspectives on the development of iron-based nanozymes are also analyzed and discussed.

Hydrogen peroxide-assisted and histidine-stabilized copper-containing nanozyme for efficient degradation of various organic dyes

Nanozymes have potential applications in many fields, and a novel copper-containing nanozyme with highly dispersity and uniformity was self-assembled for efficient degradation of various organic dyes in this work. In the nanozyme, histidine was used to coordinate with copper ions, and hydrogen peroxide was prone to Fenton-like reaction to generate hydroxylated copper oxide intermediates. The nanozyme showed good peroxidase-like activity, and also had the ability to catalyze the degradation of various organic dyes efficiently with good storage and recycling ability. Furthermore, the degradation kinetics and mechanism of nanozyme had been further studied, and found that hydroxyl radical and singlet oxygen play vital roles in the catalytic degradation process. Meanwhile, this nanozyme can efficiently degrade two organic compounds at the same time, and this system is capable of dealing with complex practical application scenarios where wastewater contains a variety of organic pollutants.

Nanozyme's catalytic activity at neutral pH: reaction substrates and application in sensing

Nanozymes exhibit their great potential as alternatives to natural enzymes. In addition to catalytic activity, nanozymes also need to have biologically relevant catalytic reactions at physiological pH to fit in the definition of an enzyme and to achieve efficient analytical applications. Previous reviews in the nanozyme field mainly focused on the catalytic mechanisms, activity regulation, and types of catalytic reactions. In this paper, we discuss efforts made on the substrate-dependent catalytic activity of nanozymes at neutral pH. First, the discrepant catalytic activities for different substrates are compared, where the key differences are the characteristics of substrates and the adsorption of substrates by nanozymes at different pH. We then reviewed efforts to enhance reaction activity for model chromogenic substrates and strategies to engineer nanomaterials to accelerate reaction rates for other substrates at physiological pH. Finally, we also discussed methods to achieve efficient sensing applications at neutral pH using nanozymes. We believe that the nanozyme is catching up with enzymes rapidly in terms of reaction rates and reaction conditions. Designing nanozymes with specific catalysis for efficient sensing remains a challenge.

Nanozyme-encoded luminescent detection for food safety analysis: An overview of mechanisms and recent applications

With the rapid growth in global food production, delivery, and consumption, reformative food analytical techniques are required to satisfy the monitoring requirements of speed and high sensitivity. Nanozyme-encoded luminescent detections (NLDs) integrating nanozyme-based rapid detections with luminescent output signals have emerged as powerful methods for food safety monitoring, not only because of their preeminent performance in analysis, such as rapid, facile, low background signal, and ultrasensitive, but also due to their strong attractiveness for future sensing research. However, the lack of a full understanding of the fundamentals of NLDs for food safety detection technologies limits their further application. In this review, a systematic overview of the mechanisms of NLDs and their applications in the food industry is summarized, which covers the nanozyme-mimicking types and their luminescent signal generation mechanisms, as well as their applications in monitoring common foodborne contaminants. As demonstrated by previous studies, NLDs are bridging the gap to practical-oriented food analytical technologies and various opportunities to improve their food analytical performance to be considered in the future are proposed.

Recent Advances in Intrinsically Fluorescent Polydopamine Materials

Fluorescence nanoparticles have gained much attention due to their unique properties in the sensing and imaging fields. Among the very successful candidates are fluorescent polydopamine (FPDA) nanoparticles, attributed to their simplicity in tracing and excellent biocompatibility. This article aims to highlight the recent achievements in FPDA materials, especially on the part of luminescence mechanisms. We focus on the intrinsic fluorescence of PDA and will not discuss fluorescent reaction with a fluorometric reagent or coupling reaction with a fluorophore, which may cause more in vivo interferences. We believe that intrinsic FPDA presents great potential in bioapplications.

A dual-emission polymer carbon nanoparticles for ratiometric and visual detection of pH value and bilirubin

Herein, we prepared a novel fluorescent polymer carbon nanoparticles by polymerizing dopamine (DA) and o-phenylenediamine (OPD) through oxidation of hydrogen peroxide. In a neutral environment, the synthesized fluorescent polymer carbon nanoparticles (PDA-OPD) exhibited two emission peaks at 460 nm and 540 nm with 400 nm excitation wavelength. In an acidic environment, the fluorescence emission peaks of PDA-OPD at 540 nm showed an obvious fluorescence quenching, and there existed a good linear relationship between the fluorescence ratio F₅₄₀/F₄₆₀ and environment pH value. In an alkaline environment, the fluorescence emission peak at 460 nm showed obvious fluorescence quenching after the addition of bilirubin, while a novel fluorescence emission peak at 560 nm emerged gradually. The PDA-OPD could be also used to detect bilirubin in the range of 0-400 μmol·L^-1.

Influence of a Polyneurotransmitter on DNA-Mediated Förster-Based Resonance Energy Transfer: A Path Leading to White Light Generation

The physiologically important biomolecule, dopamine (DA), shows strong self-oxidation and aggregation behaviors, which have been controlled and modulated to result in fluorescent polydopamine (F-PDA) nanoparticles. On the other hand, the simultaneous binding of two diverse deoxyribonucleic acid (DNA) binding probes, 4',6-diamidino-2-phenylindole dihydrochloride (DAPI) and ethidium bromide (EtBr), has been elaborately established to follow the Förster-based resonance energy transfer (FRET) pathway. The comparative understanding of this DNA-mediated FRET in three media, phosphate buffer saline (PBS) of pH 7.4, DA, and F-PDA, has concluded that the FRET efficiency in the three media follows the order: PBS > DA > F-PDA. This controlled FRET in the fluorescent F-PDA matrix serves a pivotal role for efficient white light (WL) generation with excellent Commission Internationale de l'Eclairage (CIE) parameters that match well with that of pure WL emission. The obtained WL emission has been shown to be very specific with respect to concentrations of different participating components and the excitation wavelength of the illuminating source. Furthermore, the optical properties of the WL emitting solution have been observed to be retained excellently inside the well-known agarose gel matrix. Finally, the mechanistic pathway behind such a FRET-based WL generation has been established in detail, and to the best of our knowledge, the current study offers the first and only report that discloses the influence of a fluorescent polyneurotransmitter matrix for successful generation of WL emission.

Potentiality of Nanoenzymes for Cancer Treatment and Other Diseases: Current Status and Future Challenges

Studies from past years have observed various enzymes that are artificial, which are issued to mimic naturally occurring enzymes based on their function and structure. The nanozymes possess nanomaterials that resemble natural enzymes and are considered an innovative class. This innovative class has achieved a brilliant response from various developments and researchers owing to this unique property. In this regard, numerous nanomaterials are inspected as natural enzyme mimics for multiple types of applications, such as imaging, water treatment, therapeutics, and sensing. Nanozymes have nanomaterial properties occurring with an inheritance that provides a single substitute and multiple platforms. Nanozymes can be controlled remotely via stimuli including heat, light, magnetic field, and ultrasound. Collectively, these all can be used to increase the therapeutic as well as diagnostic efficacies. These nanozymes have major biomedical applications including cancer therapy and diagnosis, medical diagnostics, and bio sensing. We summarized and emphasized the latest progress of nanozymes, including their biomedical mechanisms and applications involving synergistic and remote control nanozymes. Finally, we cover the challenges and limitations of further improving therapeutic applications and provide a future direction for using engineered nanozymes with enhanced biomedical and diagnostic applications.

The Photophysics and Photochemistry of Melanin- Like Nanomaterials Depend on Morphology and Structure

Melanin‐like nanomaterials have found application in a large variety of high economic and social impact fields as medicine, energy conversion and storage, photothermal catalysis and environmental remediation. These materials have been used mostly for their optical and electronic properties, but also for their high biocompatibility and simplicity and versatility of preparation. Beside this, their chemistry is complex and it yields structures with different molecular weight and composition ranging from oligomers, to polymers as well as nanoparticles (NP). The comprehension of the correlation of the different compositions and morphologies to the optical properties of melanin is still incomplete and challenging, even if it is fundamental also from a technological point of view. In this minireview we focus on scientific papers, mostly recent ones, that indeed examine the link between composition and structural feature and photophysical and photochemical properties proposing this approach as a general one for future research.

Nanozymes-Hitting the Biosensing "Target"

Nanozymes are a class of artificial enzymes that have dimensions in the nanometer range and can be composed of simple metal and metal oxide nanoparticles, metal nanoclusters, dots (both quantum and carbon), nanotubes, nanowires, or multiple metal-organic frameworks (MOFs). They exhibit excellent catalytic activities with low cost, high operational robustness, and a stable shelf-life. More importantly, they are amenable to modifications that can change their surface structures and increase the range of their applications. There are three main classes of nanozymes including the peroxidase-like, the oxidase-like, and the antioxidant nanozymes. Each of these classes catalyzes a specific group of reactions. With the development of nanoscience and nanotechnology, the variety of applications for nanozymes in diverse fields has expanded dramatically, with the most popular applications in biosensing. Nanozyme-based novel biosensors have been designed to detect ions, small molecules, nucleic acids, proteins, and cancer cells. The current review focuses on the catalytic mechanism of nanozymes, their application in biosensing, and the identification of future directions for the field.

In situ synthesis of fluorescent polydopamine polymer dots based on Fenton reaction for a multi-sensing platform

The development of fluorescent nanosensors has attracted extensive research interest owing to their superior optoelectronic properties. However, current fluorescent nanoprobes generally involve complicated synthesis processes, background signal disturbance, and limited analyte detection. In this work, a facile and time-saving synthetic strategy for the preparation of green emitting polydopamine polymer dots (PDA-PDs) from dopamine via Fenton reaction at room temperature was proposed for the first time. The obtained PDA-PDs possessed excellent luminescence properties, with a long-wavelength emission of 522 nm, a large Stokes shift of 142 nm, and good photostability against ionic strength and UV irradiation. The formation mechanism of fluorescent PDA-PDs is as follows: in the presence of Fe2+ and H2O2, dopamine could rapidly undergo oxidation to its quinone derivatives and further polymerize to synthesize the fluorescent PDA-PDs with the acceleration of hydroxyl radicals produced from the Fenton reaction. Thus, a versatile turn-on fluorescence sensing method was developed for the detection of multi-analytes (including Fe2+, dopamine, H2O2, and glucose) based on monitoring the intrinsic fluorescence signal of the in situ formation of PDA-PDs. This sensing method could be efficiently applied for the detection of Fe2+, dopamine, and glucose in real human serum samples. Moreover, a three-input AND molecular logic gate based on this sensing platform was designed with the fluorescence signal of PDA-PDs as the gate. Finally, the proposed PDA-PDs could have immense broad prospects in nanomaterials and biosensors.

Micro-Bio-Chemo-Mechanical-Systems: Micromotors, Microfluidics, and Nanozymes for Biomedical Applications

Wireless nano-/micromotors powered by chemical reactions and/or external fields generate motive forces, perform tasks, and significantly extend short-range dynamic responses of passive biomedical microcarriers. However, before micromotors can be translated into clinical use, several major problems, including the biocompatibility of materials, the toxicity of chemical fuels, and deep tissue imaging methods, must be solved. Nanomaterials with enzyme-like characteristics (e.g., catalase, oxidase, peroxidase, superoxide dismutase), that is, nanozymes, can significantly expand the scope of micromotors' chemical fuels. A convergence of nanozymes, micromotors, and microfluidics can lead to a paradigm shift in the fabrication of multifunctional micromotors in reasonable quantities, encapsulation of desired subsystems, and engineering of FDA-approved core-shell structures with tuneable biological, physical, chemical, and mechanical properties. Microfluidic methods are used to prepare stable bubbles/microbubbles and capsules integrating ultrasound, optoacoustic, fluorescent, and magnetic resonance imaging modalities. The aim here is to discuss an interdisciplinary approach of three independent emerging topics: micromotors, nanozymes, and microfluidics to creatively: 1) embrace new ideas, 2) think across boundaries, and 3) solve problems whose solutions are beyond the scope of a single discipline toward the development of micro-bio-chemo-mechanical-systems for diverse bioapplications.

Recent Advancements in Enzyme-Based Lateral Flow Immunoassays

Paper-based lateral-flow immunoassays (LFIAs) have achieved considerable commercial success and their impact in diagnostics is continuously growing. LFIA results are often obtained by visualizing by the naked eye color changes in given areas, providing a qualitative information about the presence/absence of the target analyte in the sample. However, this platform has the potential to provide ultrasensitive quantitative analysis for several applications. Indeed, LFIA is based on well-established immunological techniques, which have known in the last year great advances due to the combination of highly sensitive tracers, innovative signal amplification strategies and last-generation instrumental detectors. All these available progresses can be applied also to the LFIA platform by adapting them to a portable and miniaturized format. This possibility opens countless strategies for definitively turning the LFIA technique into an ultrasensitive quantitative method. Among the different proposals for achieving this goal, the use of enzyme-based immunoassay is very well known and widespread for routine analysis and it can represent a valid approach for improving LFIA performances. Several examples have been recently reported in literature exploiting enzymes properties and features for obtaining significative advances in this field. In this review, we aim to provide a critical overview of the recent progresses in highly sensitive LFIA detection technologies, involving the exploitation of enzyme-based amplification strategies. The features and applications of the technologies, along with future developments and challenges, are also discussed.

Nucleoside-based fluorescent carbon dots for discrimination of metal ions

Carbon dots (Cdots) play an important role in many biological and chemical applications. To prepare strongly fluorescent Cdots, the starting material should contain nitrogen in addition to carbon. Nucleobases are nitrogen rich with interesting metal binding properties. In this work, we prepared a series of Cdots with citrate as the carbon source, and ethylenediamine, adenosine, cytidine, thymidine or guanosine as the respective nitrogen sources. The resulting Cdots were all fluorescent with the ethylenediamine sample being the most strongly emissive. These Cdots were then tested for their metal sensitivity and all tested metal ions can quench their fluorescence. The fluorescence of the G-Cdots prepared with guanosine was quenched most efficiently by Cu2+, while the Cdots prepared with ethylenediamine were more sensitive to Hg2+. With the differential quenching by different metal ions, we prepared a sensor array to discriminate multiple metal ions, and quantified Cu2+ and Hg2+ at the same time. Our work has expanded the range of starting materials for preparing Cdots and showed that by tuning the precursor composition, Cdots with different optical and metal binding properties can be obtained, which is useful in constructing a sensing platform for a large number of metal ions.

Fabrication of Bioresource-Derived Porous Carbon-Supported Iron as an Efficient Oxidase Mimic for Dual-Channel Biosensing

Herein, we designed a new strategy for fabricating a renewable bioresource-derived N-doped hierarchical porous carbon-supported iron (Fe/NPC)-based oxidase mimic. The obtained results suggested that Fe/NPC possessed a large specific surface area (1144 m²/g) and pore volume (0.62 cm³/g) to afford extensive Fe-Nx active sites. Taking advantages of the remarkable oxidase-mimicking activity, outstanding stability, and reusability of Fe/NPC, a novel dual-channel biosensing system was strategically fabricated for sensitively determining acetylcholinesterase (AChE) through the integration of Fe/NPC and fluorescent silver nanoclusters (AgNCs) for the first time. The limits of detection for AChE can achieve as low as 0.0032 and 0.0073 U/L by the outputting fluorometric and colorimetric dual signals, respectively. Additionally, this dual-signal system was applied to analyze human erythrocyte AChE and its inhibitor with robust analytical performance. This work provides one sustainable and effective avenue to apply a bioresource for fabricating an Fe/NPC-based oxidase mimic with high catalytic performance and also gives new impetuses for developing novel biosensors by applying Fe/NPC-based enzyme mimics as substitutes for the natural enzyme.

Nanozyme's catching up: activity, specificity, reaction conditions and reaction types

Nanozymes aim to mimic enzyme activities. In addition to catalytic activity, nanozymes also need to have specificity and catalyze biologically relevant reactions under physiological conditions to fit in the definition of enzyme and to set nanozymes apart from typical inorganic catalysts. Previous discussions in the nanozyme field mainly focused on the types of reactions or certain analytical, biomedical or environmental applications. In this article, we discuss efforts made to mimic enzymes. First, the catalytic cycles are compared, where a key difference is specific substrate binding by enzymes versus non-specific substrate adsorption by nanozymes. We then reviewed efforts to engineer and surface-modify nanomaterials to accelerate reaction rates, strategies to graft affinity ligands and molecularly imprinted polymers to achieve specific catalysis, and methods to bring nanozyme reactions to neutral pH and ambient temperature. Most of the current nanozyme reactions used a few model chromogenic substrates of no biological relevance. Therefore, we also reviewed efforts to catalyze the conversion of biomolecules and biopolymers using nanozymes. By the efforts to close the gaps between nanozymes and enzymes, we believe nanozymes are catching up rapidly. Still, challenges exist in materials design to further improve nanozymes as true enzyme mimics and achieve impactful applications.

Integration of fluorescent polydopamine nanoparticles on protamine for simple and sensitive trypsin assay

As an important protease, trypsin (TRY) has been identified as a key indicator of various diseases. A simple and sensitive strategy for TRY detection by using an environment-friendly biosafe probe is significant. Herein, we introduced negatively charged fluorescent polydopamine nanoparticles (PDNPs) with 4.8 nm diameter obtained through a controllable method as an effective probe for TRY. PDNPs exhibited excellent fluorescence property but integrated with protamine (Pro) to form an aggregation-caused quenching system via a static quenching mechanism. The quenching mechanism of Pro to PDNPs revealed the significant effect of the surface charge, functional groups, and appropriate size of PDNPs on quenching process. Given the specific hydrolysis of Pro by TRY, PDNPs were released from the quenching integration of PDNPs and Pro (PDNPs-Pro) and recovered their fluorescence. Thus, a fluorescence sensor for TRY with a linear range of 0.01 and 0.1 μg/mL and a detection limit of 6.7 ng/mL was developed without the disturbing from other proteases. Compared with other TRY assays, the biosensor based on PDNPs-Pro has the advantages of simple operation, environmental friendliness, and high sensitivity. This specific controlled-synthesis PDNPs would open up a new window for the extended application of fluorescent nanomaterials in biomedicine based on fluorescence changes induced by biological interaction.

Stimuli-responsive polydopamine-based smart materials

This review provides in-depth insight into the structural engineering of PDA-based materials to enhance their responsive feature and the use of them in construction of PDA-based stimuli-responsive smart materials.

Ultrasensitive, rapid and selective sensing of hazardous fluoride ion in aqueous solution using a zirconium porphyrinic luminescent metal-organic framework

The development of a rapid and sensitive method for the detection of fluoride ion (F^-) in aqueous systems is of great significance for human health and environmental monitoring. In this study, a zirconium porphyrinic luminescent metal-organic framework (LMOF), PCN-222, was employed as a novel fluorescent probe for the ultrasensitive, rapid and selective detection of F^- in water. The PCN-222 probe was prepared by a facile solvothermal method. It exhibited good fluorescence stability and was highly stable in water. The fluorescence emission of PCN-222 could be effectively and selectively quenched by F^- due to the strong coordination affinity of F^- to the zirconium clusters in PCN-222. The proposed fluorescence method for F^- detection based on PCN-222 probe afforded a linear response range of 1-20 μmol/L and a very low detection limit (0.048-0.065 μmol/L) in reference to many reported F^- fluorescent probes. Moreover, a rapid response time (<10 s) was obtained due to the open and uniform pore structure of PCN-222 that allowed the fast diffusion of F^- to interact with the zirconium recognition sites. Finally, the PCN-222 probe was successfully applied for the fluorescence detection of F^- in real water samples. These results highlight the great application potential of LMOF in the sensing fields.

High Carbonization Temperature to Trigger Enzyme Mimicking Activities of Silk-Derived Nanosheets

Herein, it is demonstrated that N-rich carbonized silk fibroin materials (CSFs) can serve as efficient peroxidase, and oxidase mimics. Their enzyme-like activities are highly dependent on carbonization conditions. CSFs obtained at low temperatures do not exhibit significant catalytic reactivity, while their enzyme-like catalysis performance is greatly activated after high-temperature treatment. Such a phenomenon is mainly ascribed to the increase of graphitization degree and graphitic nitrogen and the emergence of disordered graphitic structures during the formation of turbostratic carbon. In addition, inspired by the excellent photothermal conversion efficiency, and temperature-dependent catalytic behavior of CSFs, near-infrared light can be used to remotely control their enzyme-like activities. More importantly, as-prepared robust silk-derived nanosheets can be applied to photothermal-catalytic cancer therapy and sensing. It is believed that such a smart artificial enzyme system will throw up exciting new opportunities for the chemical industry and biotechnology.

Synthesis of cost-effective magnetic nano-biocomposites mimicking peroxidase activity for remediation of dyes

The present study describes preparation of cellulose incorporated magnetic nano-biocomposites (CNPs) by using cellulose as base material. The prepared CNPs were characterised by SEM, EDAX, TEM, XRD, and FT-IR and found to exhibit an intrinsic peroxidase-like activity with a K_m and V_max of 550 μM and 3.8 μM/ml/min, respectively. The CNPs exhibited higher pH and thermal stability compared to commercial peroxidase. These nanocomposites were able to completely remove (i) a persistent azo dye, methyl orange at a concentration of 50 ppm, within 60 min under acidic conditions (pH 3.0) and also (ii) decolourize commercial textile dye mixture under acidic conditions within 30 min. CNP-mediated degradation of dyes into simple products was further confirmed by UV-Vis and AT-IR spectroscopy The added advantage of CNPs separation after decolourization by simple magnet due to their magnetic properties and consequent reusability makes them fairy attractive system for dye remediation from environmental samples or textile industries effluents.

Copper-tripeptides (cuzymes) with peroxidase-mimetic activity

Peroxidases are enzymes that use hydrogen peroxide to oxidize substrates such as 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ATBS). In this study, we showed that copper-tripeptide complexes ("cuzymes") also exhibited peroxidase-like activities. Different cuzymes could be formed by using various tripeptide ligands, such as GGG, GGH or HGG. However, the peroxidase-like activity of cuzymes depends on the sequence of the tripeptide (Cu-GGG > Cu-HGG > Cu-GGH). When ABTS was used as the substrate, the activity of Cu-GGG was 326 ± 1.5 U mg^-1 which was 2.5 times higher than that of horseradish peroxidase (HRP). Copper-tripeptide complexes were also used to degrade trypan blue dye. By using 0.2 mM Cu-GGG and 0.2% H₂O₂, 200 μM trypan blue could be degraded in 15 min at 50 °C. The degradation reaction followed second-order kinetics; the reaction rate was proportional to both H₂O₂ concentration and the copper-tripeptide concentration, but it was independent of the trypan blue concentration. Because copper-tripeptides catalyzed the oxidation reactions involving H₂O₂ effectively, they may have potential applications in biochemical assays and environmental remediation.

A convenient detection system consisting of efficient Au@PtRu nanozymes and alcohol oxidase for highly sensitive alcohol biosensing

Effective alcohol detection represents a substantial concern not only in the context of personal and automobile safety but also in clinical settings as alcohol is a contributing factor in a wide range of health complications including various types of liver cirrhoses, strokes, and cardiovascular diseases. Recently, many kinds of nanomaterials with enzyme-like properties have been widely used as biosensors. Herein, we have developed a convenient detection method that combines Au@PtRu nanozymes and alcohol oxidase (AOx). We found that the Au@PtRu nanorods exhibited peroxidase-like catalytic activity that was much higher than the catalytic activities of the Au and Au@Pt nanorods. The Au@PtRu nanorod-catalyzed generation of hydroxyl radicals in the presence of H₂O₂ was used to develop an alcohol sensor by monitoring the H₂O₂ formed by the oxidation of alcohol to acetaldehyde in the presence of AOx. When coupled with AOx, alcohol was detected down to 23.8 μM in a buffer solution for biological assays. Notably, alcohol was successfully detected in mouse blood samples with results comparable to that from commercial alcohol meters. These results highlight the potential of the Au@PtRu nanorods with peroxidase-like activity for alcohol detection, which opens up a new avenue for nanozyme development for biomedical applications.

Test-System for Bacteria Sensing Based on Peroxidase-Like Activity of Inkjet-Printed Magnetite Nanoparticles

Rapid detection of bacterial contamination is an essential task in numerous medical and technical processes and one of the most rapidly developing areas of nano-based analytics. Here, we present a simple-to-use and special-equipment-free test-system for bacteria detection based on magnetite nanoparticle arrays. The system is based on peroxide oxidation of chromogenic substrate catalyzed by magnetite nanoparticles, and the process undergoes computer-aided visual analysis. The nanoparticles used had a pristine surface free of adsorbed molecules and demonstrated high catalytic activities up to 6585 U/mg. The catalytic process showed the Michaelis–Menten kinetic with K_m valued 1.22 mmol/L and V_max of 4.39 µmol/s. The nanoparticles synthesized were used for the creation of inkjet printing inks and the design of sensor arrays by soft lithography. The printed sensors require no special equipment for data reading and showed a linear response for the detection of model bacteria in the range of 10⁴–10⁸ colony-forming units (CFU) per milliliter with the detection limit of 3.2 × 10³ CFU/mL.

Silica-polydopamine hybrids as light-induced oxidase mimics for colorimetric detection of pyrophosphate

In this study, silica-polydopamine hybrids (SPDA) were fabricated by a facile and one-step heating method using dopamine and (3-aminopropyl)triethoxysilane (APTES) as the reaction reagents. It was firstly found that light illuminated-SPDA could oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) to produce blue ox-TMB. The coloration process was quenched very efficiently via the addition of Cu2+. The presence of pyrophosphate ion (PPi) in the solution of light-illuminated SPDA-Cu2+-TMB induced the recovery of the coloration process. The recovery occurred because PPi coordinated with Cu2+, effectively sequestering the ion from SPDA. A calibration curve was developed that is related to the extent of absorption recovery to [PPi], making the SPDA-Cu2+-TMB system a sensitive and selective turn-on sensor for PPi detection. The limit-of-detection (LOD) for PPi was 0.06 μmol L-1 (S/N = 3) with a linear dynamic range of 0.1-30 μmol L-1 and the calibration curve of linear equation is given as: y = 0.00146x + 0.05096 (r = 0.9974). The proposed method has been successfully applied to the detection of PPi in human serum with satisfactory recovery. The simplicity, low cost, high sensitivity, good reproducibility and excellent selectivity of the PPi detection platform based on the light-induced oxidase mimicking property of SPDA makes it promising for further applications of SPDA in chemo/biosensing.

Nanozymes for medical biotechnology and its potential applications in biosensing and nanotherapeutics

Recent past years have witnessed the development of several artificial enzymes, using different materials to mimic natural enzymes with respect to their structure and functions. The nanozymes are nanomaterials possessing similar characteristics to the natural enzymes and have emerged recently as an innovative class of artificial enzymes. The nanozymes have got remarkable attention from the researchers and notable developments have been achieved owing to their unique properties compared with natural enzymes and classic artificial enzymes. In this regard, several nanomaterials have been scrutinized so far to mimic different natural enzymes for wider applications ranging from imaging, sensing, water treatment, pollutant removal, and therapeutics. The applications of nanozymes in biomedicine research are fast-growing and various nanozymes have been implicated in diagnostic medicine, targeted cancer therapy. Such abilities make them an appropriate alternative for the development of affordable, sustainable and safe diagnostic as well as therapeutic agents.

Electrochemically assisted synthesis of poly(3,4-dihydroxyphenylalanine) fluorescent organic nanoparticles for sensing applications

Facile preparation of polyDOPA-FONs is achieved via the electrochemical oxidation method.

Cobalt-based metal organic frameworks: a highly active oxidase-mimicking nanozyme for fluorescence “turn-on” assays of biothiol

With Co-MOFs as an oxidase-mimicking nanozyme, the AR oxidized product, non-fluorescent resazurin could be reduced to fluorescent resorufin by l-cysteine, which is specifically applied for fluorescence “turn-on” detection of l-cysteine.

Oxidase-Like Catalytic Performance of Nano-MnO2 and Its Potential Application for Metal Ions Detection in Water

Certain nano-scale metal oxides exhibiting the intrinsic enzyme-like reactivity had been used for environment monitoring. Herein, we evaluated the oxidase-mimicking activity of environmentally relevant nano-MnO₂ and its sensitivity to the presence of metal ions, and particularly, the use of MnO₂ nanozyme to potentially detect Cu²⁺, Zn²⁺, Mn²⁺, and Fe²⁺ in water. The results indicated the oxidase-like activity of nano-MnO₂ at acidic pH-driven oxidation of 2,6-dimethoxyphenol (2,6-DMP) via a single-electron transfer process, leading to the formation of a yellow product. Notably, the presence of Cu²⁺ and Mn²⁺ heightened the oxidase-mimicking activity of nano-MnO₂ at 25°C and pH 3.8, showing that Cu²⁺ and Mn²⁺ could modify the reactive sites of nano-MnO₂ surface to ameliorate its catalytic activity, while the activity of MnO₂ nanozyme in systems with Zn²⁺ and Fe²⁺ was impeded probably because of the strong affinity of Zn²⁺ and Fe²⁺ toward nano-MnO₂ surface. Based on these effects, we designed a procedure to use MnO₂ nanozyme to, respectively, detect Cu²⁺, Zn²⁺, Mn²⁺, and Fe²⁺ in the real water samples. MnO₂ nanozyme-based detecting systems achieved high accuracy (relative errors: 2.2-26.1%) and recovery (93.0-124.0%) for detection of the four metal ions, respectively. Such cost-effective detecting systems may provide a potential application for quantitative determination of metal ions in real water environmental samples.

Fluoride-capped nanoceria as a highly efficient oxidase-mimicking nanozyme: inhibiting product adsorption and increasing oxygen vacancies

Nanozymes aim to mimic enzyme activities using nanomaterials. Nanoceria (CeO2 nanoparticles) is an important model nanozyme for its rich redox chemistry. In particular, its oxidase-like activity allows oxidation reactions without the need of unstable and toxic H2O2. Fluoride can significantly improve its oxidase-like activity, and this work aims to understand the mechanism of fluoride-promoted catalysis. First, fluoride can adsorb on CeO2 tighter than other halides, but not as strong as phosphate as characterized by isothermal titration calorimetry (ITC). FT-IR spectroscopy indicates adsorption of fluoride likely via exchange with surface hydroxide groups. Fluoride capping inverses the surface charge of CeO2, facilitating desorption of the ABTS oxidation product, significantly increasing the turnover number. The Raman, EPR and XPS spectroscopy results demonstrate that the concentration of Ce3+ and the accompanying oxygen vacancy significantly increased upon adding F-, which can explain the enhanced catalytic activity. Finally, the electron transfer properties of fluoride-capped CeO2 were more efficient than that of the bare CeO2 as determined by a direct electrochemical measurement on a glass carbon electrode. This study has provided new insight into nanoceria, and can also further confirm the role of nanoceria as a model for engineering the surface of nanozymes.

Facile colorimetric detection of alkaline phosphatase activity based on the target-induced valence state regulation of oxidase-mimicking Ce-based nanorods

Alkaline phosphatase (ALP) is widely recognized as a significant biomarker for lots of diseases. For this reason, developing effective and simple methods to monitor ALP activity is strongly necessary. Herein, we propose a novel strategy based on the target-induced valence state regulation of oxidase-mimicking Ce-based nanorods for ALP activity sensing. The mixed-valent Ce-based material (MVCM) with a relatively high Ce(iv)/Ce(iii) ratio can exhibit good oxidase-like activity to trigger the catalytic oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue TMBox in the presence of O₂, resulting in a notable chromogenic reaction. When ALP hydrolyzes ascorbic acid phosphate into ascorbic acid (AA), the formed AA induces the partial reduction of the MVCM to one with a low Ce(iv)/Ce(iii) ratio, which shows much less activity to trigger the chromogenic reaction. According to the above principle, a facile colorimetric assay was developed for ALP activity detection, providing a linear range of 0.5-25 U L^-1 and a limit of detection of 0.1 U L^-1. Besides, the proposed strategy could offer favorable selectivity for ALP activity determination. Accurate sensing of the target in serum was demonstrated by our assay as well, revealing its promise as a reliable tool for clinical diagnosis.

Metal and metal-oxide nanozymes: bioenzymatic characteristics, catalytic mechanism, and eco-environmental applications

Phenolic contaminants (R-OH) are a category of highly toxic organic compounds that are widespread in aquatic ecosystems and can induce carcinogenic risk to wildlife and humans; natural enzymes as green catalysts are capable of step-polymerizing these compounds to produce diverse macromolecular self-coupling products via radical-mediated C-C and C-O-C bonding at either the ortho- or para-carbon position, thereby evading the bioavailability and ecotoxicity of these compounds. Intriguingly, certain artificial metal and metal-oxide nanomaterials are known as nanozymes. They not only possess the unique properties of nanomaterials but also display intrinsic enzyme-mimicking activities. These artificial nanozymes are expected to surmount the shortcomings, such as low stability, easy inactivation, difficult recycling, and high cost, of natural enzymes, thus contributing to eco-environmental restoration. This review highlights the available studies on the enzymatic characteristics and catalytic mechanisms of natural enzymes and artificial metal and metal-oxide nanozymes in the removal and transformation of R-OH. These advances will provide key research directions beneficial to the multifunctional applications of artificial nanozymes in aquatic ecosystems.

Plasma Treatment Conversion of Phenolic Compounds into Fluorescent Organic Nanoparticles for Cell Imaging

Fluorescent organic nanoparticles (FONs) are promising alternatives for biological imaging applications owing to the increasing concerns over the potential toxicity and poor degradability of inorganic particles-based probes. However, synthesis of stable, small-sized FONs in aqueous media remains challenging. Inspired by the self-polymerization chemistry of phenolic compounds, we demonstrate ultrafast synthesis of FONs (phenolic compound-derived FONs, PhFONs) from a variety of molecular building blocks including dopamine, norepinephrine, pyrogallol, and gallic acid, simply by nontherml plasma treatment at the aqueous interface. Specifically, using dopamine as the precursor, poly(dopamine) (PD)-FONs featuring a small size of 3 nm are obtained within 1 min. Compositional and structural characterizations confirm the polymeric architectures in PD-FONs. The PhFONs, with multicolor emissions, excellent biocompatibility, high stability, and size-dependent access into cell nucleus, are suitable for live cell imaging and developing nucleus-targeting imaging platforms.

Three-Dimensional Branched Crystal Carbon Nitride with Enhanced Intrinsic Peroxidase-Like Activity: A Hypersensitive Platform for Colorimetric Detection

Graphitic carbon nitride (g-C₃N₄) as a metal-free nanozyme has attracted huge attention for catalytic applications. However, the catalytic activity of pure g-C₃N₄ causes very moderate H₂O₂ activation. Herein, a novel three-dimensional (3D) branched carbon nitride nanoneedle (3DBC-C₃N₄) nanozyme has been proposed to overcome such shortcoming. This unique 3D branched structure of 3DBC-C₃N₄ facilitated effective mass transfer during catalytic reaction and induced a lightning rodlike effect to accelerate electron collection at the tip area for H₂O₂ activation. With improved H₂O₂ activation for hydroxyl radical (^•OH) generation, 3DBC-C₃N₄ showed excellent peroxidase-like activity toward 3,3',5,5'-tetramethylbenzidine oxidation in the presence of H₂O₂. As for H₂O₂, the V_max value of 3DBC-C₃N₄ was found to be 20 times higher than that of natural horseradish peroxidase. Moreover, the 3D branched structure of 3DBC-C₃N₄ offered large interface for the reversible conjugation of single-stranded DNA, which enhanced the colorimetric sensitivity. Moreover, 3DBC-C₃N₄ exhibited high sensitivity toward oxytetracycline detection, with the detection limit and quantitative limit of 1 and 50 μg/L, respectively.

Biomineralization Synthesis of the Cobalt Nanozyme in SP94-Ferritin Nanocages for Prognostic Diagnosis of Hepatocellular Carcinoma

Nanomaterials with intrinsic enzyme-like activities (nanozymes) have emerged as promising agents for cancer theranostics strategies. However, size-controllable synthesis of nanozymes and their targeting modifications are still challenging. Here, we report a monodispersed ferritin-based cobalt nanozyme (HccFn(Co₃O₄)) that specifically targets and visualizes clinical hepatocellular carcinoma (HCC) tissues. The cobalt nanozyme is biomimetically synthesized within the protein shell of the HCC-targeted ferritin (HccFn) nanocage, which is enabled by the display of HCC cell-specific peptide SP94 on the surface of ferritin through a genetic engineering approach. The intrinsic peroxidase-like activity of HccFn(Co₃O₄) nanozymes catalyzes the substrates to make color reaction to visualize HCC tumor tissues. We examined 424 clinical HCC specimens and verified that HccFn(Co₃O₄) nanozymes distinguish HCC tissues from normal liver tissues with a sensitivity of 63.5% and specificity of 79.1%, which is comparable with that of the clinically used HCC-specific marker α fetoprotein. Moreover, the pathological analysis indicates that the HccFn(Co₃O₄) nanozyme staining result is a potential predictor of prognosis in HCC patients. Staining intensity is positively correlated to tumor differentiation degree ( P = 0.0246) and tumor invasion ( P < 0.0001) and negatively correlated with overall survival ( P = 0.0084) of HCC patients. Together, our study demonstrates that ferritin is an excellent platform for size-controllable synthesis and targeting modifications of nanozymes, and the HccFn(Co₃O₄) nanozyme is a promising reagent for prognostic diagnosis of HCC.

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

An updated comprehensive review to help researchers understand nanozymes better and in turn to advance the field.

DNA-Functionalized Nanoceria for Probing Oxidation of Phosphorus Compounds

Chemical reactions without an obvious optical signal change, such as fluorescence or color, are difficult to monitor. Often, more advanced analytical techniques such as high-performance liquid chromatography and mass spectroscopy are needed. It would be useful to convert such reactions to those with changes in optical signals. In this work, we demonstrate that fluorescently labeled DNA oligonucleotides adsorbed on nanomaterials can probe such reactions, and oxidation of phosphorus-containing species was used as an example. Various metal oxides were tested, and CeO2 nanoparticles were found to be the most efficient for this purpose. Among phosphate, phosphite, and hypophosphite, only phosphate produced a large signal, indicating its strongest adsorption on CeO2 to displace the DNA. This was further used to screen oxidation agents to convert lower oxidation-state compounds to phosphate, and bleach was found to be able to oxidize phosphite. Canonical discriminant analysis was performed to discriminate various phosphorus species using a sensor array containing different metal oxides. On the basis of this, glyphosate was studied for its adsorption and oxidation. Although this method is not specific enough for selective biosensors, it is useful as a tool to produce sensitive optical signals to follow important chemical transformations.