Disclosing the Origin of Transition Metal Oxides as Peroxidase (and Catalase) Mimetics

Peroxidase mimic feroxyhyte (FeOOH) nanoparticles enabled highly specific colorimetric detection of arsenate in water and fish

Arsenic poses a serious health risk to humans. Hence, a simple and robust analytical approach for monitoring arsenic levels in water and food matrixes is required. We present a simple and rapid approach for quantifying arsenate in water and fish using feroxyhyte (FeOOH) nanoparticles as a sensor probe. The FeOOH nanoparticles showed peroxidase mimetic activity oxidizing 3,5,3'5'-tetramethylbenzidine (TMB) to a blue product (oxTMB, λmax 650 nm) in the presence of H2O2. However, arsenate's presence inhibits the peroxidase activity of FeOOH nanoparticles through binding on the catalytic active sites. Based on this principle, the presently developed method obtained a good linear response (R2, 0.99) over the range of 0.005 to 5.000 mg L-1 arsenate with 0.006 mg L-1 as the detection limit which is less than the prescribed limit (0.010 mg L-1) by WHO for drinking water. The average recoveries at different fortification levels ranged from 89.51 to 115.61 % in water and 101.11 to 106.99 % in fish muscle. The present analytical technique showed good selectivity due to pronounced peroxidase inhibition alibility (60.53-103.78 %) by arsenate than other non-target ions like PO43-, NO3-, Cr2O72-, etc. The FeOOH nanoparticles showed a promising application prospect for colorimetric detection of arsenate with a wide detection range in water and fish samples.

Ultra-small gold nanoparticle-coupled MOF-808 enabled sensitive detection of bacteria at neutral pH

Metal-organic framework (MOF)-based mimics are considered star materials to replace natural enzymes. However, their activity is generally limited to acidic conditions, which severely restricts their applications in biological systems where neutral pH is commonly required. Herein, a Zr(IV)-based MOF (MOF-808)/gold nanoparticle (AuNP) hybrid (called Hybrid-60) that shows superior peroxidase-like (POD-like) activity in both acidic and neutral media was prepared by in-situ growth of ultra-small AuNPs (UsAuNPs, ∼3.5 nm) on MOF-808. In comparison with the conventional AuNPs and MOF-808 nanozymes, Hybrid-60 demonstrated ∼8.04- and ∼6.74-time enhanced POD-like activities and superior high activity under neutral conditions, which broke the pH limitation. Furthermore, Hybrid-60 exhibited good tolerance to extreme pH value, concentrated salinity, and high-temperature environments. Taking Staphylococcus aureus (S. aureus) as a model analyte, we developed a simple immune sandwich assay using Hybrid-60 as colorimetric nanotags (ISAHC). A dual recognition strategy using anti-S. aureus antibody and concanavalin A-labeled Hybrid-60 was proposed to specifically capture and high-affinity label the target S. aureus. Then, leveraging the high POD-like activity of Hybrid-60, a simple and specific detection of S. aureus at nearly neutral pH was realized with a wide linear range (1 × 102-1 × 105 CFU/mL) and a low detection limit (32 CFU/mL). Moreover, the ISAHC method enabled one to detect the target S. aureus in human urine and serum with satisfactory recoveries from 93.8 % to 111.0 %, which indicates its clinical applicability. This study provides a new approach to develop neutral nanozymes and facilitate the point-of-care detection of bacteria.

Dual model biosensor integrated with peroxidase-like activity and self-assembly for uric acid detection

Uric acid (UA), the final product of purine metabolism, is a crucial biomarker for gout diagnostics and highly related to various metabolic diseases. Precise detection of UA levels in serum and urine enables disease diagnosis and guides treatment. Combining the advantages of colorimetry and laser desorption/ionization mass spectrometry (LDI-MS), we developed a dual-model biosensor based on hollow Cu2O@Au nanocubes (h-Cu2O@Au NCs) for UA detection. The h-Cu2O@Au NCs demonstrated excellent peroxidase (POD)-like activity and were used to rapidly detect UA by colorimetric assay, with a linear range of 0.05-2 mM and limit of detection (LOD) of 35.71 μM. Moreover, the h-Cu2O@Au NCs achieved enrichment and detection of UA via the liquid-liquid interface self-assembly-assisted LDI-MS, with a linear range of 0.01-0.5 mM, LOD of 15.6 μM, and reproducibility of <5%. In view of its advantages, the dual-model nanoplatform based on h-Cu2O@Au NCs achieved UA detection in serum samples by colorimetry assay and in urine samples by LDI-MS, obtaining results consistent with the commercial UA assay kit (72-511 μM for serum, R2 = 0.956 and 2-9 mM for urine, R2 = 0.876), presenting potential in the rapid and sensitive detection of UA in clinic.

Interactions of metal-based nanozymes with aptamers, from the design of nanozyme to its application in aptasensor: Advances and perspectives

Nanozymes, characterized by enzyme-like activity, have been extensively used in quantitative analysis and rapid detection due to their small size, batch fabrication, and ease of modification. Researchers have combined aptamers, an emerging molecular probe, with nanozymes for biosensing to address the limited reaction specificity of nanozymes. Nanozyme aptasensors are currently experiencing significant growth, offering a promising solution to the lack of rapid detection methods across various fields. Unlike traditional nanozyme research, the development of nanozyme aptasensors is challenging as it requires the design of highly active nanozymes as well as the establishment of efficient and agile interactions between aptamers and nanozymes. Therefore, this review summarizes the active species and catalytic mechanisms of various nanozymes along with classical design options, discussing the future development of nanozyme aptasensors. It is anticipated that this review will inspire researchers in this domain, leading to the design of more enzymatically active nanozymes and advanced nanozyme aptasensors.

MOF-engineered Cu2O nanozymes with boosted peroxidase-like activity for colorimetric-fluorescent dual-mode detection of deoxynivalenol

The development of a high sensitivity biosensor for the detection of highly toxic deoxynivalenol (DON) is vital for human health and food security. In this work, by integrating metal-organic frameworks (MOF) with cubic Cu2O nanoparticles (Cu2O@MOF), the nanocomposite achieved a 4.8-fold increase in specific surface area compared to pristine Cu2O, which synergistically enhanced its peroxidase-like (POD) activity through optimized substrate affinity and accelerated charge transfer. Consequently, based on the marriage properties of POD activity and fluorescence signal from Cu2O@MOF nanoparticles and carbon dots (CDs), a colorimentric-fluorescent dual-mode biosensor was constructed for DON detection. Concurrently, the competitive binding of DON with immobilized antigens on Cu2O@MOF-CDs results in antibody displacement, leading to progressive reduction of captured probes with increasing DON concentrations, thereby inducing proportional attenuation in both colorimetric and fluorescence signal intensities. Under the optimum conditions, the established biosensor achieved a detection limit of 0.0018 ng/mL for DON. Furthermore, the prepared dual-mode biosensor was successfully applied to detect DON in tap water, wheat and corn, demonstrating its practical utility for real-world applications. Overall, this work not only advances nanozyme design through MOF-mediated interface engineering but also provides a rapid, accurate, and field-deployable strategy for monitoring mycotoxins in complex matrices.

A Mo-doped carbon dot nanozyme for enhanced phototherapy in vitro

Cancer is a leading cause of death globally, and traditional treatment methods often come with non-negligible toxic side effects in its treatment, threatening patients' quality of life. Thus, developing novel, efficient, low-toxicity cancer treatment strategies is crucial. Nanozymes, as a class of powerful nanomaterials, can subtly mimic the catalytic activity of natural enzymes, making them a formidable alternative. Hypoxic molybdenum oxide (MoO3-x ), as a typical nanozyme material, possesses unique physical and chemical properties, showing great potential in fields such as cancer treatment. In this study, a simple and rapid one-pot hydrothermal synthesis method was ingeniously employed, innovatively combining molybdenum, which has high biosafety, with safflower, which exhibits anticancer pharmacological activity, to successfully prepare hypoxic molybdenum oxide (MoO3-x )-doped safflower carbon dots (H-Mo-CDs). H-Mo-CDs exhibit exceptional catalase (CAT)-like, peroxidase (POD)-like, and superoxide dismutase (SOD)-like catalytic activities and superior photothermal conversion efficiency and photostability. In vitro cellular experiments have verified their multiple therapeutic potentials in photothermal therapy (PTT), chemodynamic therapy (CDT), and photodynamic therapy (PDT), providing novel ideas and means for precise cancer treatment. This study not only paves an efficient and feasible path for the development of Mo-based nanomaterials as "smart" nanozymes but also injects new vitality and possibilities into the types and applications of nanozymes in cancer treatment.

Research progress and applications of iron-based nanozymes in colorimetric sensing of agricultural pollutants

Natural enzymes are highly valued for their efficient specificity and catalytic activity. However, their poor stability, environmental sensitivity, and costly preparation restrict their practical applications. Nanozymes are nanomaterials with superior catalytic properties that compensate for natural enzyme deficiencies. As one of the earliest developed nanozymes, iron-based nanozymes have diverse morphological structures and different simulated catalytic properties, showing promising potential for agricultural pollutant sensing. Compared with traditional detection methods, the colorimetric method based on nanozymes has the characteristics of simplicity, rapidity, and visualization, which can be used for immediate and rapid on-site detection. In this review, the catalytic types of iron-based nanozymes, such as peroxidase-like, oxidase-like, catalase-like, and superoxide dismutase-like activities, and the corresponding catalytic mechanisms are presented. The classification of iron-based nanozymes based on various structures is then discussed. Furthermore, this review focuses on the current status of iron-based nanozymes for the colorimetric detection of common agricultural pollutants, including heavy metal ions, nonmetal ions, pesticides, and pharmaceutical and personal care products. Finally, the current research status and development direction of iron-based nanozymes in sensing applications are summarized and prospected.

One-Pot and Gram-Scale Synthesis of Fe-Based Nanozymes with Tunable O2 Activation Pathway and Specificity Between Associated Enzymatic Reactions

Nanozymes have recently gained attention for their low cost and high stability. However, unlike natural enzymes, they often exhibit multiple enzyme-like activities, complicating their use in selective bioassays. Since H2O2 and O2 are common substrates in these reactions, controlling their activation-and thus reaction specificity-is crucial. Recent advances in tuning the chemical state of cerium have enabled control over H2O2 activation pathways for tunable peroxidase/haloperoxidase-like activities. In contrast, the control of O2 activation on an element in oxidase/laccase nanozymes and the impact of its chemical state on these activities remains unexplored. Herein, a facile one-pot method is presented for the gram-scale synthesis of Fe-based nanozymes with tunable compositions of Fe3O4 and Fe3C by adjusting preparation temperatures. The Fe3O4-containing samples exhibit superior laccase-like activity, while the Fe3C-containing counterparts demonstrate better oxidase-like activity. This divergent O2 activation behavior is linked to their surface Fe species: the abundant reactive Fe2+ in Fe3O4 promotes laccase-like activity via Fe3+-superoxo formation, whereas metallic Fe in Fe3C facilitates OH radical generation for oxidase-like activity. Controlled O2 activation pathways in these Fe-based nanozymes demonstrate improved sensitivity in the corresponding biomolecule detection, which should inform the design of nanozymes with enhanced activity and specificity.

Cerium oxide mimetic enzyme based colorimetric detection of potential oesophageal cancer biomarkers

Oesophageal cancer (OC) is a prevalent malignant tumor that poses a significant threat to individuals. Current mainstream detection method is endoscopy, which requires professional operators and expensive instruments. Therefore, it is crucial to develop a rapid, easy-to-operate, and low-cost detection method. In this study, an RNA colorimetric biosensor was successfully constructed using cerium oxide mimetic enzyme. The sensor is constructed on 96-well plates, which are immobilized with DNA-RNA-DNA complexes in microtiter wells when target RNA is present. This immobilization is based on the principle of base complementary pairing. The CeO2 immobilized has the unique advantage of catalyzing the bluing of 3,3',5,5'-tetramethylbenzidine (TMB) directly without the need any additional oxidant in microtiter wells. This property allows for the detection of RNA and enables the visualization of multiple sample assays. Furthermore, the RNA colorimetric sensor demonstrates good selectivity, immunity to interference, and high stability. Under optimal conditions, the sensor exhibited linearity in the range of 10-13 to 10-9 M with a detection limit of 33.26 fM. Therefore, this study presents a new detection method for oesophageal cancer screening.

Unveiling Generally-ignored Co-substrate Effect of Catalase-inherent Peroxidase Mimic for Self-verifiable Detection of High-concentration Hydrogen Peroxide

One nanoparticle possessing both peroxidase (POD) and catalase (CAT) activities is a prevalent co-substrate nanozyme system, distinct from the extensively researched cascade nanozyme system. During the sensing of hydrogen peroxide by POD, the impact of CAT is actually ignored in most studies. In this study, the CAT effect on hydrogen peroxide detection is thoroughly investigated based on POD catalysis by finely tuning the relative activity of POD and CAT. It is discovered that the CAT effect can be changed by delaying the injection of chromogenic substrate after adding hydrogen peroxide and that the linear range grows with the delayed time. Then, a theoretical mechanism showed that the time-delay mediated CAT effect magnification does not change the Vmax, but it causes Km to linearly increase with delayed time, consistent with the experiment results. Furthermore, the detection of high concentrations of hydrogen peroxide is successfully realized in contact lens care solutions by utilizing time-delay-mediated POD/CAT nanozyme. On the other hand, its linear range-tunable characteristic is used to produce multiple standard curves, then enabled self-verifying hydrogen peroxide detection. Overall, this work investigates the role of CAT in CAT-inherent POD nanozymes both theoretically and experimentally, and confirms POD/CAT nanozyme's priority in developing high-performance sensors.

Optimizing the standardized assays for determining the catalytic activity and kinetics of peroxidase-like nanozymes

Nanozymes are nanomaterials with enzyme-like catalytic properties. They are attractive reagents because they do not have the same limitations of natural enzymes (e.g., high cost, low stability and difficult storage). To test, optimize and compare nanozymes, it is important to establish fundamental principles and systematic standards to fully characterize their catalytic performance. Our 2018 protocol describes how to characterize the catalytic activity and kinetics of peroxidase nanozymes, the most widely used type of nanozyme. This approach was based on Michaelis-Menten enzyme kinetics and is now updated to take into account the unique physicochemical properties of nanomaterials that determine the catalytic kinetics of nanozymes. The updated procedure describes how to determine the number of active sites as well as other physicochemical properties such as surface area, shape and size. It also outlines how to calculate the hydroxyl adsorption energy from the crystal structure using the density functional theory method. The calculations now incorporate these measurements and computations to better characterize the catalytic kinetics of peroxidase nanozymes that have different shapes, sizes and compositions. This updated protocol better describes the catalytic performance of nanozymes and benefits the development of nanozyme research since further nanozyme development requires precise control of activity by engineering the electronic, geometric structure and atomic configuration of the catalytic sites of nanozymes. The characterization of the catalytic activity of peroxidase nanozymes and the evaluation of their kinetics can be performed in 4 h. The procedure is suitable for users with expertise in nano- and materials technology.

Highly sensitive colorimetric and paper-based detection for sildenafil in functional food based on monodispersed spherical magnetic graphene composite nanozyme

BACKGROUND

Sildenafil (SIL) is regarded as an illegal adulterant in functional foods. Some functional foods doped with SIL have posed significant concern about their safety risks. However, the facile colorimetric detection of SIL is rarely investigated.

RESULTS

Herein, we prepared a monodispersed spherical composite nanozyme (Fe3O4-NH2/GONRs), possessing excellent peroxidase-like (POD-like) and catalase-like (CAT-like) activities and strong superparamagnetic property. The enzyme-like activities of Fe3O4-NH2/GONRs can be selectively inhibited by SIL due to the synergistic effect of hydrogen bonds and π-π stacking between Fe3O4-NH2/GONRs and SIL. Leveraging this mechanism, a highly sensitive and selective colorimetric detection for SIL with a detection limit (LOD) of 0.26 ng/mL was developed. In addition, we prepared a three-dimensional paper-based analytical device (3D-PAD) for SIL colorimetric detection with naked-eyes and the semi-quantitative analysis with a LOD of 88 ng/mL.

SIGNIFICANCE

The proposed colorimetric and PAD detections demonstrated the advantages of low-cost, highly sensitive and selective, thus have promise application potential in the rapid detection of adulterated functional foods.

Cu2O nanoparticles with morphology-dependent peroxidase mimic activity: a novel colorimetric biosensor for deoxynivalenol detection

Different morphological Cu2O nanoparticles including cube, truncated cube, and octahedron were successfully prepared by a selective surface stabilization strategy. The prepared cube Cu2O exhibited superior peroxidase-like activity over the other two morphological Cu2O nanoparticles, which can readily oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to form visually recognizable color signals. Consequently, a sensitive and simple colorimetric biosensor was proposed for deoxynivalenol (DON) detection. In this biosensor, the uniform cube Cu2O was employed as the vehicle to label the antibody for the recognition of immunoreaction. The sensing strategy showed a detection limit as low as 0.01 ng/mL, and a wide linear range from 2 to 100 ng/mL. Concurrently, the approximate DON concentration can be immediately and conveniently observed by the vivid color changes. Benefiting from the high sensitivity and selectivity of the designed biosensor, the detection of DON in wheat, corn, and tap water samples was achieved, suggesting the bright prospect of the biosensor for the convenient and intuitive detection of DON in actual samples.

Sensitive colorimetric and fluorescence dual-mode detection of thiophanate-methyl based on spherical Fe3O4/GONRs composite nanozyme

Pesticide detection based on nanozyme is largely limited in terms of the variety of pesticides. Herein, a spherical and well-dispersed Fe3O4/graphene oxide nanoribbons (Fe3O4/GONRs) composite nanozyme was applied to firstly develop an enzyme-free and sensitive colorimetric and fluorescence dual-mode detection of thiophanate-methyl (TM). The synthesized Fe3O4/GONRs possess excellent dual enzyme-like activities (peroxidase and catalase) and can catalyze H2O2 to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into oxidized TMB (oxTMB). We found that Fe3O4/GONRs can adsorb TM through the synergistic effect of multiple forces, thereby inhibiting the catalytic activities of nanozyme. This inhibition can modulate the transformation of TMB to oxTMB, producing dual responses of absorbance decrease (oxTMB) and fluorescence enhancement (TMB). The limits of detection (LODs) of TM were 28.1 ng/mL (colorimetric) and 8.81 ng/mL (fluorescence), respectively. Moreover, the developed method with the recoveries of 94.8-100.8% also exhibited a good potential application in the detection of pesticides residues in water and food samples.

Copper-doped cherry blossom carbon dots with peroxidase-like activity for antibacterial applications

Safety concerns arising from bacteria present a significant threat to human health, underscoring the pressing need for the exploration of novel antimicrobial materials. Nanozymes, as a new type of nanoscale material, have attracted widespread attention for antibacterial applications owing to their ability to mimic the catalytic activity of natural enzymes. In this work, we have constructed copper-doped cherry blossom carbon dots (Cu-CDs) with excellent peroxidase-like (POD) activity using a one-pot hydrothermal method. The utilization of cherry blossom as a natural material precursor significantly enhances its biocompatibility. Furthermore, the incorporation of copper ions initiates Fenton-like reaction-triggered POD-like catalytic activity, effectively eradicating bacteria by converting hydrogen peroxide (H2O2) into hydroxyl radicals (·OH). The antibacterial test results demonstrate that Cu-CDs exhibit a bactericidal efficacy of over 90% against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). This study presents a novel environmentally friendly nanozyme material derived from natural sources, exhibiting significant antimicrobial properties and offering innovative insights for the advancement of antimicrobial materials.

Bifunctional oxidase-peroxidase mimicking Fe-Ce MOF on paper-based analytical devices to intensify luminol chemiluminescence: Application for measuring different sugars with a smartphone readout

This study presents a potent paper-based analytical device (PAD) for quantifying various sugars using an innovative bi-nanozyme made from a 2-dimensional Fe/Ce metal-organic framework (FeCe-BTC). The MOF showed excellent bifunctional peroxidase-oxidase activities, efficiently catalyzing luminol's chemiluminescence (CL) reaction. As a peroxidase-like nanozyme, FeCe-BTC could facilitate the dissociation of hydrogen peroxide (H2O2) into hydroxyl radicals, which then oxidize luminol. Additionally, it was also discovered that when reacting with H2O2, the MOF turns into a mixed-valence MOF, and acts as an oxidase nanozyme. This activity is caused by the generated Ce4+ ions in the structure of MOF that can directly oxidize luminol. The MOF was directly synthesized on the PAD and cascaded with specific natural enzymes to establish simple, rapid, and selective CL sensors for the measurement of different sugars. A cell phone was also used to record light intensities, which were then correlated to the analyte concentration. The designed PAD showed a wide linear range of 0.1-10 mM for glucose, fructose, and sucrose, with detection limits of 0.03, 0.04, and 0.04 mM, respectively. It showed satisfactory results in food and biological samples with recovery values ranging from 95.8 to 102.4 %, which makes it a promising candidate for point-of-care (POC) testing for food control and medicinal purposes.

On-demand activatable peroxidase-mimicking enzymatic polymer nanocomposite films

Nanozymes continue to attract considerable attention to minimise the dependence on expensive enzymes in bioassays, particularly in medical diagnostics. While there has been considerable effort directed towards developing different nanozymes, there has been limited progress in fabricating composite materials based on such nanozymes. One of the biggest gaps in the field is the control, tuneability, and on-demand catalytic response. Herein, a nanocomposite nanozymatic film that enables precise tuning of catalytic activity through stretching is demonstrated. In a systematic study, we developed poly(styrene-stat-n-butyl acrylate)/iron oxide-embedded porous silica nanoparticle (FeSiNP) nanocomposite films with controlled, highly tuneable, and on-demand activatable peroxidase-like activity. The polymer/FeSiNP nanocomposite was designed to undergo film formation at ambient temperature yielding a highly flexible and stretchable film, responsible for enabling precise control over the peroxidase-like activity. The fabricated nanocomposite films exhibited a prolonged FeSiNP dose-dependent catalytic response. Interestingly, the optimised composite films with 10 wt% FeSiNP exhibited a drastic change in the enzymatic activity upon stretching, which provides the nanocomposite films with an on-demand performance activation characteristic. This is the first report showing control over the nanozyme activity using a nanocomposite film, which is expected to pave the way for further research in the field leading to the development of system-embedded activatable sensors for diagnostic, food spoilage, and environmental applications.

Thiazole Functionalization of Thiosemicarbazone for Cu(II) Complexation: Moving toward Highly Efficient Anticancer Drugs with Promising Oral Bioavailability

In this work, we report the synthesis of a new thiosemicarbazone-based drug of N'-(di(pyridin-2-yl)methylene)-4-(thiazol-2-yl)piperazine-1-carbothiohydrazide (HL) featuring a thiazole spectator for efficient coordination with Cu(II) to give [CuCl(L)]2 (1) and [Cu(NO3)(L)]2 (2). Both 1 and 2 exhibit dimeric structures ascribed to the presence of di-2-pyridylketone moieties that demonstrate dual functions of chelation and intermolecular bridging. HL, 1, and 2 are highly toxic against hepatocellular carcinoma cell lines Hep-G2, PLC/PRF/5, and HuH-7 with half maximal inhibitory concentration (IC50) values as low as 3.26 nmol/mL (HL), 2.18 nmol/mL (1), and 2.54 × 10-5 nmol/mL (2) for PLC/PRF/5. While the free ligand HL may elicit its anticancer effect via the sequestration of bio-relevant metal ions (i.e., Fe3+ and Cu2+), 1 and 2 are also capable of generating cytotoxic reactive oxygen species (ROS) to inhibit cancer cell proliferation. Our preliminary pharmacokinetic studies revealed that oral administration (per os, PO) of HL has a significantly longer half-life t1/2 of 21.61 ± 9.4 h, nearly doubled as compared with that of the intravenous (i.v.) administration of 11.88 ± 1.66 h, certifying HL as an effective chemotherapeutic drug via PO administration.

Spatially Decoupled H2O2 Activation Pathways and Multi-Enzyme Activities in Rod-Shaped CeO2 with Implications for Facet Distribution

CeO2, particularly in the shape of rod, has recently gained considerable attention for its ability to mimic peroxidase (POD) and haloperoxidase (HPO). However, this multi-enzyme activities unavoidably compete for H2O2 affecting its performance in relevant applications. The lack of consensus on facet distribution in rod-shaped CeO2 further complicates the establishment of structure-activity correlations, presenting challenges for progress in the field. In this study, the HPO-like activity of rod-shaped CeO2 is successfully enhanced while maintaining its POD-like activity through a facile post-calcination method. By studying the spatial distribution of these two activities and their exclusive H2O2 activation pathways on CeO2 surfaces, this study finds that the increased HPO-like activity originated from the newly exposed (111) surface at the tip of the shortened rods after calcination, while the unchanged POD-like activity is attributed to the retained (110) surface in their lateral area. These findings not only address facet distribution discrepancies commonly reported in the literature for rod-shaped CeO2 but also offer a simple approach to enhance its antibacterial performance. This work is expected to provide atomic insights into catalytic correlations and guide the design of nanozymes with improved activity and reaction specificity.

Recent advances in metal oxide nanozyme-based optical biosensors for food safety assays

Metal oxide nanozymes are emerging as promising materials for food safety detection, offering several advantages over natural enzymes, including superior stability, cost-effectiveness, large-scale production capability, customisable functionality, design options, and ease of modification. Optical biosensors based on metal oxide nanozymes have significantly accelerated the advancement of analytical research, facilitating the rapid, effortless, efficient, and precise detection and characterisation of contaminants in food. However, few reviews have focused on the application of optical biosensors based on metal oxide nanozymes for food safety detection. In this review, the catalytic mechanisms of the catalase, oxidase, peroxidase, and superoxide dismutase activities of metal oxide nanozymes are characterized. Research developments in optical biosensors based on metal oxide nanozymes, including colorimetric, fluorescent, chemiluminescent, and surface-enhanced Raman scattering biosensors, are comprehensively summarized. The application of metal oxide nanozyme-based biosensors for the detection of nitrites, sulphites, metal ions, pesticides, antibiotics, antioxidants, foodborne pathogens, toxins, and other food contaminants has been highlighted. Furthermore, the challenges and future development prospects of metal oxide nanozymes for sensing applications are discussed. This review offers insights and inspiration for further investigations on optical biosensors based on metal oxide nanozymes for food safety detection.

Elucidating the catalytic mechanism of Prussian blue nanozymes with self-increasing catalytic activity

Although Prussian blue nanozymes (PBNZ) are widely applied in various fields, their catalytic mechanisms remain elusive. Here, we investigate the long-term catalytic performance of PBNZ as peroxidase (POD) and catalase (CAT) mimetics to elucidate their lifespan and underlying mechanisms. Unlike our previously reported Fe3O4 nanozymes, which exhibit depletable POD-like activity, the POD and CAT-like activities of PBNZ not only persist but slightly enhance over prolonged catalysis. We demonstrate that the irreversible oxidation of PBNZ significantly promotes catalysis, leading to self-increasing catalytic activities. The catalytic process of the pre-oxidized PBNZ can be initiated through either the conduction band pathway or the valence band pathway. In summary, we reveal that PBNZ follows a dual-path electron transfer mechanism during the POD and CAT-like catalysis, offering the advantage of a long service life.

A robust single compartment peroxide fuel cell using mesoporous antimony doped tin oxide as the cathode material

To date, metal oxide catalysts have not been explored as cathode materials for robust and high-performance single-compartment H2O2 fuel cells due to significant non-electrochemical disproportionation losses of H2O2 on many metal oxide surfaces. Here, for the first time, we demonstrate an acidic peroxide fuel cell with antimony doped tin oxide as the cathode and widely used Ni foam as the anode material. Our constructed peroxide fuel cell records a superior open circuit potential of nearly 0.82 V and a maximum power density of 0.32 mW cm-2 with high operational stability. The fuel cell performance is further improved by increasing the ionic strength of the electrolyte with the addition of 1 M NaCl, resulting in an increased maximum power density value of 1.1 mW cm-2.

Enhanced peroxidase-like activity of Cu-Cu2O composite film through PtPd immobilization for colorimetric glucose detection

In this study, Cu-Cu2O/PtPd nanocomposites were synthesized and characterized for their peroxidase-like enzyme activity. X-ray diffraction and energy dispersive X-ray spectroscopy analyses confirmed the successful synthesis of the nanocomposites, which exhibited a flower-like morphology and a more uniform dispersion than Cu-Cu2O. The catalytic activity of Cu-Cu2O/PtPd was evaluated using the chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB), finding that Cu-Cu2O/PtPd outperformed Cu-Cu2O. The optimal temperature and pH for the catalytic activity of Cu-Cu2O/PtPd were determined to be 40 °C and pH 4.0, respectively. A kinetic analysis revealed that Cu-Cu2O/PtPd followed Michaelis-Menten kinetics and exhibited a higher affinity toward TMB than the horseradish peroxidase enzyme. The catalytic mechanism of Cu-Cu2O/PtPd involved the generation of hydroxyl radicals, which facilitated the oxidation of TMB. Furthermore, the Cu-Cu2O/PtPd nanocomposite was successfully applied for the colorimetric detection of glucose, demonstrating a linear range of 8-90 μM, a detection limit of 2.389 μM, and high selectivity for glucose over other sugars.

Dual regulation of osteosarcoma hypoxia microenvironment by a bioinspired oxygen nanogenerator for precise single-laser synergistic photodynamic/photothermal/induced antitumor immunity therapy

The hypoxic tumor microenvironment (TME) of osteosarcoma (OS) is the Achilles' heel of oxygen-dependent photodynamic therapy (PDT), and tremendous challenges are confronted to reverse the hypoxia. Herein, we proposed a "reducing expenditure of O2 and broadening sources" dual-strategy and constructed ultrasmall IrO2@BSA-ATO nanogenerators (NGs) for decreasing the O2-consumption and elevating the O2-supply simultaneously. As O2 NGs, the intrinsic catalase (CAT) activity could precisely decompose the overexpressed H2O2 to produce O2 in situ, enabling exogenous O2 infusion. Moreover, the cell respiration inhibitor atovaquone (ATO) would be at the tumor sites, effectively inhibiting cell respiration and elevating oxygen content for endogenous O2 conservation. As a result, IrO2@BSA-ATO NGs systematically increase tumor oxygenation in dual ways and significantly enhance the antitumor efficacy of PDT. Moreover, the extraordinary photothermal conversion efficiency allows the implementation of precise photothermal therapy (PTT) under photoacoustic guidance. Upon a single laser irradiation, this synergistic PDT, PTT, and the following immunosuppression regulation performance of IrO2@BSA-ATO NGs achieved a superior tumor cooperative eradicating capability both in vitro and in vivo. Taken together, this study proposes an innovative dual-strategy to address the serious hypoxia problem, and this microenvironment-regulable IrO2@BSA-ATO NGs as a multifunctional theranostics platform shows great potential for OS therapy.

[FeIIICl(TMPPH2)][FeIIICl4]2: A Stand-Alone Molecular Nanomedicine That Induces High Cytotoxicity by Ferroptosis

Developing clinically meaningful nanomedicines for cancer therapy requires the drugs to be effective, safe, simple, cheap, and easy to store. In the present work, we report that a simple cationic Fe(III)-rich salt of [FeIIICl(TMPPH2)][FeIIICl4]2 (Fe-TMPP) exhibits a superior anticancer performance on a broad spectrum of cancer cell lines, including breast, colorectal cancer, liver, pancreatic, prostate, and gastric cancers, with half maximal inhibitory concentration (IC50) values in the range of 0.098-3.97 μM (0.066-2.68 μg mL-1), comparable to the best-reported medicines. Fe-TMPP can form stand-alone nanoparticles in water without the need for extra surface modification or organic-solvent-assisted antisolvent precipitation. Critically, Fe-TMPP is TME-responsive (TME = tumor microenvironment), and can only elicit its function in the TME with overexpressed H2O2, converting H2O2 to the cytotoxic •OH to oxidize the phospholipid of the cancer cell membrane, causing ferroptosis, a programmed cell death process of cancer cells.

Unveiling the role of inorganic nanoparticles in Earth's biochemical evolution through electron transfer dynamics

This article explores the intricate interplay between inorganic nanoparticles and Earth's biochemical history, with a focus on their electron transfer properties. It reveals how iron oxide and sulfide nanoparticles, as examples of inorganic nanoparticles, exhibit oxidoreductase activity similar to proteins. Termed "life fossil oxidoreductases," these inorganic enzymes influence redox reactions, detoxification processes, and nutrient cycling in early Earth environments. By emphasizing the structural configuration of nanoparticles and their electron conformation, including oxygen defects and metal vacancies, especially electron hopping, the article provides a foundation for understanding inorganic enzyme mechanisms. This approach, rooted in physics, underscores that life's origin and evolution are governed by electron transfer principles within the framework of chemical equilibrium. Today, these nanoparticles serve as vital biocatalysts in natural ecosystems, participating in critical reactions for ecosystem health. The research highlights their enduring impact on Earth's history, shaping ecosystems and interacting with protein metal centers through shared electron transfer dynamics, offering insights into early life processes and adaptations.

A Porphyrin-Based 3D Metal-Organic Framework Featuring [Cu8Cl6]10+ Cluster Secondary Building Units: Synthesis, Structure Elucidation, Anion Exchange, and Peroxidase-Like Activity

Herein, we report a rare example of cationic three-dimensional (3D) metal-organic framework (MOF) of [Cu5Cl3(TMPP)]Cl5 ⋅ xSol (denoted as Cu-TMPP; H2TMPP=meso-tetrakis (6-methylpyridin-3-yl) porphyrin; xSol=encapsulated solvates) supported by [Cu8Cl6]10+ cluster secondary building units (SBUs) wherein the eight faces of the Cl--based octahedron are capped by eight Cu2+. Surface-area analysis indicated that Cu-TMPP features a mesoporous structure and its solvate-like Cl- counterions can be exchanged by BF4 -, PF6 -, and NO3 -. The polyvinylpyrrolidone (PVP) coated Cu-TMPP (denoted as Cu-TMPP-PVP) demonstrated good ROS generating ability, producing ⋅OH in the absence of light (peroxidase-like activity) and 1O2 on light irradiation (650 nm; 25 mW cm-2). This work highlights the potential of Cu-TMPP as a functional carrier of anionic guests such as drugs, for the combination therapy of cancer and other diseases.

Nitrogen doped 1 T/2H mixed phase MoS2/CuS heterostructure nanosheets for enhanced peroxidase activity

Heteroatom doping and phase engineering are effective ways to promote the catalytic activity of nanoenzymes. Nitrogen-doped 1 T/2H mixed phase MoS2/CuS heterostructure nanosheets N-1 T/2H-MoS2/CuS are prepared by a simple hydrothermal approach using polyoxometalate (POM)-based metal-organic frameworks (MOFs) (NENU-5) as a precursor and urea as nitrogen doping reagent. The XPS spectroscopy (XPS) and Raman spectrum of N-1 T/2H-MoS2/CuS prove the successful N-doping. NENU-5 was used as the template to prepare 1 T/2H-MoS2/CuS with high content of 1 T phase by optimizing the reaction time. The use of urea as nitrogen dopant added to 1 T/2H-MoS2/CuS, resulted in N-1 T/2H-MoS2/CuS with an increase in the content of the 1 T phase from 80 % to 84 % and higher number of defects. N-1 T/2H-MoS2/CuS shows higher peroxidase activity than 1 T/2H-MoS2/CuS and a catalytic efficiency (Kcat/Km) for H2O2 twice as high as that of 1 T/2H-MoS2/CuS. The enhanced catalytic activity has probably been attributed to several reasons: (i) the insertion of urea during the hydrothermal process in the S-Mo-S layer of MoS2, causing an increase in the interlayer spacing and in 1 T phase content, (ii) the replacement of S atoms in MoS2 by N atoms from the urea decomposition, resulting in more defects and more active sites. As far as we know, N-1 T/2H-MoS2/CuS nanosheets have the lowest detection limit (0.16 µm) for the colorimetric detection of hydroquinone among molybdenum disulfide-based catalysts. This study affords a new approach for the fabrication of high-performance nanoenzyme catalysts.

Nanozymes: Definition, Activity, and Mechanisms

"Nanozyme" is used to describe various catalysts from immobilized inorganic metal complexes, immobilized enzymes to inorganic nanoparticles. Here, the history of nanozymes is dvescribed in detail, and they can be largely separated into two types. Type 1 nanozymes refer to immobilized catalysts or enzymes on nanomaterials, which were dominant in the first decade since 2004. Type 2 nanozymes, which rely on the surface catalytic properties of inorganic nanomaterials, are the dominating type in the past decade. The definition of nanozymes is evolving, and a definition based on the same substrates and products as enzymes are able to cover most currently claimed nanozymes, although they may have different mechanisms compared to their enzyme counterparts. A broader definition can inspire application-based research to replace enzymes with nanomaterials for analytical, environmental, and biomedical applications. Comparison with enzymes also requires a clear definition of a nanozyme unit. Four ways of defining a nanozyme unit are described, with iron oxide and horseradish peroxidase activity comparison as examples in each definition. Growing work is devoted to understanding the catalytic mechanism of nanozymes, which provides a basis for further rational engineering of active sites. Finally, future perspective of the nanozyme field is discussed.

Advances in the Application of Transition-Metal Composite Nanozymes in the Field of Biomedicine

Due to the limitation that natural peroxidase enzymes can only function in relatively mild environments, nanozymes have expanded the application of enzymology in the biological field by dint of their ability to maintain catalytic oxidative activity in relatively harsh environments. At the same time, the development of new and highly efficient composite nanozymes has been a challenge due to the limitations of monometallic particles in applications and the inherently poor enzyme-mimetic activity of composite nanozymes. The inherent enzyme-mimicking activity is due to Au, Ag, and Pt, along with other transition metals. Moreover, the nanomaterials exhibit excellent enzyme-mimicking activity when composited with other materials. Therefore, this paper focuses on composite nanozymes with simulated peroxidase activity that have been prepared using noble metals such as Au, Ag, and Pt and other transition metal nanoparticles in recent years. Their simulated enzymatic activity is utilized for biomedical applications such as glucose detection, cancer cell detection and tumor treatment, and antibacterial applications.

Deep Insight of Design, Mechanism, and Cancer Theranostic Strategy of Nanozymes

Since the discovery of enzyme-like activity of Fe3O4 nanoparticles in 2007, nanozymes are becoming the promising substitutes for natural enzymes due to their advantages of high catalytic activity, low cost, mild reaction conditions, good stability, and suitable for large-scale production. Recently, with the cross fusion of nanomedicine and nanocatalysis, nanozyme-based theranostic strategies attract great attention, since the enzymatic reactions can be triggered in the tumor microenvironment to achieve good curative effect with substrate specificity and low side effects. Thus, various nanozymes have been developed and used for tumor therapy. In this review, more than 270 research articles are discussed systematically to present progress in the past five years. First, the discovery and development of nanozymes are summarized. Second, classification and catalytic mechanism of nanozymes are discussed. Third, activity prediction and rational design of nanozymes are focused by highlighting the methods of density functional theory, machine learning, biomimetic and chemical design. Then, synergistic theranostic strategy of nanozymes are introduced. Finally, current challenges and future prospects of nanozymes used for tumor theranostic are outlined, including selectivity, biosafety, repeatability and stability, in-depth catalytic mechanism, predicting and evaluating activities.

Regulating the H2O2 Activation Pathway on a Well-Defined CeO2 Nanozyme Allows the Entire Steering of Its Specificity between Associated Enzymatic Reactions

Nanozymes are promising alternatives to natural enzymes, but their use remains limited owing to poor specificity. For example, CeO2 activates H2O2 and displays peroxidase (POD)-like, catalase (CAT)-like, and haloperoxidase (HPO)-like activities. Since they unavoidably compete for H2O2, affecting its utilization in the target application, the precise manipulation of reaction specificity is thus imperative. Herein, we showed that one can simply achieve this by manipulating the H2O2 activation pathway on pristine CeO2 in well-defined shapes. This is because the coordination and electronic structures of Ce sites vary with CeO2 surfaces, wherein the (100) and (111) surfaces display nearly 100% specificity toward POD-/CAT-like and HPO-like activities, respectively. The antibacterial results suggest that the latter surface can well-utilize H2O2 to kill bacteria (cf., the former), which is promising for anti-biofouling applications. This work provides atomic insights into the synthesis of nanozymes with improved activity, reaction specificity, and H2O2 utilization.

Modulation of two-dimensional palladium nanozyme activity to enhance chemodynamic/photothermal combined therapy for melanoma

Nanozymes are artificial enzymes that mimic natural enzyme-like activities and exhibit tremendous potential for tumor chemodynamic therapy. However, the development of novel nanozymes with superior catalytic activities for nanotheranostics remains a formidable challenge. Herein, we report a facile synthesis of monodisperse palladium nanosheets (Pd nanosheets) and their assembly on graphene oxide (GO) that enhances the catalytic activities of Pd nanoparticles. Simultaneously, the obtained nanocomposites (rGO-Pd) could be applied as a smart near-infrared (NIR) light-responsive nanotheranostic for near infrared imaging-guided chemodynamic/photothermal combined therapy. Notably, rGO-Pd exhibited high peroxidase mimicking activities, which could catalyze the conversion of intratumoral H2O2 to ˙OH. Impressively, the reactive oxygen species (ROS) generation of rGO-Pd was further remarkably enhanced by the endogenous acidity of the tumor microenvironment and the exogenous NIR light-responsive photothermal effect. These collective properties of the rGO-Pd nanozyme enabled it to be a ROS generation accelerator for photothermally enhanced tumor chemodynamic therapy. Thus, the as-developed rGO-Pd may represent a promising new type of high-performance nanozyme for multifunctional nanotheranostics toward cancer.

Recent Advances in Nanoplatform Construction Strategy for Alleviating Tumor Hypoxia

Hypoxia is a typical feature of most solid tumors and has important effects on tumor cells' proliferation, invasion, and metastasis. This is the key factor that leads to poor efficacy of different kinds of therapy including chemotherapy, radiotherapy, photodynamic therapy, etc. In recent years, the construction of hypoxia-relieving functional nanoplatforms through nanotechnology has become a new strategy to reverse the current situation of tumor microenvironment hypoxia and improve the effectiveness of tumor treatment. Here, the main strategies and recent progress in constructing nanoplatforms are focused on to directly carry oxygen, generate oxygen in situ, inhibit mitochondrial respiration, and enhance blood perfusion to alleviate tumor hypoxia. The advantages and disadvantages of these nanoplatforms are compared. Meanwhile, nanoplatforms based on organic and inorganic substances are also summarized and classified. Through the comprehensive overview, it is hoped that the summary of these nanoplatforms for alleviating hypoxia could provide new enlightenment and prospects for the construction of nanomaterials in this field.

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.

Smart Biomimetic Nanozymes for Precise Molecular Imaging: Application and Challenges

New nanotechnologies for imaging molecules are widely being applied to visualize the expression of specific molecules (e.g., ions, biomarkers) for disease diagnosis. Among various nanoplatforms, nanozymes, which exhibit enzyme-like catalytic activities in vivo, have gained tremendously increasing attention in molecular imaging due to their unique properties such as diverse enzyme-mimicking activities, excellent biocompatibility, ease of surface tenability, and low cost. In addition, by integrating different nanoparticles with superparamagnetic, photoacoustic, fluorescence, and photothermal properties, the nanoenzymes are able to increase the imaging sensitivity and accuracy for better understanding the complexity and the biological process of disease. Moreover, these functions encourage the utilization of nanozymes as therapeutic agents to assist in treatment. In this review, we focus on the applications of nanozymes in molecular imaging and discuss the use of peroxidase (POD), oxidase (OXD), catalase (CAT), and superoxide dismutase (SOD) with different imaging modalities. Further, the applications of nanozymes for cancer treatment, bacterial infection, and inflammation image-guided therapy are discussed. Overall, this review aims to provide a complete reference for research in the interdisciplinary fields of nanotechnology and molecular imaging to promote the advancement and clinical translation of novel biomimetic nanozymes.

An Atomic Insight into the Confusion on the Activity of Fe3O4 Nanoparticles as Peroxidase Mimetics and Their Comparison with Horseradish Peroxidase

Although Fe₃O₄ nanoparticles were early reported to outperform horseradish peroxidase (HRP), recent studies suggested that this material bears a very poor activity instead. Here, we resolve this disagreement by reviewing the definition of descriptors used and provide an atomic view into the origin of Fe₃O₄ nanoparticles as peroxidase mimetics. The redox between H₂O₂ and Fe(II) sites on the Fe₃O₄ surface was identified as the key step to producing OH radicals for the oxidation of colorimetric substrates. This mechanism involving free radicals is distinct from that of HRP oxidizing substrates with a radical retained on its Fe-porphyrin ring. Surprisingly, the distribution and chemical state of Fe species were found to be very different on single- and polycrystalline Fe₃O₄ nanoparticles with the latter bearing not only a higher Fe(II)/Fe(III) ratio but also a more reactive Fe(II) species at surface grain boundaries. This accounts for the unexpected gap in the catalytic constant (k_cat) observed for this material in the literature.

Surface Science of Nanozymes and Defining a Nanozyme Unit

The field of nanozyme aims to use nanomaterials to replace protein-based enzymes. Nanozymes have attracted extensive interest because of their stability, cost-effectiveness, and versatility. While the focus of the nanozyme field has mainly been the discovery of new nanozyme materials and the exploration of their analytical, biomedical, and environmental applications, the number of fundamental studies is growing. Nanozymes are related to two important fields: enzymology and heterogeneous catalysis. Although fitting nanozyme kinetic data to the Michaelis-Menten kinetics is a very common practice, using the surface science methods of heterogeneous catalysis can provide insights about their catalytic mechanisms. The definition of a nanozyme unit is critical to understanding and comparing nanozyme activities. In this perspective, we articulate the use of a surface science approach to study nanozymes and discuss the various application scenarios of using different nanozyme units.