Green production of functionalized few-layer borophene decorated with cerium-doped iron oxide nanoparticles for repeatable hydrogen peroxide detection

Functionalized few-layer borophene (FFB) was prepared using gallnut extract and coffee waste extract as natural exfoliating and stabilizing agents in an environmentally friendly ultrasonic and high shear exfoliation. Here, a facile precipitation method was employed to grow iron oxide nanoparticles doped with cerium (Ce-FeONPs) onto the surface of FFB. This intriguing combination of materials yielded Ce-FeONPs nanoparticles that exhibited exceptional peroxidase-like activity, efficiently catalyzing the conversion of 3,3',5,5'-tetramethylbenzidine (TMB) to a blue oxidized TMB (oxTMB) in the presence of hydrogen peroxide (H2O2). Additionally, the introduction of FFB contributes a reducibility effect to the catalytic oxidation of TMB, facilitating the restoration of the oxTMB to TMB. Thus, FFB-Ce-FeONPs showcase intriguing properties encompassing both oxidative and reductive characteristics, suggesting their potential as a reagent for repeated detection of H2O2. Moreover, a colorimetric sensing system enabled the liner detection of H2O2 spanning a concentration range from 0.08 to 1 mM, with a detection limit of 0.03 mM. Noteworthily, FFB-Ce-FeONPs demonstrated sustained efficacy over ten successive recycling cycles, as indicated by consistent slopes and observable color changes. In summary, this work reports the first application of nanoenzymes in repetitive H2O2 detection. Even after ten multiple cycles, the detection limit remains virtually unaltered, underscoring the robustness and enduring effectiveness of the engineered nanomaterial. The proposed simultaneous oxidation and reduction strategies for detecting H2O2 showed a commendable capability in ten cycles of H2O2 detection, thus providing a promising approach in the field of H2O2 detection.

Colorimetric and Electrochemical Dual-Mode Detection of Thioredoxin 1 Based on the Efficient Peroxidase-Mimicking and Electrocatalytic Property of Prussian Blue Nanoparticles

As a potent detection method for cancer biomarkers in physiological fluid, a colorimetric and electrochemical dual-mode sensing platform for breast cancer biomarker thioredoxin 1 (TRX1) was developed based on the excellent peroxidase-mimicking and electrocatalytic property of Prussian blue nanoparticles (PBNPs). PBNPs were hydrothermally synthesized using K3[Fe(CN)6] as a precursor and polyvinylpyrrolidone (PVP) as a capping agent. The synthesized spherical PBNPs showed a significant peroxidase-like activity, having approximately 20 and 60% lower Km values for 3,3',5,5'-tetramethylbenzidine (TMB) and H2O2, respectively, compared to those of horseradish peroxidase (HRP). The PBNPs also enhanced the electron transfer on the electrode surface. Based on the beneficial features, PBNPs were used to detect target TRX1 via sandwich-type immunoassay procedures. Using the strategies, TRX1 was selectively and sensitively detected, yielding limit of detection (LOD) values as low as 9.0 and 6.5 ng mL-1 via colorimetric and electrochemical approaches, respectively, with a linear range of 10-50 ng mL-1 in both strategies. The PBNP-based TRX1 immunoassays also exhibited a high degree of precision when applied to real human serum samples, demonstrating significant potentials to replace conventional HRP-based immunoassay systems into rapid, robust, reliable, and convenient dual-mode assay systems which can be widely utilized for the identification of important target molecules including cancer biomarkers.

Silver-palladium bimetallic nanoparticles stabilized by elm pod polysaccharide with peroxidase-like properties for glutathione detection and photothermal anti-tumor ability

Noble metal nanoparticles show good application prospects in biosensors and anti-tumor drug research. Herein, the near-spherical silver‑palladium bimetallic nanoparticles supported by elm pod polysaccharide (EPP-AgPd1.5 NPs) were prepared by using the elm pod polysaccharide (EPP). EPP acts as a stabilizer and reducing agent due to its water solubility and weak reducing ability. The particle size of EPP-AgPd1.5 NPs was 33.6 ± 5.5 nm. In addition, EPP-AgPd1.5 NPs had peroxidase-like activity to catalyze 3,3',5,5'-tetramethylbenzidine (TMB) to oxidized TMB by catalyzing H2O2 to OH. Based on the peroxidase-like activity of EPP-AgPd1.5 NPs, a method for detecting glutathione was established, and the detection limit and linear range of glutathione concentration were 0.279 μM and 0-400 μM, respectively. More importantly, the photothermal conversion efficiency of EPP-AgPd1.5 NPs reached 39.7 %, and their inhibition rate in HeLa cells reached 69.9 %. Silver‑palladium bimetallic nanoparticles stabilized by EPP had good performance in glutathione detection and anti-tumor drugs.

Design and fabrication of electrochemical sensor based on NiO/Ni@C-Fe3O4/CeO2 for the determination of niclosamide

In this study, we aimed to enhance and accelerate the electrochemical properties of a glassy carbon-based voltammetric sensor electrode. This was achieved through the modification of the electrode using a nanocomposite derived from a metal-organic framework, which was embedded onto a substrate consisting of metal oxide nanoparticles. The final product was an electrocatalyst denoted as NiO/Ni@C-Fe3O4/CeO2, tailored for the detection of the drug niclosamide. Several techniques, including FT-IR, XRD, XPS, FE-SEM, TEM, and EDS, were employed to characterize the structure and morphology of this newly formed electroactive catalyst. Subsequently, the efficiency of this electrocatalyst was evaluated using cyclic voltammetry and electrochemical impedance spectroscopy techniques. Differential pulse voltammetry was also utilized to achieve heightened sensitivity and selectivity. A comprehensive exploration of key factors such as the catalyst quantity, optimal instrumental parameters, scan rate influence, and pH effect was undertaken, revealing a well-regulated reaction process. Furthermore, the sensor's analytical performance parameters were determined. This included establishing the linear detection range for the target compound within a specified concentration interval of 2.92 nM to 4.97 μM. The detection limit of 0.91 nM, repeatability of 3.1%, and reproducibility of 4.8% of the sensor were calculated, leading to the observation of favorable stability characteristics. Conclusively, the developed electrochemical sensor was successfully employed for the quantification of niclosamide in urine samples and niclosamide tablets. This application highlighted not only the sensor's high selectivity but also the satisfactory and accurate outcomes obtained from these measurements.

Tailoring the Coordination Environment of Fe/Zn-BDC to Boost Peroxidase-like Activity for Highly Selective Detection of PFOS

Perfluorooctanesulfonic acid potassium salt (PFOS) residues in ecosystems over long periods are of increasing concern and require a selective and stable optical probe for monitoring. Herein, two functional groups (-F and -NH2) with opposite electronic modulation ability were introduced into Fe/Zn-BDC (denoted as Fe/Zn-BDC-F4 and Fe/Zn-BDC-NH2, respectively) to tailor the coordination environment of the Fe metal center, further regulating the nanozyme activity efficiently. Notably, the peroxidase-like activity is related to the coordination environment of the nanozymes and obeys the following order Fe/Zn-BDC-F4 > Fe/Zn-BDC > Fe/Zn-BDC-NH2. Based on the excellent peroxidase-like activity of Fe/Zn-BDC-F4 and the characteristics of being rich in F atoms, a rapid, selective, and visible colorimetric method was developed for detecting PFOS with a detection limit of 100 nM. The detection mechanism was attributed to various interaction forces between Fe/Zn-BDC-F4 and PFOS, including electrostatic interactions, Fe-S interactions, Fe-F bonds, and halogen bonds. This work not only offers new insights into the atomic-scale rational design of highly active nanozymes but also presents a novel approach to detecting PFOS in environmental samples.

Immobilizing nanozymes on 3D-printed metal substrates for enhanced peroxidase-like activity and trace-level glucose detection

The prevalence of 3D-printed portable biomedical sensing devices, which are fashioned mainly from plastic and polymer materials, introduces a pressing concern due to their limited reusability and consequential generation of substantial disposable waste. Considering this, herein, we pioneered a ground-breaking advancement, i.e., a 3D-printed metal substrate-based enzyme. Our inventive methodology involved the synthesis of a thermally degraded Fe-based metal-organic framework, DEG 500, followed by its deposition on a 3D-printed metal substrate composed of Ti-Al-V alloy. This novel composite exhibited remarkable peroxidase-like activity in a range of different temperatures and pH, coupled with the ability to detect glucose in real-world samples such as blood and fruit juices. The exceptional enzymatic behaviour was attributed to the diverse iron (Fe) oxidation states and the presence of oxygen vacancies, as evidenced through advanced characterization techniques. Fundamentally, we rigorously explored the mechanistic pathway through controlled studies and theoretical calculations, culminating in a transformative stride toward more sustainable and effective biomedical sensing practices.

Reaction Mechanisms and Kinetics of Nanozymes: Insights from Theory and Computation

"Nanozymes" usually refers to inorganic nanomaterials with enzyme-like catalytic activities. The research into nanozymes is one of the hot topics on the horizon of interdisciplinary science involving materials, chemistry, and biology. Although great progress has been made in the design, synthesis, characterization, and application of nanozymes, the study of the underlying microscopic mechanisms and kinetics is still not straightforward. Density functional theory (DFT) calculations compute the potential energy surfaces along the reaction coordinates for chemical reactions, which can give atomistic-level insights into the micro-mechanisms and kinetics for nanozymes. Therefore, DFT calculations have been playing an increasingly important role in exploring the mechanisms and kinetics for nanozymes in the past years. The calculations either predict the microscopic details for the catalytic processes to complement the experiments or further develop theoretical models to depict the physicochemical rules. In this review, the corresponding research progress is summarized. Particularly, the review focuses on the computational studies that closely interplay with the experiments. The relevant experimental results without DFT calculations will be also briefly discussed to offer a historic overview of how the computations promote the understanding of the microscopic mechanisms and kinetics of nanozymes.

Ceria-Based Nanozymes in Point-of-Care Diagnosis: An Emerging Futuristic Approach for Biosensing

In recent years, there has been a notable increase in interest surrounding nanozymes due to their ability to imitate the functions and address the limitations of natural enzymes. The scientific community has been greatly intrigued by the study of nanoceria, primarily because of their distinctive physicochemical characteristics, which include a variety of enzyme-like activities, affordability, exceptional stability, and the ability to easily modify their surfaces. Consequently, nanoceria have found extensive use in various biosensing applications. However, the impact of its redox activity on the enzymatic catalytic mechanism remains a subject of debate, as conflicting findings in the literature have presented both pro-oxidant and antioxidant effects. Herein, we creatively propose a seesaw model to clarify the regulatory mechanism on redox balance and survey possible mechanisms of multienzyme mimetic properties of nanoceria. In addition, this review aims to showcase the latest advancements in this field by systematically discussing over 180 research articles elucidating the significance of ceria-based nanozymes in enhancing, downsizing, and enhancing the efficacy of point-of-care (POC) diagnostics. These advancements align with the ASSURED criteria established by the World Health Organization (WHO). Furthermore, this review also examines potential constraints in order to offer readers a concise overview of the emerging role of nanoceria in the advancement of POC diagnostic systems for future biosensing applications.

Simultaneous detection of As(III/V), Cr(III/VI), and Fe(II/III) by a sensor array based on the morphology regulation of CeO2 oxidase

To develop a convenient method for simultaneous detection of As(III/V), Cr(III/VI), and Fe(II/III), three morphologies of CeO2 oxidase have been prepared. Based on the difference in oxidase activity and binding ability with substrate TMB of CeO2 of different morphologies, a 3 (Signal unit) × 6 (Target number) × 5 (Repetition) sensor array was constructed to realize simultaneous detection of six variable valence metal ions As(III/V), Cr(III/VI), and Fe(II/III). The lowest detection limit of the array for metal ions was 1.68 µg/L. The analysis of environmental samples with multiple metal ions (binary and ternary mixtures) co-existing has confirmed that the sensor array can achieve simultaneous qualitative and quantitative results for composite samples. This study not only revealed the influencing factors of crystal morphology regulation on oxidase activity, but also provided a scheme for the morphology detection of easily convertible metal ions in the field through the construction of the sensor array.

G-/C-rich ssDNA-based Fe and Cu/Fe nanoclusters with peroxidase-like activity for intracellular ROS production and cytotoxicity applications

Five G-/C-rich single-stranded DNA (ssDNA) with different sequences and lengths were templated to prepare the DNA-Cu, DNA-Fe, and bimetallic DNA-Cu/M nanoclusters (NCs). The peroxidase-like activities of these nanomaterials were studied using H₂O₂ and 3,3',5,5''-tetramethylbenzidine (TMB) as the reaction substrates in HAc-NaAc buffer. It was found that T30-G2-Fe NCs and T30-G2-Cu/Fe NCs, with a size of about 2 nm, exhibit similar and the strongest enzyme-like activity under optimal conditions. Both NCs possess a similarly high affinity to substrates, and the Michaelis-Menten constant (K_m) values to TMB and H₂O₂ are about 11 and 2-3 times lower than those of natural horseradish peroxidase (HRP), respectively. The activity of both nanozymes decreases to about 70% after being kept for one week in pH 4.0 buffer at 4 °C, which is comparable with HRP. Hydroxyl radicals (•OH) are the main reactive oxygen species (ROS) produced in the catalytic reaction. Moreover, both NCs can facilitate in situ generation of ROS in HeLa cells using endogenous H₂O₂. MTT assays indicate that the T30-G2-Cu/Fe NCs exhibit the strong selective cytotoxicity to HeLa cells over HL-7702 cells. The cellular viability is about 70% and 50% after incubating with 0.6 M NCs for 24 h without or with 2 mM H₂O₂, respectively. The current study shows that the T30-G2-Cu/Fe NCs have the potential for chemical dynamic treatment (CDT).

"Electron Transport Chain Interference" Strategy of Amplified Mild-Photothermal Therapy and Defect-Engineered Multi-Enzymatic Activities for Synergistic Tumor-Personalized Suppression

Arming activatable mild-photothermal therapy (PTT) with the property of relieving tumor thermotolerance holds great promise for overcoming traditional mild PTT limitations such as thermoresistance, insufficient therapeutic effect, and off-target heating. Herein, a mitochondria-targeting, defect-engineered AFCT nanozyme with enhanced multi-enzymatic activity was elaborately designed as a tumor microenvironment (TME)-activatable phototheranostic agent to achieve remarkable anti-tumor therapy via "electron transport chain (ETC) interference and synergistic adjuvant therapy". Density functional theory calculations revealed that the synergistic effect among multi-enzyme active centers endows the AFCT nanozymes with excellent catalytic activity. In TME, open sources of H₂O₂ can be achieved by superoxide dismutase-mimicking AFCT nanozymes. In response to the dual stimuli of H₂O₂ and mild acidity, the peroxidase-mimicking activity of AFCT nanozymes not only catalyzes the accumulation of H₂O₂ to generate ·OH but also converts the loaded 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) into its oxidized form with strong near-infrared absorption, specifically unlocking its photothermal and photoacoustic imaging properties. Intriguingly, the undesired thermoresistance of tumor cells can be greatly alleviated owing to the reduced expression of heat shock proteins enabled by NADH POD-mimicking AFCT-mediated NADH depletion and consequent restriction of ATP supply. Meanwhile, the accumulated ·OH can facilitate both apoptosis and ferroptosis in tumor cells, resulting in synergistic therapeutic outcomes in combination with TME-activated mild PTT.

Iron-Cobalt-Cerium Multimetallic Oxides Derived from Prussian Blue Precursors: Enhanced Oxygen Evolution Electrocatalysis

Exploring non-precious metal-based electrocatalysts is still challenging in 21^st century. In this work, a series of hexagonal bipyramidal Ce-based PBA materials as precursors with different Fe/Co metal ratios, namely as CeFe_x Co_1-x -PBA, are successfully constructed via co-precipitation method and converted into corresponding metal oxides (denoted as Fe_x Co_1-x CeO_y ) via thermal treatment. Then, they as electrocatalysts realize highly efficient oxygen evolution reaction (OER). Especially, as-synthesized Fe_0.7 Co_0.3 CeO_y electrocatalyst shows very low overpotentials of 320 mV at the current density of 10 mA cm^-2 and the Tafel slop of 98.4 mV dec^-1 in 1 M KOH with remarkable durability for 24 h, which was due to the synergistic effect of multi-metal FeCoCe centers. Furthermore, a two-electrode cell of Fe_0.7 Co_0.3 CeO_y /NF||Pt/C/NF realizes outstanding overall water splitting with a voltage of only 1.71 V at 10 mA cm^-2 and remarkable long-term durability, that is even superior to benchmark IrO₂ /NF||Pt/C/NF counterpart.

Light-enhanced transparent hydrogel for uric acid and glucose detection by four different analytical platforms

The lack of solid-phase media limits the portability of colorimetric sensing platforms. In this study, a series of transparent polyvinyl alcohol (PVA) hydrogels encapsulated antimony tin oxide nanoparticles (ATO NPs) and 3,3',5,5'-tetramethylbenzidine (TMB) were developed as the solid-phase sensing media for glucose and uric acid. Under the conditions of H₂O₂ and UV light, the hydrogel presented a multicatalytic ability (photo Fenton-like and peroxidase-like activities), which accelerated the oxidation of TMB, turning the hydrogel from colorless to blue and finally enhancing the detection signal. The plasticity of the hydrogel allowed it to be designed into various shapes (membrane, microsphere etc.) to adapt multiple detection platforms (a liquid/solid-phase UV spectrophotometer, a NanoPhotometer, and smartphone spectroscopy). The hydrogel sensing media exhibited excellent tunability and enhanced the photocatalytic ability. The proposed material was successfully applied to detect glucose and uric acids in real samples by four detection platforms to evaluate its practicability.

CeO2–Co3O4 nanocomposite with oxidase-like activity for colorimetric detection of ascorbic acid†

A CeO₂–Co₃O₄ nanocomposite (NC) was prepared and characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The obtained CeO₂–Co₃O₄ NC displayed biomimicking oxidase-like activity, which can catalytically oxidize the 3, 3′, 5, 5′-tetramethylbenzidine (TMB) substrate from colorless to the blue oxidized TMB (ox-TMB) product with a characteristic absorption peak at 652 nm. When ascorbic acid (AA) was present, ox-TMB would be reduced, resulting in a lighter blue and lower absorbance. On the basis of these facts, a simple colorimetric method for detection of AA was established with a linear relationship ranging from 1.0 to 500 μM and a detection limit of 0.25 μM. When this method was used to detect AA in human serum and commercially available vitamin C tablet samples, a good recovery of 92.0% to 109.0% was obtained. Besides, the catalytic oxidation mechanism was investigated, and the possible catalytic mechanism of CeO₂–Co₃O₄ NC can be described as follows. TMB is adsorbed on the CeO₂–Co₃O₄ NC surface and provides lone-pair electrons to the CeO₂–Co₃O₄ NC, leading to an increase in electron density of the CeO₂–Co₃O₄ NC. An increased electron density can improve the electron transfer rate between TMB and the oxygen absorbed on its surface to generate O₂˙⁻ and ˙O₂, which further oxidize TMB.

A CeO₂–Co₃O₄ nanocomposite (NC) was prepared and characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction.

Vanadium-Doped Porous Cobalt Oxide for Its Superior Peroxidase-like Activity in Simultaneous Total Cholesterol and Glucose Determination in Whole Blood Based on a Simple Two-Dimensional Paper-Based Analytical Device

Vanadium-doped porous Co₃O₄ (V-porous Co₃O₄) was synthesized via a simple soft-templating method and used as a superior peroxidase mimic for the simultaneous colorimetric determination of glucose and total cholesterol (TC) in whole blood samples on a two-dimensional microfluidic paper-based analytical device (2D-μPAD). The large surface area and the presence of two metals in V-porous Co₃O₄ contributed to its excellent catalytic activity toward 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) and 3,3',5,5'- tetramethylbenzidine (TMB) with Michaelis-Menten constants (K_M) of 0.1301 and 0.0141 mM, respectively. The 2D-μPAD was fabricated using simple wax screen-printing and cutting techniques. The colorimetric reactions of both glucose and TC on 2D-μPAD were simultaneously performed by adding a single drop of a whole blood sample on the sample zone made of the LF1 membrane. After the enzymatic reactions, the generated hydrogen peroxide (H₂O₂) was oxidized by V-porous Co₃O₄ to produce hydroxy radicals (^•OH), inducing ABTS and TMB to generate colored products. The generated H₂O₂ was proportional to the intensities of the green and blue products of the glucose and TC systems, respectively. The developed 2D-μPAD required a short analysis time (∼5 min) with small volumes of samples (15 μL of whole blood) whereby no sample preparation was needed. Owing to several advantages including simplicity, low cost, long-term stability, and simultaneous readout, the novel V-porous Co₃O₄ coupled with 2D-μPAD proved to be promising for practical uses as a pioneering portable device for the determination of glucose, TC, and other important biomarkers without the need of technical supports.

Iron-doped cerium/nucleotide coordination polymer as highly efficient peroxidase mimic for colorimetric detection of fluoride ion

A new coordination polymer (Ce-Fe-GMP) with excellent catalytic activity was prepared by a facile route, which was further applied to the detection of F^- with high sensitivity and selectivity. The simple doping of Fe³⁺ into the coordination network can easily modulate the mixing ratio of Ce³⁺ and Ce⁴⁺ in the presence of H₂O₂, which can extremely improve the catalytic ability of Ce-Fe-GMP. Based on the synergistic effect, the Ce-Fe-GMP with dual-active sites shows better peroxidase activity than that of Ce-GMP. In addition, we found that F^- can inhibit the peroxidase activity of Ce-Fe-GMP because of the coordination structure fragmentation and the regulation of Ce³⁺/Ce⁴⁺ ratio. Therefore, different concentrations of F^- can be detected by the colorimetric reaction based on this mechanism. The absorption at 652 nm displays a good linear relationship versus the concentration of F^- over the range 2.0 to 100.0 μM. Furthermore, F^- in real mineral-mixed samples can be measured with satisfactory results. The colorimetric strategy based on the peroxidase activity of Ce-Fe-GMP is simple and low-cost, which shows the potential applications in the field of on-site environment measurement.

Mo3Se4 nanoparticle with ROS scavenging and multi-enzyme activity for the treatment of DSS-induced colitis in mice

Ulcerative colitis (UC), as a most common inflammatory bowel disease (IBD), has become a global public health concern. Exploring novel method of treating UC is urgent and necessary. Recently, nanozyme with excellent antioxidant properties may be one useful therapeutic strategy. In this study, a two-dimensional transition metal chalcogenide (TMCs) nano flake and polyethylene glycol (PEG) modified Mo₃Se₄ nano flakes (PMNFs) was synthesized, which had multi-enzyme activity, including peroxidase, glutathione peroxidase (GPx), superoxide dismutase (SOD), and catalase (CAT). The inhibition effect of PMNFs on sodium dextran sulfate (DSS)-induced colitis was explored. UC was effectively inhibited by PMNFs in this work. PMNFs significantly reduced disease activity index (DAI) score, including weight loss, colon shorten and histopathological abnormalities. The possible mechanism of PMNFs-attenuated colitis was investigated. The results showed that PMNFs reversed DSS-induced oxidative damage, and the antioxidant pathway Nrf2-keap1 signal was activated by PMNFs. Moreover, PMNFs suppressed the expression of pro-inflammatory factors including IL-1β, TNF-α, IFN-β and IL-6 via the inactivation of TLR4/NF-κB pathway in DSS-induced colitis and LPS-treated macrophage. Furthermore, PMNFs treatment prevented the reduction of tight junction proteins (ZO-1, occludin, and claudin-1) and mucin-2 (MUC-2) as well as the up-regulation of epithelial apoptosis caused by DSS. These findings demonstrate that the PMNFs against DSS-induced colitis due to its prevention on oxidative damage, inflammation, and intestine barrier breakdown. Thus, PMNFs have a potential application in the treatment of various oxidative stress or inflammation-related diseases.

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Mo₃Se₄ nano flakes (PMNFs) can effective scavenge ROS in vivo and in vitro.

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PMNFs have multi-enzyme activity, including the peroxidase, GPx, SOD, and CAT.

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PMNFs can prevent symptoms of colitis induced by DSS in mice.

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PMNFs against DSS-induced colitis through prevention on oxidative damage, inflammation, and intestine barrier breakdown.

Enhancing enzymatic activity of Mn@Co3O4 nanosheets as mimetic nanozyme for colorimetric assay of ascorbic acid

In nanozyme-based assays, increasing enzymatic activity is very desirable for enhancing sensitivity and lowering the detection limit. In this study, novel Mn doped cobalt oxide nanosheets (Mn@Co₃O₄ NSs) were synthesized by hydrothermal process. The obtained Mn@Co₃O₄ possessed enhanced dual-enzyme mimetic, oxidase and peroxidase, and can catalytically oxidize of 3, 3', 5, 5'-tetramethylbenzidine (TMB), to a blue product of oxidized TMB. The enzyme kinetics were well-described mathematically using a common Michaelis-Menten and Lineweaver Burk model. The enzyme kinetics constant (K_m) was found to be 0.15 mM, which is relatively low comparing with pure Co₃O₄ nanosheets (0.35 mM) and natural enzyme HRP (0.434 mM). Therefore, the efficient colorimetric method was achieved for determination of H₂O₂ and ascorbic acid. The limit of detection (LOD) of H₂O₂ was 8.0 μM and the linear range was 20-200 μM based on direct turn on of the peroxidase-like activity of Mn@Co₃O₄. While, for ascorbic acid detection based on turn-off approach, the linearity range for the ascorbic acid was 1-8 μM with LOD of 0.4 μM. Moreover, the colorimetric system exhibited good stability and selectivity toward the detection of ascorbic acid effectively in real samples (vitamin C tablets) with satisfactorily accuracy and precision.

Construction of core-in-shell Au@N-HCNs nanozymes for tumor therapy

Noble metals act as nanozymes that can generate reactive oxygen species (ROS) by catalysis to induce apoptosis of tumor cells for cancer therapy. But they are easy to aggregate, which will affect their further application. Carbon materials are often used as the carrier of noble metals to improve their catalytic performance. However, designing a composite structure to build an efficient carbon/noble metal hybrid nanozyme with high catalytic performance for tumor therapy is still a significant challenge. In this work, a core-in-shell structure nanozyme composed of gold nanoparticles (AuNPs) embedded in nitrogen-doped hollow carbon nanoshells (AuNPs@N-HCNs) were fabricated, which exhibited peroxidase-like (POD-like) and oxidase-like (OXD-like) activity. Compared with core-out-of-shell structure composite, the AuNPs@N-HCNs showed a better ability to generate ROS to kill tumor cells. Furthermore, AuNPs@N-HCNs also exhibited satisfactory photothermal conversion properties, which helped build a platform for photothermal therapy. Meanwhile, the enzyme activity produced by AuNPs@N-HCNs increased significantly under light irradiation. Comparing the size of AuNPs in carbon shell, 15 nm AuNPs were better than 2 nm in both enzyme-like activities and in vivo therapeutic effect. In vitro and in vivo studies demonstrated that under the synergistic effect of light-enhancing nanozyme catalysis and photothermal therapy, AuNPs@N-HCNs could induce cancer cell apoptosis and destroy tumors effectively, which provided evidence for the feasibility of tumor catalytic-photothermal treatment.

CeO2/Co3O4@N-doped hollow carbon microspheres with improved peroxidase-like activity for the determination of quercetin

Nanozymes, as substitutes for natural enzymes, have been used to construct some fast cheap sensing platforms by virtue of the high catalytic activity. Herein, we firstly study the peroxidase-like activity of CeO₂/Co₃O₄@N-doped hollow carbon microspheres as nanozymes. In the presence of H₂O₂, CeO₂/Co₃O₄@NCH exhibits high peroxidase-like activity evaluated by the catalytic oxidation of the chromogenic substrate, 3,3',5,5'-tetramethylbenzidine (TMB) into a blue oxTMB visually in 1 min. The larger surface area, pore-like structure, and oxygen vacancies contribute to the enhanced peroxidase-like activity of CeO₂/Co₃O₄@NCH. The active species •O^2- is detected during the catalytic process. Thus, based on the excellent peroxidase-like activity of CeO₂/Co₃O₄@NCH, a facile and effective biosensor is fabricated for sensitive and selective determination of H₂O₂ and quercetin, respectively. CeO₂/Co₃O₄@NCH shows high affinity towards H₂O₂ (K_m = 4.001 mM) and TMB (K_m = 0.086 mM). The detection limit of H₂O₂ and quercetin is as low as 0.086 mM and 1.19 μM (3σ/slope), respectively. This work remarkably extends the utilization of CeO₂/Co₃O₄@NCH in designing a colorimetric sensing platform related to biosensing, food, and environmental monitoring. A fast colorimetric sensing platform for quercetin based on the excellent peroxidase-like activity of CeO₂/Co₃O₄@NCH.

Impact of Annealing Time on Structural Evolution of Pure and Dy3+-Doped CeO2 Nanopowder, Rietveld Refinement and Optical Behavior

This work reports about annealing time effect on structural and optical properties of pure and Dy[Formula: see text]-doped CeO2 nanocrystallites. The nanopowders with an average grain size 13–15[Formula: see text]nm are successfully synthesized via a conducive and neoteric Pechini-type sol–gel technique. Surface morphology, composition, band gap and photoluminescence properties of the prepared samples are examined by multifarious characterization techniques like Rietveld refinement, FESEM, HRTEM, FTIR, UV–Vis spectroscopy and PL. The effects of annealing time on structural parameters including lattice parameter, bond distance, bond angle, strain, crystallite size and texture coefficient are computed for all prepared samples which are further ensured by Rietveld refinement. In absorption spectra, blue shift in the band gap of as-prepared samples has been observed due to well-known quantum size effect, however, red shift is noticed in further annealed samples. PL emission peaks are observed in violet, blue and green regions that are devoted to various defect levels and color centers such as F, F[Formula: see text], F[Formula: see text], etc. It is suggested that defects like oxygen vacancies play vital role in tailoring the band gap of prepared samples and therefore enhance its utility in photonics and oxygen storage appliances.

A Review on Metal- and Metal Oxide-Based Nanozymes: Properties, Mechanisms, and Applications

Since the ferromagnetic (Fe3O4) nanoparticles were firstly reported to exert enzyme-like activity in 2007, extensive research progress in nanozymes has been made with deep investigation of diverse nanozymes and rapid development of related nanotechnologies. As promising alternatives for natural enzymes, nanozymes have broadened the way toward clinical medicine, food safety, environmental monitoring, and chemical production. The past decade has witnessed the rapid development of metal- and metal oxide-based nanozymes owing to their remarkable physicochemical properties in parallel with low cost, high stability, and easy storage. It is widely known that the deep study of catalytic activities and mechanism sheds significant influence on the applications of nanozymes. This review digs into the characteristics and intrinsic properties of metal- and metal oxide-based nanozymes, especially emphasizing their catalytic mechanism and recent applications in biological analysis, relieving inflammation, antibacterial, and cancer therapy. We also conclude the present challenges and provide insights into the future research of nanozymes constituted of metal and metal oxide nanomaterials.

PVP-stabilized PtRu nanozymes with peroxidase-like activity and its application for colorimetric and fluorometric glucose detection

Nanozymes have significant advantages over natural enzymes. The intrinsic peroxidase-like activity of Pt-based nanomaterials can be enhanced by alloying with other transition metals, such as Ru, that have great catalytic activity. In this study, we used polyvinylpyrrolidone (PVP) to synthesize well-dispersed and homogeneous nanostructures. PVP-stabilized Pt-Ru nanozymes (PVP/PtRu NZs) were synthesized and characterized. The PVP/PtRu NZs had an average size of 3.54 ± 0.84 nm and exhibited an intense peroxidase-like activity. The PVP/PtRu NZs were used as peroxidase mimics for colorimetric and fluorometric glucose determination by the glucose oxidase and PVP/PtRu NZs cascade reaction. In the colorimetric assay, the linearly detectable range was 0.25-3.0 mM, with an R² and limit of detection (LOD) of 0.988 and 138 μM, respectively. In the fluorometric assay, a linear relationship was found when the glucose concentration was between 5.0 and 300 μM (R² = 0.997), with an LOD of 1.11 μM. Compared to the colorimetric assay, the fluorometric assay had greater sensitivity and a lower detection limit for the determination of glucose. Moreover, the PVP/PtRu NZs had high storage stability over a month and great recovery values in human serum and artificial urine, with a range of 94-106 %. From these results, PVP/PtRu NZs are expected to be used as promising peroxidase mimics in various fields such as biosensing, pharmaceutical processing, and the food industry.

A facile strategy for synthesis of porous Cu2O nanospheres and application as nanozymes in colorimetric biosensing

Due to the unique advantages, developing a rapid, simple and economical synthetic strategy for porous nanomaterials is of great interest. In this work, for the first time, using sodium hypochlorite as a green oxidant, urea was oxidized to CO2 as a carbon source to prepare the fine-particle crosslinked Cu-precursors, which could be further reduced by sodium ascorbate into pure Cu2O nanospheres (NPs) with a porous morphology at room temperature. Interestingly, our study reveals that introduction of an appropriate amount of MgCl2 into the raw materials can tune the pore sizes and surface area, but has no influence on the phase purity of the resulting Cu2O NPs. Significantly, all the synthesized Cu2O NPs exhibited intrinsic peroxidase-like activity with higher affinity towards both 3,3,5,5-tetramethylbenzidine (TMB) and H2O2 than horseradish peroxidase (HRP) due to the highly porous morphology and the electrostatic attraction towards TMB. The colorimetric detection of glucose based on the resulting porous Cu2O NPs presented a limit of detection (LOD) of 2.19 μM with a broad linear range from 1-1000 μM, much better than many recently reported composite-based nanozymes. Meanwhile, this nanozyme system was utilized to detect l-cysteine, exhibiting a LOD value as low as 0.81 μM within a linear range from 0 to 10 μM. More interesting, this sensing system shows high sensitivity and excellent selectivity in determining glucose and l-cysteine, which is suitable for detecting serum samples with reliable results. Therefore, the present study not only develops a simple strategy to prepare Cu2O NPs with controllable porous structure, but also indicates its promising applications in bioscience and disease diagnosis.

Novel hierarchical CuNiAl LDH nanotubes with excellent peroxidase-like activity for wide-range detection of glucose

Novel hierarchical CuNiAl layered double hydroxide (CuNiAl LDH) nanotubes were prepared with in situ transformation of Al₂O₃ produced using the atomic layer deposition (ALD) method. Based on the characterizations using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), FT-IR spectrometry, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), CuNiAl LDH displays a typical nanotube-like structure consisting of uniform ultrathin nanoflakes. It is also confirmed that nitrate precursors play a crucial role in the formation of the LDH hierarchical structure. The unique hierarchical tube-like structure for CuNiAl LDH can supply more active sites and higher surface areas, leading to outstanding peroxidase mimicking property. The kinetic analyses indicate that the catalytic behavior of CuNiAl LDH follows classic Michaelis-Menten models and the affinity of CuNiAl LDH to the substrate is significantly higher than horseradish peroxidase. A simple and label-free method was developed for the colorimetric detection of glucose. As low as 2.9 μM of glucose can be detected with a broad linear range from 10 to 200 μM. The established method is also proved to be suitable for glucose detection in juice samples.

Cu2O nanocubes-grafted highly dense Au nanoparticles with modulated electronic structures for improving peroxidase catalytic performances

Based on the intermediate states of metal ions in metal oxide nanomaterials (NMs) that acted as the primary active species, the design of high-performance nanozymes has greatly stimulated current research in diverse biomedical applications. Herein, Cu₂O nanocubes-grafted highly dense Au nanoparticles (NPs) was developed as an appealing nanozyme for H₂O₂ colorimetric sensor and antioxidant detections. The obtained Au/Cu₂O heterostructures show efficient electron-transfer from metallic NPs to Cu₂O nanocubes owing to the difference of Fermi energy between two components. The modulated electronic structure of Au/Cu₂O hybrids enables them to possess enhanced peroxidase catalytic activity for the oxidation of 3, 3', 5, 5'-tetramethylbenzidine (TMB) in the presence of H₂O₂, which is about 32% higher than that of pristine Cu₂O nanocubes. Then, an excellent H₂O₂ colorimetric sensor was established by using Au/Cu₂O heterostructures with a low limit of detection (LOD) of 0.054 μM, which is much lower than the H₂O₂ allowance level of US FDA regulations (ca.15 μM, 0.05 wt%). The obtained Au/Cu₂O nanoproducts exhibit pronounced long-time stability with 95% peroxidase activity maintained after keeping them for 30 days, while residual 64.5% via Cu₂O nanocubes. Furthermore, we assessed the anti-oxidative behavior of three natural antioxidants (tannic acid, gallic acid, tartaric acid) with the LODs as low as 0.039, 0.16 and 1.55 μM, respectively, and the antioxidant capacity in the following order: tannic acid > gallic acid > tartaric acid. Therefore, it is believed that the as-prepared Au/Cu₂O nanozymes have promising potential applications in fields of biomedicine and food safety.

N,N-dicarboxymethyl Perylene-diimide modified CeCoO3: Enhanced peroxidase activity, synergetic catalytic mechanism and glutathione colorimetric sensing

N,N-dicarboxymethyl Perylene-diimide (PDI) modified CeCoO₃ nanocomposites were prepared by a two-step method. After modification with PDI molecules, the obtained PDI-CeCoO₃ nanocomposites were demonstrated to possess the heightened peroxidase-like activity, compared with that of pure CeCoO₃ nanoparticles. In the presence of H₂O₂, the heightened peroxidase-like behaviors of PDI-CeCoO₃ were evaluated by the oxidation of the colorless substrate 3,3,5,5-tetramethylbenzidine (TMB) into blue oxTMB, which was detected visually only in 4 min. Importantly, a systematic study of catalytic activity of PDI-CeCoO₃ by different means, including fluorescent probe, electrochemical data, diffuse reflection spectra together with free radical scavenger is executed, verifying that the catalytic activity were from O₂^- and electron holes (h⁺). And, the transfer of photogenerated carriers in the PDI-CeCoO₃ was the Z-scheme heterojuntion mechanism. Furthermore, the peroxidase-like activity of PDI-CeCoO₃ was significantly inhibited by Glutathione (GSH), resulting in fading of blue oxTMB. Based on this, a colorimetric assay for GSH biosensing has been developed. And, the liner range for GSH detection is from 1 to 10 μM with a detection limit of 0.658 μM. The recovery of GSH with different concentrations from 90.0% to 105.9% and the relative standard deviation (RSD) from 1.9% to 5.1%. This colorimetric sensor can be used to detect GSH in real samples.