Enhanced Fenton,photo-Fenton and peroxidase-like activity and stability over Fe_3O_4/g-C_3N_4 nanocomposites

We prepared the Fe3O4/g‐C3N4nanoparticles(NPs)through a simple electrostatic self‐assembly method with a3:97weight ratio to investigate their Fenton,photo‐Fenton and oxidative functionalities besides photocatalytic functionality.We observed an improvement of the Fenton and photo‐Fenton activities of the Fe3O4/g‐C3N4nanocomposites.This improvement was attributed to efficient charge transfer between Fe3O4and g‐C3N4at the heterojunctions,inhibition of electron‐hole recombination,a high surface area,and stabilization of Fe3O4against leaching by the hydrophobic g‐C3N4.The obtained NPs showed a higher degradation potential for rhodamine B(RhB)dye than those of Fe3O4and g‐C3N4.As compared to photocatalysis,the efficiency of RhB degradation in the Fenton and photo‐Fenton reactions was increased by20%and90%,respectively.Additionally,the horseradish peroxidase(HRP)activity of the prepared nanomaterials was studied with3,3,5,5‐tetramethylbenzidinedihydrochloride(TMB)as a substrate.Dopamine oxidation was also examined.Results indicate that Fe3O4/g‐C3N4nanocomposites offers more efficient degradation of RhB dye in a photo‐Fenton system compared with regular photocatalytic degradation,which requires a long time.Our study also confirmed that Fe3O4/g‐C3N4nanocomposites can be used as a potential material for mimicking HRP owing to its high affinity for TMB.These findings suggest good potential for applications in biosensing and as a catalyst in oxidation reactions.

Prussian Blue Nanoparticles with High Oxidase-like Activity for Simultaneous Detection of Ascorbic Acid and Alkaline Phosphatase in Real Samples

Prussian blue nanoparticles (PB NPs) have exhibited favorable anti-inflammatory effects in animal models,and have been proved to mimic three antioxidant enzymes: peroxidase,catalase,and superoxide dismutase.Whereas the oxidase-like activity of PB NPs has not been demonstrated.In this work,PB NPs with outstanding oxidase-like activity were demonstrated by virtue of the colorimetric substrate o-phenylenediamine (OPD).Interestingly,based on the oxidase-like activity,PB NPs can also catalytically oxidize vitamin B1 (VB1) to strongly fluorescent product in the physiological environment without the addition of other oxidants.Considering that VB1 has no toxicity and carcinogenicity comparing with OPD,a novel PB NPs-VB1 fluorescent sensor was further developed for simple,nontoxic,and rapid detection of ascorbic acid (AA) in foods and alkaline phosphatase (ALP) in human serum samples by taking advantage of the reduction of PB NPs to Prussian White by AA or enzymatic hydrolysate of ALP and ascorbic acid 2-phosphate (AA2P);leading to the decrease of fluorescence intensity.The extremely low detection limits of 44 nmol/L for AA and 72 mU/L for ALP as well as the excellent applicability in real samples were both obtained.Therefore,we envision that such a novel fluorescent probe would be potentially applied in the food analysis and clinic diagnosis fields.

The Mn/Yb/Er triple-doped CeO_(2) nanozyme with enhanced oxidase-like activity for highly sensitive ratiometric detection of nitrite

Long-term excessive intake of nitrite(NO_(2)^(-))poses a great threat to human health,needing a simple and fast method to detect NO_(2)-in food.Herein,via a simple and feasible strategy,Mn/Yb/Er triple-doped CeO_(2) nanozyme(Mn/Yb/Er/CeO_(2))was synthesized for highly sensitive ratiometric detection of nitrite.By doping Mn,Yb,Er into CeO_(2) lattice structure,Mn/Yb/Er/CeO_(2) nanozyme showed enhanced oxidase-like activity,obtaining a higher density of oxygen vacancy and a higher ratio of Ce^(3+)to Ce~(4+)than that of CeO_(2).The 3,3,5,5-tetramethylbenzidine(TMB)can be effectively oxidized by Mn/Yb/Er/CeO_(2) to produce the oxidized TMB(ox TMB),showing a significant absorption signal at 652 nm.Additionally,nitrite can react with ox TMB to produce yellow diazotized ox TMB,which is accompanied by an elevated absorption signal at 445 nm and a decreased absorption signal at 652 nm.Thus,based on the oxidase-mimetic activity of Mn/Yb/Er/CeO_(2) and the diazotization reaction between NO_(2)^(-)and ox TMB,a ratiometric colorimetric assay was established for NO_(2)^(-)detection in food.Furthermore,by integrating Mn/Yb/Er/CeO_(2) with a smartphone,a colorimetric smartphone-sensing platform was successfully fabricated for visualization and quantitative detection of NO_(2)^(-).Notably,this two-detection mode showed excellent sensitivity,selectivity,reliability and practicability in monitoring the NO_(2)^(-)in real samples,impling its great potential for food safety.

Silicon dioxide-protection boosting the peroxidase-like activity of Fe single-atom catalyst for combining chemo-photothermal therapy

Carbon-based single-atom catalysts(SACs)have been widely studied in the field of biomedicine due to their excellent catalytic performance.However,carbon-based SACs usually aggregate during pyrolysis,which leads to the reduction of catalytic activity.Here,we describe a method to improve the monodispersion of SACs using silicon dioxide as a protective layer.The decoration of silicon dioxide serves as a buffer layer for individual nanoparticles,which is not destroyed during the pyrolysis process,ensuring the single-particle dispersion of the nanoparticles after etching.This approach increased the hydroxyl groups on the surface of Fe-SAC(Fe-SAC-SE)and improved its water solubility,resulting in a four times enhancement of the peroxidase(POD)-like activity of Fe-SAC-SE(58.4 U/mg)than that of non-protected SACs(13.9 U/mg).The SiO_(2)-protection approach could also improve the catalytic activities of SACs with other metals such as Mn,Co,Ni,and Cu,indicating its generality for SACs preparation.Taking advantage of the high POD-like activity,photothermal properties,and large specific surface area of Fe-SAC-SE,we constructed a synergistic therapeutic system(Fe-SAC-SE@DOX@PEG)for combining the photothermal therapy,catalytic therapy,and chemotherapy.It was verified that the photothermal properties of Fe-SAC-SE@DOX@PEG could effectively improve its POD-like activity,exhibiting excellent tumor-killing performance at the cellular level.This work may provide a general approach to improve the performances of SACs for disease therapy and diagnosis.

Portable multi-amplified temperature sensing for tumor exosomes based on MnO_(2)/IR780 nanozyme with high photothermal effect and oxidase-like activity

Efficient determination of tumor exosomes using portable devices is crucial for the establishment of facile and convenient early cancer diagnostic methods. However, it is still challenging to effectively amplify the detection signal to achieve tumor exosomes detection with high sensitivity by portable devices. To address this issue, we developed a portable multi-amplified temperature sensing strategy for highly sensitive detecting tumor exosomes based on multifunctional manganese dioxide/IR780 nanosheets(MnO_(2)/IR780 NSs) nanozyme with high oxidase-like activity and enhanced photothermal performance.Inspiringly, MnO_(2)/IR780 NSs were synthesized via a facile one-step method with mild experimental conditions, which not only exhibited a stronger photothermal effect than that of MnO_(2) but also showed excellent oxidase-like activity that can catalyze the oxidation of 3',3',5',5'-tetramethylbenzidine(TMB) to generate TMB oxide(oxTMB) with a robust photothermal property, thus conjoining with MnO_(2)/IR780 NSs to further enhance the temperature signal. The present assay enables highly sensitive determination of tumor exosomes with the detection limit down to 5.1×10~3 particles/mL, which was comparable or superior to those of the most previously reported sensors. Furthermore, detection of tumor exosomes spiked in biological samples was successfully realized. More importantly, our method showed the recommendable portability, robust applicability, and easy manipulation. By taking advantages of these features,this high-performance photothermal sensor offered a promising alternative means for nondestructive early cancer diagnosis and treatment efficacy evaluation.

Enhanced oxidase-like activity of g-C_(3)N_(4) nanosheets supported Pd nanosheets for ratiometric fluorescence detection of acetylcholinesterase activity and its inhibitor

The undesirable enzymatic activity of nanozymes under near neutral p H condition and the traditional single signal output always restrict the analytical application of nanozyme-based biosensors.Herein,graphitic carbon nitride nanosheets supported palladium nanosheets composite (Pd/g-C_(3)N_(4)) with both oxidase-like activity and fluorescent property is synthesized.Notably,Pd/g-C_(3)N_(4)exhibits enhanced oxidase-like activity compared to Pd NSs under p H 7.4.By combining Pd/g-C_(3)N_(4)with o-phenylenediamine(OPD),a ratiometric fluorescence assay for acetylcholinesterase (ACh E) activity detection is developed.Pd/g-C_(3)N_(4)can catalyze oxidation of nonfluorescent OPD to fluorescent oxidized OPD (ox OPD,Em=565nm),which can quench fluorescence of g-C_(3)N_(4)supporter (Em=441 nm) through fluorescence resonance energy transfer (FRET).However,in presence of ACh E,acetylthiocholine can be hydrolyzed into thiocholine,which will block the oxidase-like activity of Pd/g-C_(3)N_(4)and then hamper the FRET process.This ratiometric fluorescence assay is also viable to screen ACh E inhibitor.This work will guide design of ratiometric fluorescence assay based on nanozymes with improved enzymatic activity.

Critical evaluation of the glucose oxidase-like activity of gold nanoparticles stabilized by different polymers

Polymer stabilizers are widely used to synthesize gold nanoparticles(Au NPs)to prevent their aggregation and improve their stability.Although stabilizers are known to greatly influence both the structure and size of Au NPs,limited efforts explore their effects on the activity of Au NPs for biocatalysis.Herein,different polymers are used as stabilizers to synthesize Au NPs.For the glucose oxidase-like activity,we find that without the spatial barrier from stabilizers,naked Au NPs show significantly high catalytic activity as well as the worst stability.Among the polymers,polyacrylic acid-stabilized Au NPs exhibit the highest activity,whose Vmax(0.74μM·s^(−1))is higher than that of the natural glucose oxidase(0.37μM·s^(−1))due to the smallest particle size(<2 nm)and the weak spatial resistance of polyacrylic acid.These variable catalytic results derive from the comprehensive effects including size,steric hindrance,and electronic effect.However,further selectivity and activity tests have exposed shortcomings.They possess universal activities for aldose oxidation,whereas cannot retain activities in typical physiological environments.Our findings highlight the role of polymer stabilizers in imposing effects on the glucose oxidase-like activity of Au NPs and provide a basis for further Au NPs engineering and applications.

Palladium Nanoparticles/Graphdiyne Oxide Nanocomposite with Excellent Peroxidase-like Activity and Its Application for Glutathione Detection

In this study, palladium nanoparticles loaded graphdiyne oxide (Pd/GDYO) nanocomposite were fabricated by in-situ reduction of palladium chloride in the dispersion of GDYO, and characte-rized by Raman spectra, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The synthesized Pd/GDYO was first found to have catalytic activities similar to those of the peroxidase enzyme, which can catalyze the oxidation of peroxidase substrate 3,3',5,5'-tetramethylbenzidine(TMB) in the presence of hydrogen peroxide(H2O2). Steady-state kinetic studies showed that the catalytic reaction of Pd/GDYO follows a ping-pong mechanism, and Pd/GDYO has a stronger activity to TMB with a Michaelis constant(Km) value of 5.32×10-4 mmol/L. Based on the shielding effect of glutathione(GSH) on the Pd/GDYO-H2O2-TMB reaction system, a colorimetric detection method for GSH was deve-loped with a wide linear range from 0.1 μmol/L to 40 μmol/L and a limit of detection of 0.1 μmol/L. The method was successfully applied for fast and accurate detection of GSH in injection powder drugs. It was expected that this peroxidase-like Pd/GDYO nano- composite would have wide applications in the fields of biomedicine and environmental chemistry.

Biocompatible carbon nitride quantum dots nanozymes with high nitrogen vacancies enhance peroxidase-like activity for broadspectrum antibacterial

Nanozyme antibacterial agents with high enzyme-like catalytic activity and strong bacteria-binding ability have provided an alternative method to efficiently disinfect drug-resistance microorganism.Herein,the carbon nitride quantum dots(CNQDs)nanozymes with high nitrogen vacancies(NVs)were mass-productively prepared by a simple ultrasonic-crushing method assisted by propylene glycol.It was found that the NVs of CNQDs were stemmed from the selective breaking of surface N-(C)_(2)sites,accounting for 6.2%.Experiments and density functional theory(DFT)simulations have demonstrated that the presence of NVs can alter the local electron distribution and extend theπ-electron delocalization to enhance the peroxidase-like activity.Biocompatible CNQDs could enter inside microorganisms by diffusion and elevate the bacteria-binding ability,which enhanced the accurate and rapid attack of·OH to the microorganisms.The sterilization rate of CNQDs against Gram-negative bacteria(E.coli),Gram-positive bacteria(S.aureus,B.subtilis),fungi(R.solani)reaches more than 99%.Thus,this work showed great potential for engineered nanozymes for broad-spectrum antibacterial in biomedicine and environmental protection.

Green synthesis of Au@WSe_(2) hybrid nanostructures with the enhanced peroxidase-like activity for sensitive colorimetric detection of glucose

We report a colorimetric method for glucose detection based on Au nanoparticle-decorated WSe_(2)(Au@WSe_(2))hybrid nanostructures.These hybrid structures are easily synthesized by simply stirring HAuCl_(4) precursor with WSe_(2) nanosheets in aqueous solution.Owing to strong synergistic catalytic effects of Au nanoparticles and WSe_(2) nanosheets,the Au@WSe_(2) hybrid nanostructures exhibit enhanced peroxidase-like activity(about 2-fold higher compared to WSe_(2) nanosheets alone)for 3,3',5,5'-tetramethylbenzidine oxidation by H_(2)O_(2).Based on the highly catalytical property,the colorimetric method for glucose detection is established by coupling glucose oxidase(GOx).The detection limit of glucose is 3.66 pM.Moreover,the proposed colorimetric method is applicable to glucose detection in serum samples and is promising for applications in biomedical fields.

One-pot synthesis of bimetallic Fe/Co incorporated silica hollow spheres with superior peroxidase-like activity

Mesoporous silica hollow spheres with a homogenous and high content distribution of Fe and Co were synthesized by a facile one-pot hydrothermal process. The sub-nanometer bimetallic components inside the silica framework facilitate the stable fixation and the open accessibility to active sites. The co-doped Fe/Co in the spheres showed excellent peroxidase-like activity and much higher catalytic performance than their monometallic-supported spheres. The synergistic effect between Fe and Co promotes the continuous formation of functional radicals during the oxidation process and thus accelerates the reaction rate. When used for colorimetric detection of hydrogen peroxide(H_(2)O_(2)), the Fe/Co incorporated silica hollow spheres show the capability of detection of H_(2)O_(2) in a wide range(10-250 μmol/L) and with the low detection limit of 0.012 ppm.

Glutamatergic CYLD deletion leads to aberrant excitatory activity in the basolateral amygdala:association with enhanced cued fear expression

Neuronal activity,synaptic transmission,and molecular changes in the basolateral amygdala play critical roles in fear memory.Cylindromatosis(CYLD)is a deubiquitinase that negatively regulates the nuclear factor kappa-B pathway.CYLD is well studied in non-neuronal cells,yet underinvestigated in the brain,where it is highly expressed.Emerging studies have shown involvement of CYLD in the remodeling of glutamatergic synapses,neuroinflammation,fear memory,and anxiety-and autism-like behaviors.However,the precise role of CYLD in glutamatergic neurons is largely unknown.Here,we first proposed involvement of CYLD in cued fear expression.We next constructed transgenic model mice with specific deletion of Cyld from glutamatergic neurons.Our results show that glutamatergic CYLD deficiency exaggerated the expression of cued fear in only male mice.Further,loss of CYLD in glutamatergic neurons resulted in enhanced neuronal activation,impaired excitatory synaptic transmission,and altered levels of glutamate receptors accompanied by over-activation of microglia in the basolateral amygdala of male mice.Altogether,our study suggests a critical role of glutamatergic CYLD in maintaining normal neuronal,synaptic,and microglial activation.This may contribute,at least in part,to cued fear expression.

Large scale and controllable preparation of W2C nanorods or WC nanodots with peroxidase-like catalytic activity

WC nanorods or WC nanodots are prepared via an easy,shape-controllable and large-scale preparation technique.Results reveal that each of the WC nanorods and WC nanodots has a peroxidase-like activity.Besides,the peroxidase-like activity of WC is the first time to be demonstrated.The catalytic efficiency of WC nanorods is much higher than that of WC nanodots and chemical condition range of WC can be wider than that of WC,which indicates that WC is likely to be used as artificial mimetic peroxidase or in-situ amplified colorimetric immunoassay.

Passive activity enhances residual control ability in patients with complete spinal cord injury

Patients with complete spinal cord injury retain the potential for volitional muscle activity in muscles located below the spinal injury level.However,because of prolonged inactivity,initial attempts to activate these muscles may not effectively engage any of the remaining neurons in the descending pathway.A previous study unexpectedly found that a brief clinical round of passive activity significantly increased volitional muscle activation,as measured by surface electromyography.In this study,we further explored the effect of passive activity on surface electromyographic signals during volitional control tasks among individuals with complete spinal cord injury.Eleven patients with chronic complete thoracic spinal cord injury were recruited.Surface electromyography data from eight major leg muscles were acquired and compared before and after the passive activity protocol.The results indicated that the passive activity led to an increased number of activated volitional muscles and an increased frequency of activation.Although the cumulative root mean square of surface electromyography amplitude for volitional control of movement showed a slight increase after passive activity,the difference was not statistically significant.These findings suggest that brief passive activity may enhance the ability to initiate volitional muscle activity during surface electromyography tasks and underscore the potential of passive activity for improving residual motor control among patients with motor complete spinal cord injury.

Size-dependent peroxidase-like catalytic activity of Fe_3O_4 nanoparticles

Peroxidase-like catalytic properties of Fe3O4 nanoparficles （NPs） with three different sizes, synthesized by chemical coprecipitation and sol-gel methods, were investigated by UV-vis spectrum analysis. By comparing Fe3O4 NPs with average diameters of 11, 20, and 150 nm, we found that the catalytic activity increases with the reduced nanoparticle size. The electrochemical method to characterize the catalytic activity of Fe3O4 NPs using the response currents of the reaction product and substrate was also developed.

Pyrophosphate-Mediated On-Off-On Oxidase-Like Activity Switching of Nanosized MnFe_(2)O_(4) for Alkaline Phosphatase Sensing

Developing reliable and facile approaches for alkaline phosphatase(ALP)sensing is important due to its role as a clinical biomarker for many diseases.In this study,we proposed a new and convenient colorimetric assay based on the pyrophosphate(PPi)-mediated oxidase-mimicking activity switching of nanosized MnFe_(2)O_(4) for the detection of ALP.The synthesized MnFe_(2)O_(4) exhibited high oxidase-like activity to catalyze the oxidation of colorless 3,3′,5,5′-tetramethylbenzidine(TMB)to its blue product TMBox in the presence of dissolved O2,leading to a color reaction rapidly and remarkably;PPi could significantly inhibit the activity of the MnFe_(2)O_(4) nanozyme via the strong interaction between PPi and the Fe(III)species in MnFe_(2)O_(4),resulting in the suppression of the TMB color reaction;when ALP was added,it hydrolyzed the PPi substrate to phosphate(Pi)that had no obvious effect on the MnFe_(2)O_(4) activity,and such that the TMB color reaction catalyzed by the nanozyme could be observed again.With the above principle,linear colorimetric determination of ALP in the scope of 0.6-55 U L−1 was achieved,giving the limit of detection down to 0.27 U L−1.Besides,the developed assay could provide selective response toward ALP against other co-existing biological species.Furthermore,reliable detection of ALP in human serum samples was verified by our assay,revealing its great promise as an effective and facile tool for ALP monitoring in clinical practice.

Ferroxidase-like activity of Au nanorod/Pt nanodot structures and implications for cellular oxidative stress

Platinum nanoparticles （NPs） are reported to mimic various anfioxidant enzymes and thus may produce a positive biological effect by reducing reactive oxygen species （ROS） levels. In this manuscript, we report Pt NPs as an enzyme mimic of ferroxidase by depositing platinum nanodots on gold nanorods （Au@Pt NDRs）. Au@Pt NDRs show pH-dependent ferroxidase-like activity and have higher activity at neutral pH values. Cytotoxicity results with human cell lines （lung adenocarcinoma A549 and normal bronchial epithelial cell line HBE） show that Au@Pt NDRs are taken up into cells via endocytosis and translocate into the endosome/lysosome. Au@Pt NDRs have good biocompatibility at NDR particle concentrations lower than 0.15 nM. However, in the presence of H202, lysosome- located NDRs exhibit peroxidase-like activity and therefore increase cytotoxicity. In the presence of FeE＋, the ferroxidase-like activity of the NDRs protects cells from oxidative stress by consuming H202. Thorough consideration should be given to this behavior when employinK Au@Pt NDRs in biological svstems.

Controlled preparation and application of glutathione capped gold and platinum alloy nanoclusters with high peroxidase-like activity

In the present study,we have prepared glutathione capped gold and platinum alloy nanoclusters(AuPtNCs)in a controlled way by employing the hydrothermal method and optimized through adjusting the ratio of raw materials,reaction temperature,and time.Compared with the corresponding monometallic gold and platinum nanoclusters,the alloy nanoclusters’catalytic activity is improved dramatically in the oxidation of 3,3,5,5-tetramethylbenzidine(TMB)in the presence of H_(2)O_(2).And the maximum velocity is calculated to be 106×10^(-8)M·s^(-1) for TMB as substrate,being much better than that of other reported metallic nanoclusters and nanoparticles.Further study shows that the high catalytic activity mainly attributes to the synergistic effect of gold and platinum.Besides,they have been applied to determine H_(2)O_(2)in the presence of TMB,which shows high sensitivity with a limit of detection(LOD)at 100 n M.The proposed method has been used to determine H_(2)O_(2)in milk and contact lens solutions,which shows very good recovery and exhibits high practical application potential.Therefore,the present study provides a new type of alloy nanoclusters with high peroxidase-like activity,which will inspire more research interests on doping and alloying with Pt to improve the catalytic activity of metal nanoclusters.

Diatomic active sites nanozymes:Enhanced peroxidase-like activity for dopamine and intracellular H2O2 detection

Considering intracellular hydrogen peroxide(H_(2)O_(2))plays pivotal roles in the regulation of serial biological processes,the in-situ detection of intracellular H2O2 has attracted an extensive attention.In the present work,an atomically dispersed diatomic active sites Nanozymes(FeN_(3)/PtN_(4)-single-atom nanozymes(SAzyme))was prepared exhibiting enhanced peroxidase-like activity.The obvious synergistic effect between Fe-Pt heteronuclear diatomic active sites was confirmed by series of characterization and density functional theory(DFT).The peroxidase-like activity of Fe-sites could be substantially enhanced by the bonded Pt-sites via the modulation effect.As a consequence,the gap between the d-band centre(εd)of Fe 3d orbitals and the Fermi energy level was narrowed and the electronic interaction could be strengthened,leading to a lower free energy barrier and a lower activation energy as well as fortified metal-O bonding in the kinetic pathway.Therefore,the constructed FeN3/PtN4-SAzyme exhibited higher peroxidase-like activity than that of FeN4-SAzyme.The FeN3/PtN4-SAzyme-assisted oxidation of 3,3',5,5'-tetramethylbenzidine(TMB)facilitated the colorimetric detection of dopamine(DA),an important biomolecule.The linear detection range and limit of detection(LOD)of DA and H_(2)O_(2) were 1-10 μM,0.01-1.0 mM and 0.109 μM,7.97 μM,respectively.In addition,the constructed SAzymes were also applied for the in-situ detection of intracellular H2O2,expanding the application scope of the newborn SAzymes.

Rod-shaped Au@PtCu nanostructures with enhanced peroxidase-like activity and their ELISA application

Pt and its based alloy nanoparticles(NPs)have been reported to demonstrate novel enzyme-like activities.Varying composition is very important to realize the optimization of their functions through the tuning of electronic structure.In this paper,our effort is focused in this direction by tailoring the electronic structure of Pt NPs via alloying with copper.Using gold nanorod(Au NR)as core,a simple method to prepare PtCu alloy shell is developed(termed as Au@PtCu NR).The introduction of copper could result in endcap-preferred growth mode owing to the lattice mismatch between alloy shell and the Au core.The variation in the electronic structure changes the substrate affinity,and enhanced affinity was found for H2O2.Besides,the designed Au@PtCu nanostructures have realized spatial separation of catalytic and recognition sites.Binding of recognition antibodies had negligible effect on their catalytic activity.Based on their peroxidaselike activity,a highly sensitive detection of human immunoglobulin G(IgG)was demonstrated in a direct enzyme-linked immunosorbent assay(ELISA)mode.The detection limit can be as low as 90 pg/mL.