Bimetal loaded graphitic carbon nitride with synergistic enhanced peroxidase-like activity for colorimetric detection of p-phenylenediamine.

In recent years, great progress has been made on the study of nanozymes with enzyme-like properties. Here, bimetallic Fe and Ni nanoclusters were anchored on the nanosheets of nitrogen-rich layered graphitic carbon nitride by one-step pyrolysis at high temperature (Fe/Ni-CN). The loading content of Fe and Ni on Fe/Ni-CN is as high as 8.0%, and Fe/Ni-CN has a high specific surface area of 121.86 m2 g-1. The Fe/Ni-CN can effectively oxidize 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2, and exhibits efficient peroxidase-like activity, leading to a 17.2-fold increase compared to pure graphitic carbon nitride (CN). Similar to the natural horseradish peroxidase (HRP), the Fe/Ni-CN nanozyme follows catalytic kinetics. The Michaelis-Menten constant (Km) value of the Fe/Ni-CN nanozyme for TMB is about 8.3-fold lower than that for HRP, which means that the Fe/Ni-CN nanozyme has better affinity for TMB. In addition, the catalytic mechanism was investigated by combination of free radical quenching experiments and density-functional theory (DFT) calculations. The results show that the high peroxidase-like activity is due to the easy adsorption of H2O2 after bimetal loading, which is conducive to the production of hydroxyl radicals. Based on the extraordinary peroxidase-like activity, the colorimetric detection of p-phenylenediamine (PPD) was constructed with a wide linear range of 0.2-30 µM and a low detection limit of 0.02 µM. The sensor system has been successfully applied to the detection of residual PPD in real dyed hair samples. The results show that the colorimetric method is sensitive, highly selective and accurate. This study provides a new idea for the efficient enhancement of nanozyme activity and effective detection of PPD by a bimetallic synergistic strategy.

Bimetallic FeMn@C derived from Prussian blue analogue as efficient nanozyme for glucose detection.

In recent decades, nanomaterial-based artificial enzymes called nanozymes have received more and more attention and have been applied in biological, chemical, medical, and other fields. In this work, bimetallic FeMn@C was synthesized by calcination from the Prussian blue analogue. The synthesized bimetallic FeMn@C exhibits efficient peroxidase-like activity. The effect of Mn doping amount, catalytic kinetics, and mechanism of FeMn@C nanozyme was further studied in detail. The results show that the peroxidase-like activity of bimetallic FeMn@C is nearly 16 times higher than that of single-metal Fe@C. The peroxidase-like activity of FeMn@C originates from its production of radicals. Compared with natural enzymes, FeMn@C nanozyme has a better affinity for the substrates. Besides, FeMn@C nanozyme has better stability than natural enzymes. Because of its strong magnetism, FeMn@C nanozyme can be recycled easily and exhibits excellent recycling performance. Based on the good affinity of FeMn@C for H2O2, a rapid and selective colorimetric assay for glucose detection is constructed, with a wide linear range of 0.01-0.75 mM and low detection limit of 4.28 µM. This sensor has been successfully applied to the determination of glucose in fruit juice, showing good selectivity and accuracy. The synthesis of bimetallic FeMn@C provides a feasible way to design nanozymes with excellent catalytic activity, high stability, and easy separation.

NiO nanobelts with exposed {110} crystal planes as an efficient electrocatalyst for the oxygen evolution reaction.

The electrocatalytic oxygen evolution reaction (OER) is necessary and challenging for converting renewable electricity into clean fuels, because of its complex proton coupled multielectron transfer process. Herein, we investigated the crystal plane effects of NiO on the electrocatalytic OER activity through combining experimental studies and theoretical calculations. The experimental results reveal that NiO nanobelts with exposed {110} crystal planes show much higher OER activity than NiO nanoplates with exposed {111} planes. The efficient OER activity of the {110} crystal planes comes from their intrinsically high catalytic ability and fast charge transfer kinetics. Density functional theory (DFT) shows that the {110} crystal planes possess a lower theoretical overpotential value for the OER, leading to a high electrocatalytic performance. This research broadens our vision to design efficient OER electrocatalysts by the selective exposure of specific crystal planes.

Surface Modification of Co3O4 Nanoplates as Efficient Peroxidase Nanozymes for Biosensing Application.

Nanomaterial-based mimetic enzymes, called nanozymes, received more and more attention in recent decades; however, their lack of biocompatibility limited the biomedical applications, which could be solved by surface modification. In this work, the Co3O4 nanoplates were modified by different functional groups, including the amino group, carboxyl group, hydroxyl group, and sulfhydryl group (NH2-Co3O4, COOH-Co3O4, OH-Co3O4, and SH-Co3O4). And the modified Co3O4 nanoplates were characterized by XRD, SEM, TEM, XPS, FTIR, TG, and the Zeta potential, verifying the successful modification of different functional groups. Their mimetic peroxidase properties and kinetics process were further studied and showed that the order of their catalytic activities was as follows: NH2-Co3O4 > SH-Co3O4 > COOH-Co3O4 > pure Co3O4 > OH-Co3O4, and the catalysis of modified Co3O4 nanozymes all followed Michaelis-Menten kinetics. The results indicated that the different functional groups changed their electron transfer ability, and further affected their catalytic activity. H2O2 detection was selected as an application model system to evaluate the modified Co3O4 nanozymes. Compared with other Co3O4 nanozymes, a wider linear range from 0.01 to 40 mmol L-1 and a lower detection limit of 1.5 µmol L-1 was constructed with NH2-Co3O4 nanozymes. The results suggested that surface modification by functional groups was a robust strategy to improve the application of Co3O4 nanozymes. The enhanced catalytic activity and good biocompatibility of modified Co3O4 nanozymes provided valuable materials for the relative application, such as medical detection and antioxidation.

In situ synthesis of molybdenum carbide/N-doped carbon hybrids as an efficient hydrogen-evolution electrocatalyst.

The development of non-precious metal based electrocatalysts for the hydrogen evolution reaction (HER) has received more and more attention over recent years owing to energy and environmental issues, and Mo based materials have been explored as a promising candidate. In this work, molybdenum carbide/N-doped carbon hybrids (Mo2C@NC) were synthesized facilely via one-step high-temperature pyrolysis by adjusting the mass ratio of urea and ammonium molybdate. The Mo2C@NC consisted of ultrasmall nanoparticles encapsulated by N-doped carbon, which had high specific surface area. They all exhibited efficient HER activity, and the Mo2C@NC with a mass ratio of 160 (Mo2C@NC-160) showed the best HER activity, with a low overpotential of 90 mV to reach 10 mA cm-2 and a small Tafel slope of 50 mV dec-1, which was one of the most active reported Mo2C-based electrocatalysts. The excellent HER activity of Mo2C@NC-160 was attributed to the following features: (1) the highly dispersed ultrasmall Mo2C nanoparticles, which exhibited high electrochemically active surface areas; (2) the synergistic effect of the N-doped carbon shell/matrix, which facilitated the electron transport.

Coral-like CeO2/NiO nanocomposites with efficient enzyme-mimetic activity for biosensing application.

Development of nanomaterials-based enzymatic mimics has gained considerable attention in recent years, because of their low cost, high stability and efficiently catalytic ability. Here, CeO2 was successfully incorporated into the coral-like NiO nanostructures assembled by nanoflakes with high surface area, forming the coral-like CeO2/NiO nanocomposites. The morphology and composition of CeO2/NiO nanocomposites were characterized by XRD, SEM, element mapping and XPS. The results of characterization showed that cerium was highly dispersed in the coral-like NiO nanostructures. The peroxidase-like activity of CeO2/NiO nanocomposites was investigated, and they exhibited enhanced peroxidase-like activity in comparison to that of pure NiO or CeO2. The catalytic activity was dependent on the cerium content, and the optimal content was 2.5%. The enhanced catalytic activity of CeO2/NiO nanocomposites arised from their high ability of electron transfer because of cerium incorporation. The catalytic performance of CeO2/NiO nanocomposites was evaluated by steady-state kinetic, which showed that the CeO2/NiO nanocomposites exhibited higher affinity for the substrates and similar catalytic efficiency compared with natural peroxidase. Based on the efficient peroxidase-like activity, CeO2/NiO was used for H2O2 determination. The constructed colorimetric H2O2 sensor had fast response for only 5min, a wide linear range from 0.05 to 40mM and a low detection limit with 0.88µM. The CeO2/NiO nanocomposites were expected to have potential applications in clinical diagnosis and biotechnology as enzymatic mimics.

Copper-incorporated SBA-15 with peroxidase-like activity and its application for colorimetric detection of glucose in human serum.

The copper incorporated SBA-15 (Cu-SBA-15) materials with different amount of Cu in framework were synthesized, and the products were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), transmission electron microscope (TEM) and N2 adsorption/desorption. The Cu contents incorporated into the framework of SBA-15 were measured by inductively coupling plasma atomic emission spectrometer (ICP-AES). Cu-SBA-15 samples were found to exhibit the peroxidase-like activity, similar to the natural peroxidase. The effect of various parameters such as the content of Cu incorporated, pH and temperature on the peroxidase-like activity was studied. Based on the peroxidase-like activity, the Cu-SBA-15 was applied to the determination of H2O2. The linear range for detecting H2O2 was from 0.8 to 60mM with a detection limit of 3.7 µM. Coupled with glucose oxidase, the Cu-SBA-15 was successfully used for the determination of glucose with the linear range of 2-80 mM and a detection limit of 5.4 µM. The determination of glucose in human serum showed high accuracy, good reproducibility, as well as high selectivity against uric acid, ascorbic acid, dopamine and glucose analogs including fructose, maltose and lactose.

Novel hierarchical NiO nanoflowers exhibiting intrinsic superoxide dismutase-like activity.

Novel hierarchical NiO nanoflowers assembled by ultrathin nanoflakes were found to exhibit intrinsic superoxide dismutase-like activity for the first time. Arising from the unique flower-like structure and the appropriate redox potential of NiII/NiIII (0.683 V vs. normal hydrogen electrode), the NiO nanoflowers had the activity parameters of IC50 = 45.2 µg mL-1 and Km = 0.043 mM, which would greatly encourage the development of NiO nanomaterial-based biomimetic superoxide dismutase.

Biogenic magnetic nanoparticles from Burkholderia sp. YN01 exhibiting intrinsic peroxidase-like activity and their applications.

A novel bacterial strain containing biogenic magnetic nanoparticles (BMNPs) was isolated from the sediments of Songhua River in Harbin, China, and was identified as Burkholderia sp. YN01. Extracted BMNPs from YN01 were characterized as pure face-centered cubic Fe3O4 with an average size of 80 nm through transmission electron microscope (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The hysteresis parameters of the BMNP samples such as Bc and Bcr and ratios Mrs/Ms were deduced as 35.6 mT, 43.2 mT, and 0.47, respectively, indicating that the BMNPs exhibit a ferromagnetic behavior. This is the first report concerning on biogenic Fe3O4 NPs produced in Burkholderia genus. Significantly, the BMNPs were proved to possess intrinsic peroxidase-like activity that could catalyze the oxidation of peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2. Kinetic analysis indicates that the catalytic behavior is in accord with typical Michaelis-Menten kinetics and follows ping-pong mechanism. The catalytic constants (K cat) were 6.5 × 10(4) s(-1) and 0.78 × 10(4) s(-1) with H2O2 and TMB as substrate, respectively, which was higher than that of horseradish peroxidase (HRP). Electron spin resonance (ESR) spectroscopy experiments showed that the BMNPs could catalyze H2O2 to produce hydroxyl radicals. The origin of peroxidase-like activity is also associated with their ability to transfer electron between electrode and H2O2 according to an electrochemical study. As a novel peroxidase mimetic, the BMNPs were employed to offer a simple, sensitive, and selective colorimetric method for H2O2 and glucose determination, and the BMNPs could efficiently catalyze the degradation of phenol and Congo red dye.

The crystal plane effect on the peroxidase-like catalytic properties of Co3O4 nanomaterials.

Nanomaterials as enzyme mimics have received considerable attention as they can overcome some serious disadvantages associated with the natural enzymes. In recently developed Co3O4 nanoparticles as peroxidase mimics, the influence of the crystal plane on the catalytic performance has not been demonstrated. In order to better understand their crystal plane-dependent catalysis, the present study was initiated using three different Co3O4 nanomaterials, nanoplates, nanorods and nanocubes, as model systems. According to HRTEM, the predominantly exposed planes of nanoplates, nanorods and nanocubes are {112}, {110} and {100} planes, respectively. The catalytic activities were explored by using H2O2 and different organic substrates as the substrates of peroxidase mimics, and were investigated in-depth by steady-state kinetics and electrochemistry methods in depth. The results show that the peroxidase-like activity increases from nanocubes to nanoplates, via nanorods. The effect of external conditions such as pH and temperature on the three nanomaterials is the same, which indicates that the difference in their catalytic activities originates from their different shapes. The peroxidase-like catalytic activities of Co3O4 nanomaterials are crystal plane-dependent and follow the order: {112} ≫ {110} > {100}. The three crystal planes have different arrangements of surface atoms, thus exhibiting different abilities of electron transfer, which induce their different peroxidase-like catalytic activities. This investigation clarifies that the peroxidase-like activity of Co3O4 nanomaterials can be enhanced by shape control. These findings show that Co3O4 nanomaterials can serve as catalyst models for designing other catalysts.

Catalase mimic property of Co3O4 nanomaterials with different morphology and its application as a calcium sensor.

The applications of inorganic nanomaterials as biomimetic catalysts are receiving much attention because of their high stability and low cost. In this work, Co3O4 nanomaterials including nanoplates, nanorods, and nanocubes were synthesized. The morphologies and compositions of the products were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The catalytic properties of Co3O4 nanomaterials as catalase mimics were studied. The Co3O4 materials with different morphology exhibited different catalytic activities in the order of nanoplates > nanorods > nanocubes. The difference of the catalytic activities originated from their different abilities of electron transfer. Their catalytic activities increased significantly in the presence of calcium ion. On the basis of the stimulation by calcium ion, a biosensor was constructed by Co3O4 nanoplates for the determination of calcium ion. The biosensor had a linear relation to calcium concentrations and good measurement correlation between 0.1 and 1 mM with a detection limit of 4 µM (S/N = 3). It showed high selectivity against other metal ions and good reproducibility. The proposed method was successfully applied for the determination of calcium in a milk sample.

Spectroscopic studies on unfolding processes of apo-neuroglobin induced by guanidine hydrochloride and urea.

Neuroglobin (Ngb), a recently discovered globin, is predominantly expressed in the brain, retina, and other nerve tissues of vertebrates. The unfolding processes of apo-neuroglobin (apoNgb) induced by guanidine hydrochloride (GdnHCl) and urea were investigated by spectroscopic methods. In the unfolding processes, apoNgb's tertiary structural transition was monitored by the changes of intrinsic fluorescence emission spectra, and its secondary structural transition was measured by the changes of far-ultraviolet circular dichroism (CD) spectra. In addition, 8-anilino-1-naphthalenesulfonic acid (ANS), a hydrophobic cluster binding dye, was also used to monitor the unfolding process of apoNgb and to explore its intermediates. Results showed that GdnHCl-induced unfolding of apoNgb was via a three-state pathway, that is, Native state (N) → Intermediate state (I) → Unfolded state (U), during which the intermediate was inferred by an increase in fluorescence intensity and the change of CD value. Gibbs free energy changes are 10.2 kJ · mol(-1) for the first unfolding transition and 14.0 kJ · mol(-1) for the second transition. However, urea-induced unfolding of apoNgb only underwent a two-state transition: Native state (N) → Partially unfolded state (P). The result showed that GdnHCl can efficiently affect the conformational states of apoNgb compared with those of urea. The work will benefit to have an understanding of the unfolding mechanism of apoNgb induced by GdnHCl and urea.

Intrinsic peroxidase-like activity and catalase-like activity of Co3O4 nanoparticles.

We demonstrate that Co(3)O(4) nanoparticles (NPs) exhibit intrinsic peroxidase-like activity and catalase-like activity. The peroxidase-like activity of the Co(3)O(4) NPs originates from their ability of electron transfer between reducing substrates and H(2)O(2), not from ˙OH radical generated. As peroxidase mimetics, Co(3)O(4) NPs were used for colorimetric determination of H(2)O(2) and glucose.

Spectroscopic study on acid-induced unfolding and refolding of apo-neuroglobin.

pH-induced unfolding and refolding of apo-neuroglobin (apo-Ngb) were investigated by UV, fluorescence, circular dichroism (CD) spectra and light scattering measurements. Results revealed that apo-Ngb became partially unfolded at around pH 5.0, with evidences from a red shift in the fluorescence spectra, a decrease in the far-UV CD and a sharp peak in the light scattering intensity. Further lowering of the pH reversed these effects, suggesting that apo-Ngb folds back to a compact state. At pH 2.0, the apo-Ngb forms a folding intermediate known as molten globule (MG), which is possessed of native-like secondary structure and almost complete loss of tertiary structure. Based on these results, the acid-induced denaturation pathway of apo-Ngb can be illustrated from the native state (N), via a partially unfolded state (U(A)) to the molten globule state (MG).