Singlet Oxygen Formation Mechanism for the H2O2-Based Fenton-like Reaction Catalyzed by the Carbon Nitride Homojunction.

The singlet oxygen (1O2) oxidation process activated by metal-free catalysts has recently attracted considerable attention for organic pollutant degradation; however, the 1O2 formation remains controversial. Simultaneously, the catalytic activity of the metal-free catalyst limits the practical application. In this study, carbon nitride (HCCN) containing an intramolecular homojunction, a kind of metal-free catalyst, exhibits excellent activity compared to g-C3N4 (CN) and crystalline carbon nitride (HCN) for tetracycline hydrochloride degradation through the H2O2-based Fenton-like reaction. The rate constant for HCCN increased about 16.1 and 8.9 times than that of CN and HCN, respectively. The activity of HCCN was enhanced, and the dominant reactive oxygen species (ROS) changed from hydroxyl radicals (•OH) to 1O2 with an increase in pH from 4.5 to 11.5. A novel formation pathway of 1O2 was revealed. This result is different from the normal reference, in which •OH is always the primary ROS in the H2O2-based Fenton-like reaction. This study may provide a possible strategy for the investigation on the nonradical oxidation process in the Fenton-like reaction.

Enhancement of the peroxidase-like activity of hollow spherical FexNi1-xS2/SC nanozymes.

Artificial nanozymes have been receiving considerable interest for their outstanding performance and wide application. However, their low activity results in a high concentration of substrates, costs, and environmental pollution. To enhance nanozymic activity, a composite, FexNi1-xS2/hollow carbon spheres (FexNi1-xS2/SC), was facilely synthesized by a solvothermal method. The response surface methodology (RSM) was used to optimize the Ni content in FexNi1-xS2/SC and the experimental conditions, where Fe0.75Ni0.25S2/SC exhibited the highest activity. The Km (Michaelis-Menten's constant) values of Fe0.75Ni0.25S2/SC are 0.025 and 0.021 mM with H2O2 and oxidized 3,3',5,5'-tetramethylbenzidine (TMB) as the substrates, respectively, which are 148 times and 20.5 times lower than those with HRP, 1.88 and 7.19 times lower than those of FeS2/SC, and 1.88 and 10.52 times lower than those of Fe0.8Ni0.2S2, meaning a strong affinity of Fe0.75Ni0.25S2/SC for the substrate. The catalytic efficiency (Kcat/Km) of Fe0.75Ni0.25S2/SC was 5.4 (H2O2) and 27.4 times (TMB), and 9.7 (H2O2) and 66.2 times (TMB) higher than those of FeS2/SC and Fe0.8Ni0.2S2, respectively. The effects of the synergistic interaction between Fe and Ni, the S-C bond formation, and the hollow carbon spheres on the activity were studied. A nanozymic mechanism was proposed. Fe0.75Ni0.25S2/SC could be used to detect cysteine (Cys) at room temperature in 1 min with a detection limit (LOD) of 0.049 µM.

Tuning Band Gap in Fe-Doped g-C3N4 by Zn for Enhanced Fenton-Like Catalytic Performance.

Multiple oxidation states of first-row transition-metal cations were always doped in g-C3N4 to enhance the catalytic activity by the synergistic action between the cations in the Fenton-like reaction. It remains a challenge for the synergistic mechanism when the stable electronic centrifugation (3d10) of Zn2+ was used. In this work, Zn2+ was facilely introduced in Fe-doped g-C3N4 (named xFe/yZn-CN). Compared with Fe-CN, the rate constant of the tetracycline hydrochloride (TC) degradation increased from 0.0505 to 0.0662 min-1 for 4Fe/1Zn-CN. The catalytic performance was more outstanding than those of similar catalysts reported. The catalytic mechanism was proposed. With the introduction of Zn2+ in 4Fe/1Zn-CN, the atomic percent of Fe (Fe2+ and Fe3+) and the molar ratio of Fe2+ to Fe3+ at the catalyst's surface increased, where Fe2+ and Fe3+ were the active sites for adsorption and degradation. In addition, the band gap of 4Fe/1Zn-CN decreased, leading to enhanced electron transfer and conversion from Fe3+ to Fe2+. These changes resulted in the excellent catalytic performance of 4Fe/1Zn-CN. Radicals •OH, •O2-, and 1O2 formed in the reaction and took different actions under various pH values. 4Fe/1Zn-CN exhibited excellent stability after five cycles under the same conditions. These results may give a strategy for synthesizing Fenton-like catalysts.

Peroxidase-like activity of hollow sphere-like FeS2/SC boosted by synergistic action of defects and S-C bonding.

Pyrite FeS2 has been applied as a peroxidase due to its easy preparation and low cost. However, the low peroxidase-like (POD) activity limited its wide application. A hollow sphere-like composite (FeS2/SC-5.3%) composed of pyrite FeS2 and sulfur-doped hollow sphere-shaped carbon was synthesized by a facile solvothermal method, where the S-doped carbon was in situ formed during FeS2 formation. The synergistic action such as the defects at the carbon surface and the formation of S-C bonding improved the nanozyme activity. The S-C bonding was a bridge between the carbon and the Fe atom in FeS2, which enhanced the electron transfer between the Fe atom and the carbon and accelerated the conversion from Fe3+ to Fe2+. The optimum experimental conditions were obtained by the response surface methodology (RSM). The POD-like activity of FeS2/SC-5.3% was significantly improved compared to that of FeS2. The Michaelis-Menten constant (Km) of FeS2/SC-5.3% is 80 times lower than that of horseradish peroxidase (HRP, natural enzyme). FeS2/SC-5.3% can be used to detect cysteine (Cys) with a limit of detection (LOD) as small as 0.061 µM at room temperature in only 1 min.

2D/3D-Shaped Fe0.8Ni0.2S2/ZIF-67 as a Nanozyme for Rapid Measurement of H2O2 and Ascorbic Acid with a Low Limit of Detection.

Metal-organic frameworks (MOFs) can be used as ideal artificial nanozymes for an open structure to transfer substances and products. The nanozymic mechanism of MOFs needs further investigation for wide application. In this manuscript, no peroxidase-like activity was found for ZIF-67 (a kind of MOF), however, it enhanced the peroxidase-like activity of the Fe0.8Ni0.2S2/ZIF-67 composite, in which Fe0.8Ni0.2S2 was the active composition. The catalytic constant (Kcat) is higher at 1.98 (for TMB) and 2.08 (for H2O2) times and the catalytic efficiency (Kcat/Km) is higher at 3.13 (for TMB) and 2.67 (for H2O2) times, than those of Fe0.8Ni0.2S2. The Km value decreased about 85 times (for H2O2) for Fe0.8Ni0.2S2/ZIF-67 than that of horseradish peroxidase (natural enzyme). The mechanism was proposed. The limit of detection of H2O2 for Fe0.8Ni0.2S2/ZIF-67 is 0.039 µM, which is lower by about 18.2 times than that of Fe0.8Ni0.2S2. Simultaneously, Fe0.8Ni0.2S2/ZIF-67 was used to rapidly detect ascorbic acid for only 1.0 min in food monitoring. This study may be important to design a new kind of nanozyme.

Synergetic adsorption and photo-Fenton degradation of methylene blue by ZnFe2O4/SiO2 magnetic double-mesoporous-shelled hollow spheres.

Adsorption and Fenton technologies have been widely employed to deal with wastewater. ZnFe2O4/SiO2 magnetic double-mesoporous-shelled hollow spheres (MDSHSs) were feasibly synthesized by a solvothermal method. The as-synthesized MDSHSs show excellent adsorption and selectivity for methylene blue (MB), which it took about only 1 min to reach the adsorption equilibrium. About 50% MB was removed by adsorption, and other 50% MB was degraded under further photo-Fenton process. Effects of experimental conditions on the adsorption and photo-Fenton process were investigated. The mechanisms of MDSHSs formation and photo-Fenton process were proposed. Total organic carbon (TOC) reduction reached as high as 90% with 60 mg/L of MB for 90 min. The experimental results indicated that MDSHSs exhibit a remarkable adsorption and catalytic activity for photo-Fenton process in a wide pH range of 3.3-11.0. Simultaneously, the composite shows an excellent stability and reusability.

Facile synthesis of Cu-CuFe2O4 nanozymes for sensitive assay of H2O2 and GSH.

Artificial enzymes have drawn substantial research interest from the scientific community due to their advantages over natural enzymes. However, majority of artificial enzymes exhibit low affinity towards H2O2, which means that a high H2O2 concentration is needed for the oxidation of a substrate such as 3,3',5,5'-tetramethylbenzidine (TMB) to blue-colored oxTMB. With this concern, Cu-CuFe2O4 was facilely synthesized, wherein, Cu0 accelerates the redox capacity of Cu-CuFe2O4 as well as the electron transfer between CuFe2O4 and H2O2. These materials induce excellent activity as a peroxidase. Cu-CuFe2O4 shows high affinity towards H2O2 with lower Michaelis-Menten constant (Km) than the reported values for ferrites and Horseradish enzyme (HRP). Moreover, it took only 5 min to detect hydrogen peroxide (H2O2) and glutathione (GSH) through a colorimetric assay using Cu-CuFe2O4. Compared with CuFe2O4, the limit of detection (LOD) is about 90-fold lower for H2O2 using Cu-CuFe2O4. In addition, Cu-CuFe2O4 shows high stability as a nanozyme. Thus, the mechanism of the peroxidase-like nanozyme Cu-CuFe2O4 is proposed.

Fabrication of FeS2/SiO2 Double Mesoporous Hollow Spheres as an Artificial Peroxidase and Rapid Determination of H2O2 and Glutathione.

Nanozymes as one of artificial enzymes show many advantages than natural enzymes. The high Michaelis-Menten constant (Km) to H2O2 is the drawback for nanozymes, which means a high H2O2 concentration to oxidize 3,3',5,5'-tetramethylbenzidine (TMB). For this problem, FeS2/SiO2 double mesoporous hollow spheres (DMHSs) were first synthesized as an artificial peroxidase through a solid reaction. The experimental results demonstrate that Fe3O4 vulcanization and DMHS formation were effective strategies to enhance affinity to H2O2 for the nanozyme. The Km of FeS2/SiO2 DMHSs (H2O2 as the substrate) is 18-fold smaller than that of FeS2 nanoparticles (NPs). The catalytic efficiency (Kcat/Km) of FeS2/SiO2 DMHSs is about 16 times higher than that of FeS2 NPs. FeS2/SiO2 DMHSs can be used as a nanozyme to sensitively and rapidly detect H2O2 and glutathione within 1 min at room temperature.

Porous 2D FeS2 nanosheets as a peroxidase mimic for rapid determination of H2O2.

Considering the threat of H2O2 to human health and environmental security, it is of great significance to develop rapid, accurate and sensitive analytical methods for H2O2 detection. In this work, porous regular hexagonal-shaped FeS2 nanosheets (NSs) with a side length of about 1 µm are employed as a peroxidase mimic to detect H2O2. The Km of the FeS2 NSs for H2O2 (0.00342 mM) is much lower than actual enzyme HRP (3.7 mM). The FeS2 NSs exhibit high sensitivity in linear range of 0.02-4.00 µM of H2O2 with a limit of detection (LOD) of 7.60 nM (S/N = 3). The Fe ion in the FeS2 NSs is the active site. No obvious effect on the determination was found when K+, NH4+, Ni2+, Mn2+, Cu2+, Zn2+, Al3+, Ba2+, Ca2+, NO3-, glucose and sucrose were added in the solution, respectively. The FeS2 NSs were also applied to rapidly detect H2O2 concentrations in different actual samples, such as lens solution, beer and disinfectant.

Facile Synthesis of 3D-Structured NiS as an Excellent Fenton-like Catalyst under Various pH through Different Mechanisms.

Two-dimensional (2D) layer-structured transition-metal sulfide shows excellent Fenton catalytic properties, which has attracted much attention. However, the release of metal cations from the catalyst induces secondary environmental pollution. In the present study, 3D-constructed NiS assembled by 2D nanosheets was synthesized by a hydrothermal method. The NiS displayed high levels of Fenton catalytic activity for the degradation of rhodamine B (RhB) under a wide pH range from 5.60 to 12.21. However, Ni2+ was released in acidic solutions, and the released Ni2+ decreased with increasing pH. The catalytic mechanisms in acidic and alkaline solutions were explored. The high Fenton catalytic activity benefits from the synergistic effect of the Ni2+ and the S2- at pH = 5.60. At pH = 12.21, the S atom accelerated the electron transfer between the Ni(III) and Ni(II) at the NiS surface. The material kept the same catalytic activity for many cycles. These results demonstrate that NiS can be used as a catalyst for the Fenton process in alkaline conditions.

In situ microcalorimetry study of ZnFe2O4 nanoparticle formation under solvothermal conditions.

Solvothermal methods have been widely used to synthesize different kinds of materials. However, only little is known about how precursor solutions react to form solid precipitates via this method. In the present study, in situ microcalorimetry is first used to investigate the formation mechanism under solvothermal conditions, where ZnFe2O4 synthesized using a solvothermal method was selected as the model sample. Some novel results are obtained, such as with the experimental temperature increase, (1) the homogeneous solution transforms to a gel containing amorphous Fe2(C2H4O2)3 and ZnC2H4O2, and NaNO3 crystals; (2) the gel dissolves; (3) α-(Fe,Zn)OOH and α-Fe2O3 are synthesized; (4) the α-(Fe,Zn)OOH transforms to α-Fe2O3; (5) Fe(2+) is formed at about 159 °C, which acts as a catalyst for the formation of Fe3O4; (6) the Fe3O4 crystals are synthesized at about 200 °C; (7) the Fe3O4 is transformed to the ZnFe2O4 with the help of NO3(-), and the reaction was kept at 200 °C for about 20 h. This study shows a facile in situ method for the investigation of reaction processes of solvothermal methods.

Magnetic and thermodynamic properties of nanosized Zn ferrite with normal spinal structure synthesized using a facile method.

Normal spinel zinc ferrite (ZnFe2O4) nanoparticles (NPs) with zero net magnetization were synthesized by a facile coprecipitation method in which two kinds of organic alkali, namely, 1-amino-2-propanol (MIPA) and bis(2-hydroxypropyl)-amine (DIPA), were used. The diameters of the ZnFe2O4 NPs were determined to be about 7 and 9 nm for samples prepared with MIPA and DIPA, respectively, and the normal spinel structure was confirmed by the magnetic property measurement at room temperature and the temperature dependence of the direct current magnetization. These results are different from those reported in the literature, where ZnFe2O4 NPs show a nonzero net magnetization. The heat capacity of the ZnFe2O4 NPs synthesized using DIPA was measured using a physical property measurement system in the temperature range from 2 to 300 K, and the thermodynamic functions were calculated based on the curve fitting of the experimental heat capacity data. The heat capacity of the ZnFe2O4 NPs was compared with that of a nanosized (Zn(0.795)Fe(0.205))[Zn(0.205)Fe(1.795)]O4 material studied in the literature, indicating that the Debye temperature of the present sample is more comparable with that of the bulk ZnFe2O4 reported by Westrum et al.

Synthesis of Ce3+ doped ZnFe2O4 self-assembled clusters and adsorption of chromium(VI).

A solvothermal synthetic route was used to prepare Ce(3+) doped Zn ferrites, where sphere-like clusters aggregated by nanosized particles were fabricated. The size of the cluster and the saturation magnetization of the sample are decreasing with the increase of Ce(3+). These samples can be easily separated from aqueous solutions by applying a magnetic field and have a high loading capacity of Cr(VI). The Cr(VI) adsorption experiments indicated that the adsorption was divided into two processes, in which the first one took place about 6h, the second one took place between 6 and 96 h. The maximum adsorption capacity for Cr(VI) was determined to be 57.24 mg/g. Langmuir model was employed to fit the adsorption isotherm, which implied the single layer adsorption. The data of SBET, external area and porous area of the samples can be used to explain these adsorption processes. And the Ce(3+) ions doped in the sample induced the increasing adsorption capacity of Cr(VI). The adsorption process can be described by the pseudo-second-order kinetic model.

Formation of self-assembled nanofiber-like Ag@PPy core/shell structures induced by SDBS.

Core-shell structured Ag@PPy nano-particles (NPs) were prepared by a facile method through a redox reaction, where silver nitrate and pyrrole were employed as reactants, sodium dodecyl benzyl sulfonate (SDBS) was used as a template to induce the formation of nanofiber-like Ag@PPy core-shell structure. Formation processes of this kind of structure were proposed. Firstly, sphere-like Ag@PPy structures were produced. Secondly, these sphere-like particles assembled to form nanofiber-like morphology. Effects of the concentration of SDBS on morphology of the sample were investigated. When the morphology of the samples changed from spherical to nanofibrous, the electrical conductivity of the sample increased from 11±2 to 165±5S/cm.

Size-controlled synthesis of rod-like α-FeOOH nanostructure.

One-dimensional goethite (α-FeOOH) nanorods were successfully fabricated by a hydrothermal route without any template. Experimental results reveal that concentrations of Fe(3+) and ethylenediaminetetraacetic disodium salt (Na2EDTA) affect the phase composition and size of the as-synthesized products. The size of the rod-like α-FeOOH increased when the concentration of Na2EDTA was increased, where Na2EDTA acts as a nucleation inhibitor. α-Fe2O3 nanoparticles were produced when the concentration of Fe(3+) was increased from 0.02 to 0.08 and 0.40 M. A possible formation mechanism was proposed based on the results of the time dependent experiments. Different electrolytes and surfactants can affect the size and the aspect ratio of the as-prepared nanorod-like α-FeOOH. Na2SO4 induced the decreasing of the size of the as-prepared sample. KCl and PVP affected the aspect ratio of the nanorods.

Preparation of stable magnetic nanofluids containing Fe3O4@PPy nanoparticles by a novel one-pot route.

Stable magnetic nanofluids containing Fe3O4@Polypyrrole (PPy) nanoparticles (NPs) were prepared by using a facile and novel method, in which one-pot route was used. FeCl3·6H2O was applied as the iron source, and the oxidizing agent to produce PPy. Trisodium citrate (Na3cit) was used as the reducing reagent to form Fe3O4 NPs. The as-prepared nanofluid can keep long-term stability. The Fe3O4@PPy NPs can still keep dispersing well after the nanofluid has been standing for 1 month and no sedimentation is found. The polymerization reaction of the pyrrole monomers took place with Fe3+ ions as the initiator, in which these Fe3+ ions remained in the solution adsorbed on the surface of the Fe3O4 NPs. Thus, the core-shell NPs of Fe3O4@PPy were obtained. The particle size of the as-prepared Fe3O4@PPy can be easily controlled from 7 to 30 nm by the polymerization reaction of the pyrrole monomers. The steric stabilization and weight of the NPs affect the stability of the nanofluids. The as-prepared Fe3O4@PPy NPs exhibit superparamagnetic behavior.

Controlling the polymorph and morphology of CaCO3 crystals using surfactant mixtures.

Inspired by mineralization in biological organisms, fabrication of higher ordered CaCO(3) crystals modified by surfactants has received much attention. In our present work, mixed surfactants of hexadecyl(trimethyl)azanium bromide and sodium dodecyl sulfate were used to mediate the nucleation and growth of CaCO(3) crystals. When the concentration of sodium dodecyl sulfate in the solution is constant (0.1 mM), the polymorph of CaCO(3) crystals changed from pure vaterite to pure aragonite with increase of the ratio of hexadecyl(trimethyl)azanium bromide to sodium dodecyl sulfate. Various morphologies of vaterite and aragonite were obtained. Based on the time-resolved experiments, we suggest that the flower-like CaCO(3) formed via aggregation of hexagon-like vaterite induced by the surfactants. More importantly, we realized that a cluster-shaped morphology of aragonite was produced through the nucleation of aragonite at the surfaces of the hexagon-like vaterite.

Structure transition from aragonite to vaterite and calcite by the assistance of SDBS.

Structure transition from aragonite to vaterite and calcite with the help of anionic surfactant sodium dodecyl benzene sulfonate (SDBS) was investigated, respectively, by a hydrothermal method. When the experimental temperature was controlled at 90 degrees C, aragonite of crystal calcium carbonate was transformed into vaterite with the assistance of SDBS. Pure vaterite was obtained as the concentration of SDBS reaches to 2.5 mM. When the experimental temperature was controlled at 120 and 150 degrees C, respectively, aragonite was transformed into calcite, and pure calcite was obtained as the concentrations of SDBS were equal to 1.0 and 2.5 mM, respectively. Possible formation mechanism of different CaCO(3) polymorphs was proposed based on the obtained experimental results.

A novel morphology of aragonite and an abnormal polymorph transformation from calcite to aragonite with PAM and CTAB as additives.

The abnormal structure conversion of CaCO3 from calcite to aragonite was investigated with a mixture of polyacrylamide (PAM) and cetyltrimethylammonium bromide (CTAB) as additives by a hydrothermal method. A novel morphology of aragonite, "magnified" cubic shape, was observed. In order to investigate the effects of PAM and experimental temperature on the morphology and phase of CaCO3, the samples were synthesized without any additives and with PAM as template at 90 and 120 degrees C, respectively. The results indicate that both the interaction between the mixed template and the CaCO3 and the elevated experiment temperature play important roles in the process of polymorph transformation.