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Keywords:

  • Iron;
  • Gold;
  • Nanostructures;
  • Electronic structure;
  • Peroxidase mimics;
  • Enzyme catalysis

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Sphere-like Fe3O4 aggregates were solvothermally prepared with ethylene glycol, sodium acetate and FeCl3·6H2O as raw materials. The sphere-like Fe3O4 aggregates provided heterogeneous growth sites for Au nanoparticles. These were obtained by reduction of HAuCl4 by sodium citrate under mild reaction conditions and the Fe3O4-Au nanocomposites were subsequently formed. The peroxidase-like activity of nanocomposites was studied with H2O2 and 3,3′,5,5′-tetramethylbenzidine as substrates. Fe3O4-Au nanocomposites exhibited better catalytic activity than pure Fe3O4 aggregates, mainly resulting from the special electronic structure at the interfaces between the sphere-like Fe3O4 aggregates and the gold nanoparticles.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

The properties of the nanomaterials generally depend on their size, morphology, components and structure, and so an effective way to improve the properties of the materials is by expanding from single to multiple components.1 Multicomponent materials can lead to multifunctionality. For example, a Co/CoSe core shell structure exhibits magnetic and optical properties and can be potentially applied as an optical reporter and a magnetic handle for bioassays.2 New functions will be produced from the interactions between different components. Gold nanoparticles deposited on a TiO2 substrate exhibited enhanced catalytic activity in the oxidation of carbon monoxide which was due to the synergistic effects between gold and metal oxide substrate.3 Thus, synthesis of multicomponent materials is still attracting the attention of researchers.

Peroxidase is one kind of redox enzyme widely found in an organism. It mainly catalyzes the decomposition of peroxides and the oxidation of some substrates. Horseradish peroxidase (HRP) is a natural peroxidase and is generally extracted from plants but it is very expensive, difficult to store and can easily become inactive.4 Therefore, searching for proper peroxidase mimetics is a great challenge for researchers seeking to understand enzymatic reactions. A series of peroxidase-like nanomaterials has been reported containing materials such as Fe3O4,5,6 graphene oxide,7 Au or Au@Pt nanocomposites,8,9 AgM(M = Pt, Au or Pd),10 single-walled carbon nanotubes,11 FeS,12 CeO2,13 Cu2O,14 Co3O415 and so on. Besides the peroxidase-like activity of Fe3O4 nanoparticle, its magnetic properties ensure it can be easily removed by using a magnet. However, the serious loss of Fe3O4 nanocrystals hinders its application though it shows high catalytic activity for the first time.[5,6,8,16,17] Bovine serum albumin (BSA) stabilized Au clusters exhibit highly intrinsic peroxidase-like activity and could be used over a wide range of pH values and temperatures. However, gold nanoparticles tend to agglomerate, resulting in a decrease in catalytic activity.8 Based upon the above considerations, Fe3O4 nanostructures aggregated by small nanocrystals not only retain the high peroxidase-like activity of small nanocrystals but also decrease losses during the magnetic separation process. In addition, the agglomeration of gold nanoparticles can be prevented by locating them on the surface of Fe3O4 nanostructures.

Sphere-like Fe3O4 aggregates were synthesized with a facile solvothermal route and subsequently the aggregates provided heterogeneous growth sites for gold nanoparticles to form Fe3O4-Au nanocomposites. The peroxidase-like activity of the nanocomposites was characterized, and the composites presented higher activity than pure Fe3O4 or gold nanoparticles because of their special electronic and surface structures.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

With ethylene glycol as a solvent, and FeCl3·6H2O and CH3COONa as raw materials, the sphere-like aggregates were solvothermally prepared at 200 °C over 10 h. From the XRD patterns (Figure 1c), all the diffraction peaks were indexed to those of inverse spinel magnetite (JCPDS no. 19–0629) which are the (220), (311), (440), (422), (511), (400), (620) and (533) reflections. There are no diffraction peaks from other impurities, indicating the pure phase of the product. According to Scherrer line width analyses on the (311) reflection, the crystalline size of the sample was estimated to be ca. 21.3 ± 0.7 nm. From the TEM image shown in Figure 1a, the products are typical aggregates with their size mainly within 200–240 nm. From the HR-TEM image (Figure 1b), it is difficult to identify a single particle. The continuously parallel lattice fringes with contrast and uniform direction indicate the crystallinity and the oriented aggregation of several nanoparticles which is consistent with reports in the literature.18 The lattice spacing of 0.42 nm corresponds to the {200} interplanar spacing of Fe3O4.

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Figure 1. TEM (a) and HR-TEM (b) images and XRD pattern (c) of Fe3O4 obtained by a solvothermal reaction.

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In the preparation of Fe3O4-Au nanocomposite, the prepared Fe3O4 aggregates, PVP, HAuCl4 and sodium citrate were used as the raw materials with deionized water as the solvent while the reaction temperature was controlled at 45 °C by using a water bath. The mild reaction conditions prevented the homogeneous nucleation of gold nanoparticles, for Au ions could be easily reduced. The well-dispersed Fe3O4 aggregates provided the growth sites for Au NPs and sodium citrate acted as reductant in the formation of Au NPs. As shown in Figure 2a, gold nanoparticles with a size of ca. 20–40 nm were successfully deposited on the surface of Fe3O4 aggregates. There are no separate gold nanoparticles, demonstrating that the homogeneous nucleation of gold nanoparticle was successfully prevented in the preparation and this is mainly due to the mild reaction conditions and the weak reducing ability of citrate. If there is no PVP in the system, the final products are prone to agglomerate, less gold nanoparticles are located on the surface of Fe3O4 aggregates and the size of gold nanoparticles is ca. 80 nm (Figure S1). PVP plays three roles in the synthesis of Au-Fe3O4 nanocomposites. First, it improves the dispersity of sphere-like Fe3O4 aggregates. Second, N-containing functional groups of PVP make it available to Au nanoparticles. Additionally, it is used as capping agent to control the size of gold nanoparticles. From Figure 2a, it seems that the Au nanoparticles are nearly spherical and careful observation of the HR-TEM image (Figure 2b) indicates that it has a threefold axis oriented parallel to the electron beam. The lattice spacings of 0.21 nm and 0.42 nm are consistent with Au (200) and Fe3O4 (200) interplanar spacings. From the XRD patterns (Figure 2c), relative to the Fe3O4 diffraction peaks (*), the products shows several additional diffraction peaks (·) which are characteristics of face centred cubic Au (JCPDS no.04–0784). According to the Scherrer equation and the full width at half maximum (FWHM) of Au (111), the gold crystalline size in the composites is estimated ca. 16.5 ± 0.3 nm. This is smaller than that observed by TEM which further demonstrates the twin crystal nature of the gold nanoparticles. The TEM and XRD analyses of the products confirm the formation of Fe3O4-Au composites. After the composites were dissolved in aqua regia and diluted with distilled water, the concentrations of iron and gold ions were determined by ICP. The gold content in the prepared composites is 14 wt.-%, which is higher than the theoretical value (10.7 wt.-%) calculated in the added materials and this might be due to the loss of Fe3O4 in the preparation process.

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Figure 2. TEM (a) and HR-TEM (b) images and XRD pattern (c) of Fe3O4-Au nanocomposites.

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FTIR and TG analysis were used to investigate the surface state of the products. As to the IR spectrum of the prepared Fe3O4 sample shown in Figure 3a, the absorptions at 3426 cm–1 and 1643 cm–1 can be ascribed to the stretching and bending vibrations of surface hydroxyl groups. The absorption at 1384 cm–1 is due to the carboxylic vibration of acetate and that at 1043 cm–1 was assigned to the vibration of the C–O bond in acetate.19 The strong vibration at 574 cm–1 is the characteristic Fe–O vibration of magnetite.20 When gold nanoparticles were deposited on magnetite, most of the IR vibration remained unchanged, indicating Fe3O4-Au composites possess similar surface groups to Fe3O4. Further IR analysis of pure Au nanoparticles prepared under the same conditions as those for preparing the composite revealed the presence of PVP on the surface of Au nanoparticles (Figure S2). In addition, the Fe–O vibration shows blue shifts to 586 cm–1, demonstrating that gold could influence the electron structure of Fe3O4. From the TG curve (Figure 3b), the gain from room temperature to 250 °C is due to the oxidation of Fe3O4 and the subsequent weight loss (ca. 0.7 wt.-%) can be ascribed to the decomposition of adsorbed acetate and the removal of hydroxyl functionalities.21 The TG curve of Fe3O4-Au nanocomposites differs little from that of sphere-like Fe3O4 aggregates. The loss (<150 °C, ca. 0.42 wt.-%) is due to the removal of adsorbed water and the weight gain (150–250 °C, ca. 0.46 wt.-%) results from the oxidation of Fe3O4.22 As the temperature increases, the decomposition of the adsorbed acetate and the loss of the bonded hydroxy groups results in a total loss of 1.7 wt.-%. From the analysis of TG curves, the result is consistent with that of IR analysis.

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Figure 3. IR spectra (a) and TG curves (b) of the samples.

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Because the XRD patterns of γ-Fe2O3 and Fe3O4 are similar, TG analyses under both air and nitrogen atmospheres were performed to study whether Fe3O4 was oxidized during the preparation and drying processes. As for the TG curve obtained under a nitrogen atmosphere shown in Figure S3, the loss below 200 °C (ca. 1.26 wt.-%) is due to the surface adsorbed water. The subsequent loss from 250 °C to 500 °C (ca. 3.14 wt.-%) results from removal of acetate and hydroxy groups bonded on the surfaces of the nanoparticles.23 In theory, the weight gain in the transformation from Fe3O4 to γ-Fe2O3 is 3.45 wt.-% and the corresponding theoretical weight gain for the composites should be 2.92 wt.-% considering the gold content. For the prepared nanocomposites, the weight loss difference under air and nitrogen atmospheres is 2.91 wt.-%, which is consistent with the theoretical value (2.92 wt.-%), demonstrating that the nanocomposites were not oxidized during the preparation and storage.

The surface electron structures of sphere-like Fe3O4 aggregates and Fe3O4-Au nanocomposites were determined from XPS patterns. As for the sphere-like Fe3O4 aggregates, the signals at 711.86 and 725.46 eV correspond to Fe2p3/2 and Fe2p1/2 levels while the signal at 530.89 eV correlates to the binding energy of O1s, which is consistent with Fe3O4 data reported in the literature.24 The absence of a satellite peak around 718 eV indicates the pure phase of Fe3O4 without γ-Fe2O3.25,26 After deposition of gold nanoparticles on the sphere-like Fe3O4 aggregates, the Fe2p3/2, Fe2p1/2 and O1s binding energies increased to 711.99, 726.99 and 532.23 eV, respectively (Figure 4a,b). The above phenomena indicate the strong electron interaction between Fe3O4 and gold nanoparticles. Generally, the binding energy of metallic Au4f7/2 is 84.00 eV.27 As for the prepared nanocomposites (Figure 4c), the Au4f7/2 binding energy is 83.21 eV which is lower than the reported value. It was reported that for Pt-Fe3O4 nanocomposites, the decrease in the Pt4f binding energy could be ascribed to the electron transferred from Fe3O4 to Pt.28 From Figure 4, the Fe2p1/2, Fe2p3/2 and O1s binding energies of Fe3O4-Au nanocomposites are higher than that of pure Fe3O4 which might result from the electron loss from Fe3O4. Therefore, the negative shift in the Au4f binding energy of the prepared nanocomposites is due to electron transfer from Fe3O4 to Au which is consistent with that reported in the literature.29,30 Comparison of XPS spectra between Fe3O4 aggregates and Fe3O4-Au nanocomposites demonstrates that the deposition of gold nanoparticles on sphere-like aggregates affected the electron structure of Fe3O4 and this is consistent with IR results.

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Figure 4. XPS patterns of sphere-like Fe3O4 aggregates and Fe3O4-Au nanocomposites. (a) Fe2p, (b) O1s, (c) Au4f.

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The M–H loops of sphere-like Fe3O4 aggregates and Fe3O4-Au nanocomposites were also measured at room temperature (Figure 5). The specific saturated magnetizations (Msat) are 85.8 and 50.9 emu g−1 for Fe3O4 and Fe3O4-Au nanocomposites. The decrease in Msat for the nanocomposites mainly results from the introduction of gold.31 In the low-field region, the hysteresis loops exist (inset in Figure 5), indicating ferromagnetic behaviour of the samples. The coercivity, Hc, for Fe3O4 aggregates and Fe3O4-Au was determined to be 33.5 and 56 Oe. In general, the surface spin canting and disorder of the nanoparticles could affect their magnetic properties.32 As for the prepared nanocomposite, its surface effect is the main factor that induces the Msat decrease.33 From the above analysis, sphere-like Fe3O4 aggregates and Fe3O4-Au nanocomposites exhibit a good magnetic response so the samples can be easily collected by a magnet and this favours recycling in practical applications.

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Figure 5. The M–H curves (a) and the magnified M–H curves in the low magnetic field (b) for sphere-like Fe3O4 aggregates and Fe3O4-Au nanocomposites.

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Enzymes that are a type of biocatalyst can catalyze special chemical reactions. The structure of most enzymes is that of a protein so that in the catalytic reaction the enzyme always exhibits specificity and they are highly efficient. However, their lack of stability means activation can easily be lost in a harsh chemical environment (e.g. nonphysiological pH, high temperature or in the presence of inhibitors).5,34 Therefore, artificial enzymes have attracted widespread attention for their stability and high efficiency over a wide range of the temperatures and acidity levels. Since Fe3O4 was reported to have peroxidase-like activity, it has been utilised as an artificial enzyme to catalyze and detect some molecules.17,35 In the presence of H2O2, Fe3O4 nanoparticles react with horseradish peroxidase [such as 3,3′-diaminobenzidine tetrahydrochloride (DAB) or TMB] to form a product similar to peroxidase and this then exhibits peroxidase-like catalytic activity.

In practical measurements, peroxidase catalyzes the oxidation of the substrates to develop a colour change in the presence of H2O2. TMB is commonly used as a peroxidase substrate because it is colourless and oxidized by H2O2 very slowly. TMB and H2O2 were selected as the reaction configuration to evaluate the catalytic activity of the prepared Fe3O4-Au nanocomposites. To acquire an optimal response, the effects of reaction temperature and pH value on the catalytic activity of the samples were investigated (Figure S4). The temperature varied from 15 °C to 60 °C and the pH value from 2 to 12. From the relative catalytic activity, the optimized pH value and temperature are 4.0 and 30 °C. It has been reported that iron ions would not leach from Fe3O4 nanoparticles in a buffer solution (pH 4.0), so H2O2 catalyzed by iron ions were excluded.6 Also, the time-dependent catalytic activities of the samples were investigated under the optimized catalytic conditions. As shown in Figure S5, Fe3O4-Au nanocomposites exhibit higher catalytic activity than pure sphere-like Fe3O4 aggregates and the catalytic activity reaches a maximum within 36 min.

In order to further investigate the peroxidase-like activity of Fe3O4-Au nanocomposites, the Michaelis constant (Km) was obtained from the slope and the intercept of the extrapolated straight line with the horizontal axis in the Lineweaver–Burk plot. As a special constant for enzymes, Km is related to the properties of the enzyme, substrate or the enzymatic reaction condition (e.g. temperature, pH or inhibitor). Km represents the affinity to the substrate for a given enzyme. The smaller Km is, the stronger the affinity between the substrate and the enzyme.36 The strong affinity indicates that at a low substrate concentration the reaction rate would reach a maximum. By changing the concentration of the substrate (TMB and H2O2), the absorbance was measured within the same time. The Km values of Fe3O4-Au nanocomposites and Fe3O4 aggregates towards different substrates were obtained and are shown in Figure 6. The Km values of Fe3O4-Au and Fe3O4 with TMB as the substrate are 0.0106 and 0.179 mM, and with H2O2 as the substrate the corresponding Km values are 0.0344 and 0.0485 mM. The Km(TMB) and Km(H2O2) values of Fe3O4-Au nanocomposites are smaller than those of sphere-like Fe3O4 aggregates, demonstrating that the nanocomposites exhibit strong affinity towards TMB and H2O2 compared with pure Fe3O4 aggregates which might be a reason for the enhanced peroxidase-like activity. Compared with HRP, the Km(TMB) and Km(H2O2) values of which are 3.7 and 0.434 mM,5 the corresponding Km values for the synthesized Fe3O4 aggregates and Fe3O4-Au nanocomposites are small, indicating the catalytic reaction can reach the maximum rate at a low concentration of TMB or H2O2.15

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Figure 6. Michaelis relationships with TMB (a, c) and H2O2 (b, d) as substrates for Au-Fe3O4 (a, b) and Fe3O4 (c, d) nanocomposites.

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Form the above experimental results, the catalytic activity of sphere-like Fe3O4 is only 51.4 % of that for Fe3O4-Au nanocomposites under the same reaction conditions, indicating deposited gold improves the catalytic activity of Fe3O4. The Zeta potentials of Fe3O4-Au and Fe3O4 are –31.62 and –25.63 mV, indicating there are more negative charges on Fe3O4-Au nanocomposites than on the pure Fe3O4 aggregates. TMB, a polyamino compound, becomes protonated in acidic solution, so more negative charges on the surface of the catalyst favour the adsorption of TMB and then promote the catalytic reaction.

In order to further investigate the reason why the deposition of gold nanoparticles improved the catalytic activity, a series of experiments was conducted. As shown in Figure 7, the reaction solution is nearly colourless without catalyst after treatment for 15 min, demonstrating that TMB is slowly oxidized by H2O2. Gold nanoparticles were prepared with reduction of HAuCl4 by sodium citrate and acted as a catalyst with the same weight to that in Fe3O4-Au nanocomposites. Under the same catalytic conditions, the catalytic activity of gold nanoparticles is 21.3 % of that for Fe3O4-Au nanocomposites which indicates that the prepared gold nanoparticles have peroxidase-like activity but its catalytic activity is lower than that of Fe3O4. Similarly, when the mixture of sphere-like Fe3O4 aggregates and gold nanoparticles with the same content as the Fe3O4-Au nanocomposites was used as the catalyst, its catalytic activity was only 38.9 % of that of the nanocomposites. From the above analyses, Fe3O4 aggregates deposited by gold nanoparticles improve the peroxidase-like activity which might result from the special electron structure in the composites. As is known, the catalytic activity of the metal oxide nanoparticles is generally driven by transferring electrons between pairs of different oxidation states of the metal ions, such as Fe2+/Fe3+ and Ce3+/Ce4+,5,38 and the nature of the peroxidase-like activity of Fe3O4-Au may originate from its ability to catalyze the decomposition of H2O2 into ·OH radicals.5 The generated ·OH radicals might be stabilized by Fe3O4-Au nanocomposites by means of partial electron exchange interactions.39 It has been reported that dumbbell-like PtPd-Fe3O4 nanocomposites exhibited enhanced catalytic activity towards electrochemical reduction of H2O2 and the catalytic enhancement was proposed to arise from the partial electron transfer from Fe3O4 to PtPd at the nanoscale interface, improving H2O2 adsorption and activation.40 In the present system, the decorating gold nanoparticles changed the electron structure at the interface which may accelerate the electron transfer. Moreover, the partial electron transfer from Fe3O4 to Au facilitates H2O2 adsorption and activation. Thus, the interaction between gold and Fe3O4 nanoparticles endows high catalytic activity upon the composites.

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Figure 7. The relative absorbance of the solutions catalyzed by different catalysts with TMB and H2O2 as substrates.

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As an inorganic material, Fe3O4-Au nanocomposites, may exhibit higher chemical and thermal stability than peroxidase, so the catalyst recycling experiments were performed and the catalytic activity was examined in each cycle. As presented in Figure S6a, the absorbance at 652 nm decreased slowly during the recycling and the catalytic activity was 84.5 % of that during the first cycle even after nine recycles (Figure S6b). From the TEM image (Figure S7), the morphology of the nanocomposites is unchanged with no obvious agglomeration. The decrease in the catalytic activity of the nanocomposites mainly results from the loss of the catalyst.6 The Fe3O4-Au nanocomposites exhibit high stability and excellent reusability in the catalytic process which is superior in practical applications.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Herein, Fe3O4-Au nanocomposites were successfully prepared and the peroxidase-like activity was investigated. Sphere-like Fe3O4 particles were aggregated by small nanoparticles in a solvothermal process while the composites were obtained from 20–40 nm gold nanoparticles depositing on the surfaces of Fe3O4 aggregates. The special electron and surface structure endowed high peroxidase-like activity and stability on the nanocomposites which have potential applications in biomedicine, environmental chemistry and medicine.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Synthesis

Synthesis of Sphere-Like Fe3O4 Aggregates: The synthesis procedure for Fe3O4 described in the literature was modified.18,37 All the reagents in the experiment were the analytical grade and used without further treatment. In the synthesis, FeCl3·6H2O (0.27 g) was dissolved in ethylene glycol (12 mL) followed by the addition of sodium acetate (0.246 g) with magnetic stirring. When the mixture turned into a transparent solution, it was transferred to a Teflon-lined autoclave and then heated to 200 °C for 10 h. After reaction, the autoclave was cooled to room temperature. The product was achieved by precipitation, washed with distilled water three times and finally dried at 60 °C for 4 h.

Synthesis of Au-Fe3O4 Composite: Into a conical flask, the above synthesized Fe3O4 (20 mg) was dispersed in deionized water (100 mL) with addition of polyvinylpyrrolidone (0.58 g, PVP, K30, Mw = 58000) and then sonicated for 30 min. After sonication, aqueous HAuCl4 (100 μL, 2 wt.-%) and aqueous sodium citrate (200 μL, 1 wt.-%) were added to above suspension and the mixture reacted in a water bath with a temperature of 45 °C for 15 min. The final product was harvested by magnetic separation and washed with distilled water.

Characterization: The X-ray diffraction (XRD) patterns of the powder samples were collected at room temperature on a Rigaku D/MAX 2200PC diffractometer with a graphite monochromator and Cu-Kα (λ = 0.15418 nm) radiation. The morphologies and microstructures of the products were characterized by using a transmission electron microscope (TEM, JEOL JEM-1100) and a high-resolution TEM (HR-TEM, JEOL JEM-2100). Before observation, the products were dispersed in ethanol. Thermal gravimetric (TG) analysis was carried out on a Netzsch STA449F3 thermal gravimetric analyzer at a heating rate of 10.0 °C min−1 under air and nitrogen atmospheres. The infrared (IR) spectra were examined on a Nicolet 5DX Fourier transform infrared (FTIR) spectrometer using the KBr pellet technique. The Zeta potential of the sample was measured on Zetasizer 3000 instrument. Au and Fe3O4 contents in the sample were analyzed by inductively coupled plasma emission spectroscopy (ICP, Thermo-Electron IRIS Intrepid II XSP). The X-ray photoelectron spectrum (XPS) was recorded on a PHI-5300 ESCA spectrometer (Perkin–Elmer) with its energy analyzer working in the pass energy mode at 35.75 eV, and the Al-Kα line was used as the excitation source. The binding energy reference was taken at 284.7 eV for the C1s peak arising from surface hydrocarbons. The hysteresis loops were conducted by using a LDJ9500 vibrating sample magnetometer at room temperature with a maximum magnetic field of 20 kOe. For magnetization measurements, the powder was pressed strongly and fixed in a small cylindrical plastic box.

Peroxidase-Like Activity Measurement: The catalytic reaction was performed at 30 °C in acetate buffer (3 mL, 0.2 M, pH = 4.0). Firstly, catalyst (34 μg) was dispersed in the above solution which was sonicated for 5 min. Then, 3,3′,5,5′-tetramethylbenzidine (150 μL, 6.2 μmol, 10 mg mL−1 in dimethylsulfoxide as solvent) and H2O2 (160 μL, 5.2 μmol, 30 wt.-%) were added as substrates. After a certain time, the absorbance of the TMB-derived oxidation product was examined at 652 nm on a UV/Vis spectrometer (Perkin–Elmer Lambda 35). The catalytic activity was evaluated by absorbance data. Michaelis constants (Km) were obtained by varying the concentrations of TMB and H2O2 at the optimal conditions.

Supporting Information (see footnote on the first page of this article): TEM image of Fe3O4-Au nanocomposites without adding PVP, FTIR spectrum of gold nanoparticles obtained at the same conditions as those for preparing Fe3O4-Au composites, TG curves of Fe3O4-Au nanocomposites at different atmospheres, catalytic activity of different samples as a function of pH, temperature and time, the catalytic comparison of samples in the stability test, and TEM image of Fe3O4-Au nanocomposites after used for nine times are presented.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

This work is supported by the National Natural Science Foundation of China (Grant 20671057), the Major State Basic Research Development Program of China (973 Program) (No. 2010CB933504), the Science Funds for Distinguished Young Scientists of Shandong Province (JQ200903), and the Natural Science Foundation of Shandong Province (No. ZR2011BZ002).

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

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