Highly Efficient Fenton and Enzyme-Mimetic Activities of Mixed-Phase

Oct 25, 2016 - VOx nanoflakes have excellent affinity toward 3,3′,5,5′-tetramethylbenzidine (TMB) for oxidation and henceforth, it can be used for...
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Highly Efficient Fenton and Enzyme-Mimetic Activities of MixedPhase VOx Nanoflakes Akif Zeb,†,‡ Xiao Xie,† Ammar B. Yousaf,† M. Imran,† Tao Wen,† Zhou Wang,† Hong-Li Guo,† Yi-Fan Jiang,† Ishtiaq A. Qazi,‡ and An-Wu Xu*,† †

Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, P.R. China ‡ Institute of Environmental Sciences and Engineering (IESE), School of Civil and Environmental Engineering (SCEE), National University of Sciences and Technology (NUST), Sector H-12, Islamabad, Pakistan S Supporting Information *

ABSTRACT: Artificial enzyme mimetics is a current research area with much interest from scientific community. Some nanomaterials have been found to possess intrinsic enzymemimetic activity. In this study, VOx nanoflakes with mixedphases are synthesized via a quick and facile one-pot synthetic process and their Fenton reaction and enzyme-mimetic activities have been studied. The results show that obtained VOx is not only highly effective Fenton reagent, completely decomposing Rhodamine B (RhB) within less than 1 min, but also exhibits excellent intrinsic peroxidase-like activity as well as H2O2 catalase-like activity. Our results suggest that this VOx nanomaterial can effectively mimic the enzyme cascade reaction of horseradish peroxidase (HRP). VOx nanoflakes have excellent affinity toward 3,3′,5,5′-tetramethylbenzidine (TMB) for oxidation and henceforth, it can be used for the colorimetric assay of glucose and H2O2. Moreover, this study indicates that VOx nanoflakes can also be used for the efficient degradation of environmental pollutants. KEYWORDS: peroxidase-like activity, VOx, catalase-like activity, hydrogen peroxide decomposition, Fenton reaction, dye degradation, enzyme kinetics

1. INTRODUCTION

evident from the name, Fenton reactions do not require light for the activation of the reactive species. Metal oxide nanoparticles possess a larger variety of enzymemimetic properties, which is similar to that of Fenton reaction. This provides an opportunity to exploit these nanoparticles in biomedical applications for the detection of several substrates. For example, nanoceria has been shown to exhibit peroxidaselike activity.10 Several other metal oxides have been observed, which exhibit Fenton and enzyme mimetic activities, both separately and in combination with other materials as nanocomposites.11−13 V2O5 nanowires exhibit an intrinsic HRP-like activity toward classical peroxidase substrates such as 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) in the presence of H2O2, while it is more effective when coupled with gold nanoparticles.14 V2O3-ordered mesoporous carbon composite has shown to have peroxidase-like activity, which was later applied in glucose colorimetric assay.13 Recently VO2 (A) and VO2 (B) have also been reported to possess intrinsic peroxidase

Vanadium oxides have been widely studied for many years because of their extensive range of applications. Vanadium oxides play an important role in several modern technological applications. For instance, they are widely used catalysts in several industrial processes including oxidation of sulfur dioxide and hydrocarbons and reduction of nitric oxide.1,2 Vanadium oxide and their derivatives are of increasing interest and have been extensively investigated due to their outstanding properties.3 The fundamentals of various oxidation mechanisms related to the structure vs performance and the nature of the active sites are still under study. The structural flexibility of vanadium oxides and its derivatives, combined with chemical and physical properties2,3 make them interesting materials for a number of other applications, such as high-energy lithium batteries, environmental remediation catalysis, hydrogen storage, organic synthesis, thermal fluids, and sensors.4−8 The photo-Fenton reaction is a kind of advanced oxidation process (AOP) for decontamination of water that involves the generation of highly oxidative radicals in the presence of hydrogen peroxide (H2O2) and metal ions (mostly Fe(II)) by the use of an external light irradiation source,9 whereas, as © XXXX American Chemical Society

Received: August 1, 2016 Accepted: October 17, 2016

A

DOI: 10.1021/acsami.6b09557 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces K ⎛ 1 1 1 ⎞ = m⎜ + ⎟ V Vmax ⎝ [C] Km ⎠

properties.15,16 VOx has different oxidation states and structures and mixed phase vanadium oxide has previously been reported to exhibit individual as well as promoting properties in various applications, including electrochemical and catalysis.17−19 However, the synergistic effect of different oxidation states of vanadium oxides for effective Fenton and enzyme-mimetic systems has not been reported to date. This inspires us to synthesize VOx nanoflakes and evaluate their Fenton and intrinsic enzyme-mimetic properties. Herein, we prepare VOx nanoflakes by a simple and facile one-pot method. Obtained VOx exhibits very fast Fenton and enzyme kinetics as compared to the previously reported single-phase vanadium oxide and other peroxidase-like mimetics. Three catalytic properties have been measured such as Fenton reaction degradation of Rhodamine B, TMB oxidation, and H2O2 catalase.

Where V is the initial reaction rate, Vmax is the maximum reaction rate, [C] is the substrate concentration, and Km is the Michaelis−Menten constant. Kinetic Measurements of H2O2 Decomposition. The catalytic activity of VOx for the decomposition of hydrogen peroxide was determined. One gram per liter of the catalyst was added to 0.02 M hydrogen peroxide and the catalyst was shaken to provide enough dispersion of the catalyst particles. The pH of the reaction mixture was not adjusted by buffer solution to avoid the reaction with the buffer medium. The pH of the reaction mixture was found to be 6.3−6.5. Solutions taken at different time intervals were filtered, and the hydrogen peroxide concentration was measured by the Ti(SO4)2 colorimetric titration method based on the formation of a yellow colored Ti(IV)-H2O2 complex.20 Twenty-one microliters of reaction solution was mixed with Ti(SO4)2 solution (25 mmol/L, 0.5 mL) and diluted with deionized water up to 5 mL. The absorption peaks were monitored with UV−vis spectrophotometer at 430 nm for Ti(IV)H2O2 complex. The initial reaction between H2O2 and VOx is considered to be the rate-limiting step. The H2O2 decomposition data were plotted by linear regression according to the following equation

2. EXPERIMENTAL SECTION Materials. Vanadium pentoxide (V2O5, analytically pure) and hydrazine monohydrate (N2H4·H2O, analytically pure) were purchased from Sigma-Aldrich. H2O2 (30 wt %), Rhodamine B (RhB) titanium disulfate (Ti(SO4)2), hydrochloric acid (HCl) and 3,3,5,5tetramethylbenzidinedihydrochloride (TMB) were purchased from Aladin Ltd.(Shanghai, China). All chemicals used in this study were commercially available, of analytical grade, and used without further purification. Preparation of VOx Nanoflakes. V2O5 and N2H4·H2O were employed as starting materials to prepare VO x nanoflakes. Concentrated HCl solution (10 mL, 1 mol) and a solution containing 0.2 mL of N2H4·H2O were added into an aqueous suspension (20 mL) containing 1 g of V2O5. The solution was kept at 60 °C for 2 h and then centrifuged. The sample was washed with ethanol several times and dried overnight in vacuum oven at 60 °C. Characterization. X-ray powder diffraction (XRD) was performed on a Rigaku (Japan) D/max-γA X-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm). The microstructures were observed by scanning electron microscope (SEM, JSM 6700F, JEOL). The X-ray photoelectron spectroscopy (XPS) was carried out on a PerkinElmer RBD upgraded PHI-5000C ESCA system. Fenton Activity of Catalyst. Degradation of Rhodamine B (RhB) in the Fenton-like process was measured at 652 nm using a Cary Bio100 UV/vis spectrometer (Varian); 0.2 g L−1 of VOx catalyst was added to 200 mL of 5 mM Rhodamine B at neutral pH, H2O2 (20 mM) was added, and the reaction was analyzed every 3 s. The data were plotted by linear regression according to the following equation ln

⎛ [H O ] ⎞ In⎜ 2 2 ⎟ = kobsdt ⎝ [H 2O2 ]o ⎠ Where kobsd is the observed first-order rate constant, whereas [H2O2] and [H2O2]0 are the concentrations of hydrogen peroxide in the solution at a given time “t” and at time zero, respectively.

3. RESULTS AND DISCUSSION VOx nanoflakes with different oxidation states were synthesized through reduction of V 2 O 5 precursor by N 2 H 4 (see Experimental Section) at a low temperature (60 °C). The process involves burst-nucleation by precursor decomposition. Upon reaching at a critical temperature, the precursor starts to decompose and creates a supersaturated reaction system with an excess of monomers. This is followed by burst-nucleation, where a large number of nuclei grow simultaneously and quickly consume the monomers, thus restraining the further growth of the particles (size focusing). Two factors determine the overall quality of nanomaterial here: the decomposition rate of the precursor (rd) and the growth rate of the grain (rg). When rd ≫ rg, it creates the supersaturated reaction system with an excess of monomer nuclei, and with increasing concentration, aggregation of the nuclei occurs. While in the other case (rd ≪ rg), the quality of monomers is not enough to produce an excess of nuclei and hence fewer smaller grains are obtained.21 When the reaction mixture is kept at room temperature, no reaction occurs. It is proposed that the precipitates arise from the decomposition of precursor when the temperature increased to 60 °C. HCl helps to dissolve V2O5 powder in the solution. Because of the characteristics of hydrazine, the pH of the solution increases very slowly and homogeneously in the reaction solution. The reducing agent produces lower valence vanadium oxide because of the stronger reducing ability of hydrazine. The hydrazine plays an important role as a coordinating ligand or reducing agent favoring the formation of reduced states of vanadium.22 The lower oxidation states of VOx were obtained by the reduction of hydrazine. The concentration of hydrazine was kept high, because at lower concentrations, oxygen in the deionized water converts lower oxidation states of vanadium into higher oxidation states at high temperature.23 The gradual and uniform rise in pH results in the nucleation and growth of final VOx nanoflakes.

Co = Kt C

Where C0 is the initial concentration of RhB (mM) and C is the concentration at different reaction times, and Kt is the pseudo-firstorder reaction rate constant (min−1). GC Measurements. Gas chromatography−mass spectroscopy (GC-MS, Trio-2000, Micromass, U.K.) with column BPX 70: size 28 m × 0.25 mm, quartz chromatographic column was used to determine the reaction intermediates. Working conditions were as follows: sampling inlet temperature, 260 °C; programming temperatures, 60−260 °C at a rate of 6 °C/min; ion source temperature, 180 °C; electron energy, 70 eV. Test of Peroxidase Activity. For peroxidase activity, 30 μL of H2O2 (30%) was added to 400 μL of sodium acetate buffer (pH 4.5) at 25 °C. Then, 40 μL of TMB (10 mM) was added into the mixture and vortexed for another 4 min. Finally, 30 μL of VOx (1 mg mL−1) was quickly added to the mixture. After addition of VOx, color changes were observed immediately. All reactions were monitored in time-scan mode at 652 nm using a Cary Bio-100 UV/vis spectrometer (Varian). The kinetic parameters were calculated based on Lineweaver−Burke plots of the double reciprocal of the Michaelis−Menten equation, as follows B

DOI: 10.1021/acsami.6b09557 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Figure 1 shows the XRD pattern of the obtained VOx nanoflakes. From XRD, it can be seen that the mixed phases

The structure and morphology of VOx samples was characterized by scanning electron microscopy (SEM) images. As shown in Figure 3, the synthesized VOx displays nanoflakes with a uniform size. The thickness is less than 20 nm and the flakes are intercrossed, providing a large surface area and porosity. Rhodamine B (RhB) was used as a model compound to test the Fenton reaction activity over VOx catalyst. Figure 4A shows the degradation of Rhodamine B (RhB) at neutral pH solution, RhB UV−vis absorption spectra were measured at 652 nm in time-scan mode. Notably, more than 95% of RhB was degraded after 30 s. From the kinetics plot shown in Figure 4B, it is found that the rate constant of this Fenton process in the VOx system reaches to about 5.79 min−1. A recently reported fastest rate constant value for photo-Fenton reaction has been observed to be 9.31 min−1 over Pt-loaded C-ZnFe2O4.27 Our VOx catalyst achieves a close value with a cheap material and without use of noble metals and light source. RhB (10 ppm concentration) can be degraded in less than 1 min. The insets in Figure 4A present digital photographs of the solution of 10 ppm RhB before the reaction (left) displaying a deep pink color and after 30 s of the Fenton reaction with a decolored solution (right). To further investigate the degradation of RhB, GC tests were performed. As shown in the Figure S1, the GC results indicate only one peak in the RhB sample before the degradation experiment. After 30 s of the reaction, the same peak almost disappeared. Both peaks have the same retention time. There are no other products formed, which indicates almost complete degradation of RhB in just 30 s. The highly efficient degradation of RhB in the VOx system is a result of heterogeneous catalytic reaction. Here, the VOx serves as an active catalyst in the generation of hydroxyl radicals (•OH) from H2O2, a reactive species that attacks the aromatic ring of organic molecules for their complete degradation. Detailed studies on reaction mechanisms for heterogeneous Fenton-like processes were previously reported.28−30 We propose that metal ions (vanadium ions in this case) react with H2O2 and H2O2 decomposes into hydroxyl ions, protons and •OH radicals. This improvement in activity of the Fenton reaction can be tentatively attributed to a key role played by the mixed phases of various oxidation states and oxygen to generate more oxidative intermediates on VOx catalyst. Although the exact mechanism for this reaction is

Figure 1. XRD pattern of as-synthesized mixed-phase VOx nanoflakes.

are present. The diffraction peaks at 17.4° and 27.7° are attributed to V2O5 (JCPDS 85−2422), while those at 25, 32.8, and 36.5° are ascribed to V2O3, and peaks at 40, 45.3, and 47.1° are assigned to VO2 phase, indicating the coexistence of V5+, V3+, and V4+ in the sample, respectively. The XRD pattern is noisy because of the low crystallinity of VOx nanoflakes. This result indicates that the vanadium in the VOx catalyst has various oxidation states. X-ray photoelectron spectroscopy (XPS) spectra of VOx nanoflakes are shown in Figure 2. The VOx catalyst mainly consists of a small portion of V3+ (V 2p3/2 at around 515.85 eV) and a large portion of V+4 and V+5 (V 2p3/2 at 516.7 and 517.7 eV, respectively).11,24 These results are in consistence with XRD results. Moreover, there is a small shift of O 1s from 530.7 eV to around 533.0 eV, which may be caused by the mixed-valence character of vanadium ions. This increases the electron cloud density on the surface. The O 1s spectrum is broad and asymmetric, indicating the presence of different oxygen species. The peaks at 531.5 and 533.1 eV can be attributed to lattice oxygen in VOx nanoflakes and surfaceadsorbed hydroxyl oxygen, respectively.25,26

Figure 2. XPS spectra of as-synthesized mixed phase VOx nanoflakes: (A) deconvoluted V 2p scan and (B) full survey. C

DOI: 10.1021/acsami.6b09557 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. Low-magnification SEM image and high-magnification SEM image (inset) of VOx nanoflakes.

Figure 4. (A) Time-dependent decoloration of RhB in the Fenton-like process analyzed at 652 nm. Experimental conditions: [catalyst] = 0.2 g L−1; [H2O2] = 20 mM; [RhB] = 5 mM, insets are photographs before (left) and after (right) 30 s of the reaction. (B) Kinetics of the same reaction.

shown in Figure 5A, VOx nanoflakes showed high intrinsic peroxidase-like activity toward TMB oxidation. The control experiments performed without H2O2 or VOx exhibit no activity toward TMB oxidation. In Figure 5B, the optimum amount of VOx nanoflakes was determined, which was about 80 μg mL−1; meanwhile, the reaction mechanism is displayed in Figure 5C. The inset shows the blue color of oxidized TMB after 30 s of the reaction. The dependence of the peroxidase-like activity on TMB concentration is shown in Figure 5D, and the dependence of the peroxidase-like activity on H2O2 concentration is displayed in Figure S2. As expected, with the increasing concentration of H2O2, the peroxidase-like activity of VOx nanoflakes increases obviously. The mechanism of peroxidase-like catalytic activity of VOx nanoflakes was further investigated using steady-state kinetics. From the control experiments shown in Figure 5A and TMB reactions at different concentrations (Figure 5D), it is found that the catalysis mediated by VOx nanoflakes is dependent on TMB

not well-established, in the case of VOx, the active sites are generally considered to be vanadium ions with the highest oxidation state (since the surface is easily oxidized), and during oxidation reactions, the active sites undergo a redox cycle, i.e., V(Reduced)−O−V(Reduced). According to a recent study, the presence of mixed VOx species during the redox cycle can greatly influence the stability of the surface species.31 Consequently, the higher activity of the VOx catalyst is due to the presence of more V5+−O−V5+ species and higher mobility of oxygen on its surface, which is strongly influenced and positively attenuated by the oxygen vacancies in other oxidation states.32,33 To investigate the intrinsic peroxidase-like activity of VOx nanoflakes, we performed other reactions. As evident in several previous studies, metal ions and even vanadium oxides exhibit optimal peroxidase-like activity at pH 4−5.14,16,17,28 Therefore, we performed all reactions in NaAc buffer solution (pH 4.5). As D

DOI: 10.1021/acsami.6b09557 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (A) UV−vis absorption spectra of TMB solutions oxidized by different systems in acetate buffer solution (pH 4.5) recorded at 60 s of the catalytic reaction. (B) Dependence of the peroxidase-like activity on the catalyst concentration. (C) Reaction mechanism for TMB oxidation with H2O2. Oxidized TMB (inset). (D) Dependence of the peroxidase-like activity on TMB concentration.

The observed rate of H2O2 decomposition (k1) was also determined. It is evident from Figure 6 that this reaction over VOx followed the first-order kinetics with a value of 9.1 × 10−3. This result suggests that VOx possesses catalase-like activity. As shown in (Table S1), the rate constant of VOx catalyst is compared with various catalysts for the decomposition of H2O2 following first-order rate kinetics.

concentration. From Figure 5D, it is also evident that the reaction is instantly triggered upon the addition of the VOx catalyst. Various reactions were performed with varied TMB concentrations, whereas keeping the other experimental conditions constant in order to get a better insight into the TMB oxidation. It is observed that the oxidation reaction catalyzed by VOx follows the Michaelis−Menten behavior toward TMB. The kinetic parameters were determined for TMB oxidation at the optimum pH of 4.5 and their comparison with previous studies of single-phase vandium oxides as well as other metal oxides is presented in Table 1. It can be seen that the Km (Michaelis constant) value of our VOx sample for TMB is 0.059 mM, which is significantly lower than that of other materials in comparison. This result implies that VOx has a strong affinity toward TMB. Table 1. Comparison of the Kinetic Parameters between Different Metal Oxide-Based Materials and Horseradish Peroxidase (HRP) with TMB as the Substrate materials

Km (mM)

Vmax (× 10−8 M s−1)

ref

HRP VOx nanoflakes VO2 (A) nanoplates VO2 (B) nanoceria Fe3O4 MNPs Co-g-C3N4 Co3O4

0.434 0.059 0.165 0.146 3.8 0.098 0.113 0.037

10 270 2.4 131 70 3.44 8.64 6.27

30 this study 14 15 10 29 34 35

Figure 6. Kinetic measurements of hydrogen peroxide decomposition catalyzed by VOx. E

DOI: 10.1021/acsami.6b09557 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces The reproducibility of mimetic peroxidase catalysis is also very important. It shows the effectiveness of the material after several recycle uses. As shown in Figure 7, in spite of the

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GC Spectrum of Rhodamine B after catalytic treatment, comparison of H2O2 decomposition with literature, the kinetic curves of VOx nanoflakes at different concentrations of H2O2 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-551-63602346. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Basic Research Program of China (2011CB933700), the National Natural Science Foundation of China (51572253, 21271165), and cooperation between NSFC and Netherlands Organization for Scientific Research (51561135011). This work is also supported by USTC and Anhui Government Scholarship programs.

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Figure 7. Determination of the reusability and reproducibility between five consecutive catalysis cycles and three different batches of VOx nanoflakes by monitoring the final activity after 1 min reaction.

(1) Weckhuysen, B. M.; Keller, D. E. Chemistry, Spectroscopy and the Role of Supported Vanadium Oxides in Heterogeneous Catalysis. Catal. Today 2003, 78, 25−46. (2) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, U.K., 1996; p 211. (3) Fan, J.; Yin, J. J.; Ning, B.; Wu, X.; Hu, Y.; Ferrari, M.; Anderson, G. J.; Wei, J.; Zhao, Y.; Nie, G. Direct Evidence for Catalase and Peroxidase Activities of Ferritine Platinum. Nanoparticles. Biomaterials 2011, 32, 1611−1618. (4) Laszczynski, N.; von Zamory, J.; Kalhoff, J.; Loeffler, N.; Chakravadhanula, V. S. K.; Passerini, S. Improved Performance of VOx-Coated Li-Rich NMC Electrodes. ChemElectroChem 2015, 2, 1768−1773. (5) Zhang, S.; Huang, J.; Yang, Y.; Li, Y.; Wang, B.; Wang, Y.; Deng, S.; Yu, G. Rapid Mechanochemical Synthesis of VOx/TiO2 as Highly Active Catalyst for HCB Removal. Chemosphere 2015, 141, 197−204. (6) Azizi, S.; Salah, M.; Nefzi, H.; Khaldi, C.; Sediri, F.; Dhahri, E.; Lamloumi, J. Structure, Volumetric Adsorption Method and Electrochemical Hydrogen Storage Properties of Vanadium Oxide Nanotubes. VOx NTs. J. Alloys Compd. 2015, 648, 244−252. (7) Fukudome, K.; Suzuki, T. Highly Selective Oxidative Dehydrogenation of Propane to Propylene over VOx-SiO2 Catalysts. Catal. Surv. Asia 2015, 19, 172−187. (8) Dey, K. K.; Bhatnagar, D.; Srivastava, A. K.; Wan, M.; Singh, S.; Yadav, R. R.; Yadav, B. C.; Deepa, M. VO2 Nanorods for Efficient Performance in Thermal Fluids and Sensors. Nanoscale 2015, 7, 6159−6172. (9) Lin, S. S.; Gurol, M. D. Catalytic Decomposition of Hydrogen Peroxide on Iron Oxide: Kinetics, Mechanism, and Implications. Environ. Sci. Technol. 1998, 32, 1417−1423. (10) Asati, M. A.; Santra, D. S.; Kaittanis, M. C.; Nath, D. S.; Perez, P. J. M. Oxidase-Like Activity of Polymer-Coated Cerium Oxide Nanoparticles. Angew. Chem., Int. Ed. 2009, 48, 2308−2312. (11) Lee, K. T.; Chuah, X. F.; Cheng, Y. C.; Lu, S. Y. Pt Coupled ZnFe2O4 Nanocrystals as a Breakthrough Photocatalyst for FentonLike Processes − Photodegradation Treatments from Hours to Seconds. J. Mater. Chem. A 2015, 3, 18578−18585. (12) Kim, M. C.; Lee, S. Y. Peroxidase-like Oxidative Activity of a Manganese Coordinated Histidyl Bolaamphiphile Self-Assembly. Nanoscale 2015, 7, 17063−17070. (13) Han, L.; Zeng, L.; Wei, M.; Li, C. M.; Liu, A. A V2O3-ordered Mesoporous Carbon Composite with Novel Peroxidase-Like Activity towards the Glucose Colorimetric Assay. Nanoscale 2015, 7, 11678− 11685.

agglomeration, the synthesized VOx nanoflakes still retain 95% of their initial activity toward TMB after several cycles. The decrease in activity can be attributed to the loss of the material during recycling process. Three different batches of the same material exhibit similar activity and reproducibility.

4. CONCLUSIONS In conclusion, mixed-phase VOx nanoflakes are prepared by a facile and one-pot synthetic method. These nanoflakes exhibit extraordinary catalytic efficiency for the Fenton reaction as well as enzyme-mimetic reactions. The one-pot synthetic pathway allows the successful assembly of nanoflakes as the Fenton catalysts that exhibit very high degradation activity toward Rhodamine B and excellent activity for H2O2 decomposition. Both processes can be employed for the fast and effective remediation of contaminated water with organic pollutants. At the same time, it has also been established that this superior efficiency of remediation is due to well-known variables such as the chemical composition, size, surface area, pore volume, crystal structure, and crystallinity of the catalyst as well as the presence of mixed phases of vanadium oxides. In addition, the high intrinsic HRP-like activity and good stability of VOx nanoflakes endows the system to serve as a robust nanoreactor for mimicking the complexities and functions of an enzymatic cascade system. By taking advantage of these unique features, this VOx system can be further applied as a robust nanoprobe for the detection of glucose as well as for the sensor for detection of H2O2. Our work is expected to provide further insight into the development of enzyme mimetics having multiple functionalities and reactivities, which could find potential applications in biocatalysis, nanobiomedicine, bioassays, and nanotechnology.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09557. F

DOI: 10.1021/acsami.6b09557 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Dealumination for Chlorinated VOC Abatement. Appl. Catal., B 2003, 41, 31−42. (34) Mu, J.; Li, J.; Zhao, X.; Yang, E.; Zhao, X. Cobalt-doped graphitic carbon nitride with enhanced peroxidase-like activity for wastewater treatment. RSC Adv. 2016, 6, 35568−35576. (35) Mu, J.; Wang, Y.; Zhao, M.; Zhang, L. Intrinsic Peroxidase-like Activity and Catalase-like Activity of Co3O4 Nanoparticles. Chem. Commun. 2012, 48, 2540−2542.

(14) Qu, K.; Shi, P.; Ren, J.; Qu, X. Nanocomposite Incorporating V2O5 Nanowires and Gold Nanoparticles for Mimicking an Enzyme Cascade Reaction and its Application in the Detection of Biomolecules. Chem. - Eur. J. 2014, 20, 7501−7506. (15) Zhang, L.; Xia, F.; Song, Z.; Webster, N. A. S.; Luo, H.; Gao, Y. Synthesis and Formation Mechanism of VO2(A) Nanoplates with Intrinsic Peroxidase-like Activity. RSC Adv. 2015, 5, 61371−61379. (16) Nie, G. D.; Zhang, L.; Lei, J. Y.; Yang, L.; Zhang, Z.; Lu, X. F.; Wang, C. Monocrystalline VO2 (B) Nanobelts: Large-Scale Synthesis, Intrinsic Peroxidase-like Activity and Application in Biosensing. J. Mater. Chem. A 2014, 2, 2910−2914. (17) Kip, B. J.; Smeets, P. A. T.; van Wolput, J. H. M. C.; Zandbergen, H. W.; van Grondelle, J.; Prins, R. Preparation And Characterization of Vanadium Oxide Promoted Rhodium Catalysts. Appl. Catal. 1987, 33, 157−180. (18) Mori, T.; Miyamoto, A.; Takahashi, N.; Fukagaya, M.; Hattori, T.; Murakami, Y. Promotion Effects of Vanadium, Niobium, Molybdenum, Tungsten, and Rhenium Oxides on Surface Reactions in the CO Hydrogenation over Ru/Al2O3 Catalyst. J. Phys. Chem. 1986, 90, 5197−5201. (19) Guerrero-Ruiz, A.; Sepulveda-Escribano, A.; Rodriguez-Ramos, I. Carbon Monoxide Hydrogenation over Carbon Supported Cobalt or Ruthenium Catalysts. Promoting Effects of Magnesium, Vanadium And Cerium Oxides. Appl. Catal., A 1994, 120, 71−83. (20) Wang, D. K.; Wang, M. T.; Li, Z. H. Fe-Based Metal Organic Frameworks for Highly Selective Photocatalytic Benzene Hydroxylation to Phenol. ACS Catal. 2015, 5, 6852−6857. (21) Guardia, P.; Pérez-Juste, J.; Labarta, A.; Batlle, X.; Liz-Marzán, L. M. Heating Rate Influence on the Synthesis of Iron Oxide Nanoparticles: The Case of Decanoic Acid. Chem. Commun. 2010, 46, 6108−6110. (22) Jin, P.; Xu, G.; Tazawa, M.; Yoshimura, K. A VO2-Based Multifunctional Window with Highly Improved Luminous Transmittance. Jpn. J. Appl. Phys. 2002, 41, L278−L280. (23) Li, W.; Ji, S.; Li, Y.; Huang, A.; Luo, H.; Jin, P. Synthesis of VO2 Nanoparticles By A Hydrothermal-Assisted Homogeneous Precipitation Approach For Thermochromic Applications. RSC Adv. 2014, 4, 13026−13033. (24) Mendialdua, J.; Casanova, R.; Barbaux, Y. XPS Studies of V2O5, V6O13, VO2 and V2O3. J. Electron Spectrosc. Relat. Phenom. 1995, 71, 249−261. (25) Werfel, F.; Brummer, O. Corundum Structure Oxides Studied by XPS. Phys. Scr. 1983, 28, 92−96. (26) Sawatzky, G. A.; Post, D. X-ray Photoelectron and Auger Spectroscopy Study of Some Vanadium Oxides. Phys. Rev. B: Condens. Matter Mater. Phys. 1979, 20, 1546−1555. (27) Lars, S.; Andersson, T. An XPS Study of Dispersion and Valence State of TiO2 Supported Vanadium Oxide Catalysts. Catal. Lett. 1990, 7, 351−358. (28) Gao, L. Z.; Zhuang, J.; Nie, L.; Zhang, J. B.; Zhang, Y.; Gu, N.; Wang, T. H.; Feng, J.; Yang, D. L.; Perrett, S.; Yan, X. Y. Intrinsic Peroxidase-Like Activity of Ferromagnetic Nanoparticles. Nat. Nanotechnol. 2007, 2, 577−583. (29) Garrido-Ramirez, E. G.; Theng, B. K. G.; Mora, M. L. Clays and Oxide Minerals as Catalysts and Nanocatalysts in Fenton-like Reactions  A Review. Appl. Clay Sci. 2010, 47, 182−192. (30) Kwan, W. P.; Voelker, B. M. Influence of Electrostatics on the Oxidation Rates of Organic Compounds in Heterogeneous Fenton Systems. Environ. Sci. Technol. 2004, 38, 3425−3431. (31) Zhu, S. J.; Song, Y. B.; Zhao, X. H.; Shao, J. R.; Zhang, J. H.; Yang, B. The Photoluminescence Mechanism in Carbon Dots (Graphene Quantum Dots, Carbon Nanodots, and Polymer Dots): Current State and Future Perspective. Nano Res. 2015, 8, 355−381. (32) Lopez-Fonseca, R.; Aranzabal, A.; Gutierrez-Ortiz, J. I.; AlvarezUriarte, J. I.; Gonzalez-Velasco, J. R. Comparative Study of the Oxidative Decomposition of Trichloroethylene over H-Type Zeolites Under Dry and Humid Conditions. Appl. Catal., B 2001, 30, 303−313. (33) Lopez-Fonseca, R.; de Rivas, B.; Gutierrez-Ortiz, J. I.; Aranzabal, A.; Gonzalez-Velasco, J. R. Enhanced Activity of Zeolites by Chemical G

DOI: 10.1021/acsami.6b09557 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX