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This experiment shows that these Au nanoflowers function as effective catalyst for the reduction of pendimethalin in the presence of NaBH4 (otherwise ...
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Catalytic Strategy for Efficient Degradation of Nitroaromatic Pesticides by Using Gold Nanoflower Kang Mao, Yinran Chen, Zitong Wu, Xiaodong Zhou,* Aiguo Shen, and Jiming Hu Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, People’s Republic of China ABSTRACT: In this contribution, we report a new type of Au nanoflower-based nitroaromatic pesticide degradation platform that is fast, efficient, and simple. We found a straightforward, economically viable, and “green” approach for the synthesis and stabilization of relatively monodisperse Au nanoflowers by using nontoxic chemical of hydroxylamine (NH2OH) without stabilizer and the adjustment of the pH environment. This experiment shows that these Au nanoflowers function as effective catalyst for the reduction of pendimethalin in the presence of NaBH4 (otherwise unfeasible if NaBH4 is the only agent employed), which was reflected by the UV/vis spectra of the catalytic reaction kinetics. Importantly, the novel degradation platform could be put in use in two different practical soil samples with satisfactory results under laboratory conditions. To demonstrate the feasibility and universality of our design, two other nitroaromatic pesticides, trifluralin, and p-nitrophenol, were selected and were successfully degraded using this degradation platform. KEYWORDS: gold nanoflower, catalytic degradation, pendimethalin, nitroaromatic pesticides, trifluralin, p-nitrophenol

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INTRODUCTION With the abundant use of pesticides and herbicides, such as pendimethalin, trifluralin, and p-nitrophenol, which they unavoidably enter the environment as a result of their necessary role in agriculture and grassland farming, soil, and water contamination due to pesticide residues unavoidably occur.1−8 One important factor contributing to the presence and potential effects on the environment is the persistence of the nitroaromatic pesticides in the environment. Hence, the need for remediation in contaminated sites has come into place. Now, most of the research on degradation of nitroaromatic pesticides has focused on microbial degradation8 as part of pesticide registration requirements in countries around the world, although photolytic breakdown6,7 is also measured in registration studies. Also, the Fenton reagent method is also available.4,5 Actually, to remediate sites contaminated with nitroaromatic pesticides well, more innovative, natural, and cost-effective approaches to degrade nitroaromatic pesticide residues are needed. In recent years, efforts have been paid to search for resolution in the field of nanotechnology, which has been emerging as a revolutionizing field of research with multiple disciplines of science like physics, chemistry, materials science, and biology.9 Metal particles in nanoscales have become a subject of intense interest in various fields stemming from their unique optical, electronic, and catalytic properties,10 which are different from their bulk counterparts and hence lead to novel applications in sensor,11,12 catalysts,13 nanoelectronic devices,14 biochemical analysis,11 and so forth. It has been experimentally demonstrated that metal nanoparticles boast good catalytic activities due to the highly reduced dimensions of their catalytically active domains for hydrogenation, hydroformylation, carbonylation,15 degradation of 4-nitrophenol,16,17 CO oxidation,18 and so forth. Although almost all metal nanoparticles boast catalytic activity, their catalytic efficiency is © 2014 American Chemical Society

greatly influenced by their variety in size, shape, and compound, based on which, the control of chemical reactions through changes in the size and shape of solid catalysts at the nanoscale has been studied. A great many chemical routines have been developed to synthesize nonclassical metal nanocrystals like nanorods, 19,20 nanowires, 21,22 nanoplates, 23−25 nanoprisms,26−28 nanostar,29 nanoflower, nanocubes,30 multipods,31 and other shapes.32 Among these shapes, nanoflowers with highly branched structures are of particular interest for catalysis, as they generally exhibit a reasonably large specific surface area and a high specific activity (that is, activity per unit surface area) owing to high densities of edges, corners, and stepped atoms present on their branches. In a number of studies, branched metal nanocrystals have exhibited great potential to be used as catalysts or electro-catalysts with substantially enhanced activity. According to El Sayed and his co-workers’ investigation of the catalytic properties of branch platinum nanocrystals, it could lower the activation energy of the reaction by 1.6 times as compared to tetrahedral nanocrystals.33 Gold nanoparticles are equally effective as heterogeneous or homogeneous catalysts, therefore, the study of nanogold catalyst has rapidly become a hot topic in chemistry.13,34,35 For the first time, we report a novel nano catalysis-based method for degrading nitroaromatic pesticide residues built on Au nanoflower platform instead of other nano shapes. A simple, facile, fast route of shape-controlled synthesis of gold nanoflower with mild reaction conditions was designed. Compared to other methods in the literature, it saves the use of any surfactants or seeds.36,37 With the existence of Au nanoflower as the catalyst, pendimethalin, which was chosen as Received: Revised: Accepted: Published: 10638

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Figure 1. (A, B) SEM and (C) TEM images of gold nanoflower that were synthesized; Mass spectral data of (D) shows the process of degradation of pendimethalin (above) to N-(1-ethylpro-pyl)-2,6-diamino-3,4-xylidine (below).

Furthermore, to verify the effectiveness of gold nanoflower as the catalyst in the degradation of nitroaromatic pesticides, additional experiments on trifluralin and p-nitrophenol, which are common nitroaromatic pesticides were carried out with satisfactory results.

the model contaminant due to its relatively long persistence in soil and its frequent use as a criterion for soil contamination by pesticide dealerships,38 was effectively degraded by sodium borohydride. Pendimethalin is the common name for N-(1ethylpro-pyl)-2,6-dinitro-3,4-xylidine, a representative member of the growing list of N-substituted 2,6-dinitroaniline herbicides used for selective control of most annual grasses and many annual broad-leaved weeds in several crops. Pendimethalin has approved uses in all countries of the European Union.39 However, according to the label of pendimethalin, even in nanomolar concentration, it is toxic to aquatic organisms such as fish and may cause long-term adverse effects in the aquatic environment. What’s more, pendimethalin has half-lives in soil usually ranging from a few days to several months.40 Plieth and Boerner41 reported approximately 10−15% remains of the 2.3 mg/kg pendimethalin 10 months after its application in the soil. Compared with previously reported ways of pendimethalin degradation, our method exhibited higher efficiency and faster reaction (within 5 min). In addition, degradation of pendimethalin in soil samples was performed to demonstrate enormous potential in the practical use of this platform.

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MATERIALS AND METHODS

Chemicals and Apparatus. Pendimethalin were purchased from Zouping Luda Pharmaceutical Co., Ltd. (Shandong, China). Trifluralin were purchased from Feng Shan Group Co., Ltd. (Jiangsu, China). pNitrophenol were purchased from Sinopharm Chemical Reagent Co.,Ltd. Two soil samples (A clay soil containing organic matter and sandy loam soil in absence of organic matter) came from Wuhan University. Other chemicals were of analytical grade and prepared with ultrapure water from a Millipore system (18.2 M resistivity). The size distribution and structure of the Au nanoflower were probed by high-resolution transmission electron microscopy (HRTEM) using a JEM-2100 (HR) operated at an acceleration voltage of 200 kV and scanning electron microscope (SEM), respectively. Subsequently, gas chromatography−mass spectrometry (GC−MS) was used to ensure the product of degradation reaction. Finally, UV−vis absorption spectrometer (UV-2550, Hitachi Co. Ltd., Japan) was employed to record UV−vis absorption spectra. 10639

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mM p-nitrophenol aqueous solution were mixed with 500 μL 15 mM NaBH4 aqueous solution respectively, then, 100 μL Au nanoflower aqueous solution was added into the mixtures. The UV−vis absorption spectrum measurements were done at room temperature.

Synthesis of Au Nanoflower. Modifications were made based on the literature37,42 and Au nanoflower has been successfully synthesized with a simple, fast process without seeds and other surfactants. First, 50 μL of HAuCl4 (wt 1%) aqueous solution was added to 2.5 mL of deionized water. Then, 10 μL of NH2OH (wt 50%) aqueous solution was injected into the solution with stirring, and after 1 min reaction, Au nanoflower dispersion was obtained. Finally, after half an hour standing, homogeneous Au nanoflower was obtained for use. UV−vis Spectra Measurements. The color change was qualitatively measured by taking UV−vis spectra with 600 μLoptical-path-length quartz cuvettes at 1 min interval in the range of 200−700 nm. The rate constant of the degradation reaction of nitroaromatic pesticides was determined by the function of the measurement of the change of absorbance at the initially observed peak at 430 nm in relation to time. All experiments were reacted and measured at room temperature. In order to investigate the feasibility of degrading pendimethalin by NaBH4 in the presence of Au nanoflower (with stable particle size100 nm) as a degradation catalyst, three 2 mL test tubes (A,B,C) have been added into 500 μL of 2.34 mM pendimethalin aqueous solution. Then, pendimethalin aqueous solution in test A and test B was respectively mixed with 500 μL of 15 mM NaBH4 aqueous solution and 500 μL ultrapure water was added into test C. 100 μL Au nanoflower was introduced into test A and 100 μL ultrapure water was added to test B and test C, respectively, to make each reaction solution 1100 μL. The final mixed solution was allowed to stand and react for 30 min at room temperature. The UV−vis absorption spectrum was measured at room temperature. To save cost and time, kinetic analysis is necessary. A 500-μL portion of 2.34 mM pendimethalin aqueous solution was mixed with 500 μL of 15 mM NaBH4 aqueous solution. After vibration, 100 μL Au nanoflower aqueous solution was added. The UV−vis absorption spectrum was measured at 1 min interval from zero to 10 min at room temperature. The rate constant of the degradation reaction of pendimethalin was determined by measuring the change in absorbance of the initially observed peak at 430 nm, for the pendimethalin, as a function of time. The pH value is a very important factor to the reaction process. The pH value of pendimethalin aqueous solution was adjusted by 1 M HCl or 1 M NaOH aqueous solution and the final concentration was 2.34 mM. 500 μL of pendimethalin aqueous solution (2.34 mM) of different pH value (pH 1, 4, 7, 11, 14) were mixed with 500 μL 15 mM of NaBH4 aqueous solution in 2.0 mL test tube respectively, and 100 μL Au nanoflower aqueous solution was added to make each solution 1.1 mL. Finally, these mixtures were detected by the UV−vis absorption spectrum at room temperature. In order to optimize volume of Au nanoflower, 500 μL of 2.34 mM pendimethalin aqueous solution were mixed with 500 μL of 15 mM NaBH4 aqueous solution. After vibration, different volumes (25−200 μL) of Au nanoflower solution were added in, respectively. Then appropriate aliquot of ultrapure water was introduced in to make each reaction solution 1.2 mL. Finally, the UV−vis absorption spectrum measurements were done at room temperature. To verify the feasibility of this new platform for pendimethalin degradation in practical samples, two soil samples (clay soil containing organic matter and sandy loam soil in absence of organic matter) are applied to degrade pendimethalin. Two 10 g samples of air-dried soil were transferred to a 100 mL flask. Then 100 mL of water was added in and stirred for about 3 h; after the soil settled, the supernatant was filtered by 0.22 μm microporous membrane. Two 4.68 mM pendimethalin portions were diluted by two soil filtrates, and the final concentration was 2.34 mM. 500 μL 2.34 mM pendimethalin was mixed with 500 μL 15 mM NaBH4 aqueous solution, then, 100 μL Au nanoflower aqueous solution was added into the mixtures. The UV− vis absorption spectrum measurements were done at room temperature. To further explore the potential application of the platform in the practical samples, the development of new degradation platform was tested by other nitroaromatic pesticides, including trifluralin and pnitrophenol. 500 μL of 1.43 mM trifluralin aqueous solution and 0.2

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RESULTS AND DISCUSSION The Proposed Mechanism. Gold-based catalysts have attracted intense interest in recent years following the discovery that small supported Au nanoparticles (NPs) can be effective catalysts for CO oxidation at low temperatures. They have been widely used and regarded as a new generation of catalysts to some reactions that are otherwise unfeasible. However, the properties of AuNPs depend strongly on their size and shape, which determine the surface structure of the particles.29 In our experiment, Au nanoflower was synthesized through NH2OH reduction of HAuCl4. To confirm the successful formation of Au nanoflower and its size and shape, the as-prepared Au nanoflower was characterized by SEM and HRTEM. As shown in Figure 1A−C, dendritic structures of Au nanoflower were synthesized, and the diameter of Au nanoflower was approximately 100 nm, exhibiting relatively narrow size distribution. Although reports of dendritic structure of gold nanoparticle are rarely seen,37,42−44 dendritic nanostructures have often been reported in the synthesis of platinum nanocrystals.45,46 According to diffusion-limited aggregation (DLA) model,33,47 small Au nanoparticle has higher energy owing to a larger surface-to-volume ratio and of a higher collision frequency associated with their greater mobility, which leads to Au nanoparticle coalescence or attachment. The probability of finding a randomly diffusing AuNPs particle around the Au core is extremely low owing to the so-called screening effect. Due to the screening effect, the tips of most advanced branches of Au core can most effectively capture the incoming and randomly diffusing gold particles, forming particle coalescence, or attachment to the tips, thus homogeneous and stable gold nanoflower solution can be obtained in the end. It can be observed that the yellow pendimethalin aqueous solution exhibited a large absorption peak at 430 nm without NaBH4 and Au nanoflower. Upon the addition of aqueous solution of NaBH4 (15 mM), the absorption peak at 430 nm of (B) and (C) remained unaltered and their solution color remained the same for a long duration, which indicated that the strong reducing agent NaBH4 alone cannot degrade pendimethalin. Interestingly, the addition of an aliquot of Au nanoflower dispersion to the reaction system caused a fading and ultimate bleaching of the yellow color of pendimethalin in aqueous solution (A), suggesting the occurrence of the degradation reaction. The UV/vis spectra in Figure 2 unambiguously verifies this conclusion as well, as the absorption band of pendimethalin at 430 nm decreases and disappears in less than 5 min after the addition of Au nanoflower. To exclude the possibility that the degradation reaction could have been activated by the superfluous NH2OH instead of Au nanoflower, NH2OH alone was added into pendimethalin and NaBH4 (15 mm) mixtures. No change in the color and position of the absorption band (at 430 nm) of pendimethalin was observed. Thus, the degradation of pendimethalin by NaBH4 has been clearly demonstrated to be activated by the Au nanoflower. The samples of pendimethalin and degredation product were characterized using gas chromatography−mass spectrometry (GC−MS) for identification of compounds. The experimental results of mass spectrum (Figure 1 D) showed that the two 10640

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area and high specific activity, gold nanoflower could adsorb hydrogen anion (H−) and provide places for the degradation of pendimethalin. Meanwhile, a large Fermi level shift caused the higher driving force of particle-mediated electron transfer from BH4− ion to nitro compounds mediated by the Au nanoflower because that existing H− belong to highly electron-injecting species.48 As seen from the principle above, pendimethalin are removed directly from the aqueous solution, leading to the formation of aromatic intermediates. Hence, this method is applicable to the degradation of all nitroaromatic pesticides rather than just a few particular nitrophenolic compounds. Kinetics for Pendimethalin Oxidation. Hydrogen anion (H−) from sodium borohydride (NaBH4) is mainly used as a principal reducing agent in pendimethalin degradation. Hydrogen anion (H−), as a nonspecific reductant, reacts with the target organic contaminants (pendimethalin) at rates close to diffusion rates in water. Thus, the kinetics for pendimethalin reducing by reduction of sodium borohydride catalyzed by Au nanoflower processes can be represented as follows:

Figure 2. (A) UV−vis absorption spectra of degradation of pendimethalin catalyzed by Au nanoflower, (B) UV−vis absorption spectra during the reduction of pendimethalin by NaBH4 without Au nanoflower, and (C) UV−vis absorption spectra of pendimethalin in aqueous solution.

dC(P)/dt = −kP′C(P) × C(NaBH4)

nitro groups of pendimethalin were reduced to two amino groups of N-(1-ethylpro-pyl)-2,6-diamino-3,4-xylidine. On the basis of the above findings, a tentative reaction pathway for degradation of pendimethalin catalyzed by Au nanoflower processes is presented as follows (Scheme 1): in this process,

(1)

A second-order rate equation can be applied to fit the data, where C(P) is the concentration of pendimethalin, C(NaBH4) is the sodium borohydride concentration, and kP′ is the secondorder rate constant for the reaction. Since the concentration of NaBH4 added in the system is much higher in comparison with that of pendimethalin, it is reasonable to assume that the concentration of BH4− remains constant during the reaction. Equation 1 can be reduced to a pseudo-first-order rate equation:

Scheme 1. Reaction Pathway for Degradation of Pendimethalin Catalyzed by Au Nanoflower Processes

dC(P)/dt = −kPC(P)

(2)

where, kP is the pseudo-first-order rate constant. In case of Au nanoflower-catalyzed degradation processes, Au nanoflower remained almost constant during the experimental time and hence eq 2 was equated in evaluating the kinetics of pendimethalin during Au nanoflower-catalyzed degradation processes. When t = 0, C(P) is equal to C(P)0, and the solution of eq 2 becomes, C(P) = C(P)0 exp( −kPt )

the main oxidizing species is H− from NaBH4. But H− is unstable in aqueous solution. Due to its large specific surface

(3)

or

Figure 3. (a) Plot of absorbance (A) against time for the catalytic reduction of pendimethalin by Au nanoflower (the insets present the reaction kinetic graph for the degradation of pendimethalin catalyzed by Au nanoflower. (a−k): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 min) and (b) Plot of −ln(A) against time for the catalytic reduction of pendimethalin by Au nanoflower (conditions: 500 μL 2.34 mM pendimethalin + 500 μL 15 mM NaBH4 + 100 μL Au nanoflower). 10641

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Figure 4. (A)Plot of the absorbance at λ max (λ max = 430 nm) of degradation pendimethalin versus time at different pH (B) Histogram of rate constant of degradation pendimethalin at different pH (pH 1, 4, 7, 11, and 14).

ln[C(P)/C(P)0 ] = −kPt

Optimal Au nanoflower volume. It is essential to carefully optimize the concentration of Au nanoflower due to its crucial role in the degradation of pendimethalin in aqueous solution and complex samples. A too low concentration of Au nanoflower could lead to low catalytic efficiency and longer time, while a too high concentration could have a slower effect on catalytic activity or cause a waste of the material. Both would result in bad rate of the degradation. Thus, the optimization of Au nanoflower concentration was performed (Figure 5). As

(4)

On the basis of the above considerations, pseudo-first-order kinetics could be used to evaluate the kinetic reaction rate of the current catalytic reaction, together with the UV/vis absorption data in Figure 3. As expected, a good linear correlation of ln(A) versus time (A is the absorption intensity of pendimethalin) was obtained (Figure 3), where k is a kinetic reaction rate constant, k = 0.8066 and b = 0.3813. The linear fitting of the experimental data gives a linear correlation coefficient of 0.998 and standard deviation of 3.75. The equation is as follows:

−ln A = kt + b Optimal pH Value. We studied the effect of pH on the removal of pendimethalin because the degradation efficiency of the pendimethalin is pH dependent.38 The changes of UV−vis absorption spectrum as a function of pH and time (Figure 4A) was taken into account. According to Figure 4A, when the pH value was increased from 1 to 11, the time of degradation was gradually prolonged, and the removal efficiency decreased. Despite the fact that velocity constant (Figure 4B) was improved slightly when the pH value was 14, it was accompanied by long degradation time and unsatisfactory removal efficiency. Under acidic conditions, protonation of nitrogen atoms of nitro of pendimethalin could easily be formed, giving them higher affinities with H− produced by NaBH4. Nevertheless, H− is unstable in acidic solution because of the presence of H+. Therefore, the degradation reaction was influenced by both the degree of protonation of nitrogen atom and the amount of H− existence. Under acidic conditions, nitrogen atoms of nitro of pendimethalin could achieve higher degree of protonation, while under alkaline conditions, H− is more stable. Due to redundant NaBH4 in the designed system, the degradation reaction had little effect on H− under acidic condition. But under alkaline conditions, the reaction platform was largely influenced by the degree of protonation. Considering all these, pH under acidic condition, which is more conducive for pendimethalin degradation, was selected in the subsequent experiments, which is in accordance with the literature.38 Since pendimethalin solution is naturally acidic (pH 4−5), no adjustments of pH were done in the experiments.

Figure 5. Plot of the absorbance at λ max of pendimethalin versus time at different volumes of Au nanoflower (a-f: 25, 50, 75, 100, 150, and 200 μL).

shown in Figure 5, the degradation time decreased gradually with the increasing concentration of Au nanoflower, and a turning point of least degradation time was observed when the volume of Au nanoflower reached 150uL (Figure 5). Therefore, an optimized volume of 150 μL Au nanoflower was chosen in the experiments. Degradation of pendimethalin in soil. To demonstrate the practical applicability of the designed strategy, we measured the UV−vis absorption spectra of soil diffusate containing pendimethalin at specific concentrations (Figure 6). It required about 7 min to complete the degradation of pendimethalin in the inorganic soil, and a little longer in organic soil yet within 10 min. Variations in pendimethalin removal efficiency between different soils were consistent with known effects from soil 10642

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Figure 6. UV−vis spectrum of degradation pendimethalin in two soils (a) Red line is the UV−vis spectrum of sandy soil leachate containing pendimethalin. Blue line is UV−vis spectrum of sandy soil leachate. Black line is UV−vis spectrum of sandy soil leachate containing pendimethalin after reaction for about 10 min. (b) Plot of the absorbance at λ max of pendimethalin degradation versus time in sandy soil. (c) Red line is UV−vis spectrum of organic soil leachate containing pendimethalin. Black line is UV−vis spectrum of organic soil leachate. Blue line is UV−vis spectrum of organic soil leachate containing pendimethalin after reaction about 10 min. (d) Plot of the absorbance at λ max of pendimethalin degradation in organic soil versus time. Note: corresponding histograms with error bar (standard deviation from the mean, n = 3).

Figure 7. UV−visible spectra of (A) trifluralin and (B) p-nitrophenol during the reduction catalyzed by Au nanoflower, [NaBH4] 2.34 mM and trifluralin 1.43 mM, p-nitrophenol 0.2 mM. Note: (a) is UV−vis spectrum before degradation reaction and (b) is UV−vis spectrum before degradation reaction. Corresponding histograms with error bar (standard deviation from the mean, n = 3).

nanoflower to design a degradation platform targeting at trifluralin and p-nitrophenol. As expected, in the absence of Au nanoflower, this platform could not function and UV−vis absorption spectrum band (trifluralin at 425 nm and pnitrophenol at 400 nm) did not change in a long duration (Figure 7); however, in the presence of Au nanoflower, the two solutions correspondingly changed from light yellow to transparency and a drop in UV−vis absorption spectrum band was observed (Figure 7). Amazingly, p-nitrophenol was degraded completely within 2 min and trifluralin within 10 min.

organic matter (i.e., removal efficiency decrease with organic content increase), which is in accordance with literature.38 Delightfully, all pendimethalin could be degraded in a short time, hence achieving satisfactory contamination soil remediation. Therefore, this new degradation platform has a great potential in pendimethalin degradation in both organic and inorganic soil with interference. Degradation of Other Nitroaromatic Pesticides. To exemplify the general applicability of our strategy to nitroaromatic pesticides, we applied the same principle and Au 10643

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Figure 8. Mass spectral data of (A) shows the process of degradation of trifluralin (above) to 2,6-2-amino-N,N-2-4-propyl-3-fluorine methylanilines (below); Mass spectral data of (B) shows the process of degradation of p-nitrophenol (above) to p-aminophenol (below). (2) Lange, J. H.; Thomulka, K. W. Use of the Vibrio harveyi Toxicity Test for Evaluating Mixture Interactions of Nitrobenzene and Dinitrobenzene. Ecotoxicol. Environ. Saf. 1997, 38, 2−12. (3) Beltran, J.; López, F. J.; Hernández, F. Solid-phase microextraction in pesticide residue analysis. J. Chromatogr. A 2000, 885, 389−404. (4) Kavitha, V.; Palanivelu, K. Degradation of nitrophenols by Fenton and photo-Fenton processes. J. Photochem. Photobiol., A 2005, 170, 83−95. (5) Kelley, R.; Gauger, W. K.; Srivastava, V. J. Application of Fenton’s Reagent As a Pretreatment Step in Biological Degradation of Polyaromatic Hydrocarbons; Institute of Gas Technology: Chicago, IL, 1990. (6) Pal, S.; Moza, P. N.; Kettrup, A. Photochemistry of pendimethalin. J. Agric. Food Chem. 1991, 39, 797−800. (7) Draper, W. M.; Crosby, D. G. Solar photooxidation of pesticides in dilute hydrogen peroxide. J. Agric. Food Chem. 1984, 32, 231−237. (8) Anhalt, J. C.; Arthur, E. L.; Anderson, T. A.; Coats, J. R. Degradation of atrazine, metolachlor, and pendimethalin in pesticidecontaminated soils: Effects of aged residues on soil respiration and plant survival. J. Environ. Sci. Health B 2000, 35, 417−438. (9) Ji, S.; Yang, Z.; Zhang, C.; Liu, Z.; Tjiu, W. W.; Phang, I. Y.; Zhang, Z.; Pan, J.; Liu, T. Exfoliated MoS2 nanosheets as efficient catalysts for electrochemical hydrogen evolution. Electrochim. Acta 2013, 109, 269−275. (10) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.; Jin, S. Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets. J. Am. Chem. Soc. 2013, 135, 10274−10277. (11) Wang, T.; Zhu, H.; Zhuo, J.; Zhu, Z.; Papakonstantinou, P.; Lubarsky, G.; Lin, J.; Li, M. Biosensor Based on Ultrasmall MoS2 Nanoparticles for Electrochemical Detection of H2O2 Released by Cells at the Nanomolar Level. Anal. Chem. 2013, 85, 10289−10295. (12) Saha, K.; Agasti, S. S.; Kim, C.; Li, X.; Rotello, V. M. Gold nanoparticles in chemical and biological sensing. Chem. Rev. 2012, 112, 2739−2779. (13) Turner, M.; Golovko, V. B.; Vaughan, O. P.; Abdulkin, P.; Berenguer-Murcia, A.; Tikhov, M. S.; Johnson, B. F.; Lambert, R. M. Selective oxidation with dioxygen by gold nanoparticle catalysts derived from 55-atom clusters. Nature 2008, 454, 981−983. (14) Querlioz, D.; Dollfus, P. The Wigner Monte-Carlo Method for Nanoelectronic Devices: A Particle Description of Quantum Transport and Decoherence; John Wiley & Sons: New York, 2013. (15) Jiang, Z. J.; Liu, C. Y.; Sun, L. W. Catalytic properties of silver nanoparticles supported on silica spheres. J. Phys. Chem. B 2005, 109, 1730−1735. (16) Narayanan, K. B.; Sakthivel, N. Synthesis and characterization of nano-gold composite using Cylindrocladium floridanum and its

The degredation of the parent pesticide were confirmed by gas chromatography−mass spectrometry (GC−MS), Figure 8A,B shows that trifluralin and p-nitrophenol were reduced to 2,6-2amino-N,N-2-4-propyl-3-fluorine methylanilines and p-aminophenol, respectively. Thus, when finished the degredation reaction, nitro groups of pesticides were reduced to amino groups of degredation product. The universality of the approach is achieved by virtue of catalyst of Au nanoflower without any change of the degradation platform structure. In summary, we have developed a simple, rapid, facile, economically viable, and “green” growth approach without the use of seeds to synthesize branched gold nanocrystals which resemble the shape of a flower through NH2OH reduction of HAuCl4. Under optimal conditions, the synthesized Au nanoflower can complete the degradation of pendimethalin in the presence of NaBH4 within 4 min. In addition, the designed strategy is well applied not only to pendimethalin degradation in practical soil samples, but also to the degradation of common nitroaromatic pesticides including trifluralin and p-nitrophenol. We believe that this novel degradation platform can be very useful in the fields of environment protection, soil contamination remediation, and sewage treatment. It has also opened a window to the wide applications of synthesized Au nanoflower in pharmaceuticals, biomedicals, and biosensors in the future.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-027-68754067; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support from the Natural Science Foundation of China (NSFC) (No. 20927003, 90913013, 41273093, and 21175101), the National Major Scientific Instruments and Device Development Project (2012YQ16000701), and the Foundatio of China Geological Survey (Grant No. 12120113015200).

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REFERENCES

(1) Gatermann, R.; Hühnerfuss, H.; Rimkus, G.; Wolf, M.; Franke, S. The distribution of nitrobenzene and other nitroaromatic compounds in the North Sea. Mar. Pollut. Bull. 1995, 30, 221−227. 10644

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dx.doi.org/10.1021/jf5034015 | J. Agric. Food Chem. 2014, 62, 10638−10645