Article pubs.acs.org/Langmuir
Preparation of Bimetallic Nanoparticles Using a Facile Green Synthesis Method and Their Application Bihua Xia, Fang He, and Lidong Li* State Key Lab for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China S Supporting Information *
ABSTRACT: A straightforward, economically viable, and green approach for the synthesis of well-stabilized Au/Ag bimetallic nanoparticles is described; this method uses nontoxic and renewable degraded pueraria starch (DPS) as a matrix and mild reaction conditions. The DPS acted as both a reducing agent and a capping agent for the bimetallic nanoparticles. Au/Ag bimetallic nanoparticles were successfully grown within the DPS matrixes, and the bimetallic structures were characterized using various methods, including high-resolution transmission electron microscopy, energy-dispersive X-ray, and X-ray diffraction. Moreover, it was shown that these DPS-capped Au/Ag bimetallic nanoparticles could function as catalysts for the reduction of 4-nitrophenol in the presence of NaBH4 and were more effective than Au or Ag monometallic nanoparticles.
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INTRODUCTION Recently, bimetallic nanoparticles have received considerable attention because of their importance for magnetic, optical, and catalytic applications in a variety of fields; their value arises from their distinctive properties, which are clearly different from those of monometallic nanoparticles.1−4 There are a number of existing methods for the synthesis of bimetallic nanoparticles;5,6 and owing to the environmentally friendly nature of the green method, this technique for the synthesis of metallic nanoparticles has attracted much attention.7−11 Various green methods have been described for the generation of different kinds of bimetallic nanoparticles in solution, including Au/Ag bimetallic nanoparticles.12,13 For example, radiation-induced synthesis,14,15 electrochemical synthesis,16−18 and sonochemical synthesis19 have been used to produce metallic nanoparticles, and these methods do not require a chemical reducing agent. However, most green synthesis methods involve the use of a reducing agent reaction to synthesize metallic nanoparticles.12,13 Mallin et al. used sodium borohydride as a reducing agent and sodium citrate as a capping agent to prepare stabilized Au/Ag bimetallic nanoparticles in water, in a room temperature environment.20 Chen et al. used hydrazine as a reducing agent, simultaneously reducing Au3+ ions and Ag+ ions in solution to form bimetallic nanoparticles.21 To simulate the synthesis of bimetallic nanoparticles in a cellular environment, Zhang et al. used the coenzyme reduced β-nicotinamide adenine dinucleotide 2′phosphate as the reducing agent in a quasi-biological system to prepare sub-5 nm Au−Ag alloy nanoparticles.22 Besides these well-defined/pure reducing agents for the synthesis of metallic nanoparticles, there are also undefined/mixture reducing agents that can produce noble metals. Sheny et al. use aqueous extract and dried leaf complex of Anacardium occidentale to synthesize © XXXX American Chemical Society
Au, Ag monometallic nanoparticles, and Au/Ag bimetallic nanoparticles.23 Mushroom extract was also used as a reducing agent for the synthesis of Au/Ag bimetallic nanoparticles.24 Although the methods for the preparation of metallic nanoparticle are mature, there is still a need for novel, facile, green synthetic systems for the preparation of bimetallic nanoparticles with nontoxic and biocompatible matrixes. Methods that use polysaccharides to produce bimetallic nanoparticles are gradually becoming more interesting for general use.25 Our group has reported a green method for the synthesis of Ag monometallic nanoparticles using tapioca starch.26 To further improve techniques for the preparation of bimetallic nanoparticles, we investigated the use of degraded pueraria starch (DPS) with good water solubility and biocompatibility for the preparation of bimetallic nanoparticles via a facile method. Here, we report a novel green method for the synthesis of Ag/Au bimetallic nanoparticles. This method used a simple procedure involving the mixing Au ions with as-prepared Ag nanoparticle seeds, which were dispersed in an aqueous solution of DPS at room temperature. In contrast with the conventional procedures, DPS acted as both the reduction agent and the capping agent in this system.27,28 This ensured the formation of well-controlled DPS-coated metallic nanoparticles in the DPS matrixes. The obtained bimetallic nanoparticles showed good catalytic activity in the reduction of 4-nitrophenol. Received: January 26, 2013 Revised: March 20, 2013
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resulting in a solution pH of ∼10.0. The final concentrations of DPS, HAuCl4, and NH3·H2O were 29 mg/mL, 0.48 mM, and 12.4 mM, respectively. The reaction mixture was then heated to 80 °C in an oil bath for 2.5 h, which led to the formation of Au nanoparticles. The Au nanoparticles obtained in the solutions were precipitated via centrifugation at 2000 rpm and were washed twice to remove excess protective agent and unreacted reactants. The final concentration of final Au(0) was 0.15 mM. Preparation of Bimetallic Au/Ag Nanoparticles. Au/Ag bimetallic nanoparticles were prepared using a replacement reaction between Ag nanoparticles seeds and Au3+ ions. 7.5 × 10−6 mol of HAuCl4 aqueous solution (300 μL, 25 mM) was dissolved in 12.5 mL of deionized water and reacted with 2 × 10−6 mol of Ag NPs colloidal solution (1 mL, 2 mM). The mixture solution was stirred (for ∼24 h) at room temperature until the color of the aqueous solution stabilized to give a red wine color. After centrifugation (at 2000 rpm), the red solution and redispersed in deionized water. Finally, we have acquired the 0.15 mM of Au/Ag nanoparticles. Catalytic Reduction Activity of Au/Ag Nanoparticles toward 4-Nitrophenol. The reduction of 4-nitrophenol was studied as a model reaction to confirm the catalytic activity of the synthesized DPS-capped Au/Ag nanoparticles. The catalytic reaction was performed in a standard quartz cuvette with a 1 cm path length. The reaction procedure was as follows: 1.0 mL of 0.015 M NaBH4 was mixed with 1.7 mL of 0.2 mM 4-nitrophenol in the quartz cuvette, leading to a color change from light yellow to yellow-green. Then 0.3 mL of the as-formed DPS-capped Au/Ag nanoparticle solution was then added to the above reaction mixture as a homogeneous catalyst. Immediately after the addition of the DPS-capped bimetallic nanoparticles solution, the UV−vis absorption spectra was recorded at 30 s intervals, in the scanning range of 200−700 nm, at room temperature (25 ± 2 °C). As control experiments, 0.3 mL of DPS reduced Ag or Au monometallic nanoparticle solutions and a physical mixture of these as-prepared Au nanoparticles and the Ag nanoparticle solution (5:1, v/v) were also used as the catalyst added to the reaction solution for the reduction 4-nitrophenol.
EXPERIMENTAL PART
Materials and Measurements. Pueraria starch was purchased from Xichuan Chunyu Geye Biotechnology Co., Ltd. (Henan, China). α-Amylase and β-amylase were purchased from J&K Scientific Ltd. Silver nitrate (AgNO3) and chloroauric acid (HAuCl4·3H2O) were purchased from Aladdin Chemistry Co., Ltd. 4-Nitrophenol, aqueous ammonia, sodium borohydride (NaBH4), acetone, and ethanol were purchased from Beijing Chemical Reagent Co., Ltd. Deionized water was obtained from a TTL-30B water purification system and was used in all experiments. All of these reagents were used without further purification, unless otherwise stated. UV−vis spectra were recorded using a Hitachi U3900 spectrophotometer with 1 cm quartz cells at room temperature. For transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) studies, the aqueous nanoparticle suspension was dropped on carbon-coated Cu grids (200 mesh) and allowed to dry overnight in air. TEM and HR-TEM images were recorded on a JEOL JEM 2100 TEM with an accelerating voltage of 200 kV. Selected area diffraction patterns were obtained using the TEM instrument. The energy-dispersive X-ray (EDX) analyzer attached to the TEM instrument was used to analyze the composition of the bimetallic nanoparticles. Fourier transform infrared spectra (FTIR) were collected using a Nicolet 170SX spectrometer in transmittance mode, in the range of 400−4000 cm−1. The aqueous suspensions of DPS or DPS-capped Au/Ag nanoparticles were added to acetone over a period of 2 h and were then filtered to remove the acetone; this operation was repeated twice. The purified DPS and DPS-capped Au/Ag nanoparticles were then airdried for 24 h to obtain a dry powder. Subsequently, ∼5 mg of the dry powder was mixed with ∼100 mg of KBr to form pellets for the FTIR studies. The average of eight scans was collected with a resolution of 4 cm−1. X-ray diffraction (XRD) analysis of the DPS-synthesized Ag nanoparticles, Au nanoparticles, and Au/Ag bimetallic nanoparticles was performed using powdered samples in air, using a TTRIII X-ray diffractometer (operating at 50 kV and 100 mA); Cu (KR = 1.540 56) was used as the X-ray source to scan the 2θ angle range of 5°−90°, and the scanning rate was 2°/min. The calcination experiment was measured by Muffle furnace (SX2-2.5-12) at 500 °C for 2 h. Preparation of DPS. The degradation experiments on pueraria starch were as follows: 15 mL of a 30 mg/mL aqueous pueraria starch solution was placed in a 50 mL single-ported beaker, and 0.03 g of αamylase and 0.03 g of β-amylase were slowly added to this solution. The reaction was performed by heating this mixture in an oil bath at 40 °C for 24 h, under stirring at 1200 rpm.29−31 After the degradation reaction had proceeded for 24 h, the mixture solution was then heated to 100 °C for 30 min to deactivate the amylase. Finally, the mixture solution was left to stand, and the sedimentary amylase was removed. Preparation of Monometallic Ag Nanoparticles. To obtain Ag nanoparticles, we first prepared a silver−ammonia solution (Ag(NH3)2OH solution), as follows: 200 μL of a 1.2 M NH3·H2O aqueous solution was slowly dropped into 2 mL of 0.12 M silver nitrate solution, and the mixture was shaken until it became colorless. Aqueous DPS dispersions containing Ag(NH3)2+ ions were then prepared by adding 300 μL of a 0.1 M Ag(NH3)2OH solution to 15 mL of a 30 mg/mL DPS aqueous solution. This mixture solution was then heated in an oil bath at 60 °C for 2.5 h to achieve the reduction of the Ag+ ions. The reduction reaction was performed in a dark environment. The formed Ag nanoparticles were centrifuged (2000 rpm) for 3 min and then redispersed in deionized water. The process of centrifugation and redispersion in an equal amount of deionized water was repeated twice to ensure better separation of the undecorated entities from the surfaces of the metal nanoparticles. The final concentration of prepared Ag nanoparticles was 2 mM. Preparation of Monometallic Au Nanoparticles. The preparation of Au nanoparticles was achieved via the reduction of a HAuCl4 aqueous solution with DPS solution in an alkaline aqueous environment. In the typical synthesis process, 15 mL of as-prepared 30 mg/mL DPS solution and 300 μL of a 25 mM HAuCl4 aqueous solution were first mixed at room temperature to give a clear light yellow solution, to which 160 μL of 1.2 M NH3·H2O was added,
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RESULTS AND DISCUSSION Pueraria starch is one of the richest starch products in China;32 it has many hydroxyl groups, a high molecular weight, renewable properties, a high swelling temperature and gelatinization temperature, strong gelation abilities, and low activity.33 It is typically used in edible films and coating materials owing to its abundance, low cost, and biodegradability.34 However, these characteristics also hinder the applications of this starch in some fields; this is especially true for green synthetic chemistry, where people typically adopt degradation or oxidization methods to modify the macrostructure of starch to enhance its usefulness for applications.35−37 There is another reason for the degradation or oxidization action: the starch can release activity groups (especially the aldehyde groups of the glucose unit) to the solution system, which can reduce Ag ions to Ag species. In this experiment, we used the degradation method to modify pueraria starch. The degradation reaction equation for pueraria starch is shown in Scheme 1 of the Supporting Information. The prepared DPS was generated through this degradation reaction. DPS is a mixture of various kinds of oligosaccharides (such as glucose, maltose, and dextrin), 38 and these oligosaccharides can reduce positive charged Ag ions to give zerovalent metallic Ag.18 Because the obtained DPS solution contained many aldehyde activity groups, and these groups could react with Ag(NH3)2+ ions to form Ag nanoparticles, similar to a mirror reaction.26 The reaction process occurred as the as-prepared silver ammonia solution and the obtained DPS solution were mixed B
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and heated to 60 °C in a dark environment; reduction occurred spontaneously, forming the DPS-Ag nanoparticle colloid within ∼2.5 h. The time evolution UV−vis absorption spectra for the Ag nanoparticles are shown in Figure 1a. We found that the
electron transfer, and there was an obvious appearance of a plasmon band at 120 min in the UV−vis spectra. After this reaction, the aldehyde groups in DPS were oxidized to carboxyl groups.44 TEM images of Au nanoparticles in 90 and 120 min are shown in Figure S3 of the Supporting Information. The number and size of Au nanoparticles in 90 min were very few and small ones; however, they became more and larger when Au nanoparticles in 120 min. This result corresponded with the UV−vis data and demonstrated the formation mechanism of Au(0). During this experiment, ammonia acted as an accelerating agent, and OH− acted as the catalyst for the reduction process.45 If no ammonia, or insufficient ammonia, was added to the reaction system, DPS failed to reduce the metallic precursor salts, as a result of its weak reduction ability under acidic, neutral, or weakly basic conditions.46 While Au(I) existed as the intermediate, their ability will undergo dismutation. The Au(0) yield can be obtained by muffle furnace experiment. We found 94% of Au ions can be converted to Au(0). Figure 2b shows a TEM image of the Au nanoparticles in 150 min. The Au nanoparticles were relatively small and well dispersed, which was in good agreement with the UV−vis absorption spectra for the Au nanoparticles. Figure S1b shows the size-distribution analysis picture of Au nanoparticles in 150 min. The diameter of Au nanoparticles was mainly distributed between 4 and 5 nm, and the standard deviation (σ) was about 0.3 nm. By using the as-synthesized monodisperse Ag nanoparticles as templates, monodisperse Au/Ag alloy nanoparticles were prepared via the galvanic replacement reaction between Ag nanoparticles and HAuCl4. Since the standard reduction potential of AuCl4−/Au (1.0 V vs standard hydrogen electrode, or SHE) is higher than that of Ag+/Ag (0.80 V vs SHE), the silver atoms in the nanoparticles were oxidized to Ag+ upon mixing with AuCl4− in the solution. The replacement reaction between the Ag nanoparticles and AuCl4− can be represented by eq 1:
Figure 1. (a) Time evolution of UV−vis absorption spectrum and (b) TEM image of DPS reduced Ag nanoparticles in 150 min.
intensity of the surface plasmon absorption (SPR) increased with time, indicating continuous reduction of the metal ions, and we also observe a slight blue-shift in the λmax of Ag nanoparticles with time, which indicated that the size of the Ag nanoparticles changed smaller. This phenomenon may be attributed to little amount of Ag nanoparticles were oxidized in aqueous solution.39,40 The reaction could also be visualized via the color changes in the resulting colloid solution (from colorless to deep brown, for the Ag nanoparticles). We also took TEM images of the DPS-reduced Ag nanoparticles in 150 min. As shown in Figure 1b, well-dispersed Ag nanoparticles with the morphology of approximately roundness were obtained in the DPS solution. The size-distribution analysis picture of Ag nanoparticles in TEM image is shown in Figure S1a; their mean diameter and standard deviation (σ) were about 26 and 1.6 nm, respectively. While for comparation, the TEM image and size-distribution analysis picture of Ag nanoparticles in 90 min were also measured (Figure S2); the size and standard deviation became larger and broaden. This result corresponded with the UV−vis spectra. While mild heating was sufficient for the DPS reduction of silver ions, the reduction of the Au3+ ions took place only in an alkaline environment. We synthesized Au nanoparticles by adding 300 μL of 25 mM HAuCl4 to 15 mL of a 30 mg/mL DPS solution, followed by the addition of 160 μL of a 12 mM ammonia aqueous solution over a period of 2.5 h, at 80 °C in a dark environment. UV−vis absorption spectra were also used to analyze the growth of the Au nanoparticles (Figure 2a). During the period before 90 min, Au(III) ions was reduced to Au(I) first41 by electron transfer between the metal ions and the hydroxyl groups or aldehyde groups,42,43 so there was no obvious plasmon band in absorption spectra. Further, more and more Au(I) ions were reduced to Au(0) nanoparticles also by
3Ag(s) + AuCl4 − → Au(s) + 3AgCl ↓(s) +Cl−
(1)
Meanwhile, the AuCl4− ions were reduced to Au atoms, which were deposited on the nanoparticle surface.13 The gold atoms then alloyed with the unreacted silver atoms under appropriate conditions to form the alloy nanoparticles. The reduced dimensions of the Ag particles and the large number of vacancy defects generated by the replacement reaction allowed the interdiffusion of the Ag and Au atoms to occur.47 Because of the SPR character of bimetallic Au/Ag nanoparticles, we could measure their absorption curve in the UV− vis region. Figure 3a shows UV−vis spectra for the as-formed
Figure 3. (a) UV−vis absorption spectra and (b) TEM image of DPScapped Au/Ag bimetallic nanoparticles. The inset shows a 5× magnified image of a DPS-capped bimetallic Au/Ag nanoparticle.
Figure 2. (a) Time evolution of UV−vis absorption spectra and (b) TEM image of DPS reduced Au nanoparticles in 150 min. C
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of the DPS-capped Au/Ag bimetallic nanoparticles; there was excellent atomic ordering within each particle. There was no lattice mismatching, as expected from the highly similar lattice constants of 0.408 for Au and 0.409 for Ag.50 Owing to the similar lattice constant values for the gold and silver, it was not possible to completely confirm the alloy nature of the nanoparticles from the HR-TEM images; thus, electron diffraction patterns were used to analyze the structure of the bimetallic Au/Ag nanoparticles (Figure 5c). The five ring patterns in the electron diffraction patterns corresponded to the 111, 200, 220, 311, and 331 reflections of the fcc structure, which indicated the Au/Ag bimetallic structure of the nanoparticles.50 To further confirm the compositional homogeneity of the nanoparticles, we also recorded XRD patterns for the monometallic Au nanoparticles, monometallic Ag nanoparticles, and bimetallic Au/Ag nanoparticles. Figure 6 shows the
Ag/Au bimetallic nanoparticles. Only one SPR band was detected for the Au/Ag nanoparticles (λmax = 545 nm), rather than two SPR bands. This observation indicated that the nanoparticles were an alloy structure with a homogeneous composition, rather than a physical mixture of monometallic Au and Ag nanoparticles.48 Figure 3b shows a TEM image of the as-formed bimetallic Au/Ag nanoparticles. Most of the particles were nearly spherical; they have similar morphology and structure. Their average diameter (d) and standard deviation (σ) were estimated to be 32 and 1.6 nm, respectively (Figure S1c); this was larger than either the monometallic Ag nanoparticles or the monometallic Au nanoparticles. This increase in the diameter might have been caused by an ingression of Au into the Ag NPs after the reduction of the Au ions, which could also be considered as a reason for the surface plasmon enhancement.49 The particles prepared here showed a lighter DPS coating banded layer; this layer contained a large amount of hydrophilic hydroxyl groups that allowed the bimetallic nanoparticles to disperse well in aqueous solution. In addition, the nanoparticles were distributed homogeneously, which indicated that there was no aggregation between the nanoparticles, further confirming that the bimetallic Au/Ag nanoparticles were coated with hydrophilic DPS molecules and were stable in aqueous solution. The composition of the nanoparticles recovered from the replacement reaction was analyzed using EDX. Figure 4 shows
Figure 6. XRD patterns for DPS synthesized Ag nanoparticles, Au nanoparticles, and bimetallic Au/Ag nanoparticles.
XRD patterns for these nanoparticles. The 2θ values for Au/Ag alloy nanoparticles were very similar to each other because Ag and Au have very similar lattice constants (JCPDS 4-0784 and 4-0783).50 The diffraction peaks at 38.2°, 44.2°, 64.4°, and 77.5° corresponded to the (111), (200), (220), and (311) reflections of cubic Ag (or Au/Ag alloy), with the exception that sharp peaks at 27° and 32° were also detected using XRD; they corresponded to the reflections of AgCl precipitation, which appeared in eq 1.51 The diffraction peaks at 38.2°, 44.2°, 64.4°, and 77.5° associated with the monomeric Ag and Au nanoparticles’ lattice planes were very weak and broad, but they were significantly enhanced in the bimetallic Au/Ag nanoparticles. This result provided further evidence that the bimetallic Au/Ag nanoparticles fabricated in the present study had a homogeneous bimetallic structure. Because the Ag nanoparticles were synthesized in situ in the DPS molecules, the DPS molecules could also serve as a host
Figure 4. EDX spectra for DPS-capped bimetallic Au/Ag nanoparticles.
the EDX spectra for the DPS-capped Au/Ag alloy nanoparticles. The proportions of Au and Ag which measured using EDX were 92.49% and 7.51%, respectively. The uniformity in the particle-to-particle composition, and the concurrent presence of both Ag and Au in each particle, confirmed that the particles had an Au/Ag alloy structure. HR-TEM was then used to analyze the formed bimetallic Au/Ag nanoparticles. Figure 5a−c shows the HR-TEM image
Figure 5. (a) HR-TEM image, (b) 2.5× magnified HR-TEM image, and (c) electron diffraction pattern for DPS-capped bimetallic Au/Ag nanoparticles. The five rings (from inner to outer) corresponded to the 111, 200, 220, 311, and 331 reflections. D
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Figure 7. Successive UV−vis absorption spectra for the reduction of 0.2 mM 4-nitrophenol by 0.015 M NaBH4 in the presence of (a) DPS-capped bimetallic Au/Ag nanoparticles, (b) without any nanoparticles, (c) DPS-reduced Ag nanoparticles, (d) DPS-reduced Au nanoparticles, and (e) DPSreduced Ag nanoparticles and DPS-reduced Au nanoparticles in the molar ratio of 1:5.
nitrophenol underwent a red shift from 317 to 400 nm (due to the generation of 4-nitrophenolate ions),44 concurrent with a significant change in the solution color from light yellow to yellow green. Over the course of 150 s, the intensity of the absorption peak at 400 nm rapidly decreased, until the peak ultimately disappeared; simultaneously, peaks appeared at 298 nm, which indicated the formation of the reduction product 4aminophenol (which is not possible if only the strong reducing agent NaBH4 is employed).44 To confirm the higher catalytic activity of the DPS-capped bimetallic Au/Ag nanoparticles, we also investigated the rate of catalysis activated by the 0.15 mM of DPS reduced monometallic Ag nanoparticles, 0.15 mM of DPS reduced monometallic Au nanoparticles, a physical mixture solution of DPS reduced monometallic Ag and Au nanoparticles (molar ratio = 1:5), and no nanoparticles. These UV−vis absorption spectra for the 4-nitrophenol reduction reaction are shown in Figure 7b−e; the successive reduction times were 66, 35, and 35 min. In a comparison of the results, we found that the DPScapped bimetallic Au/Ag nanoparticles showed prominently better catalytic activity; specifically, the activity was 26.4, 14, and 14 times higher than those associated with the DPSsynthesized monometallic nanoparticles.44−46,58,59 These results can be explained by the following two factors. First, the nanoparticles had a bimetallic composition, and structure effects, composite effects, and size effects resulted from the intimate interactions between the two metals.13,60,61 Second, a synergistic electronic effect was produced in the bimetallic nanoparticles; this effect meant that electrons could transfer from Ag to Au, leading to an increase in the electron density on the surface of the DPS-capped bimetallic Au/Ag nanoparticles and finally improving the catalytic activity.49,62 In addition, to exclude the possibility that the reduction reaction might have been activated by the pure DPS molecules instead of the bimetallic Au/Ag nanoparticles, 300 μL of an aqueous solution of DPS (30 mg/mL) alone was added to an aqueous mixture of 1.7 mL of 4-nitrophenol (0.20 mM) and 1 mL of NaBH4 (0.015 M). No change in the color and position of the absorption band (at 400 nm) for 4-nitrophenolate ion was observed. These experimental phenomena demonstrated
matrix to stabilize the metal nanoparticles. Under the continuous supply of an HAuCl4 aqueous solution, the Au ions were reduced and subsequently deposited on the surface of the Ag nanoparticles, leading to the growth of the nanoparticles into the grain size; this process could be controlled via the steric effects provided by the DPS oligosaccharides.47 Since the nanoparticles were trapped within the DPS matrix, the DPS was tightly connected with the metallic nanoparticles and endowed them with good water solubility; this ensured that the nanoparticles did not aggregate.52 The capping mechanism and the stability effects of DPS on the formation of the bimetallic Au/Ag nanoparticles are therefore topics of interest in the current investigation. To investigate the binding interactions of DPS with the Au/ Ag nanoparticles, we used FTIR spectrometry.53 The FTIR spectra for DPS, and the DPS-capped Au/Ag bimetallic nanoparticles, are shown in Figure S4. The spectra for the DPS-capped bimetallic Au/Ag nanoparticles showed a similar shape to the spectra for DPS, with the exception of the peaks at 1701, 1328, and 1234 cm−1; these peaks were significantly weaker in the spectra for the DPS-capped Au/Ag nanoparticles, probably because the vibrations of the aldehyde and hydroxyl groups in DPS were impeded by the metal nanoparticles.47 These results indicated that the DPS molecules had a good connection with the bimetallic surface.44,47 Bimetallic Au/Ag nanoparticles can have enhancing properties in a number of applications; one of the most important of these is the activation/catalysis of reactions that are otherwise unfeasible.54−56 The reduction of 4-nitrophenol by NaBH4 was chosen as a model reaction to study the catalytic performance of the bimetallic Au/Ag nanoparticles.57 Approximately 0.34 × 10−6 mol of 4-nitrophenol solution (1.7 mL, 0.2 mM) was mixed with 1.5 × 10−7 mol of NaBH4 solution (1 mL of a 0.015 M) in a 4 mL standard quartz cuvette (path length 1 cm). Then 0.45 × 10−7 mol of DPS-capped Au/Ag bimetallic nanoparticle solution (0.3 mL, 0.15 mM) was added to this reaction mixture, giving a loading of 30%. The UV−vis absorption spectrum measured during the reduction of 4-nitrophenol by NaBH4 in the presence of the DPS-capped Au/Ag bimetallic nanoparticles is shown in Figure 7a. The absorption peak associated with 4E
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(5) Scott, R. W. J.; Wilson, O. M.; Oh, S.-K.; Kenik, E. A.; Crooks, R. M. Bimetallic Palladium−Gold Dendrimer-Encapsulated Catalysts. J. Am. Chem. Soc. 2004, 126, 15583−15591. (6) Park, J. Y.; Zhang, Y.; Grass, M.; Zhang, T.; Somorjai, G. A. Tuning of Catalytic CO Oxidation by Changing Composition of Rh− Pt Bimetallic Nanoparticles. Nano Lett. 2008, 8, 673−677. (7) Choi, H. C.; Shim, M.; Bangsaruntip, S.; Dai, H. Spontaneous Reduction of Metal Ions on the Sidewalls of Carbon Nanotubes. J. Am. Chem. Soc. 2002, 124, 9058−9059. (8) Engelbrekt, C.; Sørensen, K. H.; Zhang, J.; Welinder, A. C.; Jensen, P. S.; Ulstrup, J. Green Synthesis of Gold Nanoparticles with Starch−Glucose and Application in Bioelectrochemistry. J. Mater. Chem. 2009, 19, 7839−7847. (9) He, J.; Kunitake, T.; Nakao, A. Facile In Situ Synthesis of Noble Metal Nanoparticles in Porous Cellulose Fibers. Chem. Mater. 2003, 15, 4401−4406. (10) Yang, S.; Zhou, C.; Liu, J.; Yu, M.; Zheng, J. One-Step Interfacial Synthesis and Assembly of Ultrathin Luminescent AuNPs/Silica Membranes. Adv. Mater. 2012, 24, 3218−3222. (11) Deplanche, K.; Merroun, M. L.; Casadesus, M.; Tran, D. T.; Mikheenko, I. P.; Bennett, J. A.; Zhu, J.; Jones, I. P.; Attard, G. A.; Wood, J.; Selenska-Pobell, S.; Macaskie, L. E. Microbial Synthesis of Core/Shell Gold/Palladium Nanoparticles for Applications in Green Chemistry. J. R. Soc. Interface 2012, 9, 1705−1712. (12) Wilson, O. M.; Scott, R. W. J.; Garcia-Martinez, J. C.; Crooks, R. M. Synthesis, Characterization, and Structure-Selective Extraction of 1−3 nm Diameter Au-Ag Dendrimer-Encapsulated Bimetallic Nanoparticles. J. Am. Chem. Soc. 2005, 127, 1015−1024. (13) Chen, Y.-H.; Yeh, C.-S. A New Approach for the Formation of Alloy Nanoparticles: Laser Synthesis of Gold−Silver Alloy from Gold−Silver Colloidal Mixtures. Chem. Commun. 2001, 0, 371−372. (14) Treguer, M.; Cointet, de, C.; Remita, H.; Khatouri, J.; Mostafavi, M.; Amblard, J.; Belloni, J. Dose Rate Effects on Radiolytic Synthesis of Gold−Silver Bimetallic Clusters in Solution. J. Phys. Chem. B 1998, 102, 4310−4321. (15) Cottancin, E.; Lermé, J.; Gaudry, M.; Pellarin, M.; Vialle, J.-L.; Broyer, M. Size Effects in the Optical Properties of AunAgn Embedded Clusters. Phys. Rev. B 2000, 62, 5179−5185. (16) Reetz, M. T.; Helbig, W.; Quaiser, S. A. Electrochemical Preparation of Nanostructural Bimetallic Clusters. Chem. Mater. 1995, 7, 2227−2228. (17) Kolb, U.; Quaiser, S. A.; Winter, M.; Reetz, M. T. Investigation of Tetraalkylammonium Bromide Stabilized Palladium/Platinum Bimetallic Clusters Using Extended X-ray Absorption Fine Structure Spectroscopy. Chem. Mater. 1996, 8, 1889−1894. (18) Tsai, T.-H.; Thiagarajan, S.; Chen, S.-M. Ionic Liquid Assisted One Step Green Synthesis of Au−Ag Bimetallic Nanoparticles. J. Appl. Electrochem. 2010, 40, 493−497. (19) Mizukoshi, Y.; Fujimoto, T.; Nagata, Y.; Oshima, R.; Maeda, Y. Characterization and Catalytic Activity of Core−Shell Structured Gold/Palladium Bimetallic Nanoparticles Synthesized by the Sonochemical Method. J. Phys. Chem. B 2000, 104, 6028−6032. (20) Mallin, M. P.; Murphy, C. J. Solution-Phase Synthesis of Sub-10 nm Au−Ag Alloy Nanoparticles. Nano Lett. 2002, 2, 1235−1237. (21) Chen, D.-H.; Chen, C.-J. Formation and Characterization of Au−Ag Bimetallic Nanoparticles in Water-in-Oil Microemulsions. J. Mater. Chem. 2002, 12, 1557−1562. (22) Zhang, M.-X.; Cui, R.; Zhao, J.-Y.; Zhang, Z.-L.; Pang, D.-W. Synthesis of Sub-5 nm Au−Ag Alloy Nanoparticles Using BioReducing Agent in Aqueous Solution. J. Mater. Chem. 2011, 21, 17080−17082. (23) Sheny, D. S.; Mathew, J.; Philip, D. Phytosynthesis of Au, Ag and Au−Ag Bimetallic Nanoparticles Using Aqueous Extract and Dried Leaf of Anacardium occidentale. Spectrochimica. Acta, Part A 2011, 79, 254−262. (24) Philip, D. Biosynthesis of Au, Ag and Au−Ag Nanoparticles Using Edible Mushroom Extract. Spectrochim. Acta, Part A 2009, 73, 374−381.
that the DPS synthesized bimetallic Au/Ag nanoparticles had excellent catalytic activity for the reduction of 4-nitrophenol.
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CONCLUSION In summary, a facile, economically viable, and green approach for the synthesis of well-controlled Au/Ag bimetallic nanoparticles was developed by employing DPS as a reduction agent. It is worth noting that DPS acted not only as the reducing agent but also as the capping agent in the galvanic replacement reaction for the synthesis of the bimetallic Au/Ag nanoparticles. The catalytic activity of the DPS-capped bimetallic Au/Ag nanoparticles was investigated for the reaction of reduction 4-nitrophenol. It was found that the DPS-capped bimetallic Au/Ag nanoparticles showed higher catalytic activity than DPS synthesized monometallic nanoparticles. This synthetic method for the preparation of metallic nanoparticles could be extended to other bimetallic nanoparticle systems (for example, the Pt/Ag bimetallic nanoparticle system); this is currently under further investigation. It would also be relevant to explore the possible biomedical and biosensor applications of the starch coated metal nanoparticles to take advantage of the biologically compatible, medicinal, and economical characteristics of the pueraria starch resource.
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ASSOCIATED CONTENT
S Supporting Information *
Scheme of degradation reaction of DPS, TEM image and sizedistribution analysis picture of Ag nanoparticles, TEM images of Au nanoparticles formed in 90 and 120 min, FTIR spectrum of DPS and DPS-capped bimetallic Au/Ag nanoparticles. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel +86 10 82377202; Fax +86 10 82375712; e-mail lidong@ mater.ustb.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (20904003), the Fundamental Research Funds for the Central Universities of China, the State Key Lab for Advanced Metals and Materials (2012-ZD05), the Program for Changjiang Scholars and Innovative Research Team in University, and the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-11-0576).
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REFERENCES
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