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Biosynthesis of Gold Nanoparticles Assisted by Sapindus mukorossi Gaertn. Fruit Pericarp and Their Catalytic Application for the Reduction of p‑Nitroaniline Venu Reddy, Ramulu Sri Torati, Sunjong Oh, and CheolGi Kim* Center for NanoBioEngineering & SpinTronics (nBEST) and Department of Materials Science and Engineering, Chungnam National University, Daejeon, 305-764, South Korea S Supporting Information *

ABSTRACT: We have presented a biological and eco-friendly method for the synthesis of gold nanoparticles from gold precursor (HAuCl4) using Sapindus mukorossi Gaertn. fruit pericarp (soapnut shells). We investigate the production of gold nanoparticles as a function of the concentration of HAuCl4 and the amount of soapnut shells. Average nanoparticle sizes of 9, 17, and 19 nm were obtained by using the HAuCl4 concentrations of 1, 5, and 10 mM, respectively, with a fixed amount of soapnut shells extract. The resulted gold nanoparticles are highly crystalline face-centered cubic (fcc) structures. FT−IR analysis suggests that the obtained gold nanoparticles might be stabilized through the interactions of carboxylic groups in the saponins and the carbonyl groups in the flavonoids present in the soapnut shells. These soapnut shells mediated gold nanoparticles were demonstrated to have good catalytic activity for the chemical reduction of p-nitroaniline. Don,17 Mentha piperita L.,18 Diopyros kaki Thunb.,19 pear fruit, Emblica of f icinalis Gaertn.,21 and pelargonium graveolens L’Her.22 Some of the above-mentioned methods require the plant extract employed broths obtained from boiled plant materials.15,19,22 An aqueous extract of Sapindus mukorossi Gaertn. fruit pericarp (soapnut shells) mainly composed of saponins (natural surfactants), flavonoids, and carbohydrates.23 The native people of Asia and America have long been using soapnuts for washing.24 The chemicals most commonly used as surfactants in commercial detergents release carcinogenic toxins into the environment during production. The best alternative to avoid this problem is to use soapnuts for the production of detergents because they are natural and biodegradable. Thus, soapnuts are considered and used commercially in detergents, cosmetics, and other products.25 Moreover, soapnuts have some medicinal properties such as anti-inflammatory26 and anti-microbial activities.24 In very recent years, the use of biosynthesized nanomaterials in catalytic applications has made great progress. The surface biomolecules (functional groups) of biosynthesized nanomaterials influence the catalytic behavior of nanomaterials.4,27−29 In literature, to examine the catalytic performance of gold nanoparticles, the reduction of nitro compounds with an excess amount of sodiumborohydride has often been used as a model reaction.30 For instance, Chirea et al. have studied the catalysis of p-nitroaniline reduction with gold nanowire networks.6 Kundu et al. reported that the small spherical gold nanoparticles show higher catalytic activity in p-nitroaniline reduction with respect to the spherical gold nanoparticles of

1. INTRODUCTION In recent years, noble metal nanoparticles such as gold, silver, and platinum have received much attention due to their various applications in material sciences. Among these noble metal nanoparticles, especially gold nanoparticles play an important role in various catalysis reactions due to their active surface atoms, high surface-to-volume ratio, and high surface energy properties.1 Routinely, the gold-based nanocatalysts have been synthesized by physical and chemical methods. Physical methods based on the deposition−precipitation technique require high-cost instrumental setups, optimum pH and temperatures. Also, gold recovery from the filtrate is not practical and gold capture efficiency of this method is low.2−4 In chemical methods, the gold catalysts have been synthesized by the usage of toxic molecules containing thiol functional group as stabilizing agents (e.g., dodecanthiolates) or toxic solvents (e.g., ethaline) as a medium.5,6 Thus, these reaction conditions restrict the scaling-up for industrial applications and are of environmental concern. The use of microorganisms and plant materials could be a prominent alternative to chemical and physical methods for the synthesis of gold nanoparticles in an eco-friendly manner. The use of plant materials for the synthesis of nanoparticles has advantages over the biological processes which use microorganisms, including the ability to be built up for large-scale synthesis and the elimination of elaborative processes such as the maintenance of cell cultures.7 Thus, it is a big challenge to use plant-derived products for the synthesis of gold nanoparticles in modern nanobiotechnology. A number of plant materials have been used for the reduction of gold salts into gold nanoparticles such as Coriandrum sativum L.,8 Azadirachta indica A. Juss,9 Cinnamomum zeylanicum Blume,10 Cymbopogon f lexuosus Stapf.,11 Tamarindus indica L.,12 Cinnamomum camphora (L.) J. S. Presl,13 Avena sativa L.,14 Aloe vera (L.) Burm. f,15 Terminalia Catappa L.,16 Scutellaria barbata D. © XXXX American Chemical Society

Received: July 31, 2012 Revised: December 6, 2012 Accepted: December 10, 2012

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dx.doi.org/10.1021/ie302037c | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Table 1. Catalytic Efficiency of Soapnut Shells Mediated Gold Nanoparticles with Different Core Sizes Determined in This Study no.

particle core size (nm)

time for complete the reduction (min)

number of gold nanoparticles per mL

first-order constant (k, min−1)

p-nitroaniline conversion (%)

1 2 3

9 17 19

59 72 108

1.2 ± 0.1 × 1015 1.7 ± 0.1 × 1014 1.2 ± 0.2 × 1014

4.5 × 10−2 2.6 × 10−2 1.5 × 10−2

97.1 88.8 71.6

larger sizes.31 The product of this chemical reduction, pphenylenediamine, is an attractive intermediate in the preparation of polymers, hair dyes, and rubber products.32−34 It is also used as a developing agent in the color photographic film development process. Hence, there is a great demand to develop efficient catalysts for this chemical reduction of pnitroanilines. Herein, we have demonstrated the ability of soapnut shells biomass to synthesize gold nanoparticles without the addition of external surfactant. To further strengthen this biosynthetic approach for nanomaterials, the effects of gold precursor (HAuCl4) concentration, amount of soapnut shells, and temperature on the formation of gold nanoparticles were also investigated. The catalytic activity of the biosynthesized gold nanoparticles was examined for the reduction of p-nitroaniline to p-phenylenediamine in the presence of sodium borohydride in an aqueous medium.

obtained from 1, 5, and 10 mM concentrations of HAuCl4 (keeping approximately the same number of gold nanoparticles in all the cases) were mixed. After that, different volumes of deionized water were added to the reaction mixture to nullify the dilution effect. The approximate number of nanoparticles per unit volume of three gold colloidal samples was as given in the Table 1. The approximate number of gold nanoparticles per unit volume was calculated according to Liang’s three assumptions.31 First, most of the gold nanoparticles in solution were the same shape. Second, gold nanoparticles and bulk gold have the same density. Third, the yield of gold nanoparticles was assumed to be 100%. The chemical reduction of pnitroainilne was monitored by a UV−visible spectrophotometer, using quartz cuvettes with 1-cm path length and freshly prepared solutions. After the whole reduction process was completed, the gold nanoparticles were separated from the mixture by centrifugation at 11 000 rpm, washed three times with deionized water and then reused in the next cycle. The same procedure was repeated up to 6 cycles.

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MATERIALS AND METHODS 2.1. Materials. Soapnut shells were procured from agricultural fields in India. HAuCl4, p-nitroaniline, and sodium borohydride (NaBH4) were purchased from Sigma−Aldrich (USA). 2.2. Biological Synthesis of Gold Nanoparticles. To synthesize the gold nanoparticles, 1 mL of 10 mM HAuCl4 aqueous solution was added to a 10-mL filtrate of 30% w/v soapnut shells aqueous extract. The bioreduction of the HAuCl4 is confirmed by the slow color change to a stable blue color after 8 h. In other experiments, the concentration of HAuCl4 was 5 and 1 mM. 2.3. Characterization of Gold Nanoparticles. Gold nanoparticles were characterized by a NANODROP 2000c spectrophotometer (Thermo Scientific) operated at a resolution of 1 nm using freshly prepared solutions in quartz cuvettes with 1-cm path length. X-ray diffraction (XRD) measurements were performed using a Rigaku D/MAX-RB conventional X-ray diffraction instrument operated at a voltage of 40 kV and a current of 100 mA with Cu Kα radiation. Transmission electron microscope (TEM), selected area electron diffraction pattern (SAED) and energy dispersive Xray (EDX) data were obtained on a F20 Tecnai high-resolution microscope (Philips, Netherlands). For TEM analysis, a few drops of the soapnut shells extract mediated gold solutions were placed onto a copper grid and dried overnight. 2.4. FT−IR Analysis. The soapnut shells and gold nanoparticles synthesized from soapnut shells were completely dried and direct contacted with attenuated total reflectance (ATR) tip to expose a beam of infrared light. The FT−IR spectra were collected in the transmission mode (4000−750 cm−1) using an ALPHA FT−IR instrument (Brucher Optic GmbH, Germany). 2.5. Catalysis of p-Nitroaniline. In a typical catalysis reaction, 100 μL of 1 mM p-nitroaniline, 100 μL of 10 mM NaBH4 and different volumes of gold colloidal samples

3. RESULTS AND DISCUSSION 3.1. Characterization of Soapnut Shells Mediated Gold Nanoparticles. UV−visible measurements were carried out to identify the formation of gold nanoparticles using soapnut shells. Figure 1 shows the UV−visible spectra of the

Figure 1. UV−visible spectra of HAuCl4 (a), soapnut shells aqueous extract (b), and reduced gold samples from various concentrations of HAuCl4: 1 mM (c), 5 mM (d), and 10 mM (e).

precursor HAuCl4, aqueous extract of soapnut shells and the reduced gold samples at various molar concentrations of HAuCl4. Absorption peaks were not seen for HAuCl4 (Figure 1a) or the soapnut shells extract (Figure 1b) in the spectral range of 400−800 nm, but samples with different molar concentrations of HAuCl4 in the presence of the soapnut shell extract showed a distinct broad maximum peak at ∼595 nm B



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Figure 2. TEM images of gold nanoparticles assisted by soapnut shells extract obtained at various concentrations: 1 mM (a−c), 5 mM (d−f), and 10 mM (g−i) of HAuCl4. Insets show the SAED patterns of the corresponding images.

formed clusters. The 5-mM HAuCl4 colloidal gold solution was also made up of crystalline and quasi-sphere nanoparticles with diameters of 8−27 nm, which also formed clusters (Figure 2d− f). The gold nanoparticles synthesized using 10 mM HAuCl4 formed nanoparticles of various shapes, including quasi-spheres, triangles, pentagonals, and rods (Figure 2g−i). Closer inspection of TEM images of the gold sample synthesized from 10 mM HAuCl4 showed quasi-spheres (∼92.8%) with diameters of 8−35 nm, triangles (∼3.7%) with sides of 25−52 nm, pentagonals (∼1.8%) with diameters of 35−55 nm, and rods (∼1.3%) 10−11 nm wide and 22−88 nm long, along with other shapes (∼0.4%). It has been reported earlier that the biological synthesis of gold nanoparticles from Magnolia kobus DC. requires reaction temperature of ∼95 °C to produce quasispherical shape gold nanoparticles.19 In the present study, the quasi-spherical shape gold nanoparticles were synthesized at room temperature (27 °C). The corresponding images of SAED patterns are shown in insets of Figure 2. It can be observed that the four rings in the insets of Figure 2 reflect the

(Figure 1c−e), indicating the formation of gold nanoparticles. The broad peak at 595 nm reveals that obtained gold nanoparticles are polydispersed in the solutions.35 The stability of the gold nanoparticles synthesized by soapnut shells was examined by comparing the UV−visible spectra of gold nanoparticles that were freshly prepared and stored at 4−8 °C for more than 5 months. There is no change of solution color and absorption peaks observed after 5 months of storage which would suggest that the obtained gold nanoparticles have good stability. This is another advantage of gold nanoparticles synthesized from soapnut shells: they have long stability compared to the other plant extracts.9,19 Figure 2 shows the TEM images of the gold colloidal solutions obtained using various concentrations of HAuCl4: 1, 5, and 10 mM. The dimensions of the gold nanoparticles were measured based on TEM images by analyzing more than 300 measurements. The gold nanoparticles obtained using 1 mM HAuCl4 were crystalline and quasi-sphere nanoparticles with diameters of 6−15 nm (Figure 2a−c). Some of these particles C



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gold nanoparticles is 35 nm. Closer inspection of TEM images of this sample showed spheres (∼97.1%), rods (∼0.8%), triangles (∼0.8%), and other shapes (∼1.6%). The UV−visible (Figure 4d) spectrum of this gold sample displayed slightly broad and little shift in the position of absorbance peak (560 nm) with respect to the gold nanoparticles obtained from 15% and 30% w/v amount of soapnut shells. Thus, in the present method, the use of 45% w/v amount of soapnut shells is beneficial to produce better nanoparticles. 3.3. Temperature Effect on the Nanoparticles Synthesis. A mixture of 10 mM HAuCl4 (1 mL) with 30% w/v soapnuts shells extract (10 mL filtrate) was kept at 50 °C for 8 h. Figure 5a−b displays the TEM images of the gold nanoparticles obtained by the thermal treatment at different magnifications. It can be seen that crystalline gold clusters with spherical morphologies are present in aggregation. The crystalline gold clusters size ranged from 56 to 142 nm. This indicates that the smaller gold nanoparticles (smaller particles were observed in the absence of thermal treatment) were assembled into spherical gold clusters upon thermal treatment. SAED image obtained from Figure 5b is shown in Figure 5c. It reflects the four rings (111), (200), (220), and (311) diffraction planes, respectively, which represents the fcc structure of gold nanoparticles. In the absence of thermal treatment also the structure of gold nanoparticles is in fcc structure, which were discussed earlier in SAED studies. Thus, in the present synthesis, thermal conditions affect only on the particle morphology not on the crystalline structure. The broad absorbance peak at ∼626 nm in the UV−visible spectroscopy (Figure 5d) also suggests that the obtained gold nanoparticles at 50 °C are polydispersed in solutions. 3.4. Role of Soapnut Shells. An aqueous extract of soapnut shells has a high content of saponins and flavonoids. The hydroxyl groups in these biocompounds could participate in the gold bioreduction. The bioreduction of gold(III) ions to gold (0) nanoparticles might be occurring through the oxidation of hydroxyl to carbonyl groups as shown in eq 1:

(111), (200), (220), and (311) diffraction planes, respectively, which represents the fcc structure of gold nanoparticles obtained using soapnut shells.10 Typical high-resolution TEM images show the various shapes of these nanoparticles (Supporting Information Figure S1), as well as the oriented and ordered lattice fringes of gold nanoparticles. This confirmed that the gold nanoparticles obtained from the gold sample of 10 mM HAuCl4 were in a highly crystalline state. The EDX spectrum of soapnuts mediated nanoparticles (Figure S2) confirms the presence of gold. The XRD spectrum of the powdered gold nanoparticles synthesized using an aqueous extract of soapnut shells is shown in Figure 3. Diffraction peaks at two theta = 38.27, 44.37, 64.85,

Figure 3. Representative XRD diffraction pattern of soapnut mediated gold nanoparticles. The inset indicates the mean sizes of gold particles at various molar concentrations of HAuCl4.

and 77.73° correspond to the indexed planes (111), (200), (220), and (311), respectively, which are consistent with the fcc structure of the gold.36 The average size of the gold nanoparticles was calculated from the XRD data according to the line width of the maximum intensity reflection peak by using the Scherrer equation as described earlier.37 The inset in Figure 3 shows the average sizes of the gold nanoparticles obtained from 1, 5, and 10 mM molar concentrations of HAuCl4. As the gold concentration in the soapnut shells extract increased, the average size of the nanoparticles formed also increased, with sizes of 9, 17, and 19 nm for 1, 5, and 10 mM, respectively. Such a size variation agreed with the results of the TEM studies, which were discussed earlier. 3.2. Amount of Soapnut Shells Effect on the Nanoparticles Synthesis. To investigate the effect of soapnut shells amount on the nanoparticles synthesis, we have prepared gold nanoparticles from low (15% w/v) and high (45% w/v) amounts of soapnut shells extract with respect to the 30% w/v soapnut shells extract (it was prepared in the above studies). The synthesized gold nanoparticles from a low amount of soapnut shells (15% w/v) exhibited higher aggregation and highly heterogeneous structures (data not shown). This may be due to an inadequate reduction of the gold precursor (HAuCl4). On the other hand, the gold nanoparticles obtained from the high amount of soapnut shells (45% w/v) exhibited were slightly dispersed and most of the nanoparticles were in spherical shape (Figure 4a−c). The average size of the spherical

Gold(III) + 3R − OH → Gold(0) + 3R = O + 3H+ (1)

The possible functional groups responsible for stabilization of the gold nanoparticles were studied by FT−IR. The FT−IR spectra of soapnut shells and soapnut shells mediated gold nanoparticles are shown in Figure 6. The FT−IR profiles of soapnut shells and gold nanoparticles synthesized from soapnut shells revealed that biomolecules in the soapnut shells such as saponins and flavonoids that contain functional groups might play an important role in the stabilization of gold nanoparticles during the synthetic process. The FT−IR spectra of soapnut shells (Figure 6a) shows the presence of five absorption peaks at 3315, 2875, 1701, 1604, and 1410 cm−1. The first two absorption peaks are attributed to the O−H stretching vibration of saponins or flavonoids and carboxylic O−H bond stretching vibration of saponins, respectively. The other three absorption peaks at 1701, 1604, and 1410 cm−1 are characteristic of carboxylic CO bond stretching vibration of saponins, CO stretching vibration of flavonoids, and carboxylic O−H bending vibration of saponins, respectively. In addition, the absorption peak at 1253 cm−1 indicates the C−O stretching vibration of saponins or flavonoids.38,39 The FT−IR spectra of the soapnut shells mediated gold nanoparticles (Figure 6b) showed some shifts corresponding to the stretching vibration of carboxylic O−H bond in saponins from 2875 to 2846 cm−1, the D



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Figure 4. TEM images of the gold sample reduced in aqueous 45% w/v soapnut shells extract at different magnifications: 200 nm (a), 100 nm (b), and 20 nm (c). UV−visible spectra of the gold sample reduced in aqueous 45% w/v soapnut shells (d).

Figure 5. TEM images of the gold sample reduced in aqueous extract of soapnut shells at 50 °C for different magnifications: 200 nm (a) and 20 nm (b). SAED patterns (c) of panel (b). UV−visible spectra of the gold sample reduced in aqueous extract of soapnut shells at 50 °C.

saponins from 1410 to 1453 cm−1 and the stretching vibration of C−O bond in saponins or flavonoids from 1253 to 1239

stretching vibration of CO group in flavonoids from 1604 to 1521 cm−1, the bending vibration of carboxylic O−H bond in E



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slope ln C vs time, we estimated the first-order rate constant (k) of the reaction. The gold samples produced from 1, 5, and 10 mM of HAuCl4 had the k values 4.5 × 10−2 (Figure 7a and b), 2.6 × 10−2 (Figure 7c and d), and 1.5 × 10−2 (Figure 7e and f), respectively. These results suggest that the speed of reaction was fast when using the gold sample produced from 1 mM of HAuCl4 medium when using the gold sample produced from 5 mM of HAuCl4, and slow when using the gold sample produced from 10 mM of HAuCl4 as a catalyst. All these results are schematically depicted in Figure 8. The percentage conversion of p-nitoaniline to p-phenylenediamine was ∼97.1% in the presence of the gold sample obtained from 1 mM HAuCl4, ∼88.8% in the presence of the gold sample obtained from 5 mM HAuCl4, and ∼71.6% in the presence of the gold sample obtained from 10 mM HAuCl4. This variation in reaction rate or catalytic efficiency is mainly dependent on the number of nanoparticles present in reaction mixture as well as the availability of active surface area for adsorption of reactants. In our study, we fixed approximately the same number of gold nanoparticles in all reaction mixtures by adding the different volumes of gold solutions (See Materials and Methods and Table 1). Thus, the variation in reaction rate or catalytic efficiency mostly depends on the available surface area for absorption of reactants. In the present reduction, the electron transfer occurs from BH4− to the pnitroaniline via the soapnut shells mediated gold nanoparticles. According to previous reports, many factors influence the surface area available for the absorption of reactants such as size and shape of particles as well as molecules wrapped on the nanoparticles.30,31 It was reported earlier that large gold nanoparticles have more steric interactions with BH4− and pnitroaniline compared to small gold nanoparticles,30 because more numbers of molecules were wrapped on the surface of large gold nanoparticles with respect to small gold nanoparticles. Here also, these steric interactions may be decreasing the absorption of reactants on large gold nanoparticles. Therefore, the reaction rate is fastest in small (9 nm) gold nanoparticles produced from 1 mM HAuCl4 with respect to the large gold nanoparticles produces from 5 (size 17 nm) and 10 mM (size 19 nm) HAuCl4. On the other hand, the gold nanoparticles produced from 10 mM HAuCl4 showed slow reaction rate compared to the gold nanoparticles produced from 5 mM HAuCl4. This may be because the gold sample resulting from 10 mM HAuCl4 showed various shapes of nanoparticles, such as quasi-spheres, triangles, pentagonals, and rods, whereas the gold sample for 5 mM HAuCl4 showed quasispheres. Perhaps this mixture of gold nanoparticles has a lower effective catalytic surface area than that of the quasi-sphere nanoparticles. Thus, the rate of reaction is slower in the gold sample obtained from 10 mM HAuCl4 than that from 5 mM HAuCl4. At this point, it is not sufficient to explain the exact reason for these variations in the reaction rates. Further investigations need to be carried out to clearly understand this catalytic process. In all cases the reduction of p-nitroaniline completed within 2 hours. The gold nanoparticles from the 1 mM gold precursor used as catalyst for the complete reduction of p-nitroaniline to p-phenylenediamine required time is less than 1 hour, which is lower or slightly higher than the gold nanoparticles synthesized from the chemical routes.30,31 We also examined the recyclability of gold nanoparticles obtained from 1 mM HAuCl4 as a catalyst for the reduction of p-nitroaniline with NaBH4. Figure 9 shows the reusable catalytic properties of the gold nanoparticles obtained from 1

Figure 6. FT−IR spectra of soapnut shell extract (a) and the soapnut mediated gold nanoparticles (b).

cm−1. These observations suggest that the resulting gold nanoparticles might be stabilized by functional groups present in the biomolecules of soapnut shells extract such as saponins and flavonoids through the interactions of carboxylic groups in the saponins as well as the carbonyl groups in the flavonoids. However, further investigation is needed to clearly understand the stabilization mechanism in this system. 3.5. Catalysis of p-Nitroaniline. The reaction in Scheme 1 represents the formation of p-phenylenediamine by the Scheme 1

reduction of p-nitroaniline with NaBH4 in aqueous solutions resulting from purified gold nanoparticles.The formation of pphenylenediamine in the presence of gold nanoparticles formed from soapnuts was monitored in real time by UV−visible spectroscopy. A blank experiment was carried out for the reduction of p-nitroaniline with NaBH4 in the absence of gold nanoparticles. There is a very slow decrease in the characteristic absorbance of p-nitroaniline at 378 nm after 24 h (Figure S3). The reaction was performed in the presence of soapnut shells mediated gold nanoparticles where the absorbance peak at 378 nm showed a progressive decrease, a shifting of the absorbance peak at 223 nm, and the appearance of a new absorption peak at 303 nm in Figure 7. This result indicates that the gold nanoparticles formed from soapnuts were accelerating the reduction of p-nitroaniline into p-phenylenediamine in the presence of NaBH4. The reduction rates of this reaction can be considered to be independent of the concentration of NaBH4 since this reagent was used in large excess with respect to pnitroaniline (See Materials and Methods). Therefore, this chemical reduction follows first-order kinetics. At 378 nm, the logarithm of absorbance (ln C) of p-nitroaniline decreased linearly with reaction time. From this linear regression of the F



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Figure 7. UV−visible spectra for the successive chemical reduction of p-nitroaniline with NaBH4 catalyzed by soapnut mediated gold nanoparticles obtained at various concentrations: 1 mM (a), 5 mM (c), and 10 mM (e); of HAuCl4; and plots of ln C versus time for the reduction of pnitroaniline using NaBH4 in the presence of gold samples obtained from 1 mM (b), 5 mM (d), and 10 mM (f) of HAuCl4.

4. CONCLUSIONS

mM HAuCl4. It is found that, after 6 recycling reactions, the pnitroaniline still can be converting to p-phenylenediamine in the presence of NaBH4 with a conversion rate reaching ∼86.4%. Thus, the gold nanoparticles synthesized from the soapnut shells are a potent recyclable nanocatalyst for the industrial applications.

A green and effective method has been used for the synthesis of gold nanoparticles using aqueous soapnut shells solutions. The XRD and EDX analyses confirmed the presence of pure-phase gold nanoparticles without any substantial impurities. The spectroscopic techniques (UV−visible and FT−IR), including structural (XRD) and morphological (TEM) studies, suggest G



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ASSOCIATED CONTENT

S Supporting Information *

Figure S1: HR-TEM images of the various shapes of gold nanoparticles. Figure S2: EDX spectrum of gold nanoparticles. Figure S3: UV−visible spectra for catalysis of p-nitroaniline. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-42-821-6632. Fax: +82-42-822-6272. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R32-20026).

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Figure 8. Schematic drawing of the gold nanoparticles at various molar concentrations of HAuCl4 and catalysis of p-nitroaniline in the presence of soapnut-mediated nanoparticles as catalysts.

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Figure 9. Recyclability of the gold nanoparticles obtained from 1 mM HAuCl4 as a catalyst for the reduction of p-nitroaniline with NaBH4.

that the soapnut shells played an important role in the reduction and stabilization of gold nanoparticles through the electrostatic interaction of carboxylic groups in saponins as well as the carbonyl groups in flavonoids present in the soapnut shells. The UV−visible spectroscopy results of using these gold nanoparticles as catalysts for the reduction p-nitroaniline to pphenylenediamine in the presence of sodium borohydride in an aqueous medium suggested that the gold nanoparticles from 1 mM of HAuCl4 have better catalytic properties than the gold nanoparticles from 5 and 10 mM of HAuCl4. These studies suggest that the biosynthesized gold nanoparticles have good scope for future applications in catalysis. H



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