Green Synthesis of Au Nanostructures at Room Temperature Using

DOI: 10.1021/cg9007685

Green Synthesis of Au Nanostructures at Room Temperature Using Biodegradable Plant Surfactants

2009, Vol. 9 4979–4983

Mallikarjuna N. Nadagouda,† George Hoag,‡ John Collins,‡ and Rajender S. Varma*,† †

Sustainable Technology Division, US Environmental Protection Agency, National Risk Management Research Laboratory, 26 West Martin Luther King Drive, MS 443, Cincinnati, Ohio 45268, and VeruTEK Technologies, Inc., 65 West Dudley Town Road, Suite 100, Bloomfield, Connecticut 06002

‡

Received July 6, 2009; Revised Manuscript Received September 3, 2009

ABSTRACT: One-step green synthesis of gold (Au) nanostructures is described using naturally occurring biodegradable plant surfactants such as VeruSOL-3 (mixture of D-limonene and plant-based surfactants), VeruSOL-10, VeruSOL-11, and VeruSOL-12 (individual plant-based surfactants derived from coconut and castor oils) without any special reducing agent/ capping agents. This greener method uses water as a benign solvent and surfactant/plant extract as a reducing agent. Depending upon the Au concentration used for the preparation, Au crystallizes in different shapes and sizes to form spherical, prisms, and hexagonal structures. Sizes vary from the nanometer to micrometer scale level depending on the plant extract used for preparation. Synthesized Au nanostructures were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and UV spectroscopy.

Introduction Gold nanostructures have been the focus of intense research owing to their fascinating optical, electronic, and chemical properties and promising applications in nanoelectronics, biomedicine, sensing, and catalysis.1-10 Various methods have been developed to fabricate gold nanoparticles using NaBH4,11 microwaves,12 a simple galvanic replacement reaction (transmetalation reaction),13 polymeric strands of oleylamine-AuCl complexes,14 poly (vinyl pyrrolidone) (PVP) in aqueous solutions,15 reducing agents (ascorbic acid),16 seedmediated synthesis,17 and ionic polymers.18 Wet methods often require the use of an aggressive chemical reducing agent such as sodium borohydride, hydroxylamine, and a capping agent and may additionally involve an organic solvent such as toluene or chloroform. Although these methods may successfully produce pure, well-defined metal nanoparticles, the cost of production is relatively high both materially and environmentally. Consequently, there is an unequivocal need to develop more cost-effective and environmentally benign alternatives to these existing methods. The choice of an environmentally compatible solvent system, an ecofriendly reducing agent, and a nonhazardous capping agent for stabilizing the nanoparticles are three main criteria for a “green” nanoparticle synthesis. Recently, there has been an increased emphasis on the greener production of environmentally benign and renewable materials as the respective reducing and stabilizing agents.19-22 The use of ecofriendly and biodegradable materials in the production of metal nanoparticles is important for pharmaceutical and biomedical applications. Recently, we have developed several benign methods using sugars, vitamins, biodegradable polymers, plant extracts, and microwave heating to generate various nanostructures.23,24 In continuation of our effort in this general area herein, we report a onestep method that utilizes renewable and biodegradable plant extracts at room temperature without the use of any reducing

or capping agents. Surfactants such as VeruSOL-3 (mixture of and plant-based surfactants), VeruSOL-10, VeruSOL-11, and VeruSOL-12 (individual plant-based surfactants derived from coconut and castor oils) were used without any special reducing agent/capping agents for the preparation of Au nanostructures.

D-limonene

Results and Discussion

*To whom correspondence should be addressed. E-mail: varma.rajender@ epa.gov.

The formation of gold nanostructures was obtained at room temperature followed by in situ UV-vis spectra measurements. The reaction solution containing plant extracts obtained from VeruTEK Technologies Incorporated, of Bloomfield, Connecticut, and HAuCl4 3 4H2O, was introduced into a quartz cell immediately after mixing, and the UV-vis spectra were recorded at different time intervals. The color of the solution changed gradually to light pink within 15 min after mixing. Some of the samples, however, took longer for the color formation. Figure 1 shows the timedependent spectral response obtained during the growth of Au nanostructures. The spectra recorded in the early stages show a broad peak at 550 nm, which can be assigned to the transverse component of SPR absorption. The intensity of the peak increases monotonically with time, indicating the increase in the amount of the gold products. It can be observed from Figure 1 that the intensity of the UV-vis absorption peak increases up to 2 min, and then increases exponentially due to the formation of the product. The reaction is completed within a few minutes. Figure 2 shows a typical UV-vis spectrum of gold nanostructures obtained by reducing chloroauric ions with a Au-8, Au-3, and Au-13 samples. The broad SPR bands centering at 550 nm is clearly visible, which could be attributed to the inplane dipole resonance. Similarly, the UV-vis spectra for other compositions identified in Table 1 are shown in Figures 3-5. Samples Au5, Au-10, and Au-15 (Figure 3) reveal spectra similar to Au-3, Au-8, and Au-13 (Figure 2) samples. Samples Au-1, Au-6, Au-7, Au-11, and Au-12 (Figures 4 and 5), however, do not

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Figure 1. Time-dependent Au-10 reaction after (a) 0 control, (b) 1, (c) 2, and (d) 3 min.

Figure 4. UV spectra of (a) Au-7 and (b) Au-12 samples.

Figure 2. UV spectra of (a) Au-8, (b) Au-3, and (c) Au-13 samples.



Figure 6. Representative photographic images of (a) Au-6, (a) Au7, and (c) Au-8 samples.

show the absorption at 550 nm;there may be a higher peak shift formation due to prismatic formation as observed by Mirkin et al.25 The shift at higher wavelength most likely reflects an increase in nanoprism edge length.26 The photographic image of the representative reaction products are shown in Figure 6. Representative XRD patterns of the gold nanostructures synthesized by different plant extracts are listed in Table 1 and

shown in Figure 7. A number of Bragg reflections were present which could be indexed on the basis of the face-centered cubic (fcc) gold structure. No additional impurities were found except for a broad hump around 2θ 20°. The broad hump may be from the organic moieties present in the extract. The XRD pattern clearly shows that the gold nanostructures are crystalline. In addition, the intensity of the (111) diffraction is

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Figure 7. XRD pattern of (a) Au-4, (b) Au-9, (c) Au-14, (d) Au-1, (e) Au-11, (f) Au- 5, (g) Au-10, (h) Au-8.

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Figure 8. SEM images of (a) Au-1, (b) Au-2, and (c, d) Au-4 samples.

Table 1. Different Compositions of Reaction Mixture Using Natural Plant Surfactants entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

composition

code

VeruSOL-3 2 mL þ 4 mL HAuCl4 D-limonene 2 mL þ 4 mL HAuCl4 VeruSOL-12 2 mL þ 4 mL HAuCl4 VeruSOL-10 2 mL þ 4 mL HAuCl4 VeruSOL-11 2 mL þ 4 mL HAuCl4 VeruSOL-3 2 mL þ 4 mL HAuCl4 þ 10 mL H2O D-limonene 2 mL þ 4 mL HAuCl4 þ 10 mL H2O VeruSOL-12 þ 4 mL HAuCl4þ 10 mL H2O VeruSOL-10 þ 4 mL HAuCl4þ 10 mL H2O VeruSOL-11 þ 4 mL HAuCl4þ 10 mL H2O VeruSOL-3 1 mL þ 10 mL HAuCl4 D-limonene 1 mL þ 10 mL HAuCl4 VeruSOL-12 1 mLþ 10 mL HAuCl4 VeruSOL-10 1 mL þ 10 mL HAuCl4 VeruSOL-11 1 mL þ 10 mL HAuCl4

Au-1 Au-2 Au-3 Au-4 Au-5 Au-6 Au-7 Au-8 Au-9 Au-10 Au-11 Au-12 Au-13 Au-14 Au-15

much stronger than those of the (200) and (220) diffractions. These observations indicate that the gold nanostructures formed by the reduction of Au(III) by plant extract are dominated by {111} facets, and hence more {111} planes parallel to the surface of the supporting substrate were sampled. Scanning electron microscopy (SEM) was used to understand the surface morphology of the Au nanostructures. SEM images of samples (a) Au-1; (b) Au-2; and (c, d) Au-4 samples are found in Figure 8. The samples Au-1 and Au-4 formed hexagonal and prism structures with sizes ranging from 2 to 5 μm. However, sample Au-2 did form spherical nanostructures with sizes ranging from 100 to 300 nm. It is important to note that Au-1 and Au-4 yielded few spherical nanoparticles along with prisms and hexagonal structures. Similarly, Au-11 and Au-14 samples yielded mainly prisms and hexagonal Au nanostructures with sizes ranging from 1 to 5 μm along with a small amount of spherical particles. The Au-12 sample yielded mainly spherical particles along with very few prisms (see Figure 9). Samples Au-6 and Au-9 afforded prism type structures (see Figure 10), whereas Au10 and Au-12 resulted in spherical particles with sizes ranging from 100 to 200 nm. The size and shape of the formed Au nanostructures depended upon the plant extract used. The energy dispersive X-ray spectrum (EDS) of a representative Au sample is shown in Figure 11. As expected, Au peaks were observed in the spectrum which confirms its formation.

Figure 9. SEM image of (a) Au-11, (b) Au-12, and (c, d) Au-14 samples.

Figure 10. SEM images of (a) Au-6, (b) Au-8, (c) Au-9, and (c) Au10 samples.

EDS results also support the UV spectral data wherein we observed the characteristic resonance peak at around 550 nm and the XRD pattern which were indexed to a cubic system. Typical TEM images revealing the size and morphology of the gold nanostructures are given in Figures 12 and 13. The nanostructures sizes vary from 20 nm to more than a micrometer in size depending on the extract used for preparation.

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Figure 11. Representative EDX spectra of Au nanostructures obtained using Au-6 sample.

Figure 13. TEM image of (a, b) Au-3 and (c, d) Au-4 samples.

Figure 12. TEM images of (a, b) Au-1, (c) Au-2, and (d) Au-5 samples.

Different shapes were observed including spherical and hexagonal geometries with very smooth edges. Figure 12 shows the TEM image of an isolated nanostructure obtained using respective Au-1, Au-2, and Au-5 samples. The Au-1 sample yielded interesting plate stacks, whereas Au-2 sample yielded mixed prisms, rods, and spherical particles. The Au-5 sample was observed to form only spherical nanoparticles with sizes ranging from 20 to 50 nm. Similarly, transmission electron microscopy (TEM) images of Au-3 and Au-4 at lower and higher magnification are shown in Figure 13. Au-3 samples yielded only spherical particles in contrast to Au-4 samples which primarily consisted of prisms and hexagonal structures. We have extended this strategy for generating Au nanostructures using commercially available surfactants such as butyl ammonium bromide. The reaction between butyl ammonium bromide and HAuCl4 4H2O is spontaneous and color changes from pale yellow to orange (see Figure 14 for XRD pattern). The XRD pattern after immediate reaction did not show any peaks corresponding to Au nanostructures (see Figure 14a,b). However, the overnight reacted sample had peaks which can be indexed to cubic Au pattern. The pattern was compared with JCPDF card no. 00-004-0784. Experimental Section Materials. Chloroauric acid tetrahydrate (HAuCl4 3 4H2O) and methyl ammonium bromide was obtained from Aldrich Chemical Company. Plant extracts were obtained from VeruTEK Technologies, Inc. of Bloomfield, Connecticut. VeruSOL-3 is a mixture of Dlimonene and plant-based surfactants. VeruSOL-10, VeruSOL-11, and VeruSOL-12 are individual plant-based surfactants derived from coconut and castor oils. All the chemicals were analytical grade and used without further purification. Doubly distilled water was used throughout the experiments.

Figure 14. XRD patterns of butyl ammonium bromide reduced Au nanostructures (a, b) immediately and (c) after 1 day. Synthesis of Gold Nanostructures. Different concentrations of HAuCl4 solutions were added to the solution of plant extracts at room temperature. This mixture was gently shaken, followed by rapid inversion mixing for 2 min. The composition of the reaction mixtures are shown in Table 1. Samples for UV spectroscopy measurements were reaction mixtures dispersed in distilled water. To obtain SEM images, the product was drop-casted on carbon tape and allowed to dry. TEM was performed using a JEOL-1200 EX II microscope operated at 120 kV. SEM was carried out with a fieldemission microscope (JEOL 6490 LV) operated at an accelerating voltage of 30 kV. Panalytical X-pert diffractometer with a copper KR source was used to identify crystalline phases of the lead precipitates. The tube was operated at 45 kV and 40 mA for the analyses. Scans were performed over a 2-theta ranging from 5 to 70° with a step of 0.02° and a 1-s count time at each step. Pattern analysis was performed by following ASTM procedures using the computer software Jade (Versions 8, Materials Data, Inc.), with reference to the 1995-2002 ICDD PDF-2 data files (International Center for Diffraction Data, Newtown Square, PA). UV spectra were recorded using Varian UV-visible spectrometer (model Cary 50 Conc).

Conclusions We have developed a convenient green synthesis of Au nanostructures with varying morphologies and sizes which are readily prepared from inexpensive starting materials employing biodegradable plant-based surfactants and cosolvents in water without using any capping or reducing reagent. This

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synthesis concept could ultimately enable the fine-tuning of material responses to magnetic, electrical, optical, and mechanical stimuli.

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Green Synthesis of Au Nanostructures at Room Temperature Using

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