Advanced Science, Engineering and Medicine Vol. 4, pp. 1–6, 2012 (www.aspbs.com/asem)
Copyright © 2012 by American Scientific Publishers All rights reserved. Printed in the United States of America
Green Synthesis of Gold and Silver Nanoparticles Using Achyranthes Aspera L. Leaf Extract Venkatesh Gude1 , Kalpana Upadhyaya1 , M. N. V. Prasad2 , and Nandiraju V. S. Rao1, ∗ 1 2
Department of Chemistry, Assam University, Silchar 788011, India Department of Plant Sciences, University of Hyderabad, Hyderabad 500046, India
KEYWORDS: Ag Nanoparticles, Au Nanoparticles, Achyranthes Aspera L, Synthesis.
1. INTRODUCTION As a part of promoting Green Chemistry in recent years, the green synthetic strategies1–3 adopted non-toxic chemicals, environmentally benign solvents and renewable materials in the synthesis of Au and Ag nanoparticles (NPs). Au NPs have been explored for their potential applications in biology and medicine.4–6 The tunable shape and size dependent optical properties of Au NPs have been exploited in various surface coatings7 and biomedical applications. Moreover Au NPs are bio-compatible, non toxic8 and readily binds to a large number of biomolecules such as amino acids, proteins/enzymes and DNA etc.4 Ag NPs also received considerable attention due to their physicochemical properties. Ag NPs are known to be excellent substrates for surface enhanced Raman scattering (SERS) to probe single molecules,9 as catalysts for molecular labelling10 etc. Au and Ag nanotriangles in particular have potential applications in cancer hyperthermia,11 as wave guides for electromagnetic radiation,12 SERS substrates,13 infrared absorbing optical coatings etc.7 Monodispersity of particle size and shape selectivity are the two main important issues in the nanoparticle research. The fascinating properties exhibited by ∗
Author to whom correspondence should be addressed. Email:
[email protected] Received: 8 May 2012 Revised/Accepted: 1 June 2012
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anisotropic nanostructures makes shape selective synthesis exciting.14 There are many reports over the past decade on the biological synthesis of gold and silver nanoparticles. The biosynthetic methods viz., usage of biological micro-organisms or plant extracts had emerged as simple, low cost, eco-friendly and viable alternatives to replace the synthetic chemical or physical methods. Recently biosynthesis of gold and silver nanoparticles from Candida guilliermondii15 and silver nanoparticles using moringa oleifera16 leaf extracts and their applications have been reported. The importance of the biosynthesis in the realization of gold nanoparticles from microorganisms, plants and other biological sources; the possible mechanisms in the formation of gold nanoparticles and the tuning of nanoparticle properties had been reviewed recently.17 Klaus et al.18 reported the intracellular Ag NPs formation in Pseudomonas-stutzeri, while Sastry et al. using fungi19 20 and Aloe vera, neem, lemon grass, Cinnamomum camphora plant extracts7 11 21–25 reported environment friendly methods of synthesis of noble metal NPs. In this paper we demonstrated the importance of Achyranthes aspera L. which is used for its antimicrobial, antiinflammatory, antiviral and anticarcinogenic effects. Its decoction is used against diarrhea.26–28 We have used the leaf extract for the synthesis of Au, Ag nanoparticles in quantitative yield by the reaction of aqueous chloroaurate ions and aqueous silver ions with Leaf extract. This plant possesses of phytochemical constituents like ecdysterone, betaine, 6-pentatriacontanone, etc. and exhibits important
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The importance of leaf extract of Achyranthes aspera L. as reducing agent in the synthesis of gold, silver nanoparticles (NPs) in quantitative yield, by the reaction of aqueous chloroaurate ions and aqueous silver ions with a small quantity of leaf extract is demonstrated. Changes in concentration of leaf extract can modify the phase morphology and the size of the nanoparticles can be tuned. UV-Visible spectra of the solution evidenced the variation in the dimension of anisotropic nanostructures. The formation of spherical nanostructures and nanotriangles of noble metal gold and silver nanoparticles is confirmed by transmission electron microscopy (TEM) investigations. The gold nanoparticles are poly dispersed and the average size of spherical nanoparticles is found to be 183 ± 10 nm. The average edge length of the triangle is 272 ± 20 nm. Selected area diffraction pattern (SAED) confirmed the polycrystalline molecular structure. SAED diffraction patterns of biogenic silver nanotriangles revealed the orientation of nanotriangles top surface normal to the electron beam.
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Chloroauric acid (HAuCl4 , Silver nitrate (Ag NO3 were obtained from Aldrich chemicals and used as received. Achyranthes aspera L. leaf extract: 30 g of Achyranthes aspera L. leaves thoroughly washed finely cut and boiled for 5 minutes in 100 ml double distilled water. The resulting solution filtered and used for further experiments. To 6 ml of the 10−3 M aqueous chloroaurate ions of different volumes (0.5, 0.8, 1.0, 2.0, 3.0, 4.0 ml) (or 10−3 M aqueous Ag+ ions of different volumes: 0.5, 0.8, 1.0, 2.0, 3.0, 4.0 ml) 6 ml of leaf extract was added followed by the addition of distilled water to make the solution to 10 ml in a volumetric flask and then the solution was allowed to stand for 24 hours. After 24 hours the solutions were centrifuged to obtain a residue, which was washed several times with distilled water to eliminate the unreacted biological molecules present in the extract. Here we show that at low percentage of the extract in the reaction medium more number of spherical Au NPs and Au nanotriangles formed via multiply twinned particles (MTPs), on rising the percentage of extract in the reaction medium the size of the spherical nanostructures decreasing leading to control over the optical properties in the solution and are forming big clusters leading to the shape modulated. On the other hand reaction of extract with aqueous silver ions leads to the formation of predominantly spherical NPs along with little amount of single crystalline silver nanotriangles also observed. On rising the percentage of extract in the medium the average size of the silver NPs decreasing. UV-Visible spectroscopic measurements of the synthesized Au, Ag nanoparticles with leaf extract were carried out on UV-1601PC Shimadju spectrophotometer. TEM samples of the purified Au, Ag nanoparticle solution drops placed over the carbon coated copper grids and allowed the solvent to evaporate. TEM, SAED measurements were performed on TECNAI G2 instrument operated at an accelerating voltage of 200 kV.
3. RESULTS AND DISCUSSION 3.1. Gold Nanoparticles The UV-Visible spectra of the above solutions shown in Figure 1(a), revealed the presence of gold NPs in all the mixtures and clearly they may have different size or shape (morphology). At low concentration the absorption band (∼550 nm) is originating mostly from surface plasmon vibrations of spherical gold NPs.31 32 However, with an increase in concentration, the absorption bands become broad with decreased intensity initially, shifting to longer wavelength region and then to lower wavelength region. 2
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Wavelength (nm) Fig. 1. (a) UV-visible absorption spectra recorded after 24 hours of synthesized gold NPs from different amounts of the extract of 0.5, 0.8, 1.0, 2.0, 3.0, 4.0 ml respectively treated with 6 ml of 10−3 M aqueous chloroaurate ions in a 10 ml volumetric flask by the addition of double distilled water. (b) UV-visible absorption spectra recorded as a function of time of synthesized gold NPs from 1 ml of leaf extract treated with 6 ml of 10−3 M aqueous chloroaurate ions in a 10 ml volumetric flask filled with double distilled water.
The broad absorption with decreased intensity indicates the aggregation of spherical NPs or a change in the dimension of spherical anisotropic nanostructures. To understand the Au NPs formation in the mixture, we recorded the UV-Visible spectra (Fig. 1(b)) of the solution (containing 1 ml of leaf extract and 6ml of 10−3 M aqueous chloroaurate ions in a 10 ml volumetric flask) as a function of time. Initially at 2 min, the observed broad band (∼545 nm) reflects the surface plasmon resonance of spherical gold NPs. Further the absorption intensity and broadness of the bands increased till 6 h with a small but noticeable red shift in absorption band, indicating an increase in percentage of nanoparticles in solution as a function time. However the absorption intensity of solution detained for a longer time slowly decreased which indicates a variation in the dimension of anisotropic nanostructures. The above mentioned features are confirmed by transmission electron microscopy (TEM) investigations. TEM images of the synthesized Gold NPs corresponding to 0.5, 1.0, 4.0 ml leaf extract respectively are presented in Figures 2(a)–(c) and the selected area electron Adv. Sci. Eng. Med., 4, 1–6, 2012
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diffraction pattern (SAED) of Figure 2(c) is shown in Figure 2(d). Figure 2(a) displays the formation of spherical gold NPs as well as a small percentage of triangular gold NPs in the solution containing a small amount of leaf
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Fig. 3. (a) UV-visible absorption spectra recorded after 24 hours of synthesized silver NPs with different amounts of the extract of 0.5, 1.0, 2.0, 3.0, 4.0 ml respectively treated with 6 ml of 10−3 M aqueous silver ions in a 10 ml volumetric flask by the addition of double distilled water, (b) UV-visible absorption spectra recorded as a function of time of synthesized silver NPs from 1 ml of leaf extract treated with 6 ml of 10−3 M aqueous silver ions in a 10 ml volumetric flask filled with double distilled water. Adv. Sci. Eng. Med., 4, 1–6, 2012
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Fig. 2. TEM images of gold NPs synthesized with different amounts of leaf extract (a) 0.5 ml, (b) 1.0 ml, (c) 4.0 ml and (d) selected area electron diffraction (SAED) pattern of the TEM image shown in (c).
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Fig. 4. TEM images of silver NPs synthesized with different amounts of leaf extract (a) 0.5 ml, (b) 1.0 ml, (c) 4.0 ml and (d) selected area electron diffraction (SAED) pattern of the TEM image shown in (c). Inset images in (a) and (b) silver nanotriangles and SAED pattern of nanotriangles.
image).24 It is apparent from the SEM image that spherical NPs are formed initially followed by the slow reduction and crystallization leads to the formation of MTPs as a result of their inherent stability and shape directing ability of the constituents present in the extract.33 The NPs are poly-dispersed and the average size of spherical NPs is found to be 183 ± 10 nm. The average edge length of the triangle is 272 ± 20 nm. The spherical NPs consist of dark regions surrounded by faint shell with a capping agent possibly of organic material present in leaf extract. The increase in quantity of leaf extract to 1.0 ml in solution revealed the formation of clusters of small NPs as shown in Figure 2(b). It is apparent from the careful examination of the TEM image in Figure 2(b) which revealed the contacts among faint shells surrounding the NP surfaces. These interactions are mainly because of the functional groups present in capping agent. It may be explained that the dominant attraction among the capping agents surrounding the dark region viz., non-covalent interactions like Hydrogen bonding or − interaction or dipole–dipole interactions, than 4
the attraction between NP surfaces and capping agent surface can lead to the cluster formation. Further increase in quantity of leaf extract to 4.0 ml in solution also revealed the modulation of shape of NPs (Fig. 2(c) and inset) and the importance of constituents in the extract. The selected area diffraction pattern (SAED) of the TEM image shown in Figure 2(d) revealed the concentric ring pattern reflecting the randomly oriented polycrystalline molecular structure.34 The concentric diffraction rings are indexed to reflections from the (111), (200), (220), (222) lattice planes of face centred cubic (FCC) structure of gold. 3.2. Silver Nanoparticles The UV-Visible spectra of the solutions containing silver NPs shown in Figure 3(a), revealed the presence of Ag NPs in all the mixtures. At low concentration the absorption band (∼440 nm) is originating mostly from surface plasmon vibrations of spherical silver NPs.30 31 However, with an increase in concentration, the absorption bands increased in intensity indicating the increase in population of Ag NPs. Further increase in concentration observed Adv. Sci. Eng. Med., 4, 1–6, 2012
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4. CONCLUSION The changes in concentration of leaf extract can modify the phase morphology; size of the NPs can be tuned. Even at low concentrations the formed spherical nanoparticles can be slowly transformed into gold nanotriangles via MTPs, while at high concentration clusters of small NPs Adv. Sci. Eng. Med., 4, 1–6, 2012
can be synthesized. SAED diffraction patterns of biogenic silver nanotriangles revealed the orientation of nanotriangles top surface normal to the electron beam. Acknowledgment: Venkatesh Gude and Kalpana Upadhyaya acknowledge DST, India for DST INSPIRE fellowship. We acknowledge TP Radhakrishnan and MD Prasad, University of Hyderabad, Hyderabad for providing the TEM measurements.
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sharp lines at ∼440 nm indicates the presence of silver NPs in the mixtures. To understand the Ag NPs formation in the mixture, we recorded the UV-Visible spectra (Fig. 3(b)) of the solution (containing 1 ml of leaf extract and 6 ml of 10−3 M aqueous Ag+ ions in a 10 ml volumetric flask) as a function of time. Initially at 5 min, the observed broad band (∼417 nm) reflects the surface plasmon resonance of silver NPs. Further the absorption intensity and broadness of the bands increased monotonically as a function of time with a blue shift in absorption band, indicating an increase in percentage of nanoparticles in solution as well as interaction between NPs surface and capping agent surface. However the reduction process had continued till 10 h due to low reduction potential of Ag+ ions which subsequently saturated in the formation of Ag NPs in solution. TEM images of the synthesized silver NPs corresponding to 0.5, 1, 4 ml leaf extract respectively are presented in Figures 4(a)–(c) and the selected area electron diffraction pattern (SAED) of Figure 4(c) is shown in Figure 4(d). Figure 4(a) displays the formation of spherical silver NPs with a small fraction of triangular silver NPs (also shown in inset with its SAED pattern) in the solution containing a small amount of leaf extract (0.5 ml). It is apparent from the SEM image that spherical silver NPs are polydisperse in nature with the average size of spherical NPs found to be 429 391 224 ± 10 nm. It is apparent from all the TEM images that the spherical NPs consist of dark regions surrounded by a faint thin shell with a capping agent possibly of organic material present in leaf extract. On increasing the concentration of extract the average particle size decreases. The SAED diffraction pattern of the polydisperse NPs confirmed the polycrystalline nature of Ag NPs. SAED diffraction patterns of biogenic silver nanotriangles as shown in insets of Figures 4(a)–(b) suggested the single crystalline nature of NPs.34 35 The hexagonal pattern of concentric diffraction rings could be indexed and correspond to reflections from the (111), (200), lattice planes of FCC structure of silver. This shows that triangles are highly [111] oriented with the top surface normal to the electron beam.35 The presence of the forbidden 1/3{422} reflection has also been explained as a result of stacking faults in the nanotriangles (27) and is a common feature in nanotriangles of gold and silver prepared by a variety of methods.11 36 Further work is in progress to identify the chemical components of the leaf extract which is responsible for the reduction process of gold and silver ions to NPs.
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26. A. Chakraborty, A. Brantner, and T. Mukainaka, Canc. Lett. 177, 1 (2002). 27. A. B. Gokhale, A. S. Damre, and K. R. Kulkami, Phytomedicine 5, 433 (2002). 28. W. Shibeshi, E. Makonnen, A. Debella, and L. Zerihun, Pharmacologyonline 3, 217 (2006). 29. S. Srivastav, P. Singh, G. Mishra, K. K. Jha, and R. L. Khosa, J. Nat. Prod. Plant Resour. 1, 1 (2011). 30. C. C. Barua, A. Talukdar, S. A. Begum, A. K. Handique, G. K. Handique, J. D. Roy, and B. Buragohain, Indo Global J. Pharm. Sci. 1, 13 (2011). 31. A. Henglein, J. Phys. Chem. 97, 5457 (1993).
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