Rapid synthesis of Au, Ag, and bimetallic Au core–Ag shell nanoparticles using Neem (Azadirachta indica) leaf broth

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Abstract

We report on the use of Neem (Azadirachta indica) leaf broth in the extracellular synthesis of pure metallic silver and gold nanoparticles and bimetallic Au/Ag nanoparticles. On treatment of aqueous solutions of silver nitrate and chloroauric acid with Neem leaf extract, the rapid formation of stable silver and gold nanoparticles at high concentrations is observed to occur. The silver and gold nanoparticles are polydisperse, with a large percentage of gold particles exhibiting an interesting flat, platelike morphology. Competitive reduction of Au3+ and Ag+ ions present simultaneously in solution during exposure to Neem leaf extract leads to the synthesis of bimetallic Au core–Ag shell nanoparticles in solution. Transmission electron microscopy revealed that the silver nanoparticles are adsorbed onto the gold nanoparticles, forming a core–shell structure. The rates of reduction of the metal ions by Neem leaf extract are much faster than those observed by us in our earlier studies using microorganisms such as fungi, highlighting the possibility that nanoparticle biological synthesis methodologies will achieve rates of synthesis comparable to those of chemical methods.

Introduction

There have been impressive developments in the field of nanotechnology in the recent past, with numerous methodologies formulated to synthesize nanoparticles of particular shape and size depending on specific requirements. Currently, there is a growing need to develop environmentally benign nanoparticle synthesis processes that do not use toxic chemicals in the synthesis protocol. As a result, researchers in the field of nanoparticle synthesis and assembly have turned to biological systems for inspiration. This is not surprising given that many organisms, both unicellular and multicellular, are known to produce inorganic materials either intracellularly [1] or extracellularly [2]. Some well-known examples of microorganisms synthesizing inorganic materials include magnetotactic bacteria (which synthesize magnetite nanoparticles) [3], [4], [5], diatoms (which synthesize siliceous materials) [6], [7], [8], and S-layer bacteria (which produce gypsum and calcium carbonate layers) [9], [10]. The secrets gleaned from nature have led to the development of biomimetic approaches to the growth of advanced nanomaterials.

Even though many biotechnological applications such as remediation of toxic metals employ microorganisms such as bacteria [11] and yeast [12] (the detoxification often occurring via reduction of the metal ions/formation of metal sulfides) it is only relatively recently that materials scientists have been viewing with interest such microorganisms as possible ecofriendly nanofactories [13], [14], [15], [16], [17], [18], [19]. Different species of bacteria have been shown to intracellularly synthesize metal nanoparticles [13], [14], [15], [16], [17], [18], [19] and their alloys [19]. In this laboratory, some of us have shown that apart from prokaryotic organisms (bacteria), eukaryotic organisms such as fungi may also be used to grow nanoparticles of different chemical compositions and sizes [20], [21], [22], [23], [24] including quantum dots of the technologically important CdS by enzymatic processes [24]. We have also been successful in the synthesis of fairly monodisperse gold nanoparticles of 8-nm average size using the alkalothermophilic (extremophilic) actinomycete Thermomonospora sp., [25] which is currently difficult to achieve through biological means. As can be seen from the above, the use of microorganisms in the deliberate and controlled synthesis of nanoparticles is a relatively new and exciting area of research with considerable potential for development.

While microorganisms such as bacteria, actinomycetes, and fungi continue to be investigated in metal nanoparticle synthesis, the use of parts of whole plants in similar nanoparticle synthesis methodologies is an exciting possibility that is relatively unexplored and underexploited. Even though the gold nanoparticles are considered biocompatible, chemical synthesis methods may still lead to the presence of some toxic chemical species adsorbed on the surface that may have adverse effects in medical applications. Synthesis of nanoparticles using microorganisms or plants can potentially eliminate this problem by making the nanoparticles more biocompatible. Using plants for synthesis of nanoparticles could be advantageous over other environmentally benign biological processes by eliminating the elaborate process of maintaining cell cultures. It can also be suitably scaled up for large-scale synthesis of nanoparticles. Recently, Jose-Yacaman and co-workers demonstrated the synthesis of gold and silver nanoparticles within live alfalfa plants by gold and silver ion uptake, respectively, from solid media [26], [27]. In a related report, agricultural biomass was used to reduce Cr(VI) to Cr(III) ions [28], indicating that biological methods can be very efficient in decontaminating polluted waters and soil polluted with heavy metal ions.

In this paper, we report on the synthesis of pure metallic nanoparticles of silver and gold by the reduction of aqueous Ag+ and AuCl4 ions and also the synthesis of bimetallic core–shell nanoparticles of gold and silver by simultaneous reduction of aqueous Ag+ and AuCl4 ions with the broth of Neem leaves (Azadirachta indica). Through an elaborate screening process involving a number of plants we observed that Neem leaves were potential candidates for rapid synthesis of silver and gold nanoparticles. Presented below are details of the investigation.

Section snippets

Materials and methods

Materials used for the synthesis of gold and silver nanoparticles are chloroauric acid (HAuCl4), silver nitrate (AgNO3), and Neem (Azadirachta indica) leaf broth prepared by taking 20 g of thoroughly washed and finely cut A. indica leaves in a 500-mL Erlenmeyer flask with 100 mL of sterile distilled water and then boiling the mixture for 2 min before finally decanting it.

For reduction of Ag+ ions, 5 mL of Neem leaf broth was added to 45 mL of 10−3 M aqueous AgNO3 solution. Similarly, 5 mL of

Synthesis of silver and gold nanoparticles

Formation of the metal nanoparticles by reduction of the aqueous metal ions during exposure to the broth of boiled A. indica leaves may be easily followed by UV–vis spectroscopy. It is well known that silver and gold nanoparticles exhibit yellowish-brown and ruby red colors, respectively, in water, these colors arising due to excitation of surface plasmon vibrations in the metal nanoparticles [29]. Figs. 1A and 1B show the UV–vis spectra recorded from the aqueous silver nitrate–Neem leaf broth

Conclusions

In conclusion, a process for the rapid synthesis of stable silver, gold, and bimetallic Au/Ag core–shell nanoparticles at high concentration using Neem leaf broth was demonstrated. The flavanone and terpenoid constituents of the leaf broth are believed to be the surface active molecules stabilizing the nanoparticles. The formation of pure metallic and bimetallic nanoparticles by reduction of the metal ions is possibly facilitated by reducing sugars and/or terpenoids present in the Neem leaf

Acknowledgments

S.S.S. and A.R. thank the Council of Scientific and Industrial Research (CSIR), Government of India, for financial assistance. The TEM assistance of Ms. Renu Pasricha, Physical Chemistry Division, NCL Pune, is gratefully acknowledged.

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