Biogenic synthesis of Au and Ag nanoparticles using aqueous solutions of Black Tea leaf extracts

https://doi.org/10.1016/j.colsurfb.2009.01.012Get rights and content

Abstract

We explored the application of three different aqueous solutions derived from Black Tea leaf extracts in the synthesis of Au and Ag nanoparticles. The plain tea leaf broth, as well as that containing the ethyl acetate extract of tea leaves, were found to be extremely efficient, leading to rapid formation of stable nanoparticles of various shapes: spheres, trapezoids, prisms and rods. For a given metal ion precursor, the kinetics of particle synthesis were remarkably similar in these two solutions, as evidenced from their absorption spectroscopy monitored over time. Moreover, they exhibited similar redox behavior. In contrast, with the other solution, containing the dichloromethane (CH2Cl2) extract of tea leaves, we failed to detect any nanoparticle generation under similar reaction conditions. Our results suggest that the reduction of metal ions and stabilization of the resultant particles in the first two solutions involved the same class of biomolecules. We identified these biomolecules as the tea polyphenols, including flavonoids, which were present in comparable amounts in both the tea leaf broth and ethyl acetate extract, but are absent in the CH2Cl2 extract of tea leaves. The efficiency of the tea leaf extracts towards Au and Ag nanoparticle synthesis were compared with that of a naturally occurring hydroxyflavonoid, quercetin.

Introduction

Metal nanoparticles have received considerable attention in recent years because of their unique properties and potential applications in catalysis, photonics, optoelectronics, biological tagging and pharmaceutical applications. Their performance depends critically on their size, shape and composition. Though a huge variety of chemical synthesis methods are available for metal nanoparticles, many of the reactants and starting materials used in these reactions are toxic and potentially hazardous. In contrast, synthetic methods based on naturally occurring biomaterials provide an alternative, environmental-friendly means of obtaining these nanoparticles. In recent times, several groups have achieved success in the synthesis of Ag, Au, Pd nanoparticles using extracts obtained from unicellular organisms like bacteria [1], [2] and fungi [3], [4], [5], as well as extracts of plant parts, e.g., geranium leaves [5], lemongrass [6], neem leaves [7], aloe vera [8] and others [9]. These authors have succeeded not only in the synthesis of the nanoparticles but also in obtaining particles of exotic shapes and morphologies [7]. The spectacular success in this field has opened up the prospect of developing “green-chemical” methods of synthesis of metal nanoparticles with tailor-made structural properties using benign starting materials.

In this paper, we report on the synthesis of Au and Ag nanoparticles by the reduction of aqueous Ag+ and AuCl4 ions with the help of extracts obtained from Black Tea leaves. Black Tea leaves are rich in polyphenolic compounds, especially flavonoids [10]. Theaflavins and thearubigins are the polyphenolic molecules which can be isolated from tea infusion [11]. These are catechin group of flavanols [11]. Many have pronounced anti-oxidant properties [12], [13]. However, till date, there have been no reports on the synthesis of Ag or Au nanoparticles with Black Tea leaf extracts. We have used three different extracts prepared from Black Tea leaves for the biogenic reduction of AgI and AuIII ions, which we characterized as: (i) tea leaf broth, (ii) ethyl acetate extract and (iii) CH2Cl2 extract. The leaf broth, obtained by direct filtration of an aqueous infusion of tea leaves, contains a large variety of biomolecules like caffeine, flavonoids, chlorophyll and tannic acid [12], [13]. On the other hand, the ethyl acetate extract and CH2Cl2 extract predominantly contain polyphenols (including flavonoids and tannic acids) and caffeine, respectively [14]. Thus, by using the three extracts separately, we sought to address the important problem of identifying which types of biomolecules could play the major role in metal ion reduction and subsequent formation of metal nanoparticles. Apart from using these extracts, we also tested the efficiency of quercetin, a well-known naturally occurring hydroxyflavone which is also obtained from Black Tea infusion [15] for the synthesis of the metal nanoparticles, and compared the results with the performance of the tea leaf extracts.

The formation and growth of the nanoparticles was monitored with the help of absorption spectroscopy, while their shape, size and morphologies were determined by transmission electron microscopy (TEM).

Section snippets

Experimental

Silver nitrate (AgNO3) and chloroauric acid (HAuCl4) were used as sources of AgI and AuIII ions, required for the synthesis of Ag and Au nanoparticles, respectively. The different tea leaf extracts were prepared from CTC (Curl, Tear and Crush) grade Black Tea which is available in the local market. The method of preparation was as follows. For the tea leaf broth, 180 g of dried tea leaves were boiled in 400 mL water. The resulting infusion was then filtered thoroughly and repeatedly until no

Absorption spectroscopy of nanoparticle synthesis process

The progress of the reaction between metal ions and the tea leaf extracts were monitored by recording the absorption spectra as a function of time. Fig. 2(a) and (b) show the results of the reaction between the AgI ion-containing solutions and the tea leaf broth and ethyl acetate extract of tea leaves, respectively. In Fig. 2(a), the broken curve represents the absorption spectrum of the tea leaf broth at t = 0, i.e., at the instant of addition of AgI solution. Upon stirring at 40 °C for 30 min, a

Conclusions

In an attempt to find natural, environmentally benign and easily available plant-based agents for metal nanoparticle synthesis, we have demonstrated the superb efficiency of certain Black Tea leaf extracts in the rapid synthesis of stable Ag and Au nanoparticles possessing a variety of fascinating morphologies. Based on our kinetic studies, together with evidence obtained from FTIR and cyclic voltammetry, we propose that the main biomolecules responsible for the nanoparticle synthesis were the

Acknowledgements

D. Mandal acknowledges financial support for the work from the Department of Science & Technology, India (Fast Track Project), the University Grants Commission (UGC), India, and the Council of Scientific & Industrial Research (CSIR), India. N.A. Begum acknowledges financial support from UGC. Spectroscopy and cyclic voltammetry were performed in Visva-Bharati with instruments purchased under the DST (FIST program) and UGC (X Plan program). SB and RAL thank CSIR and UGC, respectively, for

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