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Ag alloy nanoparticles using aqueous extract of mahogany (Swietenia mahogani JACQ.) leaves
Colloids and Surfaces B: Biointerfaces 82 (2011) 497–504
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Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Biogenic synthesis of Ag, Au and bimetallic Au/Ag alloy nanoparticles using aqueous extract of mahogany (Swietenia mahogani JACQ.) leaves Samiran Mondal a,b , Nayan Roy a , Rajibul A. Laskar a , Ismail Sk a , Saswati Basu b , Debabrata Mandal b , Naznin Ara Begum a,∗ a b
Bio-Organic Chemistry Lab, Department of Chemistry, Visva-Bharati University, Santiniketan 731 235, W.B., India Spectroscopy Lab, Department of Chemistry, Visva-Bharati University, Santiniketan 731 235, W.B., India
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Article history: Received 21 May 2010 Received in revised form 3 October 2010 Accepted 4 October 2010 Available online 12 October 2010 Keywords: Au and Ag nanoparticles Bimetallic Au/Ag alloy nanoparticles Mahogany leaf Swietenia mahogany JACQ. Biogenic synthesis
a b s t r a c t In this paper, we have demonstrated for the first time, the superb efficiency of aqueous extract of dried leaves of mahogany (Swietenia mahogani JACQ.) in the rapid synthesis of stable monometallic Au and Ag nanoparticles and also Au/Ag bimetallic alloy nanoparticles having spectacular morphologies. Our method was clean, nontoxic and environment friendly. When exposed to aqueous mahogany leaf extract, competitive reduction of AuIII and AgI ions present simultaneously in same solution leads to the production of bimetallic Au/Ag alloy nanoparticles. UV–visible spectroscopy was used to monitor the kinetics of nanoparticles formation. UV–visible spectroscopic data and TEM images revealed the formation of bimetallic Au/Ag alloy nanoparticles. Mahogany leaf extract contains various polyhydroxy limonoids which are responsible for the reduction of AuIII and AgI ions leading to the formation and stabilization of Au and Ag nanopaticles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Metal nanoparticles have received considerable attention in recent years because of their unique catalytic, electronic, optical and structural properties and subsequent technological applications as catalysts, sensors, nanoelectronic devices, biomedical tools and biosensors. The applicability as well as performance of these nanoparticles depends critically on their size, shape, composition and their fine structure, either as alloy or core–shell [1,2]. Therefore, the design and synthesis of nanoparticles with tailor-made structural properties is a highly challenging problem for researchers working in the field of nanoscience and nanotechnology. Though a huge variety of chemical and physical methods of synthesis are available for metal nanoparticles, these methods have several inherent drawbacks. Many of the reactants, starting materials and solvents used in the chemical synthetic methods are toxic and potentially hazardous [2,3]. Formation of toxic by-product is also a great problem associated with these methods [4]. On the other hand, in case of physical synthetic methods, there are problems due to the enormous consumption of energy required to maintain the high temperature and pressure conditions needed for these methods [5].
∗ Corresponding author. Tel.: +91 9434431810; fax: +91 3463261526. E-mail address: [email protected] (N.A. Begum). 0927-7765/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2010.10.007
One of the promising alternative synthetic routes for metal nanoparticles is biogenic synthesis, which employs non-toxic reactants derived from the biological sources ranging from unicellular organisms to higher plants. Research groups in India and abroad have achieved success in the synthesis of mostly Au and Ag nanoparticles and to some extent Pd and Pt nanoparticles using extracts obtained from unicellular organisms like bacteria [6] and fungi [7,8], as well as extracts of plant parts e.g. geranium leaves [9], lemongrass [10], neem leaves [4], and black tea leaves [2] and extracts of a plant based but animal derived biogenic materials, Indian propolis, a honey bee product [3]. The key advantage of the use of plant extracts as the biogenic agents for metal nanoparticle synthesis is their easy availability. Moreover, very simple laboratory set-up is required for the synthesis process and use of these biogenic materials potentially eliminates the elaborate process of cell culture and cell maintenance necessary for the biogenic synthesis of metal nanoparticles using unicellular organisms. Moreover, this synthetic protocol is applicable at room temperature and pressure, thus saving huge amount of energy. However, there have been relatively few cases in which attempts were made to engineer the shapes and sizes of the nanoparticles for technological applications [11]. Moreover, only a few works on the biogenic synthesis of alloy and core–shell bimetallic or trimetallic nanoparticles have been reported [4,12]. Au/Ag bimetallic nanoparticles having similar composition may show different optical responses due their different structural features e.g. alloy or core–shell. Au/Ag bimetallic alloy nanoparticles
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display a single surface plasmon resonance (SPR) band located at an intermediate position between the SPR band of monometallic Au and Ag nanoparticles, the position of the band being governed by the Au:Ag composition ratio. On the other hand, for Au/Ag bimetallic core–shell nanoparticles, two bands are observed [12,13]. The control of structural features and compositions of bimetallic nanoparticles are extensively studied by the researchers and the development of simple and versatile methods to control the structural features and compositions of bimetallic nanoparticles is an important and demanding job nowadays [13]. In this paper, we report on a clean, non-toxic and environmentally benign (green-chemical) method the synthesis of Au and Ag nanoparticles by the reduction of aqueous Ag+ and AuCl4 − ions using aqueous extracts of dried leaves of mahogany (Swietenia mahogani JACQ., family: Meliaceae). Au/Ag bimetallic nanoparticles were also synthesized by the simultaneous reduction of aqueous Ag+ and AuCl4 − ions using the same extract. This plant is locally available in our area and attracts interest because of its significant biological activities [14]. It is rich in limonoid content [14,15]. Limonoids are terpene type of phytochemicals abundant in various parts of the plants of Meliceace family to which S. mahogani JACQ. belongs [14,15]. Limonoids have wide spectrum of therapeutic effects such as antiviral, antifungal, antibacterial, antineoplastic and antimalarial [14,15]. Most of these limonoids occurring in the leaves and other part of S. mahogani JACQ. contain polyhydroxy groups [14]. Previously we have reported that polyols are basically responsible for the reduction of AuIII and AgI ions leading to the formation and stabilization of corresponding nanoparticles [2,3]. During the synthesis of Ag, Au and also Au/Ag bimetallic alloy nanoparticles, we did not use any surfactants or synthetic polymer as synthetic stabilizing agents. Moreover our synthetic protocol is also applicable at room temperature and pressure and it potentially eliminates the elaborate process of cell culture and cell maintenance necessary for the biogenic synthesis of metal nanoparticles using unicellular organisms. So a single naturally occurring and easily available material, i.e. aqueous leaf extract of S. mahogani JACQ. is playing the roles of the reducing agent and also the stabilizing agent for the rapid formation of stable metal nanoparticles with various compositions, shapes, sizes and also with high monodispersity. So it is probable that these polyhydroxy limonoid type of bioactive molecules may be attached to the surface of these synthesized nanoparticles which are actually responsible for the formation and stabilization of these nanoparticles. So the Au, Ag and bimetallic Au/Ag naoparticles synthesized by our method could have potential biomedical applications in future due to their biocompatibility. This made us interested to use aqueous extract of mahogany leaves for the synthesis of Au/Ag and bimetallic Au/Ag nanoparticles. The formation and growth of the nanoparticles were monitored with the help of absorption spectroscopy, while their shape, size and morphologies were determined by transmission electron microscopy (TEM).
(Sigma–Aldrich) were used as the source of AgI and AuIII ions required for the synthesis of Ag, Au and bimetallic Au/Ag nanoparticles. The IR spectra were taken on a Shimadzu FT-IR 8400 spectrometer. Absorption spectra were recorded on a Shimadzu UVPC-3101 spectrophotometer. Samples for transmission electron microscopy (TEM) were prepared by drop-coating the Ag and Au nanoparticle solution onto carbon-coated copper grids. The films on the grids were allowed to dry prior to the TEM measurement in a JEOL TEM2010 instrument. Cyclic voltammetry of the aqueous leaf-extracts of mahogany was carried out in a Potentiostat–galvanostat (PAR Vera Stat TMII). The scan rate was done in the potential range of −0.9 to 0.4 V vs. SCE at a scan rate of 50 mV s−1 . 2.2. Method of preparation of aqueous extracts of leaves of mahogany 1 g of the dried leaves of mahogany was boiled with 15 mL of double distilled water at 100 ◦ C for 5 min. After that, the solution was filtered and the filtrate was obtained as a clean brown solution which was used for the biogenic synthesis of Au and Ag nanoparticles as well as bimetallic Au/Ag nanoparticles. 2.3. Method of synthesis of Ag, Au and bimetallic Au/Ag alloy nanoparticles Metal nanoparticles were synthesized by adding aqueous solution of AgNO3 or HAuCl4 to aqueous extract of mahogany. In case of Ag nanoparticle synthesis, 60 L of leaf extract of mahogany was added to 5 mL of double distilled water followed by stirring for 2 min. To this solution, 30 L of 0.05 M AgNO3 solution was added so that the final concentration of AgI ions became 3 × 10−4 M. The reaction mixture was then continuously stirred at ∼40 ◦ C. Within 30 min, a yellow coloration appeared, indicating the onset Ag nanoparticle formation. The progress of the reaction was monitored by measuring the absorbance of the solution at regular intervals of time. Same procedure has been repeated at two different pH levels, 8.5 and 12.5, respectively. For the solutions at pH 12.5, upon stirring at room temperature for 5 min, a prominent peak appears at 438 nm. Upon stirring at room temperature for 25 min, a prominent peak appears at 422 nm for the solution at pH 8.5. For Au nanoparticle synthesis, a similar method has been followed, except that 50 L of 0.01 M aqueous HAuCl4 solution was used instead of AgNO3 aqueous solution. In this case, the final concentration of AuIII ions becomes 1 × 10−4 M. Within 5 min, a pink colouration was observed which indicated the onset of Au nanoparticle formation. Similarly for bimetallic Au/Ag nanoparticle synthesis, different concentrations of aqueous AgNO3 and HAuCl4 solutions were used in 5 mL of distilled water containing 60 L of aqueous extract (Table 1). 3. Results and discussions
2. Experimental
3.1. Synthesis of Ag and Au nanoparticles
2.1. General experimental procedure
UV–visible spectroscopy could be used to examine the formation of the metal nanoparticles by reduction of metal ions in aqueous solutions when exposed to the aqueous extract of mahogany leaves. Ag nanoparticles exhibit yellowish-brown color while Au nanoparticles exhibit ruby red colour in water. These colours are due to the excitation of surface plasmon vibrations in the metal nanoparticles. Fig. 1(a) shows the results of the reaction between AgI ion containing solution and the mahogany leaf extract
Leaves of mahogany are collected during the month of November 2008 from the local area. Leaves were washed with double distilled water for several times to make it free from dust and then dried under shade. After that, the leaves were crushed to into coarse powder and stored in an air-tight container at 4 ◦ C for further use. Silver nitrate (AgNO3 ) and chloroauric acid (HAuCl4 )
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Table 1 Synthesis of bimetallic Au/Ag alloy nanoparticles. System Monometallic Ag Bimetallic Au–Ag alloy Bimetallic Au–Ag alloy Bimetallic Au–Ag alloy Monometallic Au
Conc. of AgI −4
3 × 10 2.25 × 10−4 1.5 × 10−4 0.75 × 10−4 0
Conc. of AuIII
Position of SPR band (nm)
0 0.75 × 10−4 1.5 × 10−4 2.25 × 10−4 3 × 10−4
434 480 521 528 537
Fig. 1. Absorption spectra of a solution containing 3 × 10−4 M AgI ion and aqueous mahogony leaf extract at neutral pH (a), at pH = 8.5 (b) and at pH = 12.5 (c). The broken line represents the absorbance curve of the reaction medium in absence of the metal ions. Insets (i) show the change in peak absorbance and peak absorption wavelength with time, (ii) picture of the corresponding solutions after reduction.
at neutral pH as a function of time of the reaction. The broken curve represents the absorption spectrum of aqueous mahogany leaf extract at t = 0, i.e. at the instant of addition of AgI solution. Upon stirring at ∼40 ◦ C for 30 min, silver surface plasmon resonance band appears at 434 nm and steadily increases in intensity as a function of time. The inset in Fig. 1(a) demonstrates that both the absorbance as well as the wavelength of the main absorption peak of Ag nanoparticles increases steeply at early times, up to the first 200 min; after which the rate of change is reduced and finally, a saturation is observed at long times, i.e. at t ≥ 400 min. Figs. 1(b) and 2(c) show the results of the reaction between AgI ions containing solutions and mahogany leaf extract at two different pH levels, 8.5 and 12.5, respectively. In these figures, the broken curves represent the absorption spectra of mahogany leaf extract in aqueous solution at pH 8.5 and 12.5, respectively, at t = 0, i.e. at the instant addition of AgI solution. Upon stirring at room temperature for 5 min, a prominent peak appears at 438 nm for the solution at pH 12.5 and for the solution at pH 8.5, a prominent peak appears at 422 nm after 25 min. The inset in Fig. 1(b) demonstrates that, both the absorbance as well as the wavelength of the main absorption peak of Ag nanoparticles increases steeply at early times, up to the first 60 min after which the rate of change is reduced, and finally, a saturation is observed at long times, i.e. at t ≥ 85 min. Similar behavior is noticed in the case of Ag nanoparticle formation by
Fig. 2. Absorption spectra of a solution containing 1 × 10−4 M AuIII ion and aqueous mahogany leaf extract. The broken line represents the absorbance curve of the reaction medium in absence of metal ions. Insets (i) show the change in peak absorbance and peak absorption wavelength with time, (ii) picture of the corresponding solution after reduction.
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Fig. 3. (a) UV–vis spectra of Au–Ag alloy nanoparticle prepared at different Au/Ag molar ratios. [Systems (a–e) are given in Table 1]. Inset shows the colour of the corresponding nanoparticle solutions after reduction. (b) The positions of surface plasmon maximum as a function of the molar fraction of Au.
mahogany leaf extract at pH 12.5 as shown in the inset of Fig. 1(c). Here, the saturation is reached at time, t ≥ 55 min. Fig. 2 shows the results of the reaction between AuIII ion containing solution and the mahogany leaf extract as a function of time of the reaction. The broken curve represents the absorption spectrum of aqueous mahogany leaf extract at t = 0, i.e. at the instant of addition of AuIII solution. At this moment there is no peak in the range of 500–700 nm. But within 15 min of addition of AuIII solution with continuous stirring, the reaction mixture turned pink with the appearance of a main peak at 537 nm. The absorbance and peak wave length for Au nanoparticle formed by mahogany leaf extract were recorded and plotted against time, as shown in the inset of Fig. 2. It is evident that both the absorbance as well as peak wavelength shows a steep initial increase up to the first 30 min, followed by a reduction in the rate of change, leading to saturation at time: t ≥ 50 min. The increase of intensity of the SPR band with time clearly indicates the increased formation of metal nanoparticles by the reduction of AgI and AuIII ions. The gradual red-shift of the absorption peaks suggests the progressive formation of larger particles with time [16,17]. It is note-worthy that, both the absorbance and peak-shift for each solution exhibits a very similar feature which consists of an early rapid increase, followed by saturation after long time. Therefore, after a sufficiently long time, stable nanoparticle solutions are obtained. We continued to test efficacy of this extract in the synthesis of Au/Ag bimetallic nanoparticles noticing the ease and speed with which Ag and Au nanoparticles were synthesized by it. 3.2. Synthesis of bimetallic Au/Ag alloy nanoparticles AuIII and AgI ions present in the same solution are simultaneously reduced by mahogany leaf extract to form bimetallic Au/Ag alloy nanoparticles. The bimetallic Au/Ag alloy nanoparticles formation is confirmed from the fact that the optical absorption spectrum shows only one plasmon band in place of two individual bands for Au and Ag nanoparticles [12,13,18–20]. Fig. 3(a) shows normalized absorption spectra of the simultaneous reduction of the AgI and AuIII ions by mahogany leaf extract at various concentrations of AgI and AuIII ions (detailed concentration range in given in Table 1. Absorption spectra of monometallic Au and Ag nanoparticles are also incorporated in Fig. 3(a) for better comparison. The broken curves represent the absorption spectra of water extract of mahogany leaf at t = 0, i.e. at the instant of addition of AgI and AuIII ions. For system-(b), after complete reduction, the absorption maximum appears at 480 nm and for system-(c) and (d), the corresponding maxima appear at 521 nm and 528 nm, respectively. It was observed that, only one absorbance peak is obtained for each
of the bimetallic nanoparticle solutions. Moreover, in all the cases, the absorbance maxima are located at the positions intermediate of those for monometallic Ag and Au nanoparticle SPR bands at 434 and 537 nm, respectively. This is in agreement of the previous reported data [12,13]. Such absorption spectra cannot be obtained if it was a case of simple physical mixture of monometallic Ag and Au nanoparticle solution [8]. Moreover, the spectra in Fig. 3(a) do not bear a resemblance to those exhibited by the bimetallic Au/Ag core–shell nanoparticle, since two characteristic absorption peaks are observed for the bimetallic Au/Ag core–shell nanoparticle [12,13]. Moreover, as shown in Fig. 3(b), the SPR peak position of the bimetallic system was found to be gradually red-shifted in quasi-linear manner with the increase of concentration of AuIII ions; which is in agreement to previous reports of Au/Ag bimetallic alloy solutions [12,21]. The absorption spectroscopic data thus amply suggest that the simultaneous reduction of AgI and AuIII ions in aqueous mahogany extracts produce a homogeneous bimetallic alloy nanoparticle. This was further confirmed by the TEM images described below. 3.3. TEM: size and shape While the absorption spectra provide strong evidences of the formation of nanoparticles and their growth kinetics, the shape and size of the resultant naoparticles are elaborated with the help of the TEM. The TEM images in Fig. 4(a–d) confirm the formation of Ag, Au and bimetallic Au/Ag alloy nanoparticles, respectively. Fig. 4(a) and (b) are TEM images of the Ag nanoparticles formed by the aqueous leaf extract of mahogany at neutral condition and at pH 12.5, respectively. At neutral pH, particles are well separated and most of the particles are spheroidal [Fig. 4(a) (i–iv)]. But it is noteworthy that, when the pH is 12.5, although, most of the particles appear spheroidal, there are a quite few with definite anisotropic morphology. Moreover, in this case, nanoparticles form small aggregates [Fig. 4(b) (i–iv)]. On the other hand, aqueous leaf extract of mahogany at neutral condition affords a rich yield of Au nanoparticles of interesting morphologies as shown in Fig. 4(c) (i–iv). Together with spheroids, the abundance of various regular geometrical shapes e.g. triangles and hexagons are visible. Fig. 4(d) (i–iv) shows the TEM images of the bimetallic Au/Ag nanoparticles formed by the simultaneous reduction of AgI and AuIII ions in the (1:1) mixture by the aqueous extract of mahogany leaves at neutral condition. It is seen that the particles are predominantly spherical with uniform size distribution. In the close up view, well separated particles with occasional aggregations are clearly observed. TEM images of the bimetallic core/shell type structure show electron density banding with a dark Au core and a lighter
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Fig. 4. TEM images of Ag nanoparticles prepared by aq. leaf extract of mahogany at (a) neutral pH [i–iv]; (b) pH 12.5 [i–iv]; (c) TEM images of Au nanoparticles prepared by aq. leaf extract of mahogany [i–iv]; (d) TEM images of Au/Ag bimetallic alloy nanoparticles prepared by aq. leaf extract of mahogany [i–iv].
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Fig. 4. (Continued ).
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OH
OH
OH
* *
Transmittance
d
c
*
*
.
e-+H+
* *
O O
O e-+H+
Fig. 6. Scheme illustrating tentative mechanism of polyol oxidation by metal ions to ␣, -unsaturated carbonyl groups.
was observed for the same extract at pH 12.5 [Fig. 7(b)]. From the results, it is evident that the same types of biomolecules are responsible for the reduction of metal ions which are present in both the solutions.
b a
3.6. General discussion 1100
1200
1300
1400
1500
1600
1700
1800
Wavenumber(cm-1) Fig. 5. FTIR spectra of water extract of mahogany leaf (a) before reaction with metal ions and after reaction and formation of (b) Ag (c) Au and (d) Au/Ag bimetallic alloy nanoparticles. Absorption peaks in (a) are indicated by arrows, while the prominent peaks in (b), (c) and (d) are indicated by asterisks. The peak at 1390 cm−1 in (b) is due to the NO3 − ions of the precursor salt AgNO3 .
Ag shell [12]. In the TEM images of bimetallic Au/Ag nanoparticles synthesized by our method, it is clearly noticed that there is a uniform contrast for each nanoparticle; which suggests that the electron density is homogeneous within the volume of the particle. So the bimetallic nanoparticles are not core/shell type and they closely resemble to bimetallic alloy nanoparticles. These results are in strong agreement with the UV–vis spectroscopic data discussed previously and the reported data [12]. 3.4. FTIR spectra In case of leaf extract itself, the absorbance bands appear in the range of 3600–3300, 2925, 2346, 1725, 1606, 1517, 1450, 1438, 1394, 1379, 1276, 1251, 1072, 877–769 cm−1 . The IR absorption at 3600–3300, 2925, 1740–1725 cm−1 showed the presence of carbon–carbon double bond and hydroxyl and several types of carbonyl groups (e.g. cyclic ketones, lactones). These are the IR-spectroscopic evidence for the presence of polyhydroxy limonoids type of compounds in the leaf extract. Among them, the absorbance bands at 1725, 1606, 1517, 1450, 1438, 1394, 1379, 1276, 1251, 1072 cm−1 are included in the range of 1000–18000 cm−1 (Fig. 5). After biogenic reduction AgI and AuIII , these bands totally disappeared and new bands at 1742, 1652, 1613, 1515, 1454, 1379 cm−1 (in case of biogenic reduction of AgI ions) and 1739, 1650, 1516, 1451, 1379 cm−1 (in case of biogenic reduction of AuIII ions) appeared. For bimetallic nanoparticle bands appeared at 1742, 1654, 1608, 1518, 1379 cm−1 (Fig. 5). This may be due to the fact that polyols are actually responsible for the reduction of AgI and AuIII ions whereby they themselves get oxidized to ␣, -unsaturated carbonyl group or simple cyclic ketones leading to a broad peaks at 1742, 1652 cm−1 (for biogenic reduction of AgI ) and 1739, 1650 cm−1 (for biogenic reduction of AuIII ions) and at 1742, 1654 cm−1 (for bimetallic Au/Ag alloy nanoparticle formation). A tentative mechanism of this polyol oxidation has been shown in Fig. 6 [2,3]. These are in agreement with our previous findings [2,3].
We were interested in the synthesis of stable monometallic Ag, Au and bimetallic Au/Ag alloy nanoparticles by using a naturally occurring material which is locally available and environmentally benign and we have found that aqueous extract of leaves of mahogany (S. mahogani, JACQ.) served all the purposes. This single naturally occurring source is playing a strong role in the formation as well as stabilization of metal naoparticles with various chemical compositions, shapes and sizes and also with high monodispersity. In other synthetic methods, stabilization of the colloid or nanoparticle solution occurs due to the adsorption of the synthetic stabilizing agents e.g. surfactants or synthetic polymers on the surfaces of the nanoparticles which prevents the aggregation. In our work, no synthetic stabilizing agent was used. The chemical components of the aqueous extract of mahogany leaves are acting as the reducing agents and also as the stabilizing agents by adhering on the surface of the nanoparticles formed thereby preventing the aggregation and control the particle size. So our method is also important in the context of colloid and interface sciences. Furthermore, we have also proposed a probable mechanism of how these bioactive molecules are actually working. In future, proper understating and study of this may help in fine tuning of these processes leading to the formation of nanoparticles with tailor-made structural properties. In conventional chemical methods of synthesis, there is still a possibility of the adsorption of the toxic chemical entities (either reagents or by-products) on the surfaces of the nanoparticles which may cause adverse effects in medical applications [4]. The Au, Ag and bimetallic Au/Ag naoparticles synthesized by our method could have potential biomedical applications in future due to their biocompatibility as they are synthesized by a environmentally benign material having potential therapeutic applications.
0.00006
b 0.00004
Current (A)
1000
a
0.00002
0.00000
-0.00002
3.5. Redox behavior of mahogany leaf broth Cyclic voltammograms of aqueous extract of leaves of mahogany at neutral condition and at pH 12.5 are and displayed in Fig. 7. Aqueous extract at neutral condition showed a prominent reduction peak at −0.443 V [Fig. 7(a)] and a reduction peak at −0.443 V
-0.00004 -1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Potential (V) Fig. 7. Cyclic voltammograms of (a) water extract of mahogany leaf and (b) water extract of mahogany leaf at pH 12.5.
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Moreover, very simple laboratory set-up is required for our process and leaves of mahogany (S. mahogani, JACQ.) are locally available. The use of this plant based biogenic material potentially eliminates the elaborate process of cell culture and cell maintenance necessary for the biogenic synthesis of metal nanoparticles using unicellular organisms. Moreover, this synthetic protocol is applicable at room temperature and pressure, thus saving huge amount of energy. There is a special demand for the development of reliable, cost-effective and environment friendly synthetic protocol for the nanoparticles with various chemical compositions, shapes, sizes and high monodispersity for their potential technological applications and our synthetic protocol based on aqueous extract of dried leaves of mahogany (S. mahogani, JACQ.) could be a possible and promising alternative. 4. Conclusion We have demonstrated for the first time, the spectacular efficiency of a naturally available and environmentally benign plant-based product, mahogany (S. mahogani, JACQ.) leaf extract in the rapid synthesis of stable Au, Ag and bimetallic Au/Ag alloy nanoparticles possessing a wide spectrum of fascinating morphologies. The methodology which we have adopted was totally hazard free, low cost and environment friendly. The polyols, i.e. poly hydroxy limonoid type of constituents of the leaf extracts may be the surface active molecules which reduces AgI and AuIII ions to corresponding monometallic and bimetallic alloy nanoparticles and not only that, they also stabilizes these nanoparticles. In future, the present method may find potential applications in the rapid synthesis and stabilization of other monometallic and also bimetallic alloy and core–shell nanoparticles. Acknowledgements Authors are thankful to the University Grants Commission, India for the financial support (UGC M.R.P. grant no. 33-290/2007 (SR) to
N. A. B.). Thanks are also due to DST-FIST and UGC-SAP programmes of Dept. of Chemistry, Siksha Bhavana, Visva Bharati University. Authors gratefully acknowledge the help provided by Dr. A. Majee and Dr. S. Ghosh, Dept. of Chemistry, Siksha Bhavana, Visva Bharati University, for recording the IR spectra and the cyclic voltammograms, respectively. We also acknowledge Professor S. Das, Dept. of Metallurgy and Materials Engineering, IIT, Kharagpur-721302, W.B. for extending TEM facility. S.M. and I.S. thank CSIR and UGC respectively, for research fellowships. References [1] C.N.R. Rao, G.U. Kulkarni, P.J. Thomas, P.P. Edwards, Chem. Eur. J. 8 (2002) 28. [2] N.A. Begum, S. Mondal, S. Basu, R.A. Laskar, D. Mandal, Colloids Surf. B: Biointerf. 71 (2009) 113. [3] N. Roy, S. Mondal, R.A. Laskar, S. Basu, D. Mandal, N.A. Begum, Colloids Surf. B: Biointerf. 76 (2010) 317. [4] S. Shiv Shankar, A. Rai, A. Ahmad, M. Sastry, J. Colloid Interface Sci. 275 (2004) 496. [5] K.N. Thakkar, S.S. Mhatre, R.Y. Parikh, Nanomed.: Nanotechnol. Biol. Med. 6 (2010) 275. [6] A. Ahmad, S. Senapati, M.I. Khan, R. Kumar, M. Sastry, Langmuir 19 (2003) 3550. [7] P. Mukherjee, A. Ahmad, D. Mandal, S. Senapati, S.R. Sainkar, M.I. Khan, R. Ramani, R. Pasricha, P.V. Ajaykumar, M. Alam, M. Sastry, R. Kumar, Angew. Chem. Int. Ed. Engl. 40 (2001) 3585. [8] A. Ahmad, P. Mukherjee, S. Senapati, D. Mandal, M.I. Khan, R. Kumar, M. Sastry, Colloids Surf. B: Biointerf. 28 (2003) 313. [9] S. Shiv Shankar, A. Ahmad, R. Pasricha, M. Sastry, J. Mater. Chem. 13 (2003) 1822. [10] S. Shiv Shankar, A. Rai, A. Ahmad, M. Sastry, Chem. Mater. 17 (2005) 566. [11] B. Ankamwar, M. Chaudhary, M. Sastry, Syn. Reactivity in Inorg. Metal-Org. Nano-Metal Chem. 35 (2005) 19. [12] S. Shiv Shankar, A. Ahmad, A.M.I. Khan, M. Sastry, R. Kumar, Small 1 (2005) 517. [13] J.F. Sˇıanchez-Ramˇıırez, U. Pal, L. Nolasco-Hernaˇındez, J. Mendoza-Aˇı lvarez, J.A. Pescador-Rojas, J. Nanomater. (2008) 1, ID 620412. [14] M.M.G. Saad, T. Iwagawa, M. Doe, M. Nakatani, Tetrahedron 59 (2003) 8027. [15] A. Roy, S. Saraf, Biol. Pharm. Bull. 29 (2006) 191. [16] S. Link, M.A. El-Sayed, Int. Rev. Phys. Chem. 19 (2000) 409. [17] R. He, X. Qian, J. Yin, Z. Zhu, J. Mater. Chem. 12 (2002) 3783. [18] P. Malvaney, Langmuir 12 (1996) 788. [19] G.C. Papavassiliou, J. Phys. F: Metal Phys. 6 (1976) L103. [20] K. Torigoe, Y. Nakajima, K. Esumi, J. Phys. Chem. 97 (1993) 8304. [21] S. Link, Z.L. Wang, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 3529.
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Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Biogenic synthesis of Ag, Au and bimetallic Au/Ag alloy nanoparticles using aqueous extract of mahogany (Swietenia mahogani JACQ.) leaves Samiran Mondal a,b , Nayan Roy a , Rajibul A. Laskar a , Ismail Sk a , Saswati Basu b , Debabrata Mandal b , Naznin Ara Begum a,∗ a b
Bio-Organic Chemistry Lab, Department of Chemistry, Visva-Bharati University, Santiniketan 731 235, W.B., India Spectroscopy Lab, Department of Chemistry, Visva-Bharati University, Santiniketan 731 235, W.B., India
a r t i c l e
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Article history: Received 21 May 2010 Received in revised form 3 October 2010 Accepted 4 October 2010 Available online 12 October 2010 Keywords: Au and Ag nanoparticles Bimetallic Au/Ag alloy nanoparticles Mahogany leaf Swietenia mahogany JACQ. Biogenic synthesis
a b s t r a c t In this paper, we have demonstrated for the first time, the superb efficiency of aqueous extract of dried leaves of mahogany (Swietenia mahogani JACQ.) in the rapid synthesis of stable monometallic Au and Ag nanoparticles and also Au/Ag bimetallic alloy nanoparticles having spectacular morphologies. Our method was clean, nontoxic and environment friendly. When exposed to aqueous mahogany leaf extract, competitive reduction of AuIII and AgI ions present simultaneously in same solution leads to the production of bimetallic Au/Ag alloy nanoparticles. UV–visible spectroscopy was used to monitor the kinetics of nanoparticles formation. UV–visible spectroscopic data and TEM images revealed the formation of bimetallic Au/Ag alloy nanoparticles. Mahogany leaf extract contains various polyhydroxy limonoids which are responsible for the reduction of AuIII and AgI ions leading to the formation and stabilization of Au and Ag nanopaticles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Metal nanoparticles have received considerable attention in recent years because of their unique catalytic, electronic, optical and structural properties and subsequent technological applications as catalysts, sensors, nanoelectronic devices, biomedical tools and biosensors. The applicability as well as performance of these nanoparticles depends critically on their size, shape, composition and their fine structure, either as alloy or core–shell [1,2]. Therefore, the design and synthesis of nanoparticles with tailor-made structural properties is a highly challenging problem for researchers working in the field of nanoscience and nanotechnology. Though a huge variety of chemical and physical methods of synthesis are available for metal nanoparticles, these methods have several inherent drawbacks. Many of the reactants, starting materials and solvents used in the chemical synthetic methods are toxic and potentially hazardous [2,3]. Formation of toxic by-product is also a great problem associated with these methods [4]. On the other hand, in case of physical synthetic methods, there are problems due to the enormous consumption of energy required to maintain the high temperature and pressure conditions needed for these methods [5].
∗ Corresponding author. Tel.: +91 9434431810; fax: +91 3463261526. E-mail address: [email protected] (N.A. Begum). 0927-7765/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2010.10.007
One of the promising alternative synthetic routes for metal nanoparticles is biogenic synthesis, which employs non-toxic reactants derived from the biological sources ranging from unicellular organisms to higher plants. Research groups in India and abroad have achieved success in the synthesis of mostly Au and Ag nanoparticles and to some extent Pd and Pt nanoparticles using extracts obtained from unicellular organisms like bacteria [6] and fungi [7,8], as well as extracts of plant parts e.g. geranium leaves [9], lemongrass [10], neem leaves [4], and black tea leaves [2] and extracts of a plant based but animal derived biogenic materials, Indian propolis, a honey bee product [3]. The key advantage of the use of plant extracts as the biogenic agents for metal nanoparticle synthesis is their easy availability. Moreover, very simple laboratory set-up is required for the synthesis process and use of these biogenic materials potentially eliminates the elaborate process of cell culture and cell maintenance necessary for the biogenic synthesis of metal nanoparticles using unicellular organisms. Moreover, this synthetic protocol is applicable at room temperature and pressure, thus saving huge amount of energy. However, there have been relatively few cases in which attempts were made to engineer the shapes and sizes of the nanoparticles for technological applications [11]. Moreover, only a few works on the biogenic synthesis of alloy and core–shell bimetallic or trimetallic nanoparticles have been reported [4,12]. Au/Ag bimetallic nanoparticles having similar composition may show different optical responses due their different structural features e.g. alloy or core–shell. Au/Ag bimetallic alloy nanoparticles
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display a single surface plasmon resonance (SPR) band located at an intermediate position between the SPR band of monometallic Au and Ag nanoparticles, the position of the band being governed by the Au:Ag composition ratio. On the other hand, for Au/Ag bimetallic core–shell nanoparticles, two bands are observed [12,13]. The control of structural features and compositions of bimetallic nanoparticles are extensively studied by the researchers and the development of simple and versatile methods to control the structural features and compositions of bimetallic nanoparticles is an important and demanding job nowadays [13]. In this paper, we report on a clean, non-toxic and environmentally benign (green-chemical) method the synthesis of Au and Ag nanoparticles by the reduction of aqueous Ag+ and AuCl4 − ions using aqueous extracts of dried leaves of mahogany (Swietenia mahogani JACQ., family: Meliaceae). Au/Ag bimetallic nanoparticles were also synthesized by the simultaneous reduction of aqueous Ag+ and AuCl4 − ions using the same extract. This plant is locally available in our area and attracts interest because of its significant biological activities [14]. It is rich in limonoid content [14,15]. Limonoids are terpene type of phytochemicals abundant in various parts of the plants of Meliceace family to which S. mahogani JACQ. belongs [14,15]. Limonoids have wide spectrum of therapeutic effects such as antiviral, antifungal, antibacterial, antineoplastic and antimalarial [14,15]. Most of these limonoids occurring in the leaves and other part of S. mahogani JACQ. contain polyhydroxy groups [14]. Previously we have reported that polyols are basically responsible for the reduction of AuIII and AgI ions leading to the formation and stabilization of corresponding nanoparticles [2,3]. During the synthesis of Ag, Au and also Au/Ag bimetallic alloy nanoparticles, we did not use any surfactants or synthetic polymer as synthetic stabilizing agents. Moreover our synthetic protocol is also applicable at room temperature and pressure and it potentially eliminates the elaborate process of cell culture and cell maintenance necessary for the biogenic synthesis of metal nanoparticles using unicellular organisms. So a single naturally occurring and easily available material, i.e. aqueous leaf extract of S. mahogani JACQ. is playing the roles of the reducing agent and also the stabilizing agent for the rapid formation of stable metal nanoparticles with various compositions, shapes, sizes and also with high monodispersity. So it is probable that these polyhydroxy limonoid type of bioactive molecules may be attached to the surface of these synthesized nanoparticles which are actually responsible for the formation and stabilization of these nanoparticles. So the Au, Ag and bimetallic Au/Ag naoparticles synthesized by our method could have potential biomedical applications in future due to their biocompatibility. This made us interested to use aqueous extract of mahogany leaves for the synthesis of Au/Ag and bimetallic Au/Ag nanoparticles. The formation and growth of the nanoparticles were monitored with the help of absorption spectroscopy, while their shape, size and morphologies were determined by transmission electron microscopy (TEM).
(Sigma–Aldrich) were used as the source of AgI and AuIII ions required for the synthesis of Ag, Au and bimetallic Au/Ag nanoparticles. The IR spectra were taken on a Shimadzu FT-IR 8400 spectrometer. Absorption spectra were recorded on a Shimadzu UVPC-3101 spectrophotometer. Samples for transmission electron microscopy (TEM) were prepared by drop-coating the Ag and Au nanoparticle solution onto carbon-coated copper grids. The films on the grids were allowed to dry prior to the TEM measurement in a JEOL TEM2010 instrument. Cyclic voltammetry of the aqueous leaf-extracts of mahogany was carried out in a Potentiostat–galvanostat (PAR Vera Stat TMII). The scan rate was done in the potential range of −0.9 to 0.4 V vs. SCE at a scan rate of 50 mV s−1 . 2.2. Method of preparation of aqueous extracts of leaves of mahogany 1 g of the dried leaves of mahogany was boiled with 15 mL of double distilled water at 100 ◦ C for 5 min. After that, the solution was filtered and the filtrate was obtained as a clean brown solution which was used for the biogenic synthesis of Au and Ag nanoparticles as well as bimetallic Au/Ag nanoparticles. 2.3. Method of synthesis of Ag, Au and bimetallic Au/Ag alloy nanoparticles Metal nanoparticles were synthesized by adding aqueous solution of AgNO3 or HAuCl4 to aqueous extract of mahogany. In case of Ag nanoparticle synthesis, 60 L of leaf extract of mahogany was added to 5 mL of double distilled water followed by stirring for 2 min. To this solution, 30 L of 0.05 M AgNO3 solution was added so that the final concentration of AgI ions became 3 × 10−4 M. The reaction mixture was then continuously stirred at ∼40 ◦ C. Within 30 min, a yellow coloration appeared, indicating the onset Ag nanoparticle formation. The progress of the reaction was monitored by measuring the absorbance of the solution at regular intervals of time. Same procedure has been repeated at two different pH levels, 8.5 and 12.5, respectively. For the solutions at pH 12.5, upon stirring at room temperature for 5 min, a prominent peak appears at 438 nm. Upon stirring at room temperature for 25 min, a prominent peak appears at 422 nm for the solution at pH 8.5. For Au nanoparticle synthesis, a similar method has been followed, except that 50 L of 0.01 M aqueous HAuCl4 solution was used instead of AgNO3 aqueous solution. In this case, the final concentration of AuIII ions becomes 1 × 10−4 M. Within 5 min, a pink colouration was observed which indicated the onset of Au nanoparticle formation. Similarly for bimetallic Au/Ag nanoparticle synthesis, different concentrations of aqueous AgNO3 and HAuCl4 solutions were used in 5 mL of distilled water containing 60 L of aqueous extract (Table 1). 3. Results and discussions
2. Experimental
3.1. Synthesis of Ag and Au nanoparticles
2.1. General experimental procedure
UV–visible spectroscopy could be used to examine the formation of the metal nanoparticles by reduction of metal ions in aqueous solutions when exposed to the aqueous extract of mahogany leaves. Ag nanoparticles exhibit yellowish-brown color while Au nanoparticles exhibit ruby red colour in water. These colours are due to the excitation of surface plasmon vibrations in the metal nanoparticles. Fig. 1(a) shows the results of the reaction between AgI ion containing solution and the mahogany leaf extract
Leaves of mahogany are collected during the month of November 2008 from the local area. Leaves were washed with double distilled water for several times to make it free from dust and then dried under shade. After that, the leaves were crushed to into coarse powder and stored in an air-tight container at 4 ◦ C for further use. Silver nitrate (AgNO3 ) and chloroauric acid (HAuCl4 )
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Table 1 Synthesis of bimetallic Au/Ag alloy nanoparticles. System Monometallic Ag Bimetallic Au–Ag alloy Bimetallic Au–Ag alloy Bimetallic Au–Ag alloy Monometallic Au
Conc. of AgI −4
3 × 10 2.25 × 10−4 1.5 × 10−4 0.75 × 10−4 0
Conc. of AuIII
Position of SPR band (nm)
0 0.75 × 10−4 1.5 × 10−4 2.25 × 10−4 3 × 10−4
434 480 521 528 537
Fig. 1. Absorption spectra of a solution containing 3 × 10−4 M AgI ion and aqueous mahogony leaf extract at neutral pH (a), at pH = 8.5 (b) and at pH = 12.5 (c). The broken line represents the absorbance curve of the reaction medium in absence of the metal ions. Insets (i) show the change in peak absorbance and peak absorption wavelength with time, (ii) picture of the corresponding solutions after reduction.
at neutral pH as a function of time of the reaction. The broken curve represents the absorption spectrum of aqueous mahogany leaf extract at t = 0, i.e. at the instant of addition of AgI solution. Upon stirring at ∼40 ◦ C for 30 min, silver surface plasmon resonance band appears at 434 nm and steadily increases in intensity as a function of time. The inset in Fig. 1(a) demonstrates that both the absorbance as well as the wavelength of the main absorption peak of Ag nanoparticles increases steeply at early times, up to the first 200 min; after which the rate of change is reduced and finally, a saturation is observed at long times, i.e. at t ≥ 400 min. Figs. 1(b) and 2(c) show the results of the reaction between AgI ions containing solutions and mahogany leaf extract at two different pH levels, 8.5 and 12.5, respectively. In these figures, the broken curves represent the absorption spectra of mahogany leaf extract in aqueous solution at pH 8.5 and 12.5, respectively, at t = 0, i.e. at the instant addition of AgI solution. Upon stirring at room temperature for 5 min, a prominent peak appears at 438 nm for the solution at pH 12.5 and for the solution at pH 8.5, a prominent peak appears at 422 nm after 25 min. The inset in Fig. 1(b) demonstrates that, both the absorbance as well as the wavelength of the main absorption peak of Ag nanoparticles increases steeply at early times, up to the first 60 min after which the rate of change is reduced, and finally, a saturation is observed at long times, i.e. at t ≥ 85 min. Similar behavior is noticed in the case of Ag nanoparticle formation by
Fig. 2. Absorption spectra of a solution containing 1 × 10−4 M AuIII ion and aqueous mahogany leaf extract. The broken line represents the absorbance curve of the reaction medium in absence of metal ions. Insets (i) show the change in peak absorbance and peak absorption wavelength with time, (ii) picture of the corresponding solution after reduction.
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Fig. 3. (a) UV–vis spectra of Au–Ag alloy nanoparticle prepared at different Au/Ag molar ratios. [Systems (a–e) are given in Table 1]. Inset shows the colour of the corresponding nanoparticle solutions after reduction. (b) The positions of surface plasmon maximum as a function of the molar fraction of Au.
mahogany leaf extract at pH 12.5 as shown in the inset of Fig. 1(c). Here, the saturation is reached at time, t ≥ 55 min. Fig. 2 shows the results of the reaction between AuIII ion containing solution and the mahogany leaf extract as a function of time of the reaction. The broken curve represents the absorption spectrum of aqueous mahogany leaf extract at t = 0, i.e. at the instant of addition of AuIII solution. At this moment there is no peak in the range of 500–700 nm. But within 15 min of addition of AuIII solution with continuous stirring, the reaction mixture turned pink with the appearance of a main peak at 537 nm. The absorbance and peak wave length for Au nanoparticle formed by mahogany leaf extract were recorded and plotted against time, as shown in the inset of Fig. 2. It is evident that both the absorbance as well as peak wavelength shows a steep initial increase up to the first 30 min, followed by a reduction in the rate of change, leading to saturation at time: t ≥ 50 min. The increase of intensity of the SPR band with time clearly indicates the increased formation of metal nanoparticles by the reduction of AgI and AuIII ions. The gradual red-shift of the absorption peaks suggests the progressive formation of larger particles with time [16,17]. It is note-worthy that, both the absorbance and peak-shift for each solution exhibits a very similar feature which consists of an early rapid increase, followed by saturation after long time. Therefore, after a sufficiently long time, stable nanoparticle solutions are obtained. We continued to test efficacy of this extract in the synthesis of Au/Ag bimetallic nanoparticles noticing the ease and speed with which Ag and Au nanoparticles were synthesized by it. 3.2. Synthesis of bimetallic Au/Ag alloy nanoparticles AuIII and AgI ions present in the same solution are simultaneously reduced by mahogany leaf extract to form bimetallic Au/Ag alloy nanoparticles. The bimetallic Au/Ag alloy nanoparticles formation is confirmed from the fact that the optical absorption spectrum shows only one plasmon band in place of two individual bands for Au and Ag nanoparticles [12,13,18–20]. Fig. 3(a) shows normalized absorption spectra of the simultaneous reduction of the AgI and AuIII ions by mahogany leaf extract at various concentrations of AgI and AuIII ions (detailed concentration range in given in Table 1. Absorption spectra of monometallic Au and Ag nanoparticles are also incorporated in Fig. 3(a) for better comparison. The broken curves represent the absorption spectra of water extract of mahogany leaf at t = 0, i.e. at the instant of addition of AgI and AuIII ions. For system-(b), after complete reduction, the absorption maximum appears at 480 nm and for system-(c) and (d), the corresponding maxima appear at 521 nm and 528 nm, respectively. It was observed that, only one absorbance peak is obtained for each
of the bimetallic nanoparticle solutions. Moreover, in all the cases, the absorbance maxima are located at the positions intermediate of those for monometallic Ag and Au nanoparticle SPR bands at 434 and 537 nm, respectively. This is in agreement of the previous reported data [12,13]. Such absorption spectra cannot be obtained if it was a case of simple physical mixture of monometallic Ag and Au nanoparticle solution [8]. Moreover, the spectra in Fig. 3(a) do not bear a resemblance to those exhibited by the bimetallic Au/Ag core–shell nanoparticle, since two characteristic absorption peaks are observed for the bimetallic Au/Ag core–shell nanoparticle [12,13]. Moreover, as shown in Fig. 3(b), the SPR peak position of the bimetallic system was found to be gradually red-shifted in quasi-linear manner with the increase of concentration of AuIII ions; which is in agreement to previous reports of Au/Ag bimetallic alloy solutions [12,21]. The absorption spectroscopic data thus amply suggest that the simultaneous reduction of AgI and AuIII ions in aqueous mahogany extracts produce a homogeneous bimetallic alloy nanoparticle. This was further confirmed by the TEM images described below. 3.3. TEM: size and shape While the absorption spectra provide strong evidences of the formation of nanoparticles and their growth kinetics, the shape and size of the resultant naoparticles are elaborated with the help of the TEM. The TEM images in Fig. 4(a–d) confirm the formation of Ag, Au and bimetallic Au/Ag alloy nanoparticles, respectively. Fig. 4(a) and (b) are TEM images of the Ag nanoparticles formed by the aqueous leaf extract of mahogany at neutral condition and at pH 12.5, respectively. At neutral pH, particles are well separated and most of the particles are spheroidal [Fig. 4(a) (i–iv)]. But it is noteworthy that, when the pH is 12.5, although, most of the particles appear spheroidal, there are a quite few with definite anisotropic morphology. Moreover, in this case, nanoparticles form small aggregates [Fig. 4(b) (i–iv)]. On the other hand, aqueous leaf extract of mahogany at neutral condition affords a rich yield of Au nanoparticles of interesting morphologies as shown in Fig. 4(c) (i–iv). Together with spheroids, the abundance of various regular geometrical shapes e.g. triangles and hexagons are visible. Fig. 4(d) (i–iv) shows the TEM images of the bimetallic Au/Ag nanoparticles formed by the simultaneous reduction of AgI and AuIII ions in the (1:1) mixture by the aqueous extract of mahogany leaves at neutral condition. It is seen that the particles are predominantly spherical with uniform size distribution. In the close up view, well separated particles with occasional aggregations are clearly observed. TEM images of the bimetallic core/shell type structure show electron density banding with a dark Au core and a lighter
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Fig. 4. TEM images of Ag nanoparticles prepared by aq. leaf extract of mahogany at (a) neutral pH [i–iv]; (b) pH 12.5 [i–iv]; (c) TEM images of Au nanoparticles prepared by aq. leaf extract of mahogany [i–iv]; (d) TEM images of Au/Ag bimetallic alloy nanoparticles prepared by aq. leaf extract of mahogany [i–iv].
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Fig. 4. (Continued ).
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OH
OH
OH
* *
Transmittance
d
c
*
*
.
e-+H+
* *
O O
O e-+H+
Fig. 6. Scheme illustrating tentative mechanism of polyol oxidation by metal ions to ␣, -unsaturated carbonyl groups.
was observed for the same extract at pH 12.5 [Fig. 7(b)]. From the results, it is evident that the same types of biomolecules are responsible for the reduction of metal ions which are present in both the solutions.
b a
3.6. General discussion 1100
1200
1300
1400
1500
1600
1700
1800
Wavenumber(cm-1) Fig. 5. FTIR spectra of water extract of mahogany leaf (a) before reaction with metal ions and after reaction and formation of (b) Ag (c) Au and (d) Au/Ag bimetallic alloy nanoparticles. Absorption peaks in (a) are indicated by arrows, while the prominent peaks in (b), (c) and (d) are indicated by asterisks. The peak at 1390 cm−1 in (b) is due to the NO3 − ions of the precursor salt AgNO3 .
Ag shell [12]. In the TEM images of bimetallic Au/Ag nanoparticles synthesized by our method, it is clearly noticed that there is a uniform contrast for each nanoparticle; which suggests that the electron density is homogeneous within the volume of the particle. So the bimetallic nanoparticles are not core/shell type and they closely resemble to bimetallic alloy nanoparticles. These results are in strong agreement with the UV–vis spectroscopic data discussed previously and the reported data [12]. 3.4. FTIR spectra In case of leaf extract itself, the absorbance bands appear in the range of 3600–3300, 2925, 2346, 1725, 1606, 1517, 1450, 1438, 1394, 1379, 1276, 1251, 1072, 877–769 cm−1 . The IR absorption at 3600–3300, 2925, 1740–1725 cm−1 showed the presence of carbon–carbon double bond and hydroxyl and several types of carbonyl groups (e.g. cyclic ketones, lactones). These are the IR-spectroscopic evidence for the presence of polyhydroxy limonoids type of compounds in the leaf extract. Among them, the absorbance bands at 1725, 1606, 1517, 1450, 1438, 1394, 1379, 1276, 1251, 1072 cm−1 are included in the range of 1000–18000 cm−1 (Fig. 5). After biogenic reduction AgI and AuIII , these bands totally disappeared and new bands at 1742, 1652, 1613, 1515, 1454, 1379 cm−1 (in case of biogenic reduction of AgI ions) and 1739, 1650, 1516, 1451, 1379 cm−1 (in case of biogenic reduction of AuIII ions) appeared. For bimetallic nanoparticle bands appeared at 1742, 1654, 1608, 1518, 1379 cm−1 (Fig. 5). This may be due to the fact that polyols are actually responsible for the reduction of AgI and AuIII ions whereby they themselves get oxidized to ␣, -unsaturated carbonyl group or simple cyclic ketones leading to a broad peaks at 1742, 1652 cm−1 (for biogenic reduction of AgI ) and 1739, 1650 cm−1 (for biogenic reduction of AuIII ions) and at 1742, 1654 cm−1 (for bimetallic Au/Ag alloy nanoparticle formation). A tentative mechanism of this polyol oxidation has been shown in Fig. 6 [2,3]. These are in agreement with our previous findings [2,3].
We were interested in the synthesis of stable monometallic Ag, Au and bimetallic Au/Ag alloy nanoparticles by using a naturally occurring material which is locally available and environmentally benign and we have found that aqueous extract of leaves of mahogany (S. mahogani, JACQ.) served all the purposes. This single naturally occurring source is playing a strong role in the formation as well as stabilization of metal naoparticles with various chemical compositions, shapes and sizes and also with high monodispersity. In other synthetic methods, stabilization of the colloid or nanoparticle solution occurs due to the adsorption of the synthetic stabilizing agents e.g. surfactants or synthetic polymers on the surfaces of the nanoparticles which prevents the aggregation. In our work, no synthetic stabilizing agent was used. The chemical components of the aqueous extract of mahogany leaves are acting as the reducing agents and also as the stabilizing agents by adhering on the surface of the nanoparticles formed thereby preventing the aggregation and control the particle size. So our method is also important in the context of colloid and interface sciences. Furthermore, we have also proposed a probable mechanism of how these bioactive molecules are actually working. In future, proper understating and study of this may help in fine tuning of these processes leading to the formation of nanoparticles with tailor-made structural properties. In conventional chemical methods of synthesis, there is still a possibility of the adsorption of the toxic chemical entities (either reagents or by-products) on the surfaces of the nanoparticles which may cause adverse effects in medical applications [4]. The Au, Ag and bimetallic Au/Ag naoparticles synthesized by our method could have potential biomedical applications in future due to their biocompatibility as they are synthesized by a environmentally benign material having potential therapeutic applications.
0.00006
b 0.00004
Current (A)
1000
a
0.00002
0.00000
-0.00002
3.5. Redox behavior of mahogany leaf broth Cyclic voltammograms of aqueous extract of leaves of mahogany at neutral condition and at pH 12.5 are and displayed in Fig. 7. Aqueous extract at neutral condition showed a prominent reduction peak at −0.443 V [Fig. 7(a)] and a reduction peak at −0.443 V
-0.00004 -1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Potential (V) Fig. 7. Cyclic voltammograms of (a) water extract of mahogany leaf and (b) water extract of mahogany leaf at pH 12.5.
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Moreover, very simple laboratory set-up is required for our process and leaves of mahogany (S. mahogani, JACQ.) are locally available. The use of this plant based biogenic material potentially eliminates the elaborate process of cell culture and cell maintenance necessary for the biogenic synthesis of metal nanoparticles using unicellular organisms. Moreover, this synthetic protocol is applicable at room temperature and pressure, thus saving huge amount of energy. There is a special demand for the development of reliable, cost-effective and environment friendly synthetic protocol for the nanoparticles with various chemical compositions, shapes, sizes and high monodispersity for their potential technological applications and our synthetic protocol based on aqueous extract of dried leaves of mahogany (S. mahogani, JACQ.) could be a possible and promising alternative. 4. Conclusion We have demonstrated for the first time, the spectacular efficiency of a naturally available and environmentally benign plant-based product, mahogany (S. mahogani, JACQ.) leaf extract in the rapid synthesis of stable Au, Ag and bimetallic Au/Ag alloy nanoparticles possessing a wide spectrum of fascinating morphologies. The methodology which we have adopted was totally hazard free, low cost and environment friendly. The polyols, i.e. poly hydroxy limonoid type of constituents of the leaf extracts may be the surface active molecules which reduces AgI and AuIII ions to corresponding monometallic and bimetallic alloy nanoparticles and not only that, they also stabilizes these nanoparticles. In future, the present method may find potential applications in the rapid synthesis and stabilization of other monometallic and also bimetallic alloy and core–shell nanoparticles. Acknowledgements Authors are thankful to the University Grants Commission, India for the financial support (UGC M.R.P. grant no. 33-290/2007 (SR) to
N. A. B.). Thanks are also due to DST-FIST and UGC-SAP programmes of Dept. of Chemistry, Siksha Bhavana, Visva Bharati University. Authors gratefully acknowledge the help provided by Dr. A. Majee and Dr. S. Ghosh, Dept. of Chemistry, Siksha Bhavana, Visva Bharati University, for recording the IR spectra and the cyclic voltammograms, respectively. We also acknowledge Professor S. Das, Dept. of Metallurgy and Materials Engineering, IIT, Kharagpur-721302, W.B. for extending TEM facility. S.M. and I.S. thank CSIR and UGC respectively, for research fellowships. References [1] C.N.R. Rao, G.U. Kulkarni, P.J. Thomas, P.P. Edwards, Chem. Eur. J. 8 (2002) 28. [2] N.A. Begum, S. Mondal, S. Basu, R.A. Laskar, D. Mandal, Colloids Surf. B: Biointerf. 71 (2009) 113. [3] N. Roy, S. Mondal, R.A. Laskar, S. Basu, D. Mandal, N.A. Begum, Colloids Surf. B: Biointerf. 76 (2010) 317. [4] S. Shiv Shankar, A. Rai, A. Ahmad, M. Sastry, J. Colloid Interface Sci. 275 (2004) 496. [5] K.N. Thakkar, S.S. Mhatre, R.Y. Parikh, Nanomed.: Nanotechnol. Biol. Med. 6 (2010) 275. [6] A. Ahmad, S. Senapati, M.I. Khan, R. Kumar, M. Sastry, Langmuir 19 (2003) 3550. [7] P. Mukherjee, A. Ahmad, D. Mandal, S. Senapati, S.R. Sainkar, M.I. Khan, R. Ramani, R. Pasricha, P.V. Ajaykumar, M. Alam, M. Sastry, R. Kumar, Angew. Chem. Int. Ed. Engl. 40 (2001) 3585. [8] A. Ahmad, P. Mukherjee, S. Senapati, D. Mandal, M.I. Khan, R. Kumar, M. Sastry, Colloids Surf. B: Biointerf. 28 (2003) 313. [9] S. Shiv Shankar, A. Ahmad, R. Pasricha, M. Sastry, J. Mater. Chem. 13 (2003) 1822. [10] S. Shiv Shankar, A. Rai, A. Ahmad, M. Sastry, Chem. Mater. 17 (2005) 566. [11] B. Ankamwar, M. Chaudhary, M. Sastry, Syn. Reactivity in Inorg. Metal-Org. Nano-Metal Chem. 35 (2005) 19. [12] S. Shiv Shankar, A. Ahmad, A.M.I. Khan, M. Sastry, R. Kumar, Small 1 (2005) 517. [13] J.F. Sˇıanchez-Ramˇıırez, U. Pal, L. Nolasco-Hernaˇındez, J. Mendoza-Aˇı lvarez, J.A. Pescador-Rojas, J. Nanomater. (2008) 1, ID 620412. [14] M.M.G. Saad, T. Iwagawa, M. Doe, M. Nakatani, Tetrahedron 59 (2003) 8027. [15] A. Roy, S. Saraf, Biol. Pharm. Bull. 29 (2006) 191. [16] S. Link, M.A. El-Sayed, Int. Rev. Phys. Chem. 19 (2000) 409. [17] R. He, X. Qian, J. Yin, Z. Zhu, J. Mater. Chem. 12 (2002) 3783. [18] P. Malvaney, Langmuir 12 (1996) 788. [19] G.C. Papavassiliou, J. Phys. F: Metal Phys. 6 (1976) L103. [20] K. Torigoe, Y. Nakajima, K. Esumi, J. Phys. Chem. 97 (1993) 8304. [21] S. Link, Z.L. Wang, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 3529.