Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 185–192

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Green synthesis and applications of Au–Ag bimetallic nanoparticles M. Meena Kumari, John Jacob, Daizy Philip ⇑ Department of Physics, Mar Ivanios College, Thiruvananthapuram 695 015, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Alloy as well as core–shell

nanostructures are synthesized.  Study of catalytic degradation of 2-,

3-, 4-nitrophenols and methyl orange.  Alloy nanoparticles are found to form

efficient nanofluid.  The radical scavenging activity is a

promise in biomedical field.

a r t i c l e

i n f o

Article history: Received 16 May 2014 Received in revised form 9 August 2014 Accepted 23 August 2014 Available online 30 August 2014 Keywords: Au–Ag alloy nanoparticles Au–Ag core–shell nanoparticles Catalysis Thermal conductivity Nanofluid Antioxidant activity

a b s t r a c t This paper reports for the first time the synthesis of bimetallic nanoparticles at room temperature using the fruit juice of pomegranate. Simultaneous reduction of gold and silver ions in different molar ratios leads to the formation of alloy as well as core–shell nanostructures. The nanoparticles have been characterized using UV–vis spectroscopy, transmission electron microscopy, Fourier Transform Infrared Spectroscopy and X-ray diffraction. The synthesized alloy particles are used as catalysts in the reduction of 2-, 3-, 4-nitrophenols to the corresponding amines and in the degradation of methyl orange. The reduction kinetics for all the reactions follows pseudo-first order. The rate constants follow the order k4-nitrophenol < k2-nitrophenol < k3-nitrophenol. Thermal conductivity is measured as a function of volume fraction and it is observed that the incorporation of the alloy nanoparticles enhances the thermal conductivity of the base fluid (water) showing nanofluid application. The nitric oxide and hydroxyl radical scavenging activity shown by the nanoparticles promise the potential application in biomedical field. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Bimetallic nanoparticles (BMNPs) technologically excel their monometallic counterparts owing to their improved electronic, optical and catalytic properties [1]. Other than the usual morphological manipulations, the variations in the molar ratio of the individual components provide a diverse dimension in tailoring the properties of BMNPs [1,2]. Among wide range of bimetallic

⇑ Corresponding author. Tel.: +91 471 2530887; fax: +91 471 2530023. E-mail addresses: [email protected], [email protected] (D. Philip). http://dx.doi.org/10.1016/j.saa.2014.08.079 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

systems, gold (Au)–silver (Ag) nano compositions have gained significant advancement in the field of optics [3], biosensing [4], drug delivery [3] and catalysis [5]. Similar lattice constants and the facecentred crystal structure (fcc) of the gold and silver nanostructures facilitate the quite facile synthesis of Au–Ag alloy/core–shell structures [6–8]. The constrained localized Surface Plasmon Resonance (SPR) absorption for spherical Au and Ag nanoparticles (NPs) around 520 and 400 nm can be tuned continuously in the UV, visible and near IR regions by combining the two metals into a single entity with enhanced functional properties [2,9]. A number of approaches have been adopted for the synthesis of Au–Ag

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bimetallic composites which include chemical reduction by sodium borohydride [10,11], citrate [12,13], hydrazine [14,15], glucose [16], laser ablation [17,18] and biogenic synthesis [19,20]. More simple methods for the preparation of bimetallic composites is yet to be exploited. Current strategies of ‘‘green’’ concern include the use of non-hazardous chemicals, biodegradable polymers and environmentally benign solvents like plant extracts [20]. The easy availability and the protocol applicable at room temperature and pressure saving huge amount of energy are the core advantages while using plant extracts as the biogenic agents for the synthesis of metal NPs [21]. So far, only a few works on the biogenic synthesis of alloy/core shell nanostructures have been reported [20–25]. This initiated our present work where we report the room temperature synthesis of Au–Ag bimetallic nanocomposition using Punica granatum (pg) fruit juice, rich in phenolic contents and well-known for its excellent anti-oxidant properties [26–28]. In the synthesis we have not used any surfactant or synthetic polymers as stabilizing agent to procure BMNPs stable for more than 2 weeks at room temperature. BMNPs possess enhanced catalytic activity and selectivity compared to their single metallic counterparts [29,30]. Harish et al. recently reported the catalytic activity of Au–Ag alloy compositions [7]. Aromatic amines prepared by the reduction of nitrocompounds have significant industrial importance as they are widely used as the intermediates for the synthesis of dyes, pharmaceuticals and agrochemicals [31]. Also the azo dyes characterized by the presence of (N@N) group used in dyeing, textile, paper, leather, cosmetics and food processing are found to be hazardous for human health and environment [32]. Nanocatalysts supported aromatic amine formation from 4-nitrophenol [25,33] and the deterioration of certain organic pollutant dye [34] has been reported from our research group itself. Our present contribution include investigation on the catalytic activity of Au–Ag alloy NPs in the deterioration of anionic azo dye methyl orange (MO) and comparison on catalytic efficiency of alloy NPs towards the reduction of the aromatic nitro compounds 2-, 3- and 4-nitro phenols. Nanofluid consisting of solid NPs with sizes 1–100 nm suspended in base fluids, have anomalous high thermal conductivity than conventional fluids like water, oil and ethylene glycol. They eradicate the practical limit of sedimentation, clogging flow channels, cohesion, corrosion etc of micro sized suspensions and are proposed as next generation heat transfer fluids as they offer exciting new possibilities in the field of microelectronics, energy supply and transportation [35–37]. There are reports on the recent trends of Pt [38], magnetic Fe3O4 as nanofluids with water as the base fluid [35]. Au–Ag alloy nanoparticles are used in the present work to investigate the thermal enhancement of conventional fluid, water. Researchers suggest that antioxidants prevent oxidative damage caused by oxidative stress to cellular components such as DNA, proteins and lipids and lessen the risk of age related chronic diseases. Of the in vitro and in vivo evaluation of antioxidant activity, in vitro assays are simple, convenient, reproducible and cost effective [39]. Several antioxidant studies using metal NPs have been previously reported [39–44] earlier. We attempt in the current work to validate the antioxidant activity of Au–Ag alloy NPs using hydroxyl (OH⁄) and nitric oxide (NO⁄) radicals. The reactive oxygen species (ROS) stimulate auto-oxidation and thermal oxidations of lipids which are linked with aging and membrane damage in living organisms [41]. OH⁄ has a short half life and is the most reactive and damaging ROS [45]. The toxic effects of NO⁄ will increase when reacts with superoxide radicals, leading to vascular system damage resulting in conditions like inflammation, juvenile diabetes and multiple sclerosis [44]. To the best of our knowledge this is the first report on BMNPs synthesized using pg fruit juice being applied in the, catalytic, thermal and antioxidant activity.

Materials and methods Synthesis of Au–Ag bimetallic NPs Chloroauric acid (HAuCl4), silver nitrate (AgNO3) and Sodium Borohydride (SB) (NaBH4) are procured from Sigma Aldrich. Aqueous solutions of 2.9  104 M chloroauric acid and 1.17  103 M silver nitrate are used as precursors. Ripe pomegranate seeds are crushed to obtain 1 mL of fresh juice. The juice is then made up to 50 mL using de-ionized water and filtered. The filtrate is used for the simultaneous reduction of the bulk Au3+ and Ag+. 10 mL of HAuCl4 solution is made alkaline using NaOH solution of known (2 pellets of 0.2 g NaOH in 100 mL de-ionized water) concentration so as to maintain the final pH at 7.10 mL of pg fruit juice and 12.5 mL of AgNO3 are simultaneously added to the Au3+ solution with continuous stirring for 5 min to obtain colloid (gs5) in the molar ratio 1:5. The formation of BMNPs can be visualized by the onset of light reddish color within a few minutes and the reaction completed within 24 h. The Au–Ag bimetallic colloids gs4, gs3, gs2, gs1 of molar ratios 1:4, 1:3, 1:2 and 1:1 are prepared by repeating the experiment using 10, 7.5, 5 and 2.5 mL of AgNO3 respectively. We could observe the color of the colloids ranging from yellowish brown to violet indicating the formation of BMNPs nanoparticles at different molar compositions. Catalytic activity To investigate the catalytic activity of synthesized gold–silver alloy nanoparticles they are used in the reduction reaction of aromatic nitro compounds 2-, 3-, 4-nitrophenols and also in the decolourisation of an anionic mono azo dye-Methyl orange. 1 mL of 5  103 M respective nitrocompounds is mixed with 1 mL of 0.25 M SB in order to obtain aromatic amines and for MO degradation 1 mL of 1 mM MO is mixed with 1 mL of 100 mM sodium borohydride. The former solution is made up to 25 mL using deionized water, stirred for 10 min while the latter is made up to 10 mL, stirring continued for 2 min. Thereafter ample quantity of colloid is added to the mixture and stirring continued for another 3–5 min. The solution becomes colorless as the reaction proceeds. The reaction procedure is monitored using UV–Visible spectrophotometer. The percentage of degradation in each of the reaction is calculated using the following equation [46]

Percentage dye removal ¼ ððC 0  C t Þ=C 0 Þ  100

ð1Þ

Thermal conductivity For thermal conductivity measurement of Au–Ag alloy NPs the transient short-hot-wire (SHW) method is applied. The experimental setup involves SHW mounted on a Teflon cap. Alumina coated platinum wire is used as hot-wire in the measurement system. The temperature is monitored by the 2 thermo couples located at its welding spots. DI water is used as a standard liquid of known thermo-physical properties to calibrate the measurement of hotwire. To avoid temperature variations the hot wire cell is placed in a thermostatic bath kept at 303 K. The colloid at different volume ratios 0.25, 0.5, 0.75 and 1 are used for evaluating its enhancement in thermal conductivity of base fluid. In vitro antioxidant activity Hydroxyl radical scavenging activity This assay is based on the qualification of the degradation product of 2 deoxy ribose by condensation with TBA. Hydroxyl radical

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was generated by the Fe3+–ascorbate–EDTA–H2O2 system (The Fenton reaction). The reaction mixture contained in the final volume of 1 mL-2 deoxy 2 ribose (2.8 mM) KH2PO4–KOH buffer (20 mM pH 7.4), FeCl3 (100 lm), EDTA (100 lm), H2O2 (1.0 mM), ascorbic acid (100 lm) and various concentrations (12.5–200 lL) of the test sample. After incubation for 1hour at 37 °C, 0.5 mL of the reaction mixture was added to 1 mL of 2.8% TCA, then 1 mL aqueous TBA was added and the mixture was incubated at 90 °C for 15 min to develop the color. After cooling the absorbance was measured at 532 nm against an appropriate blank solution. Nitric oxide scavenging activity Nitric oxide scavenging activity has been measured spectrophotometrically. Sodium nitro prusside (5 mmol L1)in phosphate buffered saline pH 7.4, is mixed with methanol diluted samples and incubated at 25 °C for 30 min. A control without the test compound, but an equivalent amount of methanol was taken. After 30 min, 1.5 mL of the incubated solution was removed and diluted with 1.5 mL of Griess reagent (1% sulphanilamide, 2% phosphoric acid and 0.1% N-1-naphthyl ethylene diamine dihydrochloride). Absorbance of the chromophore formed during diazotization of the nitrate with sulphanilamide is measured at 546 nm and the percentage scavenging activity was measured with reference to the standard. The percentage inhibition of the radicals are measured using the equation [44]

% Inhibition ¼

control  test  100 control

ð2Þ

Instrumentation The synthesized BMNPs are optically analyzed using PerkinElmer Lambda-35 UV–Visible spectrophotometer. The morphology of the synthesized particles is better determined from images recorded using Tecnai G2 30 transmission electron microscope. FTIR spectra recorded using IR Prestige-21 Schimadzu spectrometer help to recognize the bioactive compounds in pg that are associated with the reduction and stabilization of metal ions. The crystallographic structure of the as prepared particles is probed using the XRD patterns obtained from XPERT-PRO diffractometer. The catalytic potential of synthesized metal nanoparticles in the chemical degradation of dyes is investigated using PerkinElmer Lambda-35 UV–Visible spectrophotometer. The thermal conductivity measurements are done using transient hot-wire technique and radical scavenging activity of bimetallic system is elucidated spectrophotometrically. Results and discussion Optical absorption studies The high sensitivity SPR of noble metal/alloy NPs to the size/ shape and composition provides a method to investigate the optical properties of Au–Ag bimetallic systems using UV–vis spectrometry [9,19]. Fig. 1 shows the UV–vis absorption spectra of different Au–Ag compositions. A physical mixture of the individual colloids shows two separate surface plasmon peaks corresponding to their monometallic counterparts [29]. In contrast the single SPR band for all Au–Ag compositions in Fig. 1 indicates the formation of NPs with an alloy or core–shell structure [2]. A SPR band tuning from pure Ag to pure Au as a function of Ag–Au ratio can be obtained in an alloying process whereas gold–silver core–shell structures having an electron density difference at the boundary surface between a core and a shell metal leads to the broadening and red shift of plasmon frequency in

Fig. 1. UV–vis spectra of bimetallic nanoparticles at different Ag–Au molar compositions.

the range 563–1240 nm [2,9]. In the present study, the appearance of only one intermediate plasmon band (Fig. 1) located between nano gold (540 nm) and nano silver (410 nm) is a clear indication of the formation of alloy NPs for all other Au–Ag molar ratios except 1:1. The shift of absorbance maximum in the sample at molar ratio 1:1 to higher wavelength compared to Au colloid may be attributed to the formation of core–shell structure as substantiated by previous reports [2,9,47]. The almost complete absence of SPR band of Ag NPs suggest that the formed bimetallic structure may have a thick gold core surrounded by a thin non-uniform silver shell [2]. If the shell is not thick enough the surface Plasmon of the core may effectively interact with the electromagnetic field and the Mie absorption from the core NPs can only be observed [48]. The fact that the plasmon resonance can be tuned by varying the relative size of the core and the thickness of the metallic over layer [47] put forward a future perspective of this work. It is interesting to note that plasmon maximum for Au–Ag bimetallic system red shifted almost linearly within the SPR range of pure Au and pure Ag with decrease in Ag content only for Au–Ag molar ratios 1:5, 1:4, 1:3 and 1:2. This further supports the alloy NPs are only formed in colloids gs5–gs2 [1,21,16,22,49,50]. The quasi linear dependence of absorbance maximum with Au–Ag composition and the concomitant change in color of the colloids can be seen in Fig. 2. Furthermore, in agreement with the calculated spectra of Au–Ag alloy NPs using full Mie equations [51] the absorption intensity of Au–Ag bimetallic NPs decrease with increase in Au/ Ag molar ratio and they all will be lower than those observed for pure Au and Ag. Also, as reported by Bruzzone et al. [52], the absorption intensity of core–shell structures having molar composition ratio greater than 0.5 shows an increase of absorbance. The UV–vis absorption spectra indications in our work go well with the alloy formation in colloids gs5–gs2 and core–shell in gs1. TEM analysis The UV–vis spectroscopic investigations on the formation and morphology of NPs can be more elaborately evidenced with the help of TEM analysis. Figs. 3 and 4 shows TEM images of gs5 and gs1 formed by the simultaneous reduction of Au and Ag ions respectively in the molar ratio 1:5 and 1:1 by aqueous pg fruit juice. The morphology of bimetallic NPs can be clearly seen at higher magnifications. The particles are predominantly spherical and occasional aggregations leads to a very small percentage formation of pentagonal and rod shaped structures as reported

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(2 0 0), (2 2 0) and (3 1 1) rings in the SAED patterns clearly indicate fcc crystal structure of the formed NPs [A1, A19]. The broken rings in the SAED pattern (Fig. 4(d)) can be ascribed as the increasing number of crystals having similar orientation [2]. However, a more detailed characterization using EDS and similar other techniques are required to clearly elucidate the NP morphology and composition of synthesized bimetallic systems. XRD analysis

Fig. 2. Plot of UV–vis absorbance maximum against Au–Ag composition. Inset shows color change of colloids for different Ag–Au molar ratios.

earlier [9]. The particles in Fig. 3 show uniform contrast for each NP suggesting homogenous electron density within the volume of particle with an average size 12 nm indicating alloy formation whereas in Fig. 4 the particles with different image contrasts suggest the formation of core shell structure in agreement with earlier reports [9,21]. The HR TEM image in Fig. 4(c) shows an electron density banding with dark core and a lighter shell with a thickness of about 8 nm [2,9]. These results commensurate well with the data obtained from UV–vis absorption studies and earlier reports [2,9,21,22] from which we can reasonably estimate a gold core encapsulated with a very thin silver shell. The crystalline nature of the bimetallic NPs is confirmed by the corresponding SAED patterns in the Figs. 3(c) and 4(d). The (1 1 1),

The crystalline nature of the prepared alloy NPs are further confirmed using XRD analysis. Fig. 5 shows the XRD spectrum corresponding to Au–Ag alloy. The strongest peak is observed at 38.26° which corresponds to the predominant growth in the direction of (1 1 1) plane. The (2 0 0) plane can be attributed to the peak at 44.40°. Two reflections observed at 64.56° and 77.66° are assigned to the (2 2 0) and (3 1 1) planes respectively. All the four characterization peaks for Au–Ag bimetallic system reveal that they are crystallized in fcc structure [1,14,20]. Au and Ag have very similar lattice constants and hence no lattice mismatch is observed for Au–Ag alloys and all reflections in the XRD pattern resemble to that of monometallic counterparts [29]. FTIR spectrum FTIR measurements are carried out to identify the possible biomolecules responsible for capping and efficient stabilization of the bimetallic NPs synthesized using pg. Fig. 6 shows FTIR spectra of pg fruit juice and BMNPs. The IR absorption bands at 3412 cm1, 2360 cm1, 1630 cm1and 1080 cm1 in the spectrum correspond to the OAH stretching vibration of phenolic hydroxyls [53], stretching vibrations of NH2+ and NH3+ in protein/peptide bonds

Fig. 3. (a) and (b) TEM images at different magnifications, (c) SAED pattern of colloid gs5.

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Fig. 4. (a) and (b) TEM images at different magnifications, (c) HRTEM, and (d) SAED pattern of colloid gs1.

Fig. 5. XRD pattern of BMNPs nanoparticles synthesized using pg fruit extract.

Fig. 6. FTIR spectra of (a) pomegranate fruit extract and (b) BMNPs.

[54], carbonyl stretching in proteins [53] and CAOH vibrations, respectively, of proteins [33] present in pg extract. The relative decrease in the intensity of phenolic hydroxyl stretching band in the spectrum of pg fruit extract functionalized BMNPs indicate the partial role of phenolic hydroxyls in the reduction mechanism by donating electrons and forming quinones. The appearance of band at 1380 cm1 in the functionalized spectrum of alloy correspond to the ACAOA stretching modes derived from the water soluble compounds like flavonoids and terpenoids present in the fruit juice [55]. The complete disappearance of the NH2+ and NH3+ stretching vibrations in the FTIR spectrum of BMNPs can be attributed to the breaking of amino acid residues of proteins during the reaction. Similar mechanism involving the role of phenolic

hydroxyls and proteins in the reduction and stabilization of individual metal NPs have been reported previously [53,54,56,57]. Catalytic activity In order to compare the catalytic activity of Au–Ag alloy NPs, reduction of three nitro compounds 2-, 3-, 4-nitrophenols using SB in presence of gs5 is considered [31]. On addition of SB to nitro compounds, all the three show an absorption maximum around 400 nm, characteristic of their phenolate ions [7,31,58]. The band at 400 nm remains undiminished even after the mixing of SB for several hours in the absence of NPs. This feature observed for 4 nitrophenol is reported in earlier works from our laboratory

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Fig. 7. UV–Visible spectra showing degradation of (a) 4-nitrophenol, (b) 3-nitrophenol, (c) 2-nitrophenol in presence of BMNPs, and (d) overlayed plot of ln(A/A0) v/s time.

[33,38] and is experimentally verified (figure not shown). Also as observed experimentally the reaction remains undisturbed in the sole presence of pg. On the addition of adequate aqueous suspension of gs5 NPs, the yellow color of the reaction mixture gradually diminishes with time and become colorless. The kinetics of the catalytic conversion of 2-, 3-, 4-nitrophenols using SB is periodically followed by UV–vis spectroscopy and is shown in Fig. 7. The decrease in intensity of absorption around 400 nm and emergence of new peaks at 287, 290, 300 nm each corresponding to the formation of 2-, 3-, 4-aminophenol is monitored as a function of time. The reduction of 4-nitrophenol completes in 6 min and 2-nitrophenol in 10 min while it takes 12 min for 3nitrophenol. The SB concentration is in excess during the course of the reaction and hence considered constant compared to concentration of nitro compounds. The reaction kinetics is hence assumed to follow pseudo first order and can be described by

lnðC t =C 0 Þ ¼ kt

ð3Þ

where k is the pseudo first order rate constant, t-reaction time, Ct and C0-concentration of the nitro compound at time t and 0 respectively [58]. The plot of ln(Ct/C0) against time shows a linear relation as shown in Fig. 7(d) and the slope of the linear graph directly gives the rate constant for each of the reaction. The observed rate constant is comparable to that reported for alloy composition [7]. It is noteworthy that the rate of reduction of 4-nitrophenol is significantly higher than the other two in the following order.

4NP > 2NP > 3NP

Further we have investigated the catalytic efficiency of Au–Ag alloy in the reduction of dyes by using the model degradation of MO in presence of SB. In agreement with our previous work [34] the reduction of MO by SB does not occur appreciably in the absence of metal NPs. The catalytic degradation of MO in excess SB at a wavelength 464 nm in water as medium [59] is done by successive monitoring of its UV–vis spectrum (Fig. 8). The kinetic data well fits in pseudo first order rate equation given by (3) and the rate constant 0.733 min1 is obtained and the linear plot is shown in Fig. 8(b). The percentage degradation of the reactants at the end of the reactions along with the corresponding rate constants are tabulated in Table 1. The better catalytic efficiency can be attributed to the large surface to volume ratio of NPs [60] and the improved activity of alloy NPs relative to their single counterparts [2]. The improved catalytic activity of BMNPs than that of their mono-metallic counterparts can be accounted by the electronic charge transfer effects between the nearby elements. For Au–Ag BMNPs, the ionization potential of Au and Ag is 9.22 and 7.58 eV respectively. The electronic charges could transfer from Ag atoms to Au atoms in a particle, leading to an increase in electron density on the Au, thus acting as catalytically active sites for the redox reaction to proceed in a kinetically favorable manner [50]. Thermal conductivity Thermal conductivity ratio (TCR) given by the ratio of effective thermal conductivity of the nanofluid to the thermal conductivity of the base fluid [38] is measured to evaluate the thermal

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Fig. 9. Thermal conductivity ratio v/s volume fraction of colloid gs5.

pg rich in polyphenols [26–28]. Fig. 10(a) and (b) shows the dose-response bar chart for the NO⁄ and OH⁄ scavenging activity of alloy NPs respectively. The NO⁄ and OH⁄ scavenging activity of gs5 sample correspondingly increased with increase in the concentration of the sample. The percentage inhibition of NO radical is greatly enhanced compared to the activity of pg. Also, for concentrations 25 lL, 50 lL and 100 lL the percentage inhibition is found to be comparable with that of gallic acid (GA) standard chosen. Similar effects on NO⁄ with cerium NPs [63] and silver NPs [44] are previously reported. The OH radical scavenging rate of Au–Ag alloy NPs increased from 36% to 76% with increase in concentration. The inhibition rate of OH radical becomes comparable to that of gallic acid standard

Fig. 8. UV–Visible spectra showing degradation of (a) methyl orange and (b) plot of ln(A/A0) v/s time.

Table 1 Percentage degradation of reactants. Reactant

Percentage removal of reactants at the end of the reaction

Rate constant of the reaction (min1)

4-Nitrophenol 3-Nitrophenol 2-Nitrophenol Methyl orange

95 64 75 91

0.798 0.066 0.148 0.733

performance of synthesized alloy nanoparticles gs5. TCR of gs5 recorded for different volume concentrations is shown in Fig. 9. The TCR proportionally increases with an increase in the volume fraction. Similar results are observed by Syam et al. while considering Fe2O3 nanofluids [35] and Sheny et al. [38] on considering Pt nanofluids. For an increase in volume concentration of nanofluid from 25% to 100%, 21% to 72% thermal conductivity enhancement of base fluid water is obtained at 303 K. The enhancement of thermal conductivity can be due to two mechanisms-random motions of the suspended NPs enhancing energy transport within the liquid or suspended NPs with higher TC than the base fluid alter the fluid composition and cause the heterogeneous system to have intermediate TC [61]. Antioxidant activity Plants containing polyphenols are reported to possess strong antioxidant activities [62]. Therefore the NO⁄ and OH⁄ quenching activity of Au–Ag alloy NPs are assessed and compared to that of

Fig. 10. Dose-response bar chart for the NO⁄ and OH⁄ scavenging activity of alloy NPs.

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and higher than that of pg only at 200 lL. Kanipandian et al. [41] havedone an analogous study on OH radical. In the present study bimetallic systems are found to posses antioxidant activity as determined by NO and OH radical scavenging methods. The antioxidant activity exhibited by NPs may be due to the phytochemicals present in pg, already known for their antioxidant potential [26], which form the part of synthesized NPs as capping agents [44]. But the enhanced activity of BMNPs than that of pg extract points to the obvious interaction of NPs with the free radicals which is dependent on the size, specific area and concentration [40,58]. A detailed mechanism for the scavenging activity demands a further investigation on the influence of each of these factors that remains a future perspective of the current work. Our findings suggest that BMNPs is a good platform to scavenge the ROS and thus they serve as natural source of antioxidants. Conclusions The current work deals with the spectroscopic and microscopic analysis on the synthesis of varied gold/silver compositions that lead to the formation of alloy and core–shell nanostructures. The rapid reduction of nitro compounds to aromatic amines and the efficient decolorisation of methyl orange in presence of newly synthesized BMNPs substantiate its role as an effective nanocatalyst. The excellent thermal conductivity enhancement presents these biogenic NPs as a promising candidate in many engineering applications. But the actual mechanism responsible for the conductivity enhancement requires more detailed exploration. The remarkable antioxidant activity reported for the particles proves them to be an active component in biomedical applications. It demands further research to establish the inclusion of these particles in antioxidant formulations. The future scope of the present work can also focus on the tuning of shell thickness in core–shell structures by varying the molar ratios in varied experimental conditions. Acknowledgment The authors are pleased to acknowledge Department of Chemistry, Govt. College for women, Thiruvananthapuram for FTIR spectra and NIIST, Thiruvananthapuram, for TEM measurements. References [1] A. Shah, L.U. Rahman, R. Qureshi, Z.U. Rehman, Rev. Adv. Mater. Sci. 30 (2012) 133–149. [2] D.V. Radziuk, W. Zhang, D. Shchukin, H. Mohwald, Small 6 (2010) 545–553. [3] D.X. Li, C.F. Li, A.H. Wang, Q.A. He, J.B. Li, J. Mater. Chem. 20 (2010) 7782–7787. [4] Y.W. Cao, R. Jin, C.A. Mirkin, J. Am. Chem. Soc. 123 (2001) 7961–7962. [5] C. Wang, H. Yin, R. Chan, S. Peng, S. Dai, S. Sun, Chem. Mater. 21 (2009) 433–435. [6] L. Lu, G. Burkey, I. Halaciuga, D.V. Goia, J. Colloid Interf. Sci. 392 (2013) 90–95. [7] S. Harish, R. Sabrinathan, J. Joseph, K.L.N. Phani, Mater. Chem. Phys. 127 (2011) 203–207. [8] A. Pal, S. Shah, S. Devi, Colloid. Surface A. 302 (2007) 483–487. [9] Y. Ji, S. Yang, S. Guo, X. Song, B. Ding, Z. Yang, Colloid. Surface A 372 (2010) 204–209. [10] M.P. Mallin, C.J. Murphy, Nano Lett. 2 (2002) 1235–1237. [11] N. Kometani, M. Tsubonishi, T. Fujita, K. Asami, Y. Yonezawa, Langmuir 17 (2001) 578–580. [12] S. Link, Z.L. Wang, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 3529–3533.

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Green synthesis and applications of Au-Ag bimetallic nanoparticles.

This paper reports for the first time the synthesis of bimetallic nanoparticles at room temperature using the fruit juice of pomegranate. Simultaneous...
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