Seedless synthesis of nanocomposites, optical properties, and effects of additives on their surface resonance plasmon bands

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 182 (2017) 87–94

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Seedless synthesis of nanocomposites, optical properties, and effects of additives on their surface resonance plasmon bands Zoya Zaheer ⁎, Elham Shafik Aazam Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21413, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 19 November 2016 Received in revised form 17 March 2017 Accepted 18 March 2017 Available online 3 April 2017 Keywords: Au@Ag Bimetallic Seedless Cysteine

a b s t r a c t The work describes an easy seedless competitive chemical reduction method for the synthesis of Ag@Au/Ag bimetallic nanoparticles by mixing AgNO3, HAuCl4 and cysteine. Transmission electron microscope (TEM) images show that the large number of irregular, cross-linking, and aggregated Ag@Au/Ag are formed in a reaction mixture (HAuCl4 + AgNO3 + cysteine), whereas flower-like nanocomposites are obtained in presence of cetyltrimethylammonium bromide (CTAB), which acted as a shape-directing agent. Optical images reveal that the initially reaction proceeds through formation of purple color, which changes into dark brown color with the reaction time, indicating the formation of Ag@Au/Ag nanocomposites. The Ag+ has strong tendency to form complex with cysteine. Firstly, the reduction of Ag+ ions to Ag0 occurred by the\\HS\\group of the cyste0 ine-Ag complex. Secondly, AuCl− 4 ions adsorbed on the positive surface of Ag , which undergoes reduction by potential deposition, and leads to the formation of Ag@Au/Ag bimetallic nanoparticles. Inorganic electrolytes (NaCl, NaBr, NaNO3 and Na2SO4) have significant impact on the stability and aggregation of Ag@Au/Ag nanocomposites. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Due to their wide potential applications in different fields such as electronic, magnetic optical, and surface-enhanced Raman scattering, the synthesis and characterization of mono-, and bi-metallic nanoparticles have been the subject of various investigations with and without stabilizers from more than two decades [1–10]. It has been established that bi-metallic nanoparticles of transition metals (silver, gold, zinc, copper, iron, palladium, etc.) either as alloys or as core-shell structures, often exhibit better physical and chemical properties in comparison with their mono-metallic nanoparticles [11–15]. Out of these transition metals, nanomaterials of silver and gold have intense surface resonance Plasmon absorption band at ca. 400 nm and 550 nm, respectively, in the visible region. For this reason, many chemical reduction methods have been developed for the synthesis of Ag@Au nanocomposites [16]. For example, mono-dispersed core–shell Au-Ag nanoparticles were prepared by Zhang et al. by the seed growth method in presence of shape-directing cationic CTAB surfactant [17]. Pal et al. used β-cyclodextrin as a reducing and capping agent for the synthesis of normal and inverted Au@Ag core-shell bimetallic nanoparticles [18]. Sastry and his coworkers have been used bovine serum albumin as foaming and stabilizing agent to the synthesis of gold, silver and their alloy nanoparticles [19]. Gold-coated silver and silver-coated gold composite nanoparticles have been synthesized by Yang et al. by seed growth method [20]. A ⁎ Corresponding author. E-mail address: [email protected] (Z. Zaheer).

http://dx.doi.org/10.1016/j.saa.2017.03.047 1386-1425/© 2017 Elsevier B.V. All rights reserved.

seed-growth method has been used by Bakshi and his coworkers to synthesize Au and Au-Ag bimetallic pearl necklace type nanoparticles as bioconjugate materials by using a series of phospho-glycerol and phospho-choline lipids as capping agents [21]. Generally, colloidal solution of one metal (gold or silver) was used as a seed to the reduction of second metal (Ag+ or Au3+ ions) for the preparation of core-shell bimetallic nanoparticles of Ag@Au. Liu developed a seedless method to the synthesis of spiky starshaped gold@silver bimetallic nanoparticles by using an aqueous solution of HAuCl4, AgNO3 and ascorbic acid [22]. Seedless technique is a simpler process in comparison to that of seeded growth method because seedless methods can proceeds in a one vessel to the synthesis of multi-branched nanoparticles of gold [22–25]. Khan et al. reported the synthesis of Ag@Cu bimetallic nanoparticles by seedless method. They used cysteine as a reducing agent in absence and presence of CTAB [26]. Cysteine is a proteinogenic sulfur containing amino acid has three coordination sites (HS\\, NH2\\ and \\COOH). It is well known that polar side chain amino acids provide extra coordination sites for metal complexation and acted as a tridentate ligand under different experimental conditions [27]. Gedanken reported the effect of polar side chain amino acids on the surface chemistry of Au-nanoparticles [28]. Khan et al. [29] and Chang et al. [30] suggested that the morphology of Ag-nanoparticles and Ag@Au composites strongly depends on the cysteine concentrations and pH of the working reaction mixture, respectively [31]. Tang et al. reported a method (AgNPs + citrate + HAuCl4 + NaBH4) to the synthesis of core/shell Ag@Au nanoparticles functionalized with cysteine [32]. Zhang and his coworkers also used

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cysteine as a reducing agent to the size controlled green synthesis of Agnanoparticles [33]. Out of\\NH2,\\COOH, and\\SH functional groups, the sulfur containing ligands and/or reducing agents are most susceptible to gain electrons from the oxidizing agents [34] and also possess high affinities with silver and gold ions [28–33]. The chemical literature contains abundant reports aimed towards understanding the role of amino acids (reducing and/or capping agent) in the synthesis Ag@Au nanocomposites by using seed growth method [35,36], but the synthesis of Agcore-Aushell bimetallic nanoparticles by seedless chemical reduction method involving cysteine as a reducing and/or stabilizing agent has been neglected. In this paper, the results corresponding to the title reaction in absence and presence of shape-directing CTAB are presented for the first time with a view to gaining an insight into the seedless synthesis of bimetallic nanocomposites in reaction mixture containing Ag+ ions, HAuCl4, and cysteine (sulfur containing amino acid has three potential coordination sites, acted as a mono-, bi-, and tri-dentate ligand in different pH). In addition, the effects of electrolytes on the stability of Ag@Au/Ag NPs were also discussed. 2. Experimental Section 2.1. Chemicals Cysteine (C3H7NO2S; molecular weight 121.16; oxidant; 99.9%; BDH), silver nitrate (AgNO3; molecular weight 169.87; reductant; 99.9%; BDH), Chloroauric acid (HAuCl4, molecular weight 393.79), and cetyltrimethylammonium bromide (C19H42BrN; molecular weight 364.45; stabilizer; 99.9%; Fluka) were used as received without further purification. All the glass wares were washed with aqua regia solution (HCl/HNO3, 3:1), then rinsed thoroughly with deionized and CO2 free water before use. Inorganic electrolytes (NaCl, NaBr, NaNO3, and Na2SO4) were obtained from BDH and used without further purification.

experiments were also carried out for the preparation of mono-metallic AgNPs and AuNPs under the similar concentrations of Ag+ ions, HAuCl4, cysteine and CTAB. We did not observe the appearance of any color for HAuCl4 and cysteine with and without CTAB for a short reaction time, i.e., 2 h under our experimental conditions. On the other hand, perfect transparent dark yellow color appeared for Ag+ ions, cysteine and CTAB. The shape and size of the nanoparticles depend on the reduction potentials of reactants, reaction time, and stabilizer (vide infra). 3. Results and Discussion 3.1. Morphology of Ag@Au/Ag Nanocomposites The morphology (size, shape, and the size distribution) of Ag@Au/Ag were examined using TEM, EDX and XRD. Interestingly, two type of morphology (significant amount of cross-linking, interconnected small and some unsymmetrical with a quite broad size distribution of Ag@ Au/Ag NPs) were observed in the TEM images (Fig. 1A and B). Surprisingly, a different distinguishable evolution pattern is observed (Fig. 2A to C; few Ag@Au/Ag NPs) in presence of CTAB. As can be seen in Fig. 1 (typical example), the mostly irregular anisotropic nanostructures and presence of aggregated Ag@Au/Ag clearly suggested that the cysteine acted as a reducing and capping agent, which provides the evidence for the interparticle interaction (cross-linking) and/or aggregation via S atom of cysteine [28,31]. Fig. 2 shows the adsorption of small nanoparticles on to the surface of the large nanoparticles in a unsymmetrical manner, which lead to the formation of star and/or flower-like Ag@ Au/Ag bimetallic nanoparticles in presence of CTAB. The only difference

2.2. Instruments UV–visible Recording Spectrophotometer, UV-260 Shimadzu, with 1 cm quartz cuvettes was used to monitor the progress of the reaction under different experimental conditions. Transmission electron microscope (JEOL, JEM-1011; Japan) equipped with energy dispersion X-ray spectroscopy (EDX), scanning electron microscope (SEM) (QUANTA FEG 450, FEI Company, Eindhoven, and Ni-filtered Cu Kα radiation (λ = 1.54056 Å) of a (Rigaku X-ray diffractometer, XRD) operating at 40 kV and 150 mA, Fison (VG) ESCA 210 spectrometer equipped with an MgKα X-ray source were used for the determination of morphology of as prepared Ag@Au bimetallic. For TEM and EDX analysis samples were prepared by placing a drop of the as-synthesized colloids onto a carbon-coated Cu grid followed by slow evaporation of solvent at room temperature. Fisher Scientific digital pH meter 910 fitted with a combination electrode was used for the PH measurements. FT-IR experiments were carried out on a IRPrestige-21, IRAffinity-1, FTIR-8400S, Shimadzu spectrophotometer. For sample preparation, a few drops of sol were placed on a KBr pellet and allowed to dry before IR spectra were recorded. 2.3. Synthesis of Ag@Au/Ag NPs In a typical experiment, the solution [cysteine] = 10.0 × 10− 4 mol dm− 3 was mixed in a reaction mixture containing [HAuCl4] = 10.0 × 10−4 mol dm− 3 + [Ag+] = 20.0 × 10−4 mol dm− 3 + [CTAB] 10.0 × 10− 4 mol dm− 3 (if necessary) and required amount of water for dilution. As the reaction proceeds, the resulting colorless reaction mixture, initially became purplebrown in color, and finally turns deep brown, indicating the formation of Ag@Au/Ag flower-like nanomaterials. In order to compare the morphology of as prepared bimetallic nanomaterials, a series of

Fig. 1. TEM images of Ag@Au/Ag nanocomposites. Reaction conditions: [cysteine] = 10.0 × 10−4 mol dm−3, [HAuCl4] = 10.0 × 10−4 mol dm−3, [Ag+] = 20.0 × 10−4 mol dm−3.

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Fig. 2. TEM images of CTAB capped Ag@Au/Ag nanocomposites. Reaction conditions: [cysteine] = 10.0 × 10−4 mol dm−3, [HAuCl4] = 10.0 × 10−4 mol dm−3, [CTAB] = 10.0 × 10−4 mol dm−3, [Ag+] = 20.0 × 10−4 mol dm−3.

between the aggregation conditions of Ag@Au/Ag, whose TEM images are shown on Figs. 1 and 2, is presence of CTAB in the reaction mixture at the time of nucleation and growth processes. The population of Ag@ Au/Ag particles is evidently lesser in presence of CTAB, which might be due to the strong complexation between positive head group (CH3(CH2)15N+) of CTAB and negative AuCl− 4 anion. As a result, the reduction potential of HAuCl4 decreases, which in turn decreases reaction sites as well as nucleation rates (transfer of proton from cysteine-Ag complex to gold-CTA complex). Inspection of Fig. 2(A to C) clearly shows that the corner of the star-shaped nanoparticles have a different systematic layers, black and light white, which might be due to the simultaneous reduction and/or deposition of Ag0 and Au0 on the surface of particles, and leads to the formation of Ag@Au/Ag bimetallic nanocomposites. The EDX profile of Ag@Au/Ag indicates that the as prepared composites have only Ag, Au, and S (Fig. 3(A)), indicating that the cysteine play a dual role (reducing and stabilizing agent) during the nucleation and growth processes. On the other hand, Cl peak is observed in the EDX spectra of CTAB capped Ag@Au/Ag nanocomposites (Fig. 3(B)), which might be due to the incomplete washing of the resulting Ag@Au/Ag. Both EDX spectra demonstrate the presence of Au and Ag elements

with an approximate atomic ratio of 5:1. Comparison between the two EDX spectra indicated that the cysteine has been washed out from the surface of nanomaterials in presence of CTAB [37]. The XRD results of the as synthesized Ag@Au/Ag NPs obtained in a reaction mixture containing [Ag+] (= 20.0 × 10− 4 mol dm− 3), [HAuCl4] = 10.0 × 10− 4 mol dm− 3, [cysteine] = 10.0 × 10−4 mol dm−3, and [CTAB] = 10.0 × 10−4 mol dm−3 are depicted graphically in Figs. S1 and S2 (Supplementary materials). Ag@Au/Ag NPs obtained after adding cysteine precursor, has eight characteristic peaks at 2θ = 18.2°, 28.2°, 32.5°, 38.2°, 44.3°, 55.3°, 65.5°, and 77.7°. Out of these, the four main characteristic peaks at 38.2°, 44.3°, 65.5°, and 77.7°, which respectively correspond to crystal facets of (111), (200), (220), and (311) to the pure gold phase. The other four peaks at 2θ values of 18.2°, 28.2°, 32.5°, and 55.3°, which correspond to the lattice planes of (111), (200), (311), and (511) of silver phase (JCPDS card no. 04-0783) [38]. Thus we may state confidently that the Ag@Au/Ag is composed of both Au and Ag phases. Inspection of EDX and XRD profiles clearly suggests that the no other peaks or phases are detected, indicating the high purity of the Au@Ag/Au products. Thus, we may state confidently that the Ag@Au/Ag NPs have (111), and (200) facets of silver and gold [21,39].

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Fig. 3. EDX of Ag@Au/Ag (A) and CTAB capped Ag@Au/Ag (B) nanocomposites. Reaction conditions: [cysteine] = 10.0 × 10−4 mol dm−3, [HAuCl4] = 10.0 × 10−4 mol dm−3, [Ag+] = 20.0 × 10−4 mol dm−3, [CTAB] = 10.0 × 10−4 mol dm−3,

3.2. Optical Properties of Ag@Au/Ag Nanocomposites It has been established that appearance of different color and well defined surface resonance plasmon (SRP) bands in the UV–visible spectra of Ag, Au, and Ag@Au/Ag NPs are due to the colloidal nature of sols. The shape of the spectra and peak positions strongly depend on the composition of the reactants and other experimental conditioned [21]. We recorded the optical images and UV–visible spectra of Ag@Au/Ag

NPs formation at different time intervals. Figs. 4 and 5 show a representative set of optical images and visible spectra for Ag@Au/Ag, indicating that the typical color of reaction mixture containing Ag+, HAuCl4, cysteine, and CTAB changed from colorless, purple-brown, and dark brown(Fig. 4) and there is a red shift with the reaction time (Fig. 5). The absorption spectra of pure monometallic AgNPs and AuNPs were also measured in the same way (cysteine + Ag+ or HAuCl4) for the comparison. We did not observe the formation of any prefect

Fig. 4. Optical images of Ag@Au/Ag nanocomposites at different time intervals. Reaction conditions: [cystine] = 10.0 × 10−4 mol dm−3, [HAuCl4] = 10.0 × 10−4 mol dm−3, [CTAB] = 10.0 × 10−4 mol dm−3, [Ag+] = 20.0 × 10−4 mol dm−3, and time = 10 (A), 120 (B), 180 (C), 360 (D) and 840 min (E).

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Fig. 5. Time resolved UV–visible spectra of Ag@Au/Ag nanocomposites. Reaction conditions: [cystine] = 10.0 × 10−4 mol dm−3, [HAuCl4] = 10.0 × 10−4 mol dm−3, [CTAB] = 10.0 × 10−4 mol dm−3, [Ag+] = 20.0 × 10−4 mol dm−3. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

transparent color of AuNPs during the reduction of cysteine by HAuCl4 with and without CTAB for the same reaction time (Table 1). Thus ruled out the possibility to the formation of pure AuNPs under our experimental conditions. The AgNPs exhibit an absorption band with a maximum at 425 nm (Fig. 6A), while the Ag@Au/Ag shows a broad shoulder absorption with a maximum at 475 nm (Fig. 5; blue color line). The red shift (50 nm) and increase in bandwidth in the surface plasmon absorption band of the Ag@Au/Ag relative to the pure AgNPs, which can also arise from the excitation of different multiple modes present in anisotropic particles [40]. TEM images of pure AgNPs (Fig. 6B) also provides additional evidence to the cross-linking and interparticle interactions, which can be rationalized in terms of capping action of cysteine through S atom.

Fig. 6. UV–visible spectra (A), TEM images (B) of AgNPs prepared by the reduction of Ag+ ions with cysteine in presence of CTAB. Reaction conditions: [cysteine] = 10.0 × 10−4 mol dm−3, [Ag+] = 20.0 × 10−4 mol dm−3, [CTAB] = 10.0 × 10−4 mol dm−3.

3.3. Mechanism of Ag@Au/Ag Nanocomposites Formation 3.3.1. Ag+ + Cysteine System Henglein suggested that the pH is one of the important reaction conditions because stability of the colloids, and redox potential of the reactants depends on the pH. Nucleation and growth processes can be stopped by adding the small amounts of mineral acids [2]. Table 1 also shows that the presence of CTAB is also an essential requirement to the preparation of transparent colloidal sols. The control of pH is not as straightforward in micellar solutions as in ordinary solvents. Tondre et al. [41] have advised to avoid the use of even buffer solutions to pH of micellar solutions. The pH of the working solutions were also recorded under different experimental conditions with and without CTAB,

Table 1 Effects of reactants order of mixing on the SRP of Ag@Au/Ag bimetallic nanocomposites. Reactants mixing order

Observations and morphology

HAuCl4 + H2O HAuCl4 + Cysteine AgNO3 + H2O AgNO3 + Cysteine AgNO3 + CTAB + Cysteine

Pale yellow; stable; pH = 6.7 No color change up to 2 h; pH = 5.2 Colorless; pH = 6.8 Yellowish-white turbid; pH 5.2 Yellow color; λmax = 400 nm; aggregated chain-like Pale yellow; no color change; pH = 6.7 Brown color; stable; mixed morphology; pH = 5.3 Flower-like; λmax = 450 nm; pH = 5.2

HAuCl4 + AgNO3 HAuCl4 + AgNO3 + Cysteine HAuCl4 + AgNO3 + CTAB + Cysteine

HAuCl4 and cysteine, which was found to be nearly constant with increasing [HAuCl4] and [cysteine] in presence of CTAB (Table 1). Various species of cysteine (cationic, zwitterionic, and anionic; HSCH2CH(NH+ 3 )COOH ↔ – – HSCH2CH(NH+ 3 )COO ↔ HSCH2CH(NH2)COO ) exist in an aqueous solution due to the presence of pH sensitive groups. The concentrations and reactivity of these species depends on the pH of the working solution. – Out of these species, HSCH2CH(NH+ 3 )COO is the reactive and major existing species of cysteine under our experimental conditions (Table 1; pH = 5.3). In presence of Ag+ ions, reactive species of cysteine formed a white-yellow turbidity (Eq. (1)). The appearance of perfect transparent yellow color silver sols was formed in presence of CTAB, which might be due to the shape-controlling properties of CTAB (Eq. (2)).
HSCH2 CH NH3 þ COO− þ Agþ →white yellow trubidity

ð1Þ

cysteine þ Agþ þ CTAB → AgNPs yellowcolour; λmax ¼ 410nm

ð2Þ

Cysteine has three possible coordination sites: the O-, N-, and S-centers. Reactivity of \\SH is higher in comparison \\COOH and \\NH2 groups to reduce Ag+ ions (one electron oxidation-reduction mechanism). We recorded the FT-IR spectra of pure cysteine and Ag@Au/Ag NPs to see insight in to the reactivity of cysteine functional groups and compared (Fig. 7). The peaks observed at 3089–2974, 2551, 1531, 1590, 1391, and 1424 cm−1, and are assigned to\\NH+ 3 stretching vibrations,\\SH stretching, C\\N vibrations, asymmetric COO– stretching, symmetric COO– vibrations, and CH2 stretching vibrations in pure

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HAuCl4 formed a stable bright golden color with CTAB, which might be due to the strong complexation between AuCl− 4 and positive head group of CTAB [35,43]. Appearance of color is a clear indication of water insoluble AuCl4CTA complex solubilized into micelle. As a result, CTAB protects instant reduction of AuCl4 by caging the complex inside the micelle. The various − ↔ AuCl2(OH)− species of HAuCl4 (HAuCl4 → AuCl− 4 ↔ AuCl3(OH) 2 − − ↔ AuCl(OH)3 ↔ Au(OH)4 ) exists in an aqueous solution. Under our experimental conditions (pH = 5.2) of working solution, AuCl− 4 is a major and reactive species. On the basis of above results, the following reactions are proposed for the interactions of CTAB with HAuCl4. KM

þ

CH3 ðCH2 Þ15 NðCH3 Þ3 Br ⇄ CH3 ðCH2 Þ15 N ðCH3 Þ3 þ Br− −

HAuCl4 →Hþ þ AuCl4 − fast

CTAþ þ AuCl4 → CTA−AuCl4

Fig. 7. FT-IR spectra of pure cysteine, and cysteine capped Ag@Au/Ag NPs prepared by the reduction of Ag+ + HAuCl4 with cysteine in presence of CTAB. Reaction conditions: [cysteine] = 10.0 × 10−4 mol dm−3, [Ag+] = 20.0 × 10−4 mol dm−3, [CTAB] = 10.0 × 10−4 mol dm−3. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Scheme 1. Mechanism to the formation of AgNPs.

cysteine, respectively (Fig. 7; red line) [33,42]. In Ag@Au/Ag NPs, major peaks of cysteine shifted (little blue shift), and peak intensity also decreased. Interestingly, the characteristic S\\H stretching peak was not appeared in the frequency at ca. 2551 cm−1 (Fig. 7; black line), which might be due to the oxidation of \\SH by Ag+ ions. These results are in good agreement to the observations of Zhang et al. regarding the green synthesis of AgNPs assisted by cysteine [33]. On the basis of above results and previous observations [32,33], the following mechanism is proposed to the formation of AgNPs by cysteine (Scheme 1). In Scheme 1, Eq. (3) represents the reduction of Ag+ ions by thiol (\\HS) moiety of cysteine (rate determining step), which leads to the formation of cysteine-Ag and Ag0 [33]. Ag0 under goes fast complexa2+ is formed tion with Ag+, and Ag+ 2 is formed (Eq. (4)). In Eq. (5), Ag4 after dimerization [2]. Finally, cysteine adsorbed onto the surface of Ag2+ 4 . As a result, cysteine capped AgNPs are formed (Eq. (6)) [33]. 3.3.2. HAuCl4 + Cysteine and HAuCl4 + CTAB Systems A series of experiments were performed to see insight into the reduction of HAuCl4 by cysteine under different reaction conditions. We did not observed the appearance the any color by mixing cysteine (10.0 × 10− 4 mol dm− 3) and [HAuCl4] (10.0 × 10−4 mol dm− 3) at room temperature for a short reaction time, i.e., 60 min. Thus, ruled out the formation of AuNPs (Eq. (7)).
HSCH2 CH NH3 þ COO− þ HAuCl4 →noreaction

ð7Þ

ð8Þ ð9Þ ð10Þ

3.3.3. HAuCl4 + Cysteine + CTAB + Ag+ Systems In a reaction mixture (HAuCl4 + Ag+ + cysteine + CTAB), the appearance of dark-brown purple color indicating the oxidation of cysteine-Ag complex by AuCl4-CTA (Fig. 4). From the optical images and UV–visible spectra we may state confidently that the there is a competition between the HAuCl4 (reduction potential = +1.002 V for AuCl− 4 / Aumetal), and Ag+ (reduction potential = 0.799 V for Ag+/Agmetal) to react with cysteine (reduction potential = − 0.22 V for cysteine/cystine). Ferrando et al. in his pioneering review suggested that the structure of core -shell bimetallic nanoparticles depends on the reduction potentials of the reacting metal species present in a reaction mixture [44]. Metal with higher reduction potential is reduced first and form a core, whereas the other metal species of lower reduction potential is deposited on the core as a shell. On the other hand, formation of complex between the ligand, reducing agent, and higher reduction potential metal, the core-shell structure of the bimetallic nanoparticles may be inversed [45]. In the present study, Ag+ has lower reduction potential − than AuCl− 4 . In presence of CTAB, AuCl4 ions exists mainly as CTA– AuCl4, which is more difficult to reduce than AuCl− 4 ion itself. This will 0 decreases the value of AuCl− 4 /Au standard redox potential. Therefore, Ag+ ions were first reduced by cysteine to produce AgNPs and forms a core [33,45]. Secondly, the CTA-AuCl4 complex would be reduced on the surface of the AgNPs by under potential deposition, which leads to the formation of shell and act as seeds or active sites for further growth. Finally, due to the catalytic nature of gold atoms, silver metal grew form these active sites on the surface of AuNPs, which leads to the formation of star shaped with short thorns and flower like Ag@Au/Ag nanocomposites (Fig. 2A and B) [46]. Under our experimental conditions, [Ag+] (=20.0 × 10−4 mol dm−3) and [HAuCl4] = 10.0 × 10−4 mol dm−3, respectively, were large and limited. Therefore, the optical properties of Ag@Au/Ag composites are dominated by the colloidal Ag metal [17]. On the basis of above results, the Scheme 2 mechanism is proposed to the formation of Ag@Au/Ag NPs. CTA-AuCl4 adsorbed on the surface of AgNPs, and reduction of Au3+ ions should be takes place by potential deposition, and bark-brown colored Ag@Au/Ag bimetallic nanocomposites was formed (Scheme 2).    Kad  n Ag0 þ nHAuCl4 ⇄ n Ag0 −HAuCl4 fast

Ag0 þ HAuCl4 → Ag@Au‐bimetallic

ð11Þ ð12Þ

Scheme 2 represents the adsorption of AuCl− 4 onto the positive surface of Ag0, which immediately converted to the Ag@Au/Ag bimetallic nanoparticles. Schematic adsorption of HAuCl4 on the surface of AgNPs, its reduction to Au0, and Ag growth are also summarized in Scheme 2. These results are in good agreement to the theory and

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explanations already advanced regarding the formation of core-shell bimetallic nanoparticles during the co-reduction of two metal ions in an aqueous solution [22,44–46]. Goia and Matijevic suggested that the pH of the reaction medium can have a major impact upon the redox potential of the solutes, as predicted by the Nernst equation [45]. In presence of CTAB, AuCl− 4 ions exist mainly as CTA–AuCl4, which is more difficult to reduce than AuCl− 4 ion 0 itself. This will decreases the value of AuCl− 4 /Au standard redox potential. According to the Nernst equation, the overall redox reaction and reduction potential can be written as Eqs. (13) to (15). 2cysteine‐Agþ þ CTA−AuCl4 →CTAcappedAg@Au=Ag þ cystine−Agþ

ð13Þ

ΔE ¼ ΔE0 −

  Cystine−Agþ RT In  nF ½CTA−AuCl  Cystine−Agþ 2 4

ð14Þ

ΔE ¼ ΔE0 þ

 þ 2 RT ½CTA−AuCl4  Cystine−Ag   In nF Cystine‐Agþ

ð15Þ

Zhou and his coworkers assigned the wave at −0.63 V to the reduction of the cysteine-mercury thiolate [47]. Eriksen et al. also reported the oxidation kinetics of cysteine, homocysteine, and glutathione by using pulse radiolysis and suggested that the metal-thiolate bonds could be readily oxidizes [48]. Nernst equations (Eqs. (13) and (14)) also suggests that the complete protonation and or coordination of the \\COOH and\\NH2 groups lowers the reduction potential to such extent that the oxidation site may effectively remain at the sulfur atom. 3.4. Stability of Ag@Au/Ag in Presence of Inert Salts It is well known that the organic solvents, molecular gases, nucleophilic reagents, polymers, various metal cations, anions, and analytes have significant impact on the absorbance, wavelength, and shape of the surface resonance plasmon band of colloidal silver and gold [49– 55]. Henglein and Wu reported that the cations, Ag+, Cd2 +, Hg2 +, − Ni2 +, and anions, Cl−, NO− 3 , H2PO4 affect the plasmon absorption band of AgNPs, respectively [51,53]. Therefore, a series of UV–visible spectra of resulting Ag@Au/Ag were recorded in presence of inorganic inert salts (NaCl, NaBr, NaNO3, and Na2SO4). Fig. 7 shows that the SRP remains unaffected with NO− 3 . A significant blue-shift of SRP was observed in presence of Cl− and Br−. In case of SO2− 4 , shape of spectra entirely changed, which resulted in a shift of the absorption band to longer wavelength (from 400 nm to 500 nm; red-shift) of increased intensity. On the other hand, Ag@Au/Ag intensity decreases with NO− 3 and increases with Cl− and Br−. We did not observed the appearance of any precipitate or turbidity of AgCl and AgBr indicating the complete reduction of Ag+ ions into Ag0. It should also be emphasized that the charge ion exhibits a strong on the anion is important because divalent SO2− 4

Fig. 8. UV–visible spectra of Ag@Au/Ag nanocomposites in absence and presence of inorganic inert salts (NaCl, NaBr, NaNO3, and Na2SO4). Reaction conditions: [cysteine] = 10.0 × 10−4 mol dm−3, [Ag+] = 40.0 × 10−4 mol dm−3, [HAuCl4] = 10.0 × 10−4 mol dm−3, [CTAB] = 10.0 × 10−4 mol dm−3, [inert salts] = 5.0 × 10−3 mol dm−3.

effect on the position of SRP band, whereas a much lesser influence is 2− observed with monovalent NO− 3 ions (Fig. 7). In SO4 , the negative charge is distributed at the oxygen atoms instead of localizing on a central S atom due to resonance. As a results, one SO2− 4 anion interacts with several Ag@Au/Ag NPs, which in turn, causes the aggregation of free Ag@Au/Ag. The divalent inert salt is more efficient to destabilizing the Ag@Au/Ag NPs, which can be ascribed to the strong interactions between shell of Ag@Au/Ag and anions (Fig. 8). These results are in good agreement to the observations of Chen et al. regarding the electrolytes effect (CaCl2, MgCl2 and NaCl) on the SRP of citrate coated-AgNPs [55].

4. Conclusions Our strategic is to design a simple seedless method for the synthesis of Ag@Au/Ag bimetallic nanocomposites through competitive coordination between the Ag+ ions and AuCl− 4 ions to react with cysteine with and without CTAB. It provides another rout for the synthesis of bimetallic nanocomposites, which entirely depends on the reduction potentials of the reducing agents. The morphology of the composites can be controlled by proper combination of strong and weak reducing agents. The composites would possess the optical properties of that metal which reduced onto the surface of nanoparticles. Stability of resulting nanocomposites depends on the size and polarizability of the used anion. The Ag@Au/Ag composites are stable in NaNO3 and solution but causes their aggregation in presence of NaCl, NaBr and Na2SO4.

Scheme 2. Mechanism to the formation of Ag@Au/Ag nanocomposites.

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