A Simple Route to Synthesize Cu@Ag Core–Shell Bimetallic Nanoparticles and Their Surface-Enhanced Raman Scattering Properties

Abstract

Water-dispersed Cu@Ag core–shell nanoparticles (NPs) with 15 nm-diameter Cu core and 5 nm-thick Ag shell can be synthesized by a facile one-step chemical reduction at room temperature without any protective atmosphere. To obtain a homogeneous Ag coating on Cu, the influence of [Cu/Ag] molar ratio was investigated. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed that Ag formed a dense coating on the surface of Cu, and that phase-pure spherical Cu@Ag core–shell bimetallic NPs were prepared when the [Cu/Ag] molar ratio was between 1/0.5 and 1/0.75. The time dependence of ultraviolet-visible (UV-Vis) spectra and XRD patterns of six-month stored Cu@Ag NPs showed that the as-prepared Cu@Ag NPs have a long-term antioxidant activity. Also, the surface-enhanced Raman scattering (SERS) signals had a high stability and reproducibility for the substrates. Hence, the as-prepared Cu@Ag nanostructures can be used as an efficient substrate for SERS signals.

Keywords

Cu@Ag core–shell nanoparticles surface-enhanced Raman scattering stability reproducibility

Introduction

Bimetallic core–shell nanostructures have stimulated great interest in the past decade because of their low material costs along with enhanced optical, electronic, magnetic, biological, catalytic, and surface-enhanced Raman scattering (SERS) properties in comparison with their monometallic counterparts.^1–4 On one hand, the shell layer can protect the core materials from oxidation and improve their compatibility and stability; on the other hand, the outer layer materials may also provide a platform for surface modification to obtain multifunctional materials.^5,6 Among these, Cu@Ag bimetallic nanomaterials instead of single copper nanoparticles (CuNPs) or silver nanoparticles (AgNPs) are considered to be a desired option to overcome the high cost of Ag and to prevent the spontaneous oxidation of Cu.⁷

Currently, various methods have been developed for preparing of Cu@Ag core–shell particles, such as physical vapor deposition,⁸ the pulsed wire evaporation method,⁹ electroless plating (also called spontaneous displacement reaction),¹⁰ thermal evaporation under ultrahigh vacuum,¹¹ the redox-trans-metalation method in aqueous solution,¹² the solvothermal method,¹³ the polyol process,^14,15 etc. Unfortunately, some of these synthetic processes require harsh conditions (such as an inert atmosphere), expensive equipment, complicated procedures, or even low production yield. The most successful method to prepare Cu@Ag nanoparticles (NPs) is wet chemical reduction as it provides a facile and diverse way to control the particle size and morphology.

In our research, sodium hyposulfite (Na₂S₂O₄), a cheap reducing agent much used in industry, was used to replace other reducing agents, such as sodium borohydride (NaBH₄), hydrazine hydrate (N₂H₄.H₂O), and sodium citrate (Na₃C₆H₅O₇). Gelatin has served as a dispersant to replace poly(acrylic acid) (PAA), hexadecyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), and ethylenediaminetetraacetic acid (EDTA) in many chemical reductions.¹⁶ However, the synthesis of bimetallic Cu@Ag core–shell nanostructures of high purity (no oxide layer) and concentration by chemical reduction, in which Na₂S₂O₄ works as the reducing agent and gelatin serves as the capping agent, is still a challenge.

Surface-enhanced Raman scattering (SERS) spectroscopy has become a valuable tool in the surface science, spectroscopy, and analytical chemistry fields. However, SERS, as a detection technique, should have a high stability and reproducibility for the substrates, and design and facile fabrication of reproducible SERS substrates for highly sensitive detection remain a critical challenge.¹⁷ From the viewpoint of application, it is important for the preparation of reproducible SERS substrates with large and stable enhancement of the Raman signal. It is well known that the SERS enhancement of colloidal AgNPs is intrinsically much higher than that of CuNPs.¹² Thus, it is expected that the reproducibility and the SERS performance of colloidal CuNPs could be further improved by coating a dense silver shell onto the surface of CuNPs to form a core–shell structure.

Here, we report a facile, low-cost, and one-step synthetic method to synthesize monodisperse Cu@Ag bimetallic colloidal NPs in aqueous solution at room temperature without any protective atmosphere. The uniform and dense Ag shell can be deposited on the surfaces of the CuNPs via chemical reduction in which Na₂S₂O₄ is utilized as a reduction agent and gelatin as a dispersant. Furthermore, the as-prepared Cu@Ag core–shell NPs have long-term stability, which is of great importance in practical application as SERS substrates.

Experimental section

Materials and Reagents

Copper(II) sulfate pentahydrate (CuSO₄.5H₂O), silver nitrate (AgNO₃), Na₂S₂O₄, sodium hydroxide (NaOH), and crystal violet (CV) were obtained from Sinopharm Chemical Reagent Co. Ltd., China. Gelatin was purchased from Guangdong Chem. Co. All chemicals were used directly without further purification.

Synthesis of Cu@Ag Core–Shell NPs

Firstly, CuNPs were synthesized according to the process in our previously published research.¹⁶ Herein, 5 mL of CuSO₄.5H₂O (0.2 M) was added in 10 mL gelatin (0.01 g/mL) solution to form the oxidizing solution. Then a 5 mL aqueous solution containing 0.87 g Na₂S₂O₄ and 1.3 mL NaOH (5 M), which functioned as reducing agent, was added rapidly to the solution, and the wine-reddish solution was stirred for 10 min to obtain the colloidal CuNPs solution. Subsequently, without any treatment, a 5 mL silver ammonia solution was directly added dropwise to the colloidal CuNPs solution, and the suspension was stirred mechanically for 15 min to ensure sufficient reaction. It should be pointed out that the reactor was kept in the aqueous solution at room temperature without any inert atmosphere. The chemical reduction reaction involved can be expressed as follows:

{Cu}_{(aq)}^{2 +} + gelatin (gel)_{(aq)} \to [Cu (gel)_{(aq)}^{2 +}]

(1)

Cu (gel)_{(aq)}^{2 +} + S_{2} O_{4}^{2 -} + 4 OH - \to gelatin - cappedCu + 2 {SO}_{3}^{2 -} + 2 H_{2} O

(2)

Cu + 2 Ag + \to Ag + Cu 2 +

(3)

2 [Ag ({NH}_{3})_{2}] + + S_{2} O_{4}^{2 -} + 4 OH - \to 2 Ag + 2 {SO}_{3}^{2 -} + 2 H_{2} O + {3 NH}_{3}

(4)

Via the above transmetalation reaction, a uniform and dense Ag shell can be coated onto the CuNPs surface by properly controlling the [Cu/Ag] molar ratio.^18,19 Here, the [Cu/Ag] molar ratio was controlled by the concentration of the silver ammonia solution with a constant quantity of CuNPs.

Characterization

Ultraviolet-visible (UV-vis) absorbance spectra were recorded using a Shimadzu UV-2500 spectrophotometer in a 1 cm optical path quartz cuvette over the 200–800 nm range. Transmission electron microscopy (TEM) micrographs were obtained using a JEM-2000EX high-resolution transmission electron microscope operating at an acceleration voltage of 160 kV. The TEM samples were obtained by casting a drop of the as-prepared Cu@Ag NP suspension onto a carbon-coated copper grid and then drying them in air. The crystal structures of the as-prepared nanostructures were also obtained by Bruker D8 Advance X-ray diffraction (XRD) using Cu K_α radiation. Both ordinary Raman and SERS spectra were recorded on a Renishaw Invia Raman microscope excited by an argon ion laser beam (514.5 nm, 20 mW). To record the Raman spectrum of CV adsorbed on the Cu@Ag NPs, the Cu@Ag substrate aqueous solution (2 × 10⁻⁴M) was mixed with an equal volume of CV aqueous solution (2 × 10⁻⁵M). After 24 h of adsorption, the sample was measured. More details on the measurement can be found in our previous research.²⁰

Results and discussion

Characterization of Cu@Ag Core–Shell NPs

In our present study, we found that the homogeneous Ag shell on the CuNPs surface, which prevented the core material from being oxidized, was strongly affected by the [Cu/Ag] molar ratio. X-ray diffraction (XRD) analysis was performed to determine the composition and crystal structure of the synthetic Cu@Ag NPs. Figure 1 shows the XRD results of the as-prepared samples at different [Cu/Ag] molar ratios. The patterns obtained experimentally were identified through comparison with the standard Cu and Ag patterns. The diffraction peaks at 2θ values of 43.2°, 50.3°, and 74.1° are associated with the (111), (200), and (220) planes of face-centered cubic (FCC) Cu (JCPDS NO. 85-1326), and the peaks at 38.2°, 44.4°, and 64.6° correspond to the (111), (200), and (220) crystalline planes of Ag (JCPDS NO. 87-0719).^10,21

Figure 1.

XRD patterns of Cu@Ag NPs synthesized at different [Cu/Ag] molar ratios.

It can be seen that both Cu and Ag peaks can be found in all the samples, and the relatively intensities of the Cu(111) and Cu(200) peaks decreased compared to those of the Ag(111) and Ag(200) peaks with the decrease of [Cu/Ag] molar ratio from 1:0.25 to 1:0.75. This phenomenon can be explained by adding AgNO₃ which helps to form the Ag shell coating on the CuNPs core surface. On the other hand, as shown in Figure 1, XRD patterns show no diffraction peaks of copper oxides except those of pure Cu and Ag when the [Cu/Ag] molar ratio is 1:0.5 and 1:0.75. Furthermore, no diffraction peaks of alloy are detected, which shows that the as-prepared nanostructures are core–shell bimetallic composites rather than Cu–Ag alloys.^22,23 However, in the case of [Cu/Ag] molar ratios of 1:0.25, 1:1, and 1:1.5, the formation of Cu₂O phase was observed in the XRD patterns, which showed that the as-prepared samples had been oxidized.

In order to gain more details about the microstructure of the Cu@Ag core–shell NPs, typical TEM and high-resolution TEM (HRTEM) images were obtained from the samples prepared at different [Cu/Ag] molar ratios, as illustrated in Figure 2. As can be seen clearly from Figure 2a, for samples prepared at a [Cu/Ag] molar ratio of 1:0.25, the core CuNP surfaces were incompletely coated by Ag shells. This is attributable to the fact that the generated AgNPs are not sufficient to fully cover the CuNP surfaces. With a decrease of the [Cu/Ag] molar ratio to 1:0.5 and 1:0.75, the formation of a dense and uniform Ag shell was revealed on the CuNP surface (Figure 2b and c), which is also in agreement with the XRD results. However, on further decrease of the [Cu/Ag] molar ratio to 1:1, the core CuNP surfaces were only partially coated by Ag shells, as shown in Figure 2d. This can be explained by the fact that when an excess amount of AgNO₃ is added, the Ag⁺ ions are then reduced to AgNPs on the surfaces of the CuNPs core with the aid of CuNPs and Na₂S₂O₄ which acts as reducing agent. The AgNPs thus generated quickly and agglomerated to form island structures as a result of surface, interface, and strain energies of the Ag film.²⁴ These observations indicate that the CuNP core is partially oxidized with decreasing [Cu/Ag] molar ratio, and the formation of Cu₂O phase was experimentally confirmed using XRD analysis (Figure 1d and 1e). Moreover, the experiment of Chang et al. proceeded to show that,²⁵ when the [Cu/Ag] molar ratio was further decreased to 1:2, the NPs comprised a Ag core and a Cu shell.²⁵ So the optimum condition of the [Cu/Ag] molar ratio was determined to between 1/0.5 and 0.75.

Figure 2.

TEM images of Cu@Ag NPs prepared with different [Cu/Ag] molar ratios: (a) 1:0.25; (b) 1:0.5; (c) 1:0.75; (d) 1:1.

To further identify the structural nature of the Cu@Ag core–shell NPs prepared at a [Cu/Ag] molar ratio of 1:0.75, further insights into the morphology and microstructure of the samples were obtained using TEM and HRTEM, and the corresponding images are shown in Figure 3. As shown in Figure 3a, the as-prepared Cu@Ag core–shell NPs were found to be spherical in shape with an average particle size of 35 nm, and roughly monodispersed without any aggregation. Figure 3b and 3c shows the HRTEM images of a single Cu@Ag core–shell structure with about 15 nm-diameter Cu core and 5 nm-thick Ag shell. From the HRTEM lattice image of the Cu–Ag sample (Figure 3d), the as-prepared sample was crystalline rather than a single crystal, and the as-marked interplanar distance of the fringes were measured to be 0.236 nm and 0.209 nm, corresponding to the (111) lattice spacings of FCC Cu and FCC Ag, respectively.²² These results are also consistent with the XRD data and those reported previously for Cu@Ag core–shell NPs prepared using different methods.¹¹

Figure 3.

As-prepared Cu@Ag core–shell NPs using a [Cu/Ag] molar ratio of 1:0.75: (a) TEM image; (b, c) HRTEM images of a single Cu@Ag NP; (d) HRTEM lattice image.

The above results confirm that the microstructure of the Cu@Ag core–shell NPs is strongly influenced by the [Cu/Ag] molar ratio, and the uniform Ag shell is fully coated on the CuNPs surfaces when the optimum condition of the [Cu/Ag] molar ratio is determined to between 1/0.5 and 0.75. When the optimum condition of the [Cu/Ag] molar ratio is between 1/0.5 and 0.75 the uniform Ag shell is fully coated on the CuNP surfaces, which prevents the CuNP core materials from being oxidized.

The XRD and UV-vis measurements were also performed after several months of storage to study the stability of the Cu@Ag core–shell NPs prepared at [Cu/Ag] molar ratio of 1:0.75. The results obtained for the stored samples are given in Figures 4 and 5. As can be seen in Figure 4, there are no diffraction peaks of copper oxides and alloy but only those of pure Cu and Ag after six-month storage.^22,23

Figure 4.

XRD patterns of freshly prepared and six-month stored Cu@Ag NPs: (a) freshly prepared Cu@Ag NPs; (b) six-month stored Cu@Ag NPs.

Figure 5.

UV-Vis absorption spectra of Cu@Ag NPs after several months storage.

It is reported that spherical AgNPs typically exhibit localized surface plasmon resonance (LSPR) at around 410 nm.²⁶ In our previous report, the LSPR band of the CuNPs was centered at 575 nm.¹⁶ Colloidal Cu@Ag particles display two bands corresponding to the LSPRs of Cu and Ag, respectively, and the Cu band is shifted to low wavelength.^27,28 As can be seen in Figure 5, the Cu@Ag NPs have a maximum in their absorption at 550 nm (LSPR of Cu) and 400 nm (LSPR of Ag), respectively, and the Cu band is shifted from 575 nm to a lower wavelength (550 nm) as expected. This result further indicates that the as-prepared samples are Cu@Ag NPs.

As seen also in Figure 5, the absorption intensity of the samples decreases due to the slight aggregation of the Cu@Ag NPs during several months of storage. However, the two absorption peaks remained unchanged and no precipitation occurred after several months of storage under ambient conditions, clearly indicating that the Cu@Ag NPs have high stability. So the as-prepared Cu@Ag NPs have excellent antioxidant properties, which will not affect their potential application.

SERS Activity

Substrates with good stability and reproducibility are of crucial importance for the use of SERS as a routine detection technique. To test whether the as-prepared Cu@Ag colloidal NPs are able to give reproducible and deterministic SERS signals, a low concentration of the CV target molecule was used. The SERS spectra of CV molecules from three randomly selected positions on the Cu@Ag core–shell NP substrate, and also the SERS spectra of CV on Cu@Ag NPs after different storage times, are shown in Figure 6.

Figure 6.

Comparison of SERS spectra of CV on freshly prepared and six-month stored Cu@Ag NPs (A – freshly prepared samples, B – six-month stored samples): (a) Raman spectra of CV with the intensities multiplied by 10; (b) a series of SERS spectra of CV collected on three randomly selected spots of Cu@Ag NPs.

As shown in Figure 6a, the intensities of all these bands of CV are very weak even when they are magnified 10 times. It is noted that the band at about 917 cm⁻¹ is attributed to the ring skeletal vibrations of CV.²⁹ The C–H in-plane bending vibrations and the N–phenyl stretching appear at about 1175 and 1375 cm⁻¹.³⁰ The bands at 1447, 1591, and 1620 cm⁻¹ are assigned to ring C–C stretching.³¹ Detailed peak frequency assignments and the calculated I_SERS/I_normal values are shown in Table 1. In the SERS spectra of CV adsorbed on Cu@Ag colloidal NPs (Figure 6b), the relative intensities of the bands mentioned above are much stronger than those of CV. Furthermore, a new band at 1533 cm⁻¹ is observed, which can be attributed to the ring C–C stretching vibrations of CV.

Table 1.

Assignment and the SERS EF of several normal Raman (NR) peaks of CV on freshly prepared Cu@Ag NPs.

Shift (cm⁻¹)		Assignment	I _SERS /I _normal				EF
NR	SERS	Assignment	b1	b2	b3	b_ave	EF
1179	1180	C–H bending vibrations	55.8	58.1	57.9	57.3	1.0 × 10⁴
1371	1375	N–phenyl stretching	55.7	55.7	56.1	55.8	0.97 × 10⁴
1449	1447	ring C–C stretching	58.1	58.9	57.5	58.2	1.0 × 10⁴
1544	1544	ring C–C stretching	56.7	55.4	53.1	55.1	0.96 × 10⁴
1586	1588	ring C–C stretching	54.9	55.2	54.3	54.8	0.95 × 10⁴
1619	1621	ring C–C stretching	53.1	52.5	52.2	52.6	0.92 × 10⁴

In order to estimate reproducibility of the SERS signals, the calculated I_SERS/I_normal approximations of the freshly prepared samples are summarized in Table 1. The average I_SERS/I_normal value is 55.6, and the relative standard deviation (RSD) of I_SERS/I_normal of the bands at 1179, 1371, 1449, 1544, 1586, and 1619 cm⁻¹ are all <20% at 3.1%, 0.3%, 4.7%, 0.9%, 1.4%, and 5.4%, respectively, again supporting the great enhancement effect and high reproducibility of this substrate. In addition, the I_SERS/I_normal approximations of the six-month stored samples were calculated. The average I_SERS/I_normal value is 51.9, and the RSD of I_SERS/I_normal of all the bands increased, but all of them are all <20%. These results also indicate that the CV probe molecules are uniformly distributed on the surface of the Cu@Ag core–shell substrates, and the reproducibility of the substrate is relatively better than our previous research about the SERS of CuNPs.¹⁶ So the colloidal Cu@Ag core–shell NPs can serve as an excellent SERS substrate to detect any analyte in trace amounts.

To evaluate the SERS activity of the as-prepared Cu@Ag core–shell colloids, the enhancement factor (EF) can be calculated using the following equation:³²

EF = (I_{SERS} / I_{normal}) (N_{bulk} / N_{ads})

(5)

where I_SERS and I_normal are the SERS intensity of CV on a Cu@Ag core–shell substrate and the normal Raman (NR) intensity of bulk CV, respectively, and N_bulk and N_ads are the numbers of CV molecules illuminated by the laser light to obtain the corresponding NR and SERS spectra. Here N_bulk and N_ads were calculated using the absorption intensity of CV solution in the presence and absence of Cu@Ag NPs, and the results are shown in Figure 7. For CV, the maximum absorption peak is at 585 nm and the peaks at 250 nm and 300 nm are negligible. From Figure 7, it can be seen that the Cu@Ag NP colloid solution (with a concentration of 10⁻⁴M) has a maximum in its absorption at 400 nm, but the enlarged picture in the top right corner of Figure 7 indicates that the absorption intensity at 585 nm is too low. Thus, the UV-vis absorbance of a 10⁻⁵M concentration of CV solution and a 10⁻⁵M concentration of CV adsorbed on a 10⁻⁴M concentration of Cu@Ag core–shell substrate solution at 585 nm were measured and the intensity was 0.347 and 0.345, respectively. So the N_bulk/N_ads ratio was about 1.74 × 10². Thus, the EF values of this system were calculated, as summarized in Table 1. The average SERS EFs of our as-prepared colloidal Cu@Ag core–shell NPs were estimated to be up to 10⁴, which is higher than that of CuNPs in our previous research.¹² When the possible spectral overlap of Cu@Ag NPs and CV is considered, the N_bulk/N_ads ratio is larger than 1.74 × 10². These results indicate that colloidal Cu@Ag core–shell systems are able to give reproducible and deterministic SERS signals and could be used as an active SERS substrate.

Figure 7.

UV-vis absorption spectra of (a) CV, (b) CV solution in the presence of Cu@Ag NPs, and (c) Cu@Ag NPs.

Conclusions

In this work, Cu@Ag core–shell NPs were successfully prepared in a one-step process using successive sodium hyposulfite reduction of metal salts in aqueous solution at room temperature without any inert atmosphere. To obtain a homogeneous Ag coating on CuNPs, the influence of [Cu/Ag] molar ratio on coatings was investigated. In the synthesized process, [Cu/Ag] molar ratios play an important role in forming the uniform and dense Ag shell on the surface of the CuNPs. From the structure and the composition of the analyses of the samples prepared at different [Cu/Ag] molar ratios, the optimum condition of the [Cu/Ag] molar ratio varies between 1/0.5 and 1/0.75 for a 15 nm-diameter Cu core and 5 nm-thick uniform Ag shell structure. In these synthetic conditions, the as-prepared nanomaterials are shown to be stable over several months, and the long-term stability is of the great importance in practical application. The results of the SERS activity of the freshly prepared and six-month stored nanostructures indicate that the CV probe molecules are uniformly distributed on the substrate, and the reproducibility of SERS signals is very high. The average SERS enhancement factors were estimated to be up to 10⁴, and the as-prepared Cu@Ag core–shell NPs can serve as an excellent SERS substrate. This study also has significant implications for the preparation of other bimetallic core–shell nanostructures, such as Au@Co, Au@Ni, and Au@Ni.

Footnotes

Conflict of Interest

None declared.

Funding

This work was supported by the Natural Science Foundation of Anhui Province (grant number 1508085SMB209), the Anhui Provincial Key Science Foundation for Outstanding Young Talent (grant number 2013SQRL022ZD), Zhejiang Province Key Laboratory of Soldering & Brazing Materials and Technology (grant number 1402), and the Graduate Innovation Foundation of Anhui University of Technology (grant number 2014075).

References

Bao

J.F.

Ren

Yao

J.L.

R.A.

Tian

Z.Q.

“Synthesis and Characterization of Au@Co and Au@Ni Core–Shell Nanoparticles and Their Applications in Surface-Enhanced Raman Spectroscopy”. J. Phys. Chem. C 2007; 111: 345–350.

She

Chen

Zhang

Wang

Peng

D.L.

“Structure, Optical and Magnetic Properties of Ni@Au and Au@Ni Nanoparticles Synthesized via Non-Aqueous Approaches”. J. Mater. Chem 2012; 22: 2757–2765.

Ding

Yang

Liu

“A Simple Method to Prepare the Magnetic Ni@Au Core–Shell Nanostructure for the Cycle Surface Enhanced Raman Scattering Substrates”. J. Raman Spectrosc 2013; 44: 987–993.

Hwang

E.T.

Lee

Y.W.

Park

H.C.

Kwak

D.H.

Kim

D.M.

“Synthesis of Pt-Rich@Pt–Ni Alloy Core–Shell Nanoparticles Using Halides”. RSC Adv 2015; 5: 8301–8306.

W.F.

You

L.J.

J.M.

Guo

J.J.

Wang

C.C.

“Toward Designer Magnetite/Polystyrene Colloidal Composite Microspheres with Controllable Nanostructures and Desirable Surface Functionalities”. Langmuir 2012; 28: 3271–3278.

Yong

Zhang

Geng

Zhang

“Fabrication and Characterization of 1-D Fe₃O₄/P(NIPAM–MAA–MBA) Nanochains with Thermo- and pH-Responsive Shell for Controlled Release for Phenolphthalein”. J. Mater. Sci 2015; 50: 3083–3090.

Mancier

Rousse-Bertrand

Dille

Michel

Fricoteaux

“Sono- and Electrochemical Synthesis and Characterization of Copper Core–Silver Shell Nanoparticles”. Ultrasonics Sonochem 2010; 17: 690–696.

Cazayous

Langlois

Oikawa

Ricolleau

Sacuto

“Cu–Ag Core–Shell Nanoparticles: A Direct Correlation between Micro-Raman and Electron Microscopy”. Phys. Rev. B: Condens. Matter Mater. Phys 2006; 73. 113402.

Safaee

Sarkar

D.K.

Farzaneh

“Superhydrophobic Properties of Silver-Coated Films on Copper Surface by Galvanic Exchange Reaction”. Appl. Surf. Sci 2008; 254: 2493–2498.

10.

Peng

Y.H.

Yang

C.H.

Chen

K.T.

Popuri

S.R.

Lee

C.H.

Tang

B.S.

“Study on Synthesis of Ultrafine Cu–Ag Core–Shell Powders with High Electrical Conductivity”. Appl. Surf. Sci 2012; 263: 38–44.

11.

Zhang

Yuan

Wang

Yang

“Core/Shell Cu@Ag Nanoparticle: A Versatile Platform for Colorimetric Visualization of Inorganic Anions”. ACS Appl. Mater. Interfaces 2011; 3: 4092–4100.

12.

Chen

L.Y.

Zhang

Fujita

Chen

M.W.

“Surface-Enhanced Raman Scattering of Silver@Nanoporous Copper Core–Shell Composites Synthesized by an In Situ Sacrificial Template Approach”. J. Phys. Chem. C 2009; 113: 14195–14199.

13.

Huang

Yang

Zhao

Wang

“Solvothermal Preparation and Characterization of Cu–Ag Nanoparticles”. Res. Chem. Intermediat 2014; 40: 1957–1964.

14.

Tsuji

Hikino

Tanabe

Yamaguchi

“Synthesis of Ag@Cu Core-Shell Nanoparticles in High Yield Using a Polyol Method”. Chem. Lett 2010; 39: 334–336.

15.

Tsuji

Hikino

Sano

Horigome

“Preparation of Cu@Ag Core–Shell Nanoparticles Using a Two-Step Polyol Process under Bubbling of N₂ Gas”. Chem. Lett 2009; 38: 518–519.

16.

Mao

A.Q.

Ding

M.L.

Jin

X.L.

Cai

Chen

Zhang

T.Y.

“Direct, Rapid Synthesis of Water-Dispersed Copper Nanoparticles and their Surface-Enhanced Raman Scattering Properties”. J. Mol. Struct 2015; 1079: 396–401.

17.

Sinha

Depero

L.E.

Alessandri

“Recyclable SERS Substrates Based on Au-Coated Zno Nanorods”. ACS Appl. Mater. Interfaces 2011; 3: 2557–2563.

18.

Duan

Liu

You

Wei

Liu

“Anodic Behavior of Carbon Supported Cu@Ag Core–Shell Nanocatalysts in Direct Borohydride Fuel Cells”. J. Power Sourc 2015; 293: 292–300.

19.

Hai

H.T.

Takamura

Koike

“Oxidation Behavior of Cu–Ag Core–Shell Particles for Solar Cell Applications”. J. Alloys Compd 2013; 564: 71–77.

20.

Mao

A.Q.

Jin

X.L.

Wei

X.Q.

Yang

G.Q.

“Rapid, Green Synthesis and Surface-Enhanced Raman Scattering Effect of Single-Crystal Silver Nanocubes”. J. Mol. Struct 2012; 1021: 158–161.

21.

Zhao

Zhang

Song

“Simple and Eco-Friendly Preparation of Silver Films Coated on Copper Surface by Replacement Reaction”. Appl. Surf. Sci 2012; 258: 7430–7434.

22.

Hua

Guo

C.Y.

C.L.

“A Highly Sensitive Non-Enzymatic Glucose Sensor Based on Bimetallic Cu–Ag Superstructures”. Biosens. Bioelectron 2015; 63: 339–346.

23.

Rahman

L.U.

Qureshi

Yasinzai

M.M.

Shah

“Synthesis and Spectroscopic Characterization of Ag–Cu Alloy Nanoparticles Prepared in Various Ratios”. C. R. Chim 2012; 15: 533–538.

24.

Choi

T.S.

Levitin

Hess

D.W.

“Mechanistic Considerations in Plasma-Assisted Etching of Ag and Au Thin Films”. ECS J. Solid State Sci. Technol 2013; 2: 275–281.

25.

Chang

K.K.

Lee

G.J.

Min

K.L.

Chang

K.R.

“A Novel Method to Prepare Cu@Ag Core–Shell Nanoparticles for Printed Flexible Electronics”. Powder Technol 2014; 263: 1–6.

26.

Panacek

Kvítek

Prucek

Kolar

Vecerova

Pizurova

Sharma

V.K.

Nevecna

Zboril

“Silver Colloid Nanoparticles: Synthesis, Characterization, and Their Antibacterial Activity”. J. Phys. Chem. B 2006; 110: 16248–16253.

27.

Muzikansky

Nanikashvili

Grinblat

Zitoun

“Ag Dewetting in Cu@Ag Monodisperse Core–Shell Nanoparticles2. J. Phys. Chem. C 2012; 117: 3093–3100.

28.

Zhao

Zhang

Zhao

“Fabrication of Cu–Ag Core–Shell Bimetallic Superfine Powders by Eco-Friendly Reagents and Structures Characterization”. J. Solid State Chem 2011; 184: 2339–2344.

29.

Mclellan

J.M.

Siekkinen

Chen

Xia

“Comparison of the Surface-Enhanced Raman Scattering on Sharp and Truncated Silver Nanocubes”. Chem. Phys. Lett 2006; 427: 122–126.

30.

E.C.L.

Blackie

Meyer

Etchegoin

P.G.

“Surface-Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study”. J. Phys. Chem. C 2007; 111: 13794–13803.

31.

Sancı

Volkan

“Surface-Enhanced Raman Scattering (SERS) Studies on Silver Nanorod Substrates”. Sens. Actuators, B 2009; 139: 150–155.

32.

Zhang

D.H.

Liu

X.H.

Wang

“Synthesis of Single-Crystal Silver Slices with Predominant (111) Facet and their SERS Effect”. J. Mol. Struct 2011; 985: 82–85.