Ultrastable, Uniform, Reproducible, and Highly Sensitive Bimetallic

Article pubs.acs.org/JPCC

Ultrastable, Uniform, Reproducible, and Highly Sensitive Bimetallic Nanoparticles as Reliable Large Scale SERS Substrates Mohammad Y. Khaywah,†,‡ Safi Jradi,† Guy Louarn,∥ Yvon Lacroute,§ Joumana Toufaily,‡ Tayssir Hamieh,‡ and Pierre-M. Adam*,† †

Laboratoire de Nanotechnologie et d’Instrumentation Optique, Institut Charles Delaunay,UMR 6281 CNRS, Université de Technologie de Troyes, 12 rue Marie Curie - CS 42060, 10004 Troyes, France ‡ Laboratoire de Matériaux, Catalyse, Environnement et Méthodes Analytiques (MCEMACHAMSI), Faculté des Sciences, Section 1, Université Libanaise, Hadath, Mont-Liban, Beyrouth, Liban ∥ Institut des Materiaux Jean Rouxel (IMN), CNRS-Université de Nantes, 2 rue de la Houssinière, Nantes, France § Laboratoire de Physique, Optique Submicronique, Université de Bourgogne, 9 Avenue A. Savary BP 47870, F-21078 Dijon, France S Supporting Information *

ABSTRACT: A strong interest exists in developing surfaceenhanced Raman spectroscopy (SERS) substrates that uniformly enhance Raman signals of chemical and biological molecules over large scales while reaching the detection limit of trace concentrations. Even though the resonant excitation of localized surface plasmons of single or assembled metallic nanoparticles used in SERS substrates can induce large electromagnetic fields, these substrates display a SERS activity which suffers from poor reproducibility, uniformity, and stability, preventing them from being reliable for applications. In this work, we have developed self-supported large scale Ag/Au bimetallic SERS-active substrate with a high density of nanoparticles and uniform hot spots. The resultant substrates are very stable under ambient conditions, providing unchanging Raman enhancement signals even after one year of fabrication, due to the protective Au shell on the bimetallic nanoparticles. The Ag/Au bimetallic substrate exhibits remarkable SERS enhancement for nonresonant molecules, permitting the detection of trace concentrations reaching 10−13 mol/ L.

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INTRODUCTION Noble metal nanoparticles (NPs) have drawn attention in the past decades due to their high potential optical, 1−4 electronic,5−7 catalytic,8−13 and magnetic properties.14,15 More recently, bimetallic NPs have received considerable attention due to their unique properties that are superior to those of monometallic NPs.16−18 The properties of bimetallic NPs, integrated with the synergistic effects of monometallic NPs, broaden the application fields for this type of nanomaterial. As an example, bimetallic NPs often have shown superior performances to their monometallic counterparts in the catalytic19−22 and photovoltaic domains.23 Silver and gold noble metal NPs have imposed themselves in surface-enhanced Raman spectroscopy (SERS),24−32 which was discovered in 1974. It is widely known that Ag NPs provide larger enhancement than Au NPs in Raman intensities due to their sharper plasmonic peaks and stronger interparticle near-field coupling effects. Unfortunately, Ag NPs are not as stable as Au NPs in biological environments due to the fast sulfuration and surface oxidation of Ag.33,34 Therefore, an approach where both metals are combined in Ag/Au bimetallic nanostructures has © XXXX American Chemical Society

appeared to be a solution in order to sum the merits of both Ag optical enhancing properties and Au chemical surface properties. In recent years, Ag/Au bimetallic NPs have been fabricated by several synthesis procedures. Colloidal fabrication under the galvanic replacement principle is the most popular method used due to its simplicity, adaptability, and low cost.9,35−37 Diverse types of Ag/Au bimetallic NPs were produced using this strategy like Au@Ag core−shell nanocubes,38 Ag/Au bimetallic nanorods,39 and Ag−Au nanowires.40 However, colloidal NPs suffer from lack of homogeneity and reproducibility, resulting in an important drawback for SERS, since they form aggregates when deposited on substrates. This complication can be avoided by functionalization of the nanoparticles, but this process will affect the SERS properties of the NPs, removing the first-layer enhancement effect and thus diminishing the enhancement due to an increased distance between the particle Received: May 22, 2015 Revised: October 26, 2015

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DOI: 10.1021/acs.jpcc.5b04914 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Additionally, thermal annealing of the last layer will increase the interparticle separation distance between the nanoparticles, thus reducing their SERS activity. In detail, commercial glass slides (20 mm × 20 mm)2 were purchased from Carl Roth. Prior to metal evaporation, the glass substrates were washed with mixture of detergent (Decon 90) and ddH2O (1:9 volume ratio) in an ultrasonic water bath at 50 °C for 20 min and then dried within a N2 stream. The metal evaporation is conducted in a vacuum evaporator (Plassys MEB400, France) using the thermal effect mode with an evaporation rate of 0.03 nm/s monitored using a built-in quartz crystal sensor. The working pressure was 1.0 × 10−6 Torr. Extinction Spectra. A home-built confocal extinction measurement system was used to record the LSPR spectra in the UV−vis−NIR range from 200 to 1100 nm. The optical setup contained a white light source, two optical fibers (one for illumination and other for collection), and an adjustable sample holder. It should be noted that due to the optical fibers and lens combinations used in the confocal setup, the incident beam with a size of 10 μm was focused at normal incidence onto the substrates during all the measurements. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed at room temperature on an Axis Nova instrument from Kratos Analytical spectrometer with a monochromatic Al Kα line (1486.6 eV) source operated at 300 W. Survey spectra were acquired at pass energies of 80 eV. The core level spectra (C 1s, Au 4f, and Ag 3d) were acquired with an energy step of 0.1 eV and using a constant pass energy mode of 20 eV to obtain data in a reasonable experimental time (energy resolution of 0.48 eV). XPS data were acquired for a minimum of three areas per sample. Data analysis was performed using CasaXPS software. SERS Experiments. A Dilor Jobin-Yvon Spex instrument from Horiba equipped with a 632.8 nm laser, a CCD camera, and a 10× (NA: 0.3) objective in a reflection configuration was used. All shown SERS spectra were acquired independently, at full laser power (2 mW), and employing 10 s of acquisition time. For the SERS measurements a 20 μL drop was deposited on the substrate, and the spectra acquisitions were rapidly conducted before the drop was dried. It should be noted that the drying operation could cause a nonuniform stacking of the analytes over the substrate surface, resulting undesirably in preferential coating on the SERS hot spot regions consequently hindering a correct characterization of the substrate.42,43 In addition, the substrate needs to be validated in solution in order to claim that a SERS substrate is useful as a molecular sensor. Therefore, all our SERS measurements were made in the wet environment after depositing the 20 μL drop by exactly 1 min to avoid the solvent evaporation effect. Scanning Electron Microscopy. the SEM images were obtained by Schottky Jeol 6500F scanning electron microscope, with 1.5 nm resolution at 15 kV, secondary electron image, and accelerating voltage of 0.5 kV to 30. High-Resolution Transmission Electron Microscope. HRTEM images were obtained on a Hitachi H9000 NAR (electron beam emitted from LaB6 filament), operating at an accelerating voltage of 300 kV and direct magnification of 200 000 from 60 K to 300 K.

and the analyte. Another type of method for fabricating the Ag/ Au bimetallic NPs is the top-down lithography technique. Au/ Ag alloy nanostructures fabricated by electron-beam lithography have been previously reported.41 However, it is an expensive, complex, time-consuming, and microscale manufacturing technique. If one considers the ultimate goal of SERS to be used as a routine application for identifying traces of molecules, an ideal SERS substrate would thus combine stability, homogeneity, and reproducibility while keeping a high enhancement to reach a high sensitivity. To reach this goal, we have followed the thermally annealed thin metallic film strategy to fabricate the desired substrate, which is a relatively simple way of fabrication. Despite the higher cost for the thermal annealing strategy compared to colloidal nanoparticles, however, this technique produces large-scale substrates which is a key feature for application-oriented SERS substrate. It is known that bimetallic NPs properties are directly related to their compositions and geometric distributions. Considering that we chose thermal annealing of thin metallic film as a method for bimetallic NPs fabrication, we thus needed at least two evaporation layers. Searching for the appropriate Ag/Au bimetallic NPs as reliable SERS substrates, we have fabricated these bimetallic NPs with double and triple and quadruple layers. We have found that bimetallic NPs made of triple layers provide better SERS enhancement due to their smaller interparticles distances compared to two-layer substrates. Substrates made of quadruple-layer NPs were discarded because the percolation threshold was reached. In this work we reveal key relationships governing the LSPR spectral location of the bimetallic NPs and its SERS sensitivity. Additionally, we highlight the highly Raman signal enhancing triple-layer Ag/Au bimetallic NPs, namely Au−Ag/Au core/ shell NPs, where the core is a mixture of Au and Ag covered with a pure Au shell to provide stability. This stability was tested by comparing the optical properties of these NPs to chemically unprotected Au/Ag core/shell NPs. The synthesized Ag/Au bimetallic NPs substrates were characterized using the scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM), and extinction spectroscopy to determine the localized surface plasmon resonance (LSPR), in addition to X-ray photoelectron spectroscopy (XPS). Finally, the performances of such new structures have been tested and compared by SERS experiments as well as the stability of their Raman responses as a function of time.

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EXPERIMENTAL PART Bimetallic Nanoparticles Fabrication. The fabrication of the substrates started by evaporating a thin metallic layer to form metallic islands followed by thermal annealing (T) which leads to the formation of metallic NPs. The Au NPs were simply fabricated by evaporating 2 nm gold film followed by thermal annealing at 250 °C for 20 s. The double-layered Ag/ Au bimetallics were fabricated by evaporating 2 nm of the first metallic layer followed by thermal annealing at 250 °C for 20 s using a hot plate. Second, 2 nm film of the second metallic layer was evaporated. The triple-layered Ag/Au bimetallic nanoparticles were prepared similarly to the double-layer NPs; however, three metallic evaporations were conducted where each two evaporations were separated by thermal annealing at the same parameters. For the Au−Ag/Au core/shell NPs, it is critical not to perform thermal annealing for the last gold layer to avoid thermal diffusion from the protective gold shell.

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DISCUSSION The SEM images of Au NPs (single-layer NPs) and Au/Ag core/shell NPs (double-layer bimetallic NPs) are illustrated in Figures 1A and 1B, respectively. The NPs size distribution of B


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Figure 1. SEM images of the (A) monolayer nanoparticles and (B) Ag/Au bilayer bimetallic nanoparticles; size distribution corresponding to the SEM images for the (C) monolayer nanoparticles and (D) Ag/Au bilayer bimetallic nanoparticles.

the single- and double-layer NPs are obtained using the public software ImageJ developed at the National Institutes of Health, having average sizes of 11 and 16 nm (Figures 1C and 1D, respectively). XPS has been used to verify the composition of the double-layered bimetallic nanoparticles, shown in Figure S1. Concerning the calibration, the binding energy of the C 1s hydrocarbons peak was set at 284.8 eV (Figure S2). We focus on the Ag 3d and Au 4f pairs for both Au-T-Ag and Ag-T-Au nanoparticles. The Ag 3d pair of peaks Ag 3d/2 and Ag 3d5/2 are obtained at 374 and 368 eV, respectively. The Au 4f pair of peaks Au 4f3/2 and Au 4f7/2 are obtained at 87.9 and 84.2 eV, respectively. In order to ensure the evaporated metal thickness on each substrate, we calculate the percentage of gold and silver metals for the Au-T-Ag and Ag-T-Au substrates. The percentages are calculated by integrating the area under the Au 4f peak and Ag 3d peak. Table 1 illustrates the full width at half-maximum and the area under each peak for the Au-T-Ag and Ag-T-Au samples with the calculated percentage of each metal. Au-T-Ag nanoparticles were fabricated by evaporating 2 nm gold followed by 2 nm silver separated by thermal annealing. The estimated metallic percentage would be 50% gold and 50% silver. The calculated percentage based on XPS measurements are 57.8% silver and 42.2% gold. Ag-T-Au nanoparticles are fabricated by the successive evaporation of 2 nm silver and gold metals; the calculated metallic percentages based on XPS peaks are 43.9% for silver and 56.1% for gold.

Table 1. Ag 3d and Au 4f Peak Positions, FWHM, Area, and the Percentage of Au and Ag in the Au/Ag Double-Layer Bimetallic Nanoparticles

Au-T-Ag NPs Ag-T-Au NPs

name

position (nm)

FWHMa (nm)

area

at. %b

Ag 3d Au 4f Ag 3d Au 4f

368.2 84.2 368.2 84.2

2.435 2.556 2.492 2.584

58248.0 44299.6 85011.2 113470.0

57.8 42.2 43.9 56.1

a

FWHM is the full width at half-maximum. bAt. % is the metallic percentage of Au and Ag.

The difference obtained between the estimated and the calculated percentage arises from the fact that XPS is a surface effective technique. Since the last evaporated metal is silver in Au-T-Ag NPs and gold in Ag-T-Au NPs, this explains the higher silver percentage for Au-T-Ag NPs and higher gold percentage for Ag-T-Au NPs. After verifying the metallic composition of the double-layer bimetallic nanoparticles, the LSPR for these NPs in addition to the monometallic doublelayer NPs were measured (Figure 2A). Starting by doubled monometallic substrates, the Ag-T-Ag nanoparticles have a localized surface plasmon resonance at 469 nm in addition to a broad secondary resonance at 625 nm. The Au-T-Au nanoparticles have an LSPR at 628 nm. As for double-layer C


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Figure 2. (A) Localized surface plasmonic resonance of the double-layer nanoparticles, (B) SERS response of the BPE molecule of concentration 10−5 M over the double-layered nanoparticles, and (C) the relation between the absolute value of the difference between the Raman excitation wavelength and the LSPR resonance Δλ.

presents a blue-shift of approximately 11 cm−1 in the SERS spectrum of BPE deposited on Au NPs. These results suggest that the molecule of BPE interacts strongly with the surface of the Au NPs through the nitrogen atom corresponding to the pyridyl ring.43 The Ag-T-Ag substrate shows the lowest enhancement between the four samples, whereas Au-T-Ag and Ag-T-Au nanoparticles provide moderate enhancement. The highest SERS signal is obtained on the Au-T-Au substrate. In order to evaluate the effect of the localized surface plasmon position on the Raman enhancements, the SERS intensities of the benzene ring stretching (1605 cm−1) from BPE molecules are plotted versus Δλ (the absolute value of the difference between the Raman excitation wavelength (632.8 nm) and the LSPR resonance) for each of the four samples (Figure 2C). Clearly from the Figure 2C, we observe an inversely proportional relationship between the SERS intensity and Δλ. The substrate exhibiting the closest LSPR spectral position to the excitation wavelength provides the highest enhancement in Raman intensities, which is the case for the Au-T-Au substrate. Even though silver nanoparticles are known to provide higher enhancements than their gold counterparts,48−50 however, the Au-T-Au substrate which is constituted of pure gold only has showed the higher enhancement. This can be explained by two contributions. First, the Au-T-Au NPs are characterized with LSPR spectral position closer to the excitation wavelength. Second, the bimetallic NPs with the closest LSPR position (AuT-Ag NPs) have a broader peak than the Au-T-Au NPs, which decreases its enhancing ability. Searching for more tunable

bimetallic nanoparticles, Au-T-Ag and Ag-T-Au substrates have LSPR spectral positions at 599 and 710 nm, respectively. For the double-evaporation system, the particles deposited from the first evaporation followed by thermal annealing can be referred to as the as-synthesized core particles. The metallic vapor has a higher tendency to bound to the metal than to the substrate;44 this leads to the formation of a shell over the core particles. For core/shell nanoparticles with shell thickness less than 4 nm, the plasmonic influence of the core material increases with decreasing the shell thickness. 45−47 Knowing that the evaporated shell thickness in our substrates is 2 nm, this means that the shell thickness is small enough to allow the plasmonic influence of the core material to take place. Figure 2B represents the SERS measurements for the 1,2-bis(4pyridyl)ethylene (BPE) solution of 10−5 M concentration as a nonresonant probe molecule over the four prepared doublelayered monometallic and bimetallic nanoparticle. The SERS spectra were baseline corrected using an automated algorithm in the Raman spectroscopy software, where an example on the raw spectra is given in Figure S3. The Raman spectrum for 0.1 M BPE in ethanol−water 1:2 v/v is compared to the SERS spectrum for 10−5 M BPE solution over Au-T-Au NPs (Figure S6). The peak at 994 cm−1 corresponds to the ring breathing mode of BPE pyridine. A blue-shift of 21 cm−1 was observed in the SERS spectra of BPE, which includes the vibrational movement of the pyridyl nitrogen atom. Similarly, the vibrational signature observed at 1596 cm−1 corresponds to the C−N stretching mode of the pyridyl ring. This band D


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Figure 3. SEM image (A) and the size distribution correspondent to the SEM image (B) of the Ag/Au triple-layer bimetallic nanoparticles.

in LSPR is obtained. Similarly, Au-T-Ag-T-Ag NPs and Au-TAg-T-Au NPs exhibit metallic ratios of 2/3Ag:1/3Au and 1/ 3Ag:2/3Au, respectively. The red-shift observed for the Au-TAg-T-Au NPs at LSPR of 599 nm compared to Au-T-Ag-T-Ag NPs at LSPR of 533 nm could be attributed to the increase in its gold content. Ag-T-Au-T-Ag and Au-T-Ag-T-Ag have the same metallic ratio of 2/3Ag and 1/3Au, but with different evaporation order. The Ag-T-Au-T-Ag has a LSPR position at 504 nm while that of Au-T-Ag-T-Ag NPs is at 533 nm. This red-shift observed for the Au-T-Ag-T-Ag reveals a higher effect of the first layer than the second layer on the LSPR peak position. In other words, the presence of gold in the first layer affects the plasmonic resonance more than its presence in the second layer. Similarly, by comparing Ag-T-Au-T-Au and Au-TAg-T-Au NPs having LSPR positions at 560 and 599 nm, a higher effect is observed when silver is present in the first layer than in the second layer (blue-shift). More importantly, the stronger effect of the third evaporated layer can be revealed after comparing Au-T-Ag-T-Ag NPs with Au-T-Ag-T-Au NPs or Ag-T-Au-T-Ag NPs with Ag-T-Au-T-Au NPs, having LSPR differences of 66 and 56 nm, respectively. Thus, the layer composition effect on the LSPR spectral position follows this ascending order: middle, first, and then the third layer. Surface plasmons only occur at the interface between a metal and a dielectric. An existence condition for surface plasmons is that the two adjacent media forming the interface must have opposite sign permittivities. Thus, no surface plasmon can exist at the interface between two metals. We thus believe that the resonance observed for three-layer particles is mainly controlled by the coupling between the plasmon in the upper air−metal interface and the plasmon in the lower metal−glass interface, the intermediate layer playing a less important role in the position of the peak. SERS measurements have been carried out on the four triple-layered bimetallic substrates to verify if the triple-layered system succeeded in providing bimetallic substrate containing silver and having LSPR position in the range of the excitation wavelength for high enhancement ability in SERS (Figure 4B). The Au-T-Ag-T-Ag substrate shows the lowest SERS signal, whereas Ag-T-Au-T-Ag provides slightly higher enhancement signal. Ag-T-Au-T-Au nanoparticles have moderate signal; meanwhile, the highest SERS signal is obtained over the Au-T-Ag-T-Au substrate. Similarly to double-layered system, an inversely proportional relation

bimetallic nanoparticles which can provide Ag/Au bimetallic NPs having LSPR in resonance with the excitation wavelength, triple-layer bimetallic NPs were fabricated. The triple-layer bimetallic NPs with average size of 22 nm in diameter and 49% of surface coverage (Figure 3) exhibited more uniform size and shape distribution when compared to the single- and double-layer NPs having respectively 11, 16 nm and 29, 41% as average size in diameter and surface coverage in percentage (Table S1). Similarly to the double-layered bimetallic nanoparticles, the XPS technique was used to ensure the composition of the three-layered bimetallic nanoparticles, shown in Figures S5 and S6. Table 2 contains the full width at Table 2. Ag 3d and Au 4f Peaks Positions, FWHM, Area, and the Percentage of Au and Ag in the Au/Ag Triple-Layer Bimetallic Nanoparticles nanoparticles

name

position

FWHMa

area

at. %b

Au-T-Ag-T-Ag

Ag 3d Au 4f Ag 3d Au 4f Ag 3d Au 4f Ag 3d Au 4f

368.8 84.2 368.2 84.2 368.2 84.2 368.2 84.2

2.522 2.640 2.606 3.013 2.674 2.673 2.522 2.687

168614 15797 134050 47851 2820 21090 27754 92522

91.7 8.3 74.5 25.5 11.8 88.2 23.8 76.2

Ag-T-Au-T-Ag Ag-T-Au-T-Au Au-T-Ag-T-Au a

FWHM is the full width at half-maximum. bAt. % is the metallic percentage of Au and Ag.

half-maximum and the area under each peak for the Au-T-AgT-Ag, Ag-T-Au-T-Ag, Ag-T-Au-T-Au, and Au-T-Ag-T-Au substrates with the relative atomic percentage of each metal. Again, the difference between the estimated and the measured percentage is related to the surface effective property of the the XPS technique. Figure 4A shows the LSPR spectral positions for each of the four triple-layer bimetallic substrates. For studying the effect of the evaporated percentage of each metal on the LSPR position, we compare the substrates having different metallic percentages of gold and silver. For Ag-T-AuT-Ag and Ag-T-Au-T-Au substrates, they have metallic ratios of 2/3Ag:1/3Au and 1/3Ag:2/3Au, respectively, with an LSPR position at 504 nm for the Ag-T-Au-T-Ag nanoparticles and 560 nm for Ag-T-Au-T-Au nanoparticles. This shows that with increasing gold percentage within the nanoparticle, a red-shift E


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Figure 4. (A) Localized surface plasmonic resonance of the triple-layer bimetallic nanoparticles, (B) SERS response of the BPE molecule of concentration 10−5 M over the triple-layered nanoparticles, and (C) the relation between the absolute value of the difference between the Raman excitation wavelength and the LSPR resonance Δλ.

between the SERS intensity and Δλ was obtained also for the three-layered system (Figure 4C). The Au-T-Ag-T-Au system exhibits the smallest Δλ with an LSPR spectral position at 599 nm providing the highest SERS signal. To analyze the enhancing ability of the Au-T-Ag-T-Au NPs, we compare the Au-T-Ag-T-Au bimetallic NPs with pure triple monometallic silver NPs Ag-T-Ag-T-Ag and triple monometallic gold nanoparticles Au-T-Au-T-Au (Figure S7). By comparing the Au-T-Ag-T-Au NPs with the Au-T-Au-T-Au NPs, we can observe a slight blue-shift of 6 nm for the Au-T-Ag-T-Au NPs, which confirms the hypothesis we mentioned earlier that the second layer have the lowest effect on the LSPR position. Therefore, by substituting the second evaporated metal in AuT-Au-T-Au by silver nearly did not largely affect the LSPR position. However, higher enhancement is obtained over the Au-T-Ag-T-Au in comparison with the Au-T-Au-T-Au substrate (Figure S8). This higher enhancement can be attributed to the presence of silver in system having LSPR spectral position in resonance with the Raman excitation wavelength. The size distribution of the nanoparticles in the two substrates based on their SEM images shows an average diameter of 19 and 22 nm for the Au-T-Au-T-Au (Figure S9) and Au-T-Ag-T-Au substrates, respectively. The two NPs types are comparable in size; meanwhile, the Au-T-Ag-T-Au NPs provide much higher SERS enhancement in comparison to the Au-T-Au-T-Au NPs. Second, comparing the Au-T-Ag-T-Au NPs with the Ag-T-AgT-Ag NPs confirms further the effect of LSPR position on the SERS enhancement ability, where the pure silver system has a

lower SERS signal than Au-T-Ag-T-Au substrate (Figure S8) because its spectral position lies at 509 nm which is far from Raman excitation wavelength (Figure S7). The highest enhancement obtained over the Au−Ag/Au NPs (Au-T-AgT-Au) is hence related to the presence of silver in a bimetallic structure having LSPR position in resonance with the Raman excitation wavelength. Knowing that the highest SERS enhancement has been obtained over the Au-T-Ag-T-Au substrate, we aimed to study this substrate in more detail, willing to understand the reason for such an enhancement. The HRTEM image in Figure 5A shows a single 15.6 nm/2.3 nm Au-T-Ag-T-Au NP. The core size of the NP is in good harmony to the average size of double-layer NPs. Furthermore, the shell is 2.3 nm thick, which corresponds to the final evaporated thickness of gold layer. As shown in the HRTEM image, the Au−Ag core is a bimetallic structure of neither core/ shell nor alloy type. However, the structure includes mixture zones of both Au and Ag metals. The obtained metallic distribution in the core based on the HRTEM image (using ImageJ) is in good agreement with the expected one, as the obtained Au and Ag percentages are 48.8% and 51.2%, respectively. Consequently, the HRTEM image confirms that the core is composed of 2 nm gold and 2 nm silver (see Figure S10 and Table S2). Figure 5B shows the LSPR of the Au−Ag/Au core/shell NPs in comparison to the LSPR to the as-synthesized Au NPs and Au−Ag NPs. The LSPR peak wavelengths are respectively at F


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Figure 5. (A) HRTEM image of the Au−Ag/Au core/shell nanoparticles. (B) Extinction spectra of the Au NPs, Au−Ag NPs, and Au−Ag/Au NPs. (C) SERS response of BPE on the Au−Ag/Au NPs and Au−Ag NPs freshly prepared and after 1 year from preparation.

the decrease in broadening for the Au−Ag/Au NPs compared to the Au−Ag NPs. As mentioned before, the aim of our work is to fabricate SERS substrate characterized with high enhancement factor associated with high stability under physiological conditions. Thus, a SERS study was made to evaluate the efficiency of the Au−Ag/Au NPs substrate in comparison to the unprotected Au−Ag NPs substrate by Au shell. Figure 5C shows a strong SERS signal for BPE on the Au−Ag/Au NPs substrate when compared to the moderate enhancement obtained on the Au− Ag NPs substrate, whereas no SERS signal was obtained for the Au NPs. The reasons for such a high enhancement can be attributed to different key factors. Primarily, it is due to the presence of Ag in the NPs, since SERS signals are only obtained from the two substrates containing NPs with Ag. Indeed, it is well-known that Ag NPs have higher enhancement factor for Raman scattering than Au NPs.48−50 Second, related to the LSPR peak positions, Au−Ag/Au NPs and Au−Ag NPs have LSPR at 599 nm, while a LSPR at 577 nm for the Au NPs. This red-shift in LSPR for the Au−Ag/Au and Au−Ag NPs makes it closer to the Raman excitation wavelength which is 632.8 nm,

577 nm for Au NPs and at 599 nm for both Au−Ag NPs and Au−Ag/Au core/shell NPs. We first noticed a red-shift in the LSPR peak position for the Au−Ag NPs compared to the Au NPs. This red-shift can be attributed to the increase in size of the Au−Ag nanoparticles. However, the presence of silver should reversely affect the LSPR peak position by blue-shifting it. A third effect affecting the LSPR shift is the decrease in the interparticle distance due to an increased density of the Au−Ag nanoparticles. This interparticle reduced distance increases near-field coupling between particles thus inducing a red-shift. The global red-shift implies that increased size of NPs and decreased interparticle distance preside over the presence of silver. For Au−Ag/Au NPs compared to Au−Ag we should observe a red-shift induced by simultaneous increased size, increased gold composition, and decreased interparticles distances. As indeed we do not observe any shift we argue that a compensating blue-shift effect must be present. On the SEM images the Au−Ag/Au NPs show a better sphericity, removing the existence of two distinct LSPR modes, thus blueshifting the extinction peak. The better sphericity also explains G


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Figure 6. (A) SERS spectra for BPE solutions of different concentrations ranging from 10−5 to 10−13 M over Au−Ag/Au core/shell NPs substrate. (B) SERS intensity of the 1605 cm−1 band as a function of the BPE concentration obtained over Au−Ag/Au core/shell NPs substrate; the error bars represent the standard deviation of the average Raman intensity obtained for ten SERS measurements.

above results, thermal annealing of thin film strategy provides us with highly reproducible substrates for SERS measurements. In order to evaluate the sensitivity of the Au−Ag/Au core/ shell bimetallic NPs for low concentration detection, the SERS responses for BPE solutions at different concentrations ranging from 10−13 to 10−5 M are recorded. Figure 6A shows that the SERS signal decreases in intensity with decreasing the BPE concentration, where the lowest concentration of BPE that can be detected over the Au−Ag/Au core/shell NPs is 10−13 M. Thus, we can estimate our detection threshold to a concentration equal to 10−13 M of BPE solution. For a further investigation of the quantitative analytical detection potentials for the Au−Ag/Au core/shell NPs, the evolution of the SERS intensity for the 1605 cm−1 band is plotted versus the BPE concentration (Figure 6B). The error bars represent the standard deviation of the average Raman intensity obtained on ten SERS measurements. From the linear fit, it can be observed that the Au−Ag/Au core/shell NPs substrate exhibits a linear detection range from 10−5 to 10−13 M with a correlation coefficient R2 = 0.9602. Thus, Au−Ag/Au core/shell bimetallic NPs have considerable potential as quantitative SERS sensors. We now consider the enhancement factor (EF) for the Au− Ag/Au core/shell NPs. The enhancement factor (EF) of the Au−Ag/Au core/shell NPs was calculated by comparing the SERS signal obtained for the 10−5 M solution and the Raman signal obtained for the 0.1 M solution. The intensity of the 1605 and 1636 cm−1 bands have been estimated together since they are close to each other and thus are easier to be treated as a single band. The enhancement factor per molecule is defined as EF = (ISERS × NVol)/(IRS × NSurf),51 where ISERS is the integrated intensity of the BPE band under consideration recorded for the 10−5 M solution over the Au−Ag/Au core/ shell NPs and IRS the integrated intensity of the same Raman band obtained for 0.1 M solution over reference glass substrate (Figure S14); NSurf is the number of molecules constituting the first 10 nm under the laser spot area, and NVol is the number of molecules excited within the volume of the laser waist for the 0.1 M BPE solution. Using the ×10 microscope objective, the area of the laser spot is about 7 μm2. Thus, the value for NSurf is estimated from the approximative cylinder volume of the laser waist with 10 nm height which is the working distance of the SERS effect, thus leading to NSurf of about 4 × 102 molecules. The value of NVol is estimated from the volume of the laser waist, which is assumed to be that of a cone of apex angle given

thus inducing a better matching of the LSPR with the Raman excitation wavelength. When now we compare the SERS signals for the two bimetallic substrates, as mentioned before the Au− Ag/Au NPs have lower interparticle distance. It leads to a stronger near-field coupling effect between the neighboring NPs creating more efficient hotspots on this substrate in comparison to the Au−Ag NPs, thus creating a stronger SERS signal. To evaluate the stability of the substrates, we restudied the SERS capacities on the three substrates one year after fabrication and after the first SERS results, using the same probe molecule and following the same experimental procedure. Figure 5C shows nearly the same SERS signal for the Au−Ag/Au NPs after one year whereas for Au−Ag NPs SERS signal appears to fade almost totally. The reason for such results can be related to the sulfuration of silver in the case of Au−Ag NPs, while this problem is solved in the Au−Ag/Au NPs since silver is encapsulated and protected by the Au shell. For monitoring the SERS signal homogeneity all over the substrate, SERS measurements were made for 10 different drops (5 μL) of BPE solution (10−5 M) placed on 10 different positions, as shown in Figure S11. The 10 different drops were distributed all over the substrate to study the homogeneity of the hole substrate area (2 cm × 2 cm). The SERS measurement on each drop was made separately and after exactly 1 min after drop deposition to iliminate the effect of solvent evaporation on the SERS measurements. By comparing the SERS intensities of the benzene ring stretching (1605 cm−1), one can calculate the relative standard deviation of the measurements which is 2% for Au−Ag/Au core/shell NPs (Table S3). To study the reproducibility of the Au−Ag/Au core/shell NPs, we compared five different substrates fabricated separately. Figure S12 shows the LSPR spectra for the five substrates. Table S4 illustrates the Gaussian peak fit for the LSPR spectra, in addition to the relative standard deviation (RSD) of the LSPR position and width. The obtained RSD are 1.3% and 6.2% for the LSPR peak and width, respectively. The SERS signals obtained over the five substrates were compared to follow the enhancing reproducibility of the Au−Ag/Au core/shell NPs. The SERS signals of the BPE solution (10−5 M) with acquisition time of 5 s are illustrated Figure S13. Table S5 shows the SERS intensity for the benzene ring stretching (1605 cm−1) in addition to the calculated relative standard deviation which is 8%. Based on the H


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(3) Rycenga, M.; Hou, K. K.; Cobley, C. M.; Schwartz, A. G.; Camargo, P. H. C.; Xia, Y. Probing the Surface-Enhanced Raman Scattering Properties of Au-Ag Nanocages at Two Different Excitation Wavelengths. Phys. Chem. Chem. Phys. 2009, 11, 5903−5908. (4) Pellegrini, G.; Bello, V.; Mattei, G.; Mazzoldi, P. Local-Field Enhancement and Plasmon Tuning in Bimetallic Nanoplanets. Opt. Express 2007, 15, 10097−10102. (5) Henglein, A. Physicochemical Properties of Small Metal Particles in Solution: “Microelectrode” Reactions, Chemisorption, Composite Metal Particles, and the Atom-to-Metal Transition. J. Phys. Chem. 1993, 97, 5457−5471. (6) Henglein, A. Small-Particle Research: Physicochemical Properties of Extremely Small Colloidal Metal and Semiconductor Particles. Chem. Rev. 1989, 89, 1861−1873. (7) Alivisatos, A. P. Perspectives on the Physical Chemistry of Semiconductor Nanocrystals. J. Phys. Chem. 1996, 100, 13226−13239. (8) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Marković, N. M. Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability. Science 2007, 315, 493−497. (9) Zhang, Q. Z.; Lee, J. Y.; Yang, J.; Boothroyd, C.; Zhang, J. Size and Composition Tunable Ag−Au Alloy Nanoparticles by Replacement Reactions. Nanotechnology 2007, 18, 245605. (10) Maksimuk, S.; Yang, S.; Peng, Z.; Yang, H. Synthesis and Characterization of Ordered Intermetallic PtPb Nanorods. J. Am. Chem. Soc. 2007, 129, 8684−8685. (11) Yang, S.; Peng, Z.; Yang, H. Platinum Lead Nanostructures: Formation, Phase Behavior, and Electrocatalytic Properties. Adv. Funct. Mater. 2008, 18, 2745−2753. (12) Yen, C.-W.; Lin, M.-L.; Wang, A.; Chen, S.-A.; Chen, J.-M.; Mou, C.-Y. CO Oxidation Catalyzed by Au−Ag Bimetallic Nanoparticles Supported in Mesoporous Silica. J. Phys. Chem. C 2009, 113, 17831−17839. (13) Liu, J.-H.; Wang, A.-Q.; Chi, Y.-S.; Lin, H.-P.; Mou, C.-Y. Synergistic Effect in an Au−Ag Alloy Nanocatalyst: CO Oxidation. J. Phys. Chem. B 2005, 109, 40−43. (14) Wang, C.; Daimon, H.; Onodera, T.; Koda, T.; Sun, S. A General Approach to the Size- and Shape-Controlled Synthesis of Platinum Nanoparticles and Their Catalytic Reduction of Oxygen. Angew. Chem., Int. Ed. 2008, 47, 3588−3591. (15) Yano, K.; Nandwana, V.; Chaubey, G. S.; Poudyal, N.; Kang, S.; Arami, H.; Griffis, J.; Liu, J. P. Synthesis and Characterization of Magnetic FePt/Au Core/Shell Nanoparticles. J. Phys. Chem. C 2009, 113, 13088−13091. (16) Cai, S.; Wang, D.; Niu, Z.; Li, Y. Progress in Organic Reactions Catalyzed by Bimetallic Nanomaterials. Chin. J. Catal. 2013, 34, 1964− 1974. (17) Mu, R.; Fu, Q.; Xu, H.; Zhang, H.; Huang, Y.; Jiang, Z.; Zhang, S.; Tan, D.; Bao, X. Synergetic Effect of Surface and Subsurface Ni Species at Pt−Ni Bimetallic Catalysts for CO Oxidation. J. Am. Chem. Soc. 2011, 133, 1978−1986. (18) Duan, S.; Wang, R. Bimetallic Nanostructures with Magnetic and Noble Metals and Their Physicochemical Applications. Prog. Nat. Sci. 2013, 23, 113−126. (19) Wang, S.; Zhang, D.; Ma, Y.; Zhang, H.; Gao, J.; Nie, Y.; Sun, X. Aqueous Solution Synthesis of Pt−M (M = Fe, Co, Ni) Bimetallic Nanoparticles and Their Catalysis for the Hydrolytic Dehydrogenation of Ammonia Borane. ACS Appl. Mater. Interfaces 2014, 6, 12429− 12435. (20) Wang, J. L.; Ando, R. A.; Camargo, P. H. C. Investigating the Plasmon-Mediated Catalytic Activity of AgAu Nanoparticles as a Function of Composition: Are Two Metals Better than One? ACS Catal. 2014, 4, 3815−3819. (21) Zhang, J.; Teo, J.; Chen, X.; Asakura, H.; Tanaka, T.; Teramura, K.; Yan, N. A Series of NiM (M = Ru, Rh, and Pd) Bimetallic Catalysts for Effective Lignin Hydrogenolysis in Water. ACS Catal. 2014, 4, 1574−1583. (22) Cai, S.; Duan, H.; Rong, H.; Wang, D.; Li, L.; He, W.; Li, Y. Highly Active and Selective Catalysis of Bimetallic Rh3Ni1 Nano-

by the numerical aperture (NA) of the microscope objective and the height of the focusing scope; for the ×10 objective used with NA 0.3, this apex angle is 27 and the focusing scope is approximately 18 μm, thus giving the waist volume of 1600 μm3. The estimated volume of the laser waist leading to a NVol value of about 1 × 1011 molecules and assuming that all the molecules excited in this volume are equally involved in the spontaneous Raman intensity. The calculated enhancement factor is EF = (ISERS × NVol)/(IRS × NSurf) = (491642 × 1011)/ (86994 × 4 × 102) = 6 × 109. In summary, large scale and stable Au−Ag/Au core/shell bimetallic NPs with considerable potential as quantitative SERS sensor were synthesized by a simple and low cost way of fabrication. The bimetallic substrates exhibit remarkable SERS enhancement for a nonresonant molecule reaching detection threshold of concentration 10−13 mol/L. The high Raman signal enhancement obtained over these NPs is attributed to the presence of a high number of efficient hotspots. These hotspots originate in the smaller interparticle gaps as attested in SEM images. The Au−Ag/Au core/shell NPs proved their stability due to the presence of a protective Au shell, which was proved by the TEM image. The chemical compositions for the as-synthesized Au NPs, the Au−Ag bimetallic NPs, and the Au−Ag/Au core/shell NPs were confirmed using the XPS technique. The results presented here emphasize the importance of these nanostructures in the SERS domain. Additionally, the high stability, tunability, homogeneity, and reproducibility of these bimetallic nanoparticles encourage investigations in other nanostructures-based domains, photovoltaic and photocatalytic devices as examples. Employing the nanoparticles in the photovoltaic device could enhance the short circuit photocurrent and improve the power conversion efficiency. In particular, the LSPR-mediated catalytic activity of our highly stable and sensitive Au−Ag/Au bimetallic NPs will be investigated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04914. Figures S1−S15 and Tables S1−S4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.-M.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Nicolas Gautier and the microscopy center of IMN for their technical support. Mohammad Yehia Khaywah and Pierre-Michel Adam acknowledge the COST Action MP1302 NanoSpectroscopy and the NanoMat regional platform.

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REFERENCES

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Ultrastable, Uniform, Reproducible, and Highly Sensitive Bimetallic

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