Ag CoreâShell Superstructures with Tunable Surface

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Bimetallic Au/Ag Core-Shell Superstructures with Tunable Surface Plasmon Resonance in NIR and High Performance SERS Liwei Dai, Liping Song, Youju Huang, Lei Zhang, Xuefei Lu, Jiawei Zhang, and Tao Chen Langmuir, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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Bimetallic Au/Ag Core-Shell Superstructures with Tunable Surface Plasmon Resonance in NIR and High Performance SERS Liwei Dai, Liping Song, Youju Huang∗, Lei Zhang, Xuefei Lu, Jiawei Zhang, Tao Chen* Ningbo Institute of Material Technology and Engineering, Key Laboratory of Marine Materials and Related Technologies, Chinese Academy of Science, Ningbo 315201 ABSTRACT: Due to the larger surface area and the synergistic effects between two noble metals, the bimetallic superstructures exhibit enhanced distinctive optical, catalytic, photothermal performances and SERS “hotspot” effect, and thus attracted great interest in various applications. Compared with the common Pd, Pt hierarchical structures coated onto Au nanoparticles, easily synthesized via fast autocatalytic surface growth arising from intrinsic properties of Pd and Pt metals, precisely controlling the hierarchical Ag growth onto Au NPs is rarely reported. In our present study, the reducing agent, dopamine dithiocarbamate (DDTC) was covalently capped onto the first metal core (Au) to delicately control the growth model of second metal (Ag). This results in heterogeneous nucleation and growth of Ag precursor on surface of Au NRs, and further formation of corn-like bimetallic Au/Ag core-shell superstructures, which

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E-mail: [email protected].

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E-mail: [email protected].

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usually can’t be achieved from the traditional epitaxial growth. The thickness of hierarchical Ag shell was finely tuned in a size range from 8 to 22 nm by simply varying the amount ratio between Ag ions and DDTC capped on Au NRs core. The tunable Ag shell leads to anisotropic bimetallic Au/Ag core-shell superstructures, displaying two distinctive plasmonic resonances in near infrared region (NIR). Particularly, the longitudinal surface plasmon resonance exhibits a broadly tunable range from 840 to 1277 nm. Additionally, the rich hot spots from obtained Au/Ag superstructures significantly enhanced surface-enhanced Raman scattering (SERS) performance. KEYWORDS: bimetallic, Au/Ag superstrucutres, site-specific reduction, plasmonics, SERS 1. Introduction Bimetallic nanomaterials have been the subject of intensive research due to their unique catalytic,1-3 optical,4-6 magnetic,7-9 and collective electronic characteristics10-12 and various potential applications in fluorescence enhancement, catalysts, imaging, electrochemical energy, and biomedicine.13-18 Compared to monometallic nanoparticles (NPs), bimetallic nanomaterials intend to display enhanced properties and some newly generated functional performances because of the unique interparticle interactions between individual NPs and their synergistic effects. For instance, Yang and co-worker19 used Pd seeds to direct the epitaxial overgrowth of Pt, and the obtained Pd/Pt core-shell nanocubes showed the superior electrochemical catalytic activity due to the synergistic effect between Pd and Pt. Tilley and co-workers20 demonstrated the Pd/Au nanostructures can display the localized surface plasmon resonance (LSPR) absorption maximum in near infrared region (NIR) for enhanced suitability in photothermal hyperthermia therapy, due to the interaction and the electronic coupling between Au and Pd.


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Apart from “composition” dependent properties of bimetallic NPs, the atoms stacking and spatial arrangement of outer metals play crucial roles in the physicochemical performances and technological applications. In particular, outer atoms assembled three dimensional (3D) superstructures easily achieve the larger surface area, rich SERS hotspots and related enhanced properties, which were widely explored in broad areas of nanotechnologies. Recently, Li and coworkers21 reported an Au/Pd core-shell superstructure that exhibited higher catalytic properties than common individual Au and Pd due to the highly branched structures. Barheer and coworkers22 presented an Au/Pt core-shell multi-branched superstructure with both high SERS activity and catalytic performance. Zubarev and co-workers23 prepared polymer-functionalized bimetallic Au/Pt superstructure NRs that offered a unique possibility to prepare soluble Pt nanostructures capable of catalyzing reactions in organic media. Compared with the common Pd or Pt hierarchical structures coated onto Au NPs via fast autocatalytic surface growth at the beginning of the reaction,24-27 precisely controlling the other metals without capabilities for fast autocatalytic reaction to form the hierarchical structures onto Au NPs remains a great challenge. For instance, previous works28-32 reported the formation of Au/Ag core-shell nanostructures with uniform solid Ag continuous layer. However, to the best of our knowledge, precisely controlling the discontinuous Ag overgrowth onto Au NPs is rarely reported. In chemical wet synthesis of noble metal NPs, it is well known that surfactants and reducing agents are homogeneously dispersed in solution to control the nucleation and growth of NPs. This conventional method, to a certain extent, limits the precise control of the chemical kinetics. Recently, we reported33 a new approach to easily synthesize core-shell superstructures by covalently bonding surfactants onto the surfaces of Au seeds, which resulted in discontinuously exposed surfaces for Au atoms disposition. Reducing agent is another crucial factor to control the rate of atom creation and


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deposition onto the seeds. Compared with conventional homogeneous dispersion of reducing agent in reaction solution, covalently bonding reducing agent onto the core seeds would be a unique approach to achieve the Au/Ag core-shell superstructures. In this work, we bond the reducing agent (DDTC) to the first metal core (Au) by Au-S bonds, and thus to control the discontinuous growth mode of the second metal ions (Ag+), achieving Au/Ag core-shell superstructures. Systematic study indicated that the silver ions underwent a heterogeneous nucleation and growth process that completely different from traditional epitaxial growth. More importantly, the thickness of hierarchical Ag shell was finely tuned in a size range from 8 to 22 nm by simply varying the amount ratio between Ag ions and DDTC capped on Au NRs core. The tunable Ag shell leads to anisotropic bimetallic Au/Ag coreshell superstructures displaying two distinctive plasmonic resonances in near infrared region (NIR), particularly, the longitudinal surface plasmon resonance exhibits a broadly tunable range from 840 to 1277 nm. Additionally, the rich hot spots from obtained Au/Ag superstructures significantly enhanced surface-enhanced Raman scattering (SERS) performance. 2. Results and Discussion Synthesis of bimetallic Au/Ag core-shell superstructures. Au nanorods (NRs), with an average diameter of 25 nm and an average length of 85 nm (Figure 1c), were synthesized according to a classical seed-growth method.34, 35 The uniformity and yield were dramatically improved by using binary surfactant mixtures. The representative UV-Vis absorption spectra in Figure S1 showed two clear peaks around 510 nm and 840 nm, corresponding to the plasmon resononce of transverse modes and longitudinal modes, respectively. The obtained Au NRs were employed as the seeds for the synthesis of Au/Ag core-shell nanostructures. Figure 1a, b schematically illustrated two growth modes of Ag on surface of Au NRs. In a conventional


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method for Ag overgrowth on Au NR seeds (Figure 1a), both reducing agent (ascorbic acid) and Ag precursor (AgNO3) were uniformly dispersed in reaction solution, which resulted into the homogeneous reduction of silver ions. By controlling the reaction rate, the reduced Ag atoms intended to equivalently accumulate in the whole surfaces of Au NR seeds, leading to the formation of a continuous Ag layer as the shell on Au NR, which was consistent with previous reports.36, 37 TEM image (Figure 1d) showed that the transverse axis of synthetic NRs became wider than original Au NRs, due to the formation of Ag shell. In our present method (Figure 1b), a synthetic small molecule, dopamine dithiocarbamate (DDTC) containing functional catechol and thiol groups was used as a new type of reducing agent instead of widely used ascorbic acid. Thiol groups were used to strongly bond on the surfaces of Au NR seeds by Au-S bonds. Catechol groups displayed the reductive property for in situ reduction of Au ions. The surface chemistry property of Au NRs was characterized before and after DDTC adsorption. As shown in Figure S2a, the CTAC-coated Au NRs exhibited positive potential around 36 mV due to the positive electrical property of CTAC. Figure S2b showed that the Zeta potential reduced to around 4 mV because of the weak negative electrical property of DDTC, which suggested the successful adsorption of DDTC on the surface of Au NRs via the Au-S bond. Importantly, DDTC plays two decisive factors differing from ascorbic acid. Firstly, DDTC was covalently bound onto Au NR seeds, rather than freely dispersed in reaction solution as ascorbic acid without strong interaction with Au NR seeds. Secondly, DDTC was not homogeneously dispersed in solution, only anchored on the Au NR surfaces. These two distinctive features lead to a new growth model for Au/Ag core-shell superstructures. As shown in Figure 1b, only these silver ions, closely touched with DDTC can be reduced into Ag atoms. Because of the superadhesive feature of DDTC,38 the reduced Ag atoms preferentially deposited onto the Au NR


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seeds, nearby the reduction sites of DDTC. The anchored Ag atoms further acted as nucleus for the successive deposition of reduced Ag atoms, and eventually formed small nanoparticles onto the surfaces of Au NR seeds, resulting in the ultimate formation of Au/Ag core-shell superstructures. TEM image (Figure 1e) clearly exhibited the nanostructure. It was worth noting that the shell was discontinuous and contained a lot of small Ag particles, rather than the conventional continuous Ag shell, suggesting the inhomogeneous deposition of Ag atoms at the specific sites of the Au NR surfaces. The discontinuous Ag shell coated onto Au NRs makes them like corn in shape. While, in the solution, there was not any new particles appeared, due to the absence of reducing agents. This would significantly improve the uniformity and yield of Au/Ag core-shell superstructures.


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Figure 1. Schematic illustration of two growth modes of Ag using AA (a) and DDTC (b) as reducing agents. The corresponding TEM images of original Au NRs (c), Au/Ag core-shell NRs with continuous Ag shell (d), and Au/Ag core-shell with discontinuous Ag shell (e). The scale bar represents 100 nm. Figure 2a showed the low-magnification TEM image of bimetallic Au/Ag core-shell superstructures, indicating almost all of particles were corn-like in shape, and the number percentage approached 100% without purification. The corresponding SEM image (Figure 2b) clearly showed the 3D morphology of corn-like NRs. The novel bimetallic superstructures in the blue dashed outline of Figure 3a were characterized using the HAADF-STEM elemental analysis. Figure 3b-c exhibited the clear color contrast of Au (green) and Ag (red), confirming


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the successful formation of the core-shell structure. The tips composed of numerous Ag NPs could also be easily resolved under HAADF-STEM due to the difference in bright and dark contrast between Au and Ag (Figure 3d).

Figure 2. Low magnification TEM (a) and SEM (b) images of bimetallic Au/Ag core-shell superstructures prepared by using 60 µL of 0.01 M AgNO3. The scale bar represents 100 nm.

Figure 3. TEM (a) and STEM (d) images of bimetallic Au/Ag core-shell superstructures, corresponding HAADF-STEM elemental mapping images: the green (b) and red (c) colors stand for gold and silver, respectively. The scale bar represents 100 nm. Tunning the thickness of corn-like NRs. The discontinuous Ag shell of superstructures could be easily tuned under identical experimental conditions (i.e., seed, temperature, reducing


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agent, and reagent concentrations) except for varying the amount of Ag precursor (AgNO3). Figure 4a-f showed TEM images of the bimetallic Au/Ag core-shell superstructures, synthesized in the presence of 10, 20, 40, 60, 80 and 100 µL of 0.01 M AgNO3 solution, respectively. The shapes of nanorods gradually changed from cylindrical to corn-like as the increase of AgNO3. As shown in Figure 4a, when 10 µL of 0.01 M AgNO3 was added into the growth solution, only part of the gold surface was covered with Ag NPs. As the amount of AgNO3 was increased from 10 µL to 100 µL, all of Au NRs were completely encased in Ag shell due to the nucleation and growth of Ag on the surfaces of the Au NRs. The average thickness of discontinuous Ag shell increased from 8 nm to 22 nm by controlling the amount of AgNO3 solution. However, much bigger amount of AgNO3 solution would not continuously increase the thickness of shell (Figure S3A-B), indicating that there was a cut-off value of AgNO3 amount for the reaction with the reducing agent (DDTC) adsorbed on Au NRs. Figure S3C showed the trend of the thickness with the increase of the Ag+ concentration, and exhibited that the maximum thickness of the shell was around 22 nm. The corresponding SEM images of superstructures with different thicknesses revealed the evolution of the morphology and also confirmed the uniformity and high yield of Au-Ag core-shell superstructures (Figure S4a-f).


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Figure 4. Representative TEM images of bimetallic Au/Ag core-shell superstructures prepared by using (a) 10, (b) 20, (c) 40, (d) 60, (e) 80, (f) 100 µL of 0.01 M AgNO3, respectively. The scale bars represent 100 nm. It is well known that Au and Ag nanoparticles exhibit LSPR in the visible region, and the desired design of novel structures for extension of plasmon resonances in NIR would expand their practical applications in bio-systems, as NIR can avoid or significantly reduce potential damage to biological tissues and cells.39 The LSPR properties of the Au/Ag core-shell superstructures were investigated via the variation of the shell thickness owing to the geometrydependent plasmonic characteristics. Each spectrum of superstructures exhibited two obvious absorption peaks, suggesting the anisotropic characteristics (Figure 5A). However, it was surprising to find that the longitudinal LSPR peak of superstructures remarkably red-shifted to the near infrared region (NIR) instead of blue-shifting as the increase of the Ag thickness in previous reports.28, 40, 41 As shown in Figure 5B, the longitudinal LSPR peaks were red-shifted from 1041 to 1277 nm in NIR when the thickness of discontinuous Ag shell was increased from


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8 to 22 nm, which could be attributed to the plasmonic percolation phenomenon.42,

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plasmon of the nanostructure will undergo a red-shift procedure when the volume fraction of the discontinuous Ag shells increases, resulting in the transition in the real part of dielectric function. More importantly, the plasmonic percolation behavior was found to be independent of the metal type in the discontinuous shell.44 In addition to the change of the plasmon, the color of the aqueous suspension also changed from red to dark-brown when the thickness of Ag shell was increased from 8 to 22 nm (inset in Figure 5 A).

Figure 5. Representative UV-vis-NIR absorption spectra of bimetallic Au/Ag core-shell superstructures (A) with different discontinuous Ag shell thickness: (a) 8, (b) 12, (c) 15, (d) 18, (e) 20, (f) 22 nm and corresponding inset photographs of solution color. (B) The plot of longitudinal absorption peak intensity versus thickness of Ag shell. High performance SERS of bimetallic Au/Ag superstructures. The synergistic effects between two noble metals and rich “hotspots” from the superstructure of Ag shell make Au/Ag superstructures highly attractive for SERS detection.45 Rhodamine 6G (R6G) was used as the analyte molecule in the SERS studies. As a demonstration, we compared the Raman spectra of three representative samples, monometallic Au NRs with aspect ratio 3.4 and bimetallic Au/Ag core-shell NRs with smooth and rough shells around 8 nm, respectively. Figure 6 revealed several well-resolved Raman signals of active substrates at 612 cm-1 (C-C-C in-plane bending), 773 cm-1 (C-H out-of-plane bending), 1181 cm-1 (C-H in-plane bending) and 1311 cm-1 (C-O-C


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stretching), and C-C stretching modes for aromatic rings exhibited peaks at 1361 cm-1, 1508 cm1

, 1572 cm-1 and 1649 cm-1.46-48

The absolute value of the enhancement factor (EF) was

estimated by using the formula: EF = (ISERS/IBULK) × (NBULK/NSERS),49 where ISERS and NSERS were the intensity of the band and number of the adsorbed molecules for SERS, respectively. IBULK and NBULK were the intensity of the band and number of molecules for free R6G. According to this calculated definition, the intensity of the 1649 cm-1 peak was used for the estimation due to the aromatic C-C stretch vibration (see supporting information), and the EF of Au/Ag core-shell NR with rough shell was approximately 1.97 × 108, which was much higher than monometallic Au NRs (4.02 × 106) and Au/Ag core-shell NR with smooth shell (5.12 × 107, Figure 6). The uniformity of SERS signal was the major concern for any SERS substrates. Hence, we recorded SERS signals from 10 random sites and the results indicated that the variation of the intensity on main characteristic peaks was fairly uniform (Figure 7a). In order to directly compare the variation error, the intensity at 612 cm-1 was listed in Figure 7b, showing the relative standard deviation (RSD) was 6.62%, which was much lower than those in previous reports.50-52


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Figure 6. Raman spectra of Au NRs (black), Au/Ag core-shell NRs with smooth shell (blue) and rough shell (red) covered substrates. Both of their thickness of the shell was around 8 nm. The exposure time was 1 s, laser wavelength was 532 nm, and laser power was 0.12 mW.

Figure 7. SERS spectra of Au/Ag core-shell NRs with rough shell acquired from 10 random sites on SERS substrate (a) and the corresponding bar chart for the peak intensity at 612 cm-1 from 10 random sites on SERS substrate (b). Exposure time was 1 s, laser wavelength was 532 nm, and laser power was 0.12 mW. 3. Conclusions We have demonstrated a simple, robust strategy to prepare bimetallic Au/Ag core-shell superstructures by covalently binding the reducing agent (DDTC) to the surfaces of Au NR seeds. The anchored reducing agents in-stiu reduced Ag ions into atoms, and further led to the heterogeneous nucleation and growth modes of overgrowth on the surface of Au NRs, resulting


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in the ultimate formation of Au/Ag core-shell superstructures. Meanwhile, the thickness of superstructures can be finely tuned from 8 nm to 22 nm by simply adjusting the amount of Ag precursor (AgNO3). The intriguing corn-like structures exhibit LSPR absorption in NIR and high-performance SERS, making them highly attractive in practical bio-applications.

4. Experimental Section Materials: Tetrachloroauric acid (HAuCl4•4H2O), cetyltrimethylammonium bromide (CTAB), and dopamine were purchased from Sigma-Aldrich. Silver nitrate and carbon disulfide were purchased from Aladdin Industrial Corporation in Shanghai. Sodium borohydride, hydrogen chloride, and L-ascorbic acid were obtained from Sinopharm Chemical Reagent Co., Ltd. Sodium oleate was supplied by TCI (Shanghai) Development Co., Ltd. Instrument: UV-Vis-NIR absorption spectra were recorded by Lambda 950 UV-Vis-NIR spectrophotometer from Perkin-Elmer Instrument Co. Ltd US and the structures of gold nanorods were observed by transmission electron microscopy (TEM), which was conducted on the JEOL JEM2010 electron microscope. Raman spectra were recorded by Renishaw inVia Reflex Confocal micro Raman spectrometer from Renishaw. Synthesis of gold nanorods: In order to synthesize uniform Au NRs, the seed solution for Au NRs growth was prepared as follows: 0.1 mL of 25 mM HAuCl4 was dissolved in 4.9 mL deionized water and then was mixed with 5 mL of 0.2 M CTAB in a 20 mL glass bottle. Subsequently, 100 mL of fresh 0.006 M NaBH4 was prepared and 1 mL of the NaBH4 solution was rapidly injected to the HAuCl4-CTAB solution under vigorous stirring (1200 rpm). The stirring was stopped after 2 min and the seed solution colour changed from yellow to brownish


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yellow. Finally, the solution was placed without disturbance at room temperature for 30 min before use. The Au NRs growth solution was prepared as follows: 7.0 g (0.077 M) of CTAB and 1.234 g (0.016M) of sodium oleate were dissolved in 250 mL of warm (50 oC) deionized water. The mixed solution was cooled down to 30 oC and 18 mL AgNO3 solution (4.0 mM) was added. The mixture was placed without undisturbed at 30 oC for 15 min and then 250 mL of HAuCl4 (1.0 mM) solution was added under magnetic stirring (700 rpm). The solution became colourless after 90 min and 2.1 mL of HCl solution (37 wt % in water, 12.1 M) was added under magnetic stirring (400 rpm) for 15 min. Then 1.25 mL of ascorbic acid (AA) solution was added and the solution was vigorously stirred (1200 rpm) for 30 s after which 0.4 mL of seed solution was injected into the growth solution with another stir (1500 rpm) for 30 s. In the end, the solution was left undisturbed at 30 oC for 12 h for Au NRs growth. The obtained Au NRs were centrifuged at 8000 rpm for 10 min and then the precipitate was renewedly dispersed in 80 mM CTAC solution. This process was repeated three times and the final solution (CTAC-coated Au NRs) was placed without any disturbance for the future use. Synthesis of Dopamine dithiocarbamate: Dopamine dithiocarbamate (DDTC) was synthesized according to previous report.53 Briefly, 80 µL CS2 (0.549 mM) and 10 µL triethylamine were added in dopamine (0.1896 g, 0.549 mM) and then mixture was sonicated for 5 min at room temperature to acquire the ideal assembly of DDTC. Synthesis of bimetallic Au/Ag core-shell NR: Au/Ag core-shell with smooth shell was synthesized according to previous report.28 In order to obtain bimetallic Au/Ag core-shell superstructures, 0.8 mL DDTC was added into 1 mL CTAC-coated Au NRs solution and


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sonicated for 2 min. The mixture was mildly stirred for 12 h to ensure the sufficient adsorption of DDTC through Au-S bond. Subsequently, 100 µL AgNO3 (0.01M) was added to the mixture and sonicated for 1 min. Finally, the resultant solution was left undisturbed at 65 oC for 4 h for Ag shell growth. Preparation and characterization of SERS samples: The concentrated sample solution was uniformly dropped on the silicon wafer (0.5 × 0.5 cm2) and dried for 6 h at 25 oC. In order to test the feasibility of active SERS substrate, R6G was used as the Raman model probe. Briefly, 10 µL of R6G solution in ethanol (0.1μM) was dropped on the silica wafer. After drying at 25 o

C, substrates were washed with deionized water and absolute ethanol for several times to

remove the free R6G molecules. Raman spectra were collected on a Renishaw inVia Reflex Confocal micro Raman spectrometer from Renishaw using a red LED laser (532 nm). Exposure time was 1 s and laser power was 0.12 mW through 20× aperture ( NA = 0.4) for all spectra. Enhancement Factor (EF) = (ISERS/IBULK) × (NBULK/NSERS), where ISERS and NSERS are the intensity of the band and number of the adsorbed molecules for SERS, respectively. IBULK and NBULK are the intensity of the band and number of molecules for free R6G.

Acknowledgments We gratefully acknowledge the Natural Science Foundation of China (Grant Nos. 51473179, 21404110, 51603219), Excellent Youth Foundation of Zhejiang Province of China (Grant No. LR14B040001), Ningbo Science and Technology Bureau (Grant Nos. 2014B82010, 2015A610036, and 2015C110031), Youth Innovation Promotion Association of Chinese


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Academy of Sciences (Grant No. 2016268), and Key Research Program of Frontier Science, Chinese Academy of Sciences (QYZDB-SSW-SLH036). Supporting Information Available: UV-Vis spectra of synthesized Au NRs, Zeta potential of Au NRs measured before and after DDTC absorption, TEM images of bimetallic Au/Ag coreshell superstructures, SEM images of bimetallic Au/Ag core-shell superstructures, and normal Raman spectra of R6G.

REFERENCES AND NOTES 1. Ghosh Chaudhuri, R.; Paria, S., Core/shell nanoparticles: classes, properties, synthesis mechanisms, characterization, and applications. Chem. Rev. 2012, 112, (4), 2373-433. 2. Wei, Z.; Sun, J.; Li, Y.; Datye, A. K.; Wang, Y., Bimetallic catalysts for hydrogen generation. Chem. Soc. Rev. 2012, 41, (24), 7994-8008. 3. Yu, W.; Porosoff, M. D.; Chen, J. G., Review of Pt-based bimetallic catalysis: from model surfaces to supported catalysts. Chem. Rev. 2012, 112, (11), 5780-817. 4. Cortie, M. B.; McDonagh, A. M., Synthesis and optical properties of hybrid and alloy plasmonic nanoparticles. Chem. Rev. 2011, 111, (6), 3713-35. 5. Andolina, C. M.; Dewar, A. C.; Smith, A. M.; Marbella, L. E.; Hartmann, M. J.; Millstone, J. E., Photoluminescent gold-copper nanoparticle alloys with composition-tunable near-infrared emission. J. Am. Chem. Soc. 2013, 135, (14), 5266-9. 6. Mayer, M.; Scarabelli, L.; March, K.; Altantzis, T.; Tebbe, M.; Kociak, M.; Bals, S.; Garcia de Abajo, F. J.; Fery, A.; Liz-Marzan, L. M., Controlled Living Nanowire Growth: Precise Control over the Morphology and Optical Properties of AgAuAg Bimetallic Nanowires. Nano Lett. 2015, 15, (8), 5427-37.


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7. Li, Q.; Wu, L.; Wu, G.; Su, D.; Lv, H.; Zhang, S.; Zhu, W.; Casimir, A.; Zhu, H.; Mendoza-Garcia, A.; Sun, S., New approach to fully ordered fct-FePt nanoparticles for much enhanced electrocatalysis in acid. Nano Lett. 2015, 15, (4), 2468-73. 8. Sachan, R.; Malasi, A.; Ge, J.; Yadavali, S.; Krishna, H.; Gangopadhyay, A.; Garcia, H.; Duscher, G.; Kalyanaraman, R., Ferroplasmons: intense localized surface plasmons in metalferromagnetic nanoparticles. ACS Nano 2014, 8, (10), 9790-8. 9. Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A., Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices. Science 2000, 287, (5460), 198992. 10. Passarelli, N.; Perez, L. A.; Coronado, E. A., Plasmonic interactions: from molecular plasmonics and Fano resonances to ferroplasmons. ACS Nano 2014, 8, (10), 9723-8. 11. Link, S.; Wang, Z. L.; El-Sayed, M. A., Alloy Formation of Gold−Silver Nanoparticles and the Dependence of the Plasmon Absorption on Their Composition. J. Phys. Chem. B 1999, 103, (18), 3529-3533. 12. Tsao, Y. C.; Rej, S.; Chiu, C. Y.; Huang, M. H., Aqueous phase synthesis of Au-Ag coreshell nanocrystals with tunable shapes and their optical and catalytic properties. J. Am. Chem. Soc. 2014, 136, (1), 396-404. 13. Zaera, F., Nanostructured materials for applications in heterogeneous catalysis. Chem. Soc. Rev. 2013, 42, (7), 2746-62. 14. Seo, W. S.; Lee, J. H.; Sun, X.; Suzuki, Y.; Mann, D.; Liu, Z.; Terashima, M.; Yang, P. C.; McConnell, M. V.; Nishimura, D. G.; Dai, H., FeCo/graphitic-shell nanocrystals as advanced magnetic-resonance-imaging and near-infrared agents. Nat. Mater. 2006, 5, (12), 971-6. 15. You, H.; Yang, S.; Ding, B.; Yang, H., Synthesis of colloidal metal and metal alloy nanoparticles for electrochemical energy applications. Chem. Soc. Rev. 2013, 42, (7), 2880-904. 16. Lu, Y.; Zhao, Y.; Yu, L.; Dong, L.; Shi, C.; Hu, M. J.; Xu, Y. J.; Wen, L. P.; Yu, S. H., Hydrophilic Co@Au yolk/shell nanospheres: synthesis, assembly, and application to gene delivery. Adv. Mater. 2010, 22, (12), 1407-11. 17. Zhang, L.; Dai, L.; Rong, Y.; Liu, Z.; Tong, D.; Huang, Y.; Chen, T., Light-triggered reversible self-assembly of gold nanoparticle oligomers for tunable SERS. Langmuir 2015, 31, (3), 1164-71. 18. Nooney, R. I.; Stranik, O.; McDonagh, C.; MacCraith, B. D., Optimization of plasmonic enhancement of fluorescence on plastic substrates. Langmuir 2008, 24, (19), 11261-7. 19. Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P., Shaping binary metal nanocrystals through epitaxial seeded growth. Nat. Mater. 2007, 6, (9), 692-7. 20. McGrath, A. J.; Chien, Y. H.; Cheong, S.; Herman, D. A.; Watt, J.; Henning, A. M.; Gloag, L.; Yeh, C. S.; Tilley, R. D., Gold over Branched Palladium Nanostructures for Photothermal Cancer Therapy. ACS Nano 2015, 9, (12), 12283-91. 21. Zhou, Y.; Wang, D.; Li, Y., Pd and Au@Pd nanodendrites: a one-pot synthesis and their superior catalytic properties. Chem. Commun. (Camb) 2014, 50, (46), 6141-4. 22. Cui, Q.; Shen, G.; Yan, X.; Li, L.; Mohwald, H.; Bargheer, M., Fabrication of Au@Pt multibranched nanoparticles and their application to in situ SERS monitoring. ACS Appl. Mater. Interfaces 2014, 6, (19), 17075-81. 23. Khanal, B. P.; Zubarev, E. R., Polymer-functionalized platinum-on-gold bimetallic nanorods. Angew. Chem. Int. Ed. Engl. 2009, 48, (37), 6888-91.


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Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

24. Chen, J.; Herricks, T.; Geissler, M.; Xia, Y., Single-crystal nanowires of platinum can be synthesized by controlling the reaction rate of a polyol process. J. Am. Chem. Soc. 2004, 126, (35), 10854-5. 25. Ciacchi, L. C.; Pompe, W.; De Vita, A., Initial nucleation of platinum clusters after reduction of K2PtCl4 in aqueous solution: A first principles study. J. Am. Chem. Soc. 2001, 123, (30), 7371-7380. 26. Song, Y.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H.; Jiang, Y.; Brinker, C. J.; van Swol, F.; Shelnutt, J. A., Controlled synthesis of 2-D and 3-D dendritic platinum nanostructures. J. Am. Chem. Soc. 2004, 126, (2), 635-45. 27. Colombi Ciacchi, L.; Pompe, W.; De Vita, A., Growth of Platinum Clusters via Addition of Pt(II) Complexes: A First Principles Investigation. J. Phys. Chem. B 2003, 107, (8), 17551764. 28. Huang, Y.; Ferhan, A. R.; Kim, D. H., Tunable scattered colors over a wide spectrum from a single nanoparticle. Nanoscale 2013, 5, (17), 7772-5. 29. Qu, Y.; Cheng, R.; Su, Q.; Duan, X., Plasmonic enhancements of photocatalytic activity of Pt/n-Si/Ag photodiodes using Au/Ag core/shell nanorods. J. Am. Chem. Soc. 2011, 133, (42), 16730-3. 30. Becker, J.; Zins, I.; Jakab, A.; Khalavka, Y.; Schubert, O.; Sonnichsen, C., Plasmonic focusing reduces ensemble linewidth of silver-coated gold nanorods. Nano Lett. 2008, 8, (6), 1719-23. 31. Chiang, C.; Huang, M. H., Synthesis of Small Au-Ag Core-Shell Cubes, Cuboctahedra, and Octahedra with Size Tunability and Their Optical and Photothermal Properties. Small 2015, 11, (45), 6018-25. 32. Samal, A. K.; Polavarapu, L.; Rodal-Cedeira, S.; Liz-Marzan, L. M.; Perez-Juste, J.; Pastoriza-Santos, I., Size tunable Au@Ag core-shell nanoparticles: synthesis and surfaceenhanced Raman scattering properties. Langmuir 2013, 29, (48), 15076-82. 33. Huang, Y.; Dandapat, A.; Kim, D. H., Covalently capped seed-mediated growth: a unique approach toward hierarchical growth of gold nanocrystals. Nanoscale 2014, 6, (12), 6478-81. 34. Ye, X.; Zheng, C.; Chen, J.; Gao, Y.; Murray, C. B., Using binary surfactant mixtures to simultaneously improve the dimensional tunability and monodispersity in the seeded growth of gold nanorods. Nano Lett. 2013, 13, (2), 765-71. 35. Huang, Y.; Wu, L.; Chen, X.; Bai, P.; Kim, D.-H., Synthesis of Anisotropic Concave Gold Nanocuboids with Distinctive Plasmonic Properties. Chem. Mater. 2013, 25, (12), 24702475. 36. Fan, F. R.; Liu, D. Y.; Wu, Y. F.; Duan, S.; Xie, Z. X.; Jiang, Z. Y.; Tian, Z. Q., Epitaxial growth of heterogeneous metal nanocrystals: from gold nano-octahedra to palladium and silver nanocubes. J. Am. Chem. Soc. 2008, 130, (22), 6949-51. 37. Sanchez-Iglesias, A.; Carbo-Argibay, E.; Glaria, A.; Rodriguez-Gonzalez, B.; PerezJuste, J.; Pastoriza-Santos, I.; Liz-Marzan, L. M., Rapid epitaxial growth of Ag on Au nanoparticles: from Au nanorods to core-shell Au@Ag octahedrons. Chem.-Eur. J. 2010, 16, (19), 5558-63. 38. Zhang, L.; Xiao, P.; Lu, W.; Zhang, J.; Gu, J.; Huang, Y.; Chen, T., Macroscopic Ultrathin Film as Bio-Inspired Interfacial Reactor for Fabricating 2D Freestanding Janus CNTs/AuNPs Hybrid Nanosheets with Enhanced Electrical Performance. Adv. Mater. Interfaces 2016, 3, (15), 1600170.


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Page 20 of 21

39. Huang, Y.; Ferhan, A. R.; Gao, Y.; Dandapat, A.; Kim, D. H., High-yield synthesis of triangular gold nanoplates with improved shape uniformity, tunable edge length and thickness. Nanoscale 2014, 6, (12), 6496-500. 40. Jiang, R.; Chen, H.; Shao, L.; Li, Q.; Wang, J., Unraveling the evolution and nature of the plasmons in (Au core)-(Ag shell) nanorods. Adv. Opt. Mater. 2012, 24, (35), OP200-7. 41. Liu; Guyot-Sionnest, P., Synthesis and Optical Characterization of Au/Ag Core/Shell Nanorods. J. Phys. Chem. B 2004, 108, (19), 5882-5888. 42. Isichenko, M. B., Percolation, statistical topography, and transport in random media. Rev. Mod. Phys. 1992, 64, (4), 961-1043. 43. Nan, C. W.; Shen, Y.; Ma, J., Physical Properties of Composites Near Percolation. Ann. Rev. Mater. Res. 2010, 40, (1), 131-151. 44. Chen, H.; Wang, F.; Li, K.; Woo, K. C.; Wang, J.; Li, Q.; Sun, L. D.; Zhang, X.; Lin, H. Q.; Yan, C. H., Plasmonic percolation: plasmon-manifested dielectric-to-metal transition. ACS Nano 2012, 6, (8), 7162-71. 45. Xu, B. B.; Wang, L.; Ma, Z. C.; Zhang, R.; Chen, Q. D.; Lv, C.; Han, B.; Xiao, X. Z.; Zhang, X. L.; Zhang, Y. L.; Ueno, K.; Misawa, H.; Sun, H. B., Surface-plasmon-mediated programmable optical nanofabrication of an oriented silver nanoplate. ACS Nano 2014, 8, (7), 6682-92. 46. Lee, J.; Seo, J.; Kim, D.; Shin, S.; Lee, S.; Mahata, C.; Lee, H. S.; Min, B. W.; Lee, T., Capillary force-induced glue-free printing of Ag nanoparticle arrays for highly sensitive SERS substrates. ACS Appl. Mater. Interfaces 2014, 6, (12), 9053-60. 47. Contreras-Caceres, R.; Dawson, C.; Formanek, P.; Fischer, D.; Simon, F.; Janke, A.; Uhlmann, P.; Stamm, M., Polymers as Templates for Au and Au@Ag Bimetallic Nanorods: UV–Vis and Surface Enhanced Raman Spectroscopy. Chem. Mater. 2013, 25, (2), 158-169. 48. Bechger, L.; Koenderink, A. F.; Vos, W. L., Emission Spectra and Lifetimes of R6G Dye on Silica-Coated Titania Powder. Langmuir 2002, 18, (6), 2444-2447. 49. Rao, V. K.; Radhakrishnan, T. P., Tuning the SERS Response with Ag-Au NanoparticleEmbedded Polymer Thin Film Substrates. ACS Appl. Mater. Interfaces 2015, 7, (23), 12767-73. 50. Gong, Z.; Du, H.; Cheng, F.; Wang, C.; Wang, C.; Fan, M., Fabrication of SERS swab for direct detection of trace explosives in fingerprints. ACS Appl. Mater. Interfaces 2014, 6, (24), 21931-7. 51. Polavarapu, L.; Porta, A. L.; Novikov, S. M.; Coronado-Puchau, M.; Liz-Marzan, L. M., Pen-on-paper approach toward the design of universal surface enhanced Raman scattering substrates. Small 2014, 10, (15), 3065-71. 52. Chen, J.; Huang, Y.; Kannan, P.; Zhang, L.; Lin, Z.; Zhang, J.; Chen, T.; Guo, L., Flexible and Adhesive Surface Enhance Raman Scattering Active Tape for Rapid Detection of Pesticide Residues in Fruits and Vegetables. Anal. Chem. 2016, 88, (4), 2149-55. 53. Kailasa, S. K.; Wu, H. F., One-pot synthesis of dopamine dithiocarbamate functionalized gold nanoparticles for quantitative analysis of small molecules and phosphopeptides in SALDIand MALDI-MS. Analyst 2012, 137, (7), 1629-38.


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Ag CoreâShell Superstructures with Tunable Surface

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