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1 Electronic supplementary information for: Charge-transfer-induced suppression of galvanic replacement and synthesis of double shell nanoparticles for highly uniform, robust and sensitive bioprobes Dao Thi Ngoc Anh, Prerna Singh, Cheshta Shankar, Derrick Mott and Shinya Maenosono School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa , Japan 1

2 Experimental details A. Chemicals Silver nitrate (AgNO 3 ) %, gold tetrachloroaurate trihydrate (HAuCl 4 3H 2 O) 99.9%, trisodium citrate trihydrate (SC) 99.0%, sodium chloride (NaCl) 99.5%, and common solvents were obtained from Aldrich. We obtained 3-amino-1,2,4-triazole-5-thiol (ATT) 98.0% from Tokyo Chemical Industry. Water was purified with a Millipore Direct-Q system (18.2 MΩ). B. Instrumentation and measurements Synthesized NPs were characterized by transmission electron microscopy (TEM), scanning TEM (STEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), UV-visible spectroscopy (UV-Vis), and Raman spectroscopy. TEM analysis was performed on a Hitachi H-7100 transmission electron microscope operated at 100 kv. The core-shell structure was further confirmed by using a JEOL JEM-ARM200F scanning transmission electron microscope, which incorporates a spherical aberration corrector, with an EDS detector. Samples for TEM, and EDS mapping were prepared by dropping the suspended NPs onto a carbon coated copper grid and drying overnight in air. Raman spectra were obtained with an Ar + ion laser (wavelength nm, power 50 mw), using a Horiba-Jobin Yvon Ramanor T64000 triple monochromator equipped with a CCD detector. The nonpolarized Raman scattering measurements were set under a microscope sample holder using a 180 o backscattering geometry at room temperature. The laser spot diameter was 1 μm. An acquisition time of 60s per spectrum was used with averaging of three spectra per analysis area. XPS analysis was carried out on a Shimadzu Kratos AXIS-ULTRA DLD high performance XPS system. Photoelectrons were excited by monochromated Al K α radiation. Detection was done with a delay-line detector (DLD) and a concentric hemispherical analyzer (CHA). The X-ray tube was operated at 150 W. The pass energy of the CHA was 20 ev for narrow-scan spectra. The analyzed area on the specimen surface was μm 2 and was located in the center of the irradiated region. For the sample preparation, the precipitated NPs were deposited on carbon tape and dried in air. The instrument was operated at a vacuum level of Torr. UV-Vis spectra were collected in the range of 300 to 1100 nm using a Perkin-Elmer Lambda 35 UV-Vis spectrometer. C. Au NP synthesis The Au seeds were prepared by the citrate-reduction procedure of Frens. 17 A 45 ml solution containing 0.01g HAuCl 4 3H 2 O was stirred and heated to boiling, and then 5 ml of a 1% SC solution was added to the boiling solution. The solution was further boiled for 1 hour and then left to cool to room temperature. D. Au@Ag core@shell NP synthesis The as-synthesized citrate-capped Au NPs were used as core particles in preparation of Au@Ag core@shell NPs. The Au NP dispersion (20 ml) was brought to reflux with stirring until boiling, and then 5 ml aqueous solution of AgNO 3 20 mm and SC (5 ml, 13.5 mm) were simultaneously added dropwise. The reaction solution was refluxed for 20 minutes and then left to cool to room temperature and used for further experiments. The Ag shell thickness of Au@Ag core@shell NPs was controlled by adding different amounts (0.3, 0.7, 1.8, and 3.2 ml) of 20 mm AgNO 3. E. (Au@Ag)@Au double shell NP synthesis For the synthesis of (Au@Ag)@Au double shell NPs, a 20 ml as-synthesized Au@Ag 3.6 core@shell NP dispersion was refluxed with stirring until boiling, and then HAuCl 4 3H 2 O (0.1 ml, 15 mm) and SC (5 ml, 13.5 mm) were simultaneously added dropwise. The reaction solution was refluxed for 10 minutes and then left to cool to room temperature. 2

3 Table S1. Mean diameter and shell thicknesses of NPs Type of NPs x (nm) y (nm) D (nm) Au 14.4 ± 0.7 Au@Ag ± ± 0.7 Au@Ag ± ± 1.2 Au@Ag ± ± 0.8 Au@Ag ± ± 0.9 Double shell NPs with a thin Au second shell 3.6 ± ± 1.2 Double shell NPs with a thick Au second shell 3.9 ± ± 1.9 x: Ag first shell thickness, y: Au second shell thickness Table S2. Mean diameter of NPs before and after adding NaCl Type of NPs D (nm) Before adding NaCl 3 hours after adding NaCl Ag 17.1 ± 2.2 Totally destroyed Au@Ag ± ± 1.6 (Au@Ag 3.6 )@Au 21.9 ± ± 1.6 Normalized Absorbance Wavelength (nm) Figure S1. Normalized UV-Vis spectra for as-synthesized NPs: Au (solid line), Au@Ag (circles), (Au@Ag)@Au (squares), and Ag (triangles) NPs. 3

4 Figure S2. TEM images for Au and NPs synthesized using an increasing amount of Ag in the shell. As-synthesized Au NPs (a), NPs with different shell thickness: 0.4 (b), 1 (c), 2.2 (d), and 3.6 (e) nm. Figure S3. UV-Vis spectra for as-synthesized nanoparticles: Au seeds and Au@Ag core-shell NPs with different shell thicknesses. 4

5 Figure S4. (a-c) STEM-HAADF images and (d-f) EDS elemental mapping images of double shell NPs with a thick Au second shell (1.2 nm): Overlay (d) of Ag L edge (e) and Au M edge (f). Figure S5. (a) XPS spectra of Au, Au@Agx (x = 0.4, 1.0, 2.2 and 3.6), and (Au@Ag3.6)@Au double shell NPs, respectively from the bottom to top. (b) Magnified XPS spectra focusing on the Au 4f7/2 peak. 5

6 Figure S6. TEM images of (a) Ag, (b) and (c) NPs 3 hours after adding NaCl. Figure S7. Additional TEM images of (a) Au@Ag3.6 and (b) (Au@Ag3.6)@Au NPs 3 hours after adding NaCl. Figure S8. Raman spectra of Au@Agx [from bottom to top, x = 0.4, 1, 2.2, and 3.6 nm (blue curve)] and (Au@Ag3.6)@Au (red curve) NP assemblies created by using ATT. 6

7 Expanded Explanation of the Relationship between Charge Transfer and Galvanic Replacement The galvanic replacement reaction is driven by the difference in the electrochemical potential between the two metals, with one serving as the cathode and the other as the anode. Regarding the general preparation of NPs, the reduction potential of AuCl 4 /Au (0.99 V vs SHE) is more positive than that of AgCl/Ag (0.22 V vs SHE). 1 Hence, Ag NPs serve as sacrificial templates being oxidized by HAuCl 4 according to Equation (1). This reaction is initiated locally at a high-energy site (e.g., surface step, point defect, or hole in the capping layer) 2 rather than over the entire surface. In the case that Au@Ag NPs are used as cores, however, the Ag shells are expected to have a higher electron density than Ag NPs due to electron transfer from the Au core to the Ag shell. The electron rich Ag shell results in a negative oxidation state, Ag δ, and thus, may lead to effectively suppress the galvanic replacement reaction. Xia and coworkers also claimed a similar result in which a higher potential was required to oxidize Ag atoms contained in a Ag-Au alloy. 1 It was also reported that when the molar ratio of Au to Ag is more than 0.17, the galvanic reaction was hindered, indicating that a higher Au content can protect Au-Ag alloy NPs against galvanic etching. 3 Very possibly, the addition of Au changes the Ag reduction potential and alters the oxidative relationship between Ag 0 and AuCl 4. With respect to core@shell NPs, Liz Marzán and coworkers have synthesized Au@Ag, Au@Ag@Au and finally Au@Ag@Au@Ag multishell NPs using a similar synthetic approach to the present scheme. 4 In their approach however, the deposited intermediate Ag shell thickness was much greater than our own (ca. 32 nm), which resulted in the formation of hollow structures with partial alloying when the Au shell was deposited. In our own study, the intermediate Ag shell thickness is limited to the range where the charge transfer phenomenon takes place, allowing the ability to create Au@Ag@Au NPs without significant alloying or defects in the structure. 3Ag(s) + HAuCl 4 (aq) Au(s) + 3AgCl(s) + HCl(aq) (1) 1. S. E. Skrabalak, J. Chen, Y. Sun, X. Lu, L. Au, C. M. Cobley, and Y. Xia, Acc. Chem. Res. 41, 1587 (2008). 2. Z. L. Wang, J. Phys. Chem. B 104, 1153 (2000). 3. X. Gong, Y. Yang, and S. Huang, J. Phys. Chem. C 114, (2010). 4. B. R-González, A. Burrows, M. Watanabe, C. J. Kiely, and L. M. L. Marzán, J. Mater. Chem. 15, 1755 (2005). 7