-Supporting Information- An Unconventional Role of Ligand in Continuously Tuning of Metal-Metal Interfacial Strain Yuhua Feng, Jiating He, Hong Wang, Yee Yan Tay, Hang Sun, Liangfang Zhu, and Hongyu Chen*, Division of Chemistry and Biological Chemistry, Nanyang Technological University, Singapore 637371, and School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 Email: hongyuchen@ntu.edu.sg ; Web: http://www.ntu.edu.sg/home/hongyuchen/ S1
Experiment section Materials and methods: All chemical reagents were used as purchased without further purification. Hydrogen tetrachloroaurate(iii) hydrate (HAuCl 4 3H 2 O), 99.9% (metal basis Au 49%) was purchased from Alfa Aesar; sodium citrate tribasic dihydrate (99.0%, Sigma), AgNO 3 (99%, Sigma), 4- mercaptobenzoic acid (90%, Aldrich), 4-aminothiophenol (97%, Aldrich), 1,2-dipalmitoyl-sn-glycero-3- phosphothioethanol (Sodium salt, Avanti Polar Lipids, Inc.), Amphiphilic diblock copolymer polystyreneblock-poly(acrylic acid) (PS 144 -b-paa 28, M n = 15000 for the polystyrene block and M n = 1600 for the poly(acrylic acid) block, M w /M n = 1.11) was purchased from Polymer Source, Inc.; Deionized water (resistance > 18.2 M cm -1 ) was used in all reactions. Copper specimen grids (300 mesh) with formvar/carbon support film (referred to as TEM grids in the text) were purchased from Beijing XXBR Technology Co. UV-Vis spectra were collected on a Cary 100 UV-Vis spectrophotometer. TEM images were collected from a JEM-1400 (JEOL) Transmission Electron Microscopy operated at 100 kv. (NH 4 ) 6 Mo 7 O 24 was used as the negative stain, so that the polymer shells appear white against the stained background. High resolution TEM (HRTEM) image was taken from JEOL 2100 F Field Emission Transmission Electron Microscope at 200 kv. Raman spectra were collected from the sample solution in a 4 ml glass vial on a PeakSeeker Pro spectrometer (Raman Systems Inc.) using a red laser ( = 785 nm) at 290 mw. PSPAA encapsulation of metal nanoparticles (NPs). 1 The as-synthesized NPs solution was directly used in the encapsulation step. To 200 L of NPs solution, 800 L of DMF was added and mixed by vortexing for 10 s, then 80 L of PS 144 -b-paa 28 solution (8 mg/ml in DMF) was added and vortexed, followed by the addition of 40 L of ligand L solution (2 mg/ml in EtOH) (the final concentration of L is 120 M). The mixture was heated at 110 ºC for 2 h in an oil bath to induce the polymer self-assembly. The ligand L is virtually SERS inactive; its thiol group coordinated to the Au surface rendering it hydrophobic. The polystyrene blocks attached to the hydrophobic Au surface, whereas the poly(acrylic acid) blocks S2
dissolved in solution facing outwards. After cooling, the reaction mixture was diluted by water to de-swell and immobilize the polymer shells; the resulting NPs were isolated by centrifugation and then re-dispersed in water. Figure S1 UV-Vis absorption spectra of (Au@MBIA)@Ag dimeric NPs before and after PSPAA encapsulation. Synthesis of monodisperse AuNPs. The AuNPs used as seeds (d av = 40 nm) was synthesized by the citrate-reduction method. 2 The large AuNPs (d av = 50 nm) was synthesized by a seeded mediated growth method derived from the citrate-reduction method. In a typical procedure, 0.5 ml HAuCl 4 solution (10 mg/ml) was added to 50 ml H 2 O in a 1 L round bottle flask equipped with a condenser. The reaction mixture was refluxed for 30 min in an oil bath. Then, 0.75 ml sodium citrate solution (1% wt) was added into the boiling HAuCl 4 solution. The color of the resulting solution changed from red to dark gray to black and then to purple. About 10 min later, the solution changed to red. Monodispersed AuNPs of 40 nm were formed at this stage. The following procedures were carried out using this seed solution without isolation step. S3
After refluxed for another 15 min, 50 ml boiled water was added into the red AuNPs solution, followed by the dropwise addition of 0.1 ml 6.6 mg/ml NaOH solution and the quick addition of 0.5 ml sodium citrate solution (1% wt) and 0.5 ml HAuCl 4 solution (10 mg/ml). The mixture was heated for 20 min to completely reduce the HAuCl 4 and form Au layer on the seed NP surface. In the third cycle, 100 ml H 2 O, 0.2 ml NaOH, 1 ml citrate and 1 ml HAuCl 4 were added in sequence via the same way. The addition cycle (each cycle using 100 ml H 2 O, 0.2 ml NaOH, 1 ml sodium citrate and 1 ml HAuCl 4, followed by 20 min incubation at refluxing) was repeated twice, followed by additional 6 cycles (each cycle using 0.2 ml NaOH, 1 ml sodium citrate and 1 ml HAuCl 4, followed by 20 min incubation at refluxing). To clarify, in total, using the 40 nm seed solution, there were 5 cycles with the addition of water, and 6 cycles without addition of water. The resulting solution was cooled to room temperature in the oil bath. This stock solution of AuNPs was stored at room temperature and used for the following syntheses. A small aliquot of the AuNPs was encapsulated in polymer shells and then characterized by TEM (Figure S2). Using the NanoMeasure software (available at website, June 25 th, 2011), the shown 94 particles gave an average size of 50 nm in diameter. S4
Figure S2 TEM image, UV absorption spectrum and size distribution of the synthesized 50 nm AuNPs. The calculated concentration of the as-synthesized 50 nm AuNPs. The concentration of the assynthesized AuNPs solution can be estimated from the total amount of Au used in the synthesis, the density of Au, and the volume of each AuNP. Here, we assume that the as-synthesized 50 nm AuNPs are ideally uniform in size and shape. S5
Total weight of HAuCl 4 in the as-synthesized 50 nm AuNP solution = 100 mg Total weight of Au in the as-synthesized 50 nm AuNP solution = 50 mg Weight of each 50 nm AuNP = Au V AuNP = 1.26 10-12 mg Total number of 50 nm AuNPs = 50 mg / 1.26 10-12 mg = 3.97 10 13 particles = 6.59 10-11 mol Total volume of the synthesis solution = 422 ml Concentration of the as-synthesized 50 nm AuNP solution = 6.59 10-11 mol / 0.42 L = 15.69 10-11 M = 156.9 pm Synthesis of the Au@Ag core-shell NPs. The Au@Ag core-shell nanoparticles of 45 nm in diameter were synthesized following a literature report. 3 Because we cannot synthesize 50 nm pure AgNPs in high monodispersity, we use the Au@Ag NPs with 15 nm Au core as a replacement. In a typical synthesis, 10 ml 15 nm AuNPs was added 50 ml H 2 O. After being heated to boiling, 500 L 1% sodium citrate and 500 L AgNO 3 (10 mg/ml) was added quickly under vigorous stirring and the resulting solution was heated for 0.5 h during which the color changed from red to orange. After 0.5 h, the same amount of sodium citrate and AgNO 3 were added and heated for the other 0.5 h. In the third round, 480 L sodium citrate and 480 L AgNO 3 were added and the resulting solution was heated for 1 h and cooled to room temperature. Then 21.9 ml H 2 O was added into the solution. Based on the TEM images in Figure S3, by using NanoMeasure software, the shown 49 particles gave an average size of 45 nm in diameter. For convenience, the core-shell Au@Ag NPs will be called as AgNPs in the following. S6
Absorbance (a.u.) 1.0 0.8 0.6 0.4 0.2 0.0 400 500 600 700 800 Wavelength (nm) Figure S3 TEM image, UV absorption spectrum and size distribution of the synthesized 45 nm Au@Ag NPs. Synthesis of the Au-Ag hybrid NPs. To 1 ml as-synthesized citrate-stabilized 50 nm AuNPs solution, different amount of MBIA ligand (1, 5, 20 and 150 µm) was added under vortexing. After the solution was incubated at room temperature or at 60 S7
ºC for 2 h, 60 L hydroquinone (10 mm in water) and 60 L AgNO 3 (10 mm in water) were added in sequence into the above solution under vortexing, the mixture was placed undisturbed at room temperature for at least 2 h to complete the reduction of AgNO 3. Synthesis of the Ag-Ag dimers. This hybrid NPs were synthesized by following the same procedure above for the preparation of Au-Ag NPs, with 20 µl MBIA ligands incubated with 1 ml AgNPs at 60 ºC for 2 h before Ag deposition. Synthesis of Au-Ag-Ag and Ag-Ag-Ag trimers. In this synthesis, Au-Ag and Ag-Ag heterodimers were used as seeds. The as-synthesized Au@Ag and Ag@Ag were incubated at room temperature for 2 h after being synthesized, then 60 L hydroquinone (10 mm in water) and 60 L AgNO 3 (10 mm in water) were added in sequence into the above solution under vortexing, the mixture was placed undisturbed at room temperature for at least 2 h to complete the reduction of AgNO 3. Synthesis of Au-Ag hybrid NPs without pre-incubation of Au seeds with MBIA ligands. In this synthesis, 1 ml citrate-aunps were mixed with 60 L hydroquinone (10 mm in water). To this solution, a mixture of 20 L MBIA (1 mm in EtOH) and 60 L AgNO 3 (10 mm in water) was added under vortexing, the mixture was placed undisturbed at room temperature for at least 2 h to complete the reduction of AgNO 3. Reduction of AgNO 3 in the presence of the mixture of citrate-au and Au@MBIA NPs as seeds. In this synthesis, 0.5 ml of citrate-au NPs were mixed with 60 L of hydroquinone (10 mm in water). This solution was mixed with 0.5 ml of Au@MBIA NPs (prepared by incubating 0.5 ml Au seeds with 10 M MBIA at 60 ºC for 2 h). Immediately after this, 60 L of AgNO 3 (10 mm in water) was added S8
under vortexing. The resulting solution was placed undisturbed at room temperature for at least 2 h to complete the reduction of AgNO 3. TEM images before and after PSPAA encapsulation:. As shown in Figure S14-17, there was no obvious difference before and after the polymer encapsulation. Because ligand L and PSPAA were added after Ag growth, it is unlikely that they would affect the deposition of Ag (we know the completion of Ag reduction from the color changes). In our previous papers on the purification of dimers, the PSPAA shells were self-assembled after the aggregation of NPs. The encapsulation same method can be used for encapsulating single-particles as reported in our earlier papers. The nice point of using polymer shells is that it is very easy to distinguish the boundaries of the NPs, which is particularly important for discerning anisotropic NPs such as the heterodimers. For example, with NPs contacting each other, it is hard to tell dimers from tetramers. Moreover, we would not be able to tell if the aggregation occurred during Ag growth, PSPAA encapsulation, or drying. S9
Figure S4 Large area TEM image of the concentric core-shell Au@Ag hybrid NPs of sample i. S10
Figure S5 Large area TEM image of the eccentric core-shell Au-Ag hybrid NPs of sample ii. S11
Figure S6 Large area TEM image of the acorn Au-Ag hybrid NPs of sample iii. S12
Figure S7 Large area TEM image of the heterodimer Au-Ag hybrid NPs of sample iv. S13
Figure S8 Large area TEM images of the Au-Ag-Ag trimers as shown in Figure 4a. S14
Figure S9 TEM image of Au-Ag-Ag trimers prepared by a 2nd cycle of Ag reduction in the presence of Au-Ag acorn NPs (sample iii). The double formation of acorn structures is not as recognizable as that in Figure S8, but two Ag domains can be roughly identified. The 2nd Ag domain rarely formed on the Au domain, even though the Au surface is covered with a thin Ag layer. In some cases, the Au-Ag-Ag domains form triangular conformation, making it difficult to identify the growth sites. Such trimers were not counted. S15
Figure S10 TEM and enlarged TEM images showed in Figure 4f-g of maintext. S16
Figure S11 (a-c) TEM images of Au@Ag hybrid NPs with different Ag domain sizes. The samples were prepared by incubating the same Au seeds with 20 µl MBIA (1 mm) at 60 ºC for 2 h, but adding different amount of AgNO 3 and reductant. (d) UV-Vis absorption spectra of samples a-c, which showed absorption red-shift with increasing Ag domain size. S17
Figure S12 TEM image of Au-Ag hybrid NPs prepared from 15 nm Au seeds, by incubating them with 20 M MBIA at 60 ºC for 2 h before Ag growth. S18
Figure S13 (a-c) TEM images showing the same Au-Ag hybrid NPs of sample ii, iii, and iv as shown in Figure 1c (arrow marks the line scan direction which is different from that in the main text); (d-f) Au and Ag elements EDS line scan corresponding to the while lines in a-c. Circles mark the thin Ag layer on the surface of Au. S19
Figure S14 TEM images of core-shell NPs (i) with and without PSPAA encapsulation (isolated after Ag growth without adding L or PSPAA). Please note that the sample without PSPAA encapsulation was directly prepared without purification. The organic matter was likely the excess hydroquinone or citrate or residue MBIA. Figure S15. TEM image of eccentric core-shell NPs (ii) with and without PSPAA encapsulation. S20
Figure S16. TEM image of acorn-type NPs (iii) with and without PSPAA encapsulation. Figure S17. TEM image of heterodimer structure (iv) with and without PSPAA encapsulation. S21
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