Supporting Information

Transcription

1 Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, Supporting Information for Small, DOI: /smll Engineering Hot Nanoparticles for Surface-Enhanced Raman Scattering by Embedding Reporter Molecules in Metal Layers Yuhua Feng, Yong Wang, Hong Wang, Tao Chen, Yee Yan Tay, Lin Yao, Qingyu Yan, Shuzhou Li, * and Hongyu Chen*

2 -Supporting Information- DOI: /smll Engineering "Hot" Nanoparticles of Surface-Enhanced Raman Scattering by Embedding Reporter Molecules in Metal Layers** Yuhua Feng, Yong Wang, Hong Wang, Tao Chen, Yee Yan Tay, Lin Yao, Qingyu Yan, Shuzhou Li,*, and Hongyu Chen* [*] Yuhua Feng, Yong Wang, Hong Wang, Tao Chen, Lin Yao and Prof. Hongyu Chen Division of Chemistry and Biological Chemistry Nanyang Technological University 21 Nanyang Link, Singapore Fax: (+65) Web: Dr. Yee Yan Tay, Prof. Qingyu Yan and Prof. Shuzhou Li Division of Materials Science and Engineering Nanyang Technological University Singapore

3 -Supporting Information- Scheme S1. Molecules used in this study: L, 1,2-dipalmitoyl-sn-glycero-3- phosphothioethanol; PSPAA, polystyrene-block-poly(acrylic acid) (PS 154 -b-paa 49 ); MBA, 4- mercaptobenzoic acid; MPAA, 4-mercaptophenylacetic acid; ATP, 4-aminothiophenol. 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-mercaptophenylacetic acid (97%, Aldrich), 4-aminothiophenol (97%, Aldrich), 1,2-dipalmitoyl-sn-glycero-3- phosphothioethanol (Sodium salt, Avanti Polar Lipids, Inc.), amphiphilic diblock copolymer polystyrene-block-poly(acrylic acid) (PS 154 -b-paa 49, M n = for the polystyrene block and M n = 3500 for the poly(acrylic acid) block, M w /M n = 1.15) was purchased from Polymer Source, Inc.; Deionized water (resistance > 18.2 MΩ 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

4 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 in all TEM images reported in this paper, 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 (λ = 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 µl of NPs solution, 800 µl of DMF was added and mixed by vortexing for 10 s, then 80 µl of PS 154 -b-paa 49 solution (8 mg/ml in DMF) was added and vortexed, followed by the addition of 40 µl of ligand L solution (2 mg/ml in EtOH). The final concentration of L is 120 µm which is significantly higher than the 4.5 µm of MBA used in the synthesis of 1@Ag. 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 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. After the repeated isolation by centrifugation, a small amount of the NPs were lost in the discarded supernatant and in the form of aggregates at the bottom of eppendorf tubes. Thus, the concentration of the NPs cannot be precisely determined. In addition, the Ag absorption - 3 -

5 shoulder decreased slightly after the polymer encapsulation (Figure S1), possibly due to the etching by the excess ligand L. Figure S1 (a) Normalized UV-Vis absorption spectra of NPs (i) before and (ii) after PSPAA encapsulation. UV-Vis absorption spectra of (b) 62 nm Au and (c) 74 nm NPs (before MBA treatment) that used to prepare reference NP-1 and NP-2. As shown in Figure 1f in the main text, the UV-Vis absorption peak of the was red-shifted (542 nm) compared with citrate stabilized AuNPs in water (522 nm). This is consistent with the decrease in surface plasmon resonance energy with the increasing of refractive index of the surrounding medium. [2] Synthesis of monodisperse Au NPs (d av = 62 nm). The AuNPs used as seeds (d av = 15 nm) was synthesized by the citrate-reduction method. [3] The large AuNPs (d av = 62 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, 1.5 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 - 4 -

6 and then to purple. About 10 min later, the solution changed to red. Monodispersed AuNPs of 15 nm were formed at this stage. The following procedures were carried out using this seed solution without isolation step. Figure S2 (a) TEM image of the synthesized 62 nm AuNPs; (b) the report of NanoMeasure software about particle distribution. 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 14 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 15 nm seed solution, there were 4 cycles with the addition of water, and 14 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 - 5 -

7 temperature and used for the following syntheses. All of the NP-1, NP-2, and NPs reported in this paper were derived from this stock solution. 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 26 particles gave an average size of 62 nm in diameter. The calculated concentration of the as-synthesized 62 nm AuNPs. The concentration of the as-synthesized 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 assynthesized 62 nm AuNPs are ideally uniform in size and shape. Total weight of HAuCl 4 in the as-synthesized 62 nm AuNP solution = mg Total weight of Au in the as-synthesized 62 nm AuNP solution = 91.1 mg Weight of each 62 nm AuNP = ρ Au V AuNP = mg Total number of 62 nm AuNPs = 91.1 mg / mg = particles = mol Total volume of the synthesis solution = 441 ml Concentration of the as-synthesized 62 nm AuNP solution = mol / 0.44 L = M = pm Synthesis of the reference Au@Ag core-shell NPs. The Au@Ag core-shell nanoparticles of 74 nm in diameter were synthesized following a literature report. [4] In a typical synthesis, to 10 ml the as-synthesized 62 nm AuNPs solution (see above), 750 µl AgNO 3 (10 mg/ml) was added. The resulting solution was heated to 110 ºC. Then, 500 µl sodium citrate (1% wt) - 6 -

8 was added quickly under vigorous stirring, the solution was heated for 1 h, during which the color changed from red to orange. If we define the concentration of Au seed (62 nm AuNPs) as C 0, the concentration of the synthesized Au@Ag NPs is 0.89C 0. That is, the concentration of the as-synthesized Au@Ag NPs is 127 pm. Based on the TEM images in Figure S3, by using NanoMeasure software, the mean particle size is 74 nm in diameter. Synthesis of the (Au@MBA)@Ag core-shell NPs. To 1 ml as-synthesized citrate-stabilized 62 nm AuNPs solution, 8 µl 4-MBA ligand (0.57 mm in 0.25 M NaOH) was added under vortex. After the solution was incubated overnight at room temperature, 30 µl hydroquinone (10 mm in water) and 30 µl AgNO 3 (10 mm in water) were added in sequence into the above solution under vigorous stirring, the mixture was incubated at room temperature for 12 hrs to complete the reduction of AgNO 3. The concentration of the resulting NPs was estimated to be 0.94C

9 Figure S3 (a, c) TEM image of the synthesized NPs; (b, d) the reports of NanoMeasure software on the particle distribution. As shown in Figure 1f in the main text, the UV-Vis absorption peak of the NPs is slightly blue shifted (542 nm) compared with MBA capped AuNPs in water (536 nm). This blue shift was consistent with that reported in the literatures. [4-5] Based on the TEM image in Figure S4, by using NanoMeasure software, we measured 40 particles, the mean particle size is 72 nm in diameter

10 Figure S4 TEM image of the synthesized NPs (after polymer encapsulation) and a NanoMeasure software report of particle distribution. Synthesis of (Au@MBA)@Ag core-shell NPs with different Ag shell thickness. Three samples (1 ml as synthesized 62 nm AuNPs solution) were marked as i, ii and iii: 1) To i, ii and iii), 8 µl 4-MBA ligand (0.057 mm in 0.25 M NaOH) was added under vortex. After incubated overnight at room temperature, 15 µl hydroquinone (10 mm) and 15 µl AgNO 3 (10 mm) were added in sequence into the above solution under vigorous stirring. The mixture was incubated at room temperature overnight to complete the reduction of AgNO 3. 2) To ii and iii, 30 µl hydroquinone and 30 µl AgNO 3 were added to reduce a second Ag layer on the surface of (Au@MBA)@Ag NPs, the solutions were incubated overnight. 3) To sample iii, another 30 µl hydroquinone and 30 µl AgNO 3 were added to reduce a third Ag layer. Based on the TEM images in Figure S5, by using NanoMeasure software, we measured (i) 46 particles, the mean particle size is about 67 nm in diameter; (ii) 23 particles, the mean particle size is about 72 nm in diameter; (iii) 25 particles, the mean particle size is about 82 nm in diameter

11 Figure S5 TEM image of the synthesized NPs with different thickness Ag shell and the corresponding NanoMeasure software report of particle distribution: (a, b) sample i; (c, d) sample ii; (e, f) sample iii

12 NaCl induced aggregation of and Three samples (A, B, C) were prepared as follows: 1 ml as-synthesized 62 nm AuNPs solution were incubated with 8 µl of MBA ligand (0.057 mm in 0.25 M NaOH) overnight at room temperature. Sample A was directly used for aggregation. Sample B and C were coated a thin layer of Ag by the same procedure mentioned above. To sample C, another 4 µl of MBA was added and incubated overnight at room temperature. For aggregation, 18, 16 and 14 µl 2.5 M NaCl were added quickly into sample A, B and C, respectively. The red color of the three samples turned to dark within 1 min. Raman spectra were collected right after the NaCl addition. The photographs were taken quickly after Raman measurement, after that, the UV- Vis absorption spectra were collected. The calculation of SERS enhancement factor: We employed the MBA Raman peak at 1073 cm -1 whose intensity is the strongest in its Raman and SERS spectra to calculate the SERS enhancement factor of NP-1, NP-2 and 1@Ag NPs. The calculation is based on the following equation: EF = (I SERS C bulk )/(I bulk C SERS ) Where I SERS and I bulk are the Raman intensities of the same 1073 cm -1 peak for NPs (NP-1 or NP-2 or 1@Ag) and pure MBA solution, C SERS and C bulk are the concentrations of MBA on (or in) NPs (NP-1 or NP-2 or 1@Ag) and in pure solution

13 For the selection of I bulk, we made a M MBA solution in 5 M NaOH. By stepwise diluting, we got titration plots of SERS intensities vs [MBA] (Figure S6). Within this titration plots, the I bulk should be in the range with fine linear ship between Raman intensity and [MBA]. Thus we selected Raman intensity corresponding to mm as I bulk. a b c Figure S6 Linear fit for Raman intensity vs [MBA] in different concentration range of (a) 0 to 160 mm; (b) 0 to 310 mm; (c) 0 to 620 mm, among which 0 to 160 mm range show fine linear relation. As discussed in the main text, the MBA ligands present in the solution were not enough for full cover the AuNP surface. Thus, we use the total concentration of MBA ligand in NP-1 (or NP-2 or 1@Ag) solution as the C SERS. Based on this method, the calculated SERS EF for MBA on NP-1 is ; for MBA on NP-2 is ; and for MBA on 1@Ag NPs is Raman spectra data processing. We use peak height to compare the Raman intensity of in the sample solution. All Raman intensities reported in this work are calculated using this method. The formula is: Peak Height y 1 (y 2 + y 3 )/2 (Scheme S2)

14 A(x 1, y 1 ) B(x 2, y 2 ) Peak Height y 1 -(y 2 +y 3 )/2 C(x 3, y 3 ) Scheme S2 The calculation of Raman peak height. Figure S7 (a) SERS and (b) UV-Vis spectra of (i) Au@MPAA; (ii) (Au@MPAA)@Ag NPs

15 Submitted to Figure S8 TEM image of NPs Figure S9 (a) SERS and (b) UV-Vis spectra of (i) (ii) NPs. After Ag coating on the surface of NPs, enhanced SERS signals could be obtained

16 Figure S10 TEM image of NPs Figure S11 SERS spectra of (a) before (red) and after (blue) overnight incubation at room temperature (overlapped, no difference was observed); (b) before (i) and after (ii) overnight incubating at 60 ºC; (c) before (blue) and after (green) polymer encapsulation at 110 ºC for 2 h

17 Upon heating or laser irradiation, ATP is known to be oxidized to give an azo compound with stronger SERS intensity. For NPs, incubation at room temperature overnight didn t cause increase in SERS intensity (Figure S11a). After being incubated at 60 ºC for overnight, the SERS intensity was enhanced (Figure S11b). After Ag coating on the surface of Au@ATP NPs, enhanced SERS signals could be obtained (Figure S9). After PSPAA encapsulation at 110 ºC for 2 h, the SERS intensity was greatly enhanced (Figure S11c). In addition to the increase of in overall SERS intensity, some weak Raman peaks of Au@ATP were selectively enhanced, which were called as b2 mode of ATP molecule. This result is consistent with a recent report, which showed that oxidation of ATP led to its dimerization to give an azo compound, DMAB, which have 100 folds stronger Raman cross-section than that of the ATP monomer. [6] Unlike ATP molecules, MBA, MPAA are stable upon heating and we did not observe increase in SERS intensity after polymer encapsulation

18 Figure S12 TEM image of the NPs with small free Ag NPs. In the presence of excess MBA (950 µm, which is significantly higher than the 4.5 µm used in the main text), the AuNPs aggregated. Nevertheless, after the reduction of AgNO 3 by hydroquinone, we did not observe the formation of Ag layer on the AuNPs (shown here were only isolated AuNPs not representative of the whole sample). Instead, the Ag formed small NPs in the solution

19 Submitted to Figure S13 Large area TEM images of the NPs shown in Figure 1b

20 References [1] H. Chen, S. Abraham, J. Mendenhall, S. C. Delamarre, K. Smith, I. Kim, C. A. Batt, ChemPhysChem 2008, 9, [2] Y. Kang, T. A. Taton, Angew. Chem. Int. Ed. 2005, 44, [3] G. Frens, Nature-Physical Science 1973, 241, [4] W. Xie, L. Su, P. Donfack, A. G. Shen, X. D. Zhou, M. Sackmann, A. Materny, J. M. Hu, Chem. Commun. 2009, [5] D. K. Lim, I. J. Kim, J. M. Nam, Chem. Commun. 2008, [6] Y. F. Huang, H. P. Zhu, G. K. Liu, D. Y. Wu, B. Ren, Z. Q. Tian, J. Am. Chem. Soc. 2010, 132,