MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key

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1 Supporting Information Smart Ag Nanostructures for Plasmon-Enhanced Spectroscopies Chao-Yu Li, Meng Meng, Sheng-Chao Huang, Lei Li, Shao-Rong Huang, Shu Chen, Ling-Yan Meng, Rajapandiyan Panneerselvam, San-Jun Zhang, Bin Ren, Zhi-Lin Yang, Jian-Feng Li,*, and Zhong-Qun Tian MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, and Department of Physics, Xiamen University, Xiamen , China. State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai , China. S1.Experimental Section S1.1 Materials Chloroauric acid tetrahydrate (HAuCl 4 4H 2 O), sodium citrate, sodium borohydride (NaBH 4 ), L-ascorbic acid (AA), pyridine (Py) were purchased from Sinopharm Chemical Reagent Co. Ltd.; Anhydrous silver perchlorate (AgClO 4 ), (3-aminopropyl)trimethoxysilane (ATPMS), tetraethoxysilane (TEOS) and 4-aminothiophenol (PATP) were purchased from Alfa Aesar; Triethoxypropylsilane (TEPS) was purchased from Tokyo Chemical Industry; Sodium silicate solution, silver wire, rhodamine isothiocyanate (RITC), N, N-diisopropylethylamine (DIPEA) and 3-aminopropyltriethoxysilane (APTS) were purchased from Sigma-Aldrich. All reagents were used as received without further purification. Deionised Milli-Q water (~18.2 MΩcm) was used throughout the study. S1

2 S1.2 Instruments and simulations The morphology and structure of nanostructures were characterized by SEM (Hitachi S-4800) and TEM (Tecnai F30 and JEOL JEM-2100); The extinction spectra were measured with UV-Vis spectrophotometer (SHIMADZU UV-2550). The SHINERS and SERS experiments were carried out using an XploRA Raman instrument (Jobin Yvon-Horiba, France) which is equipped with 532 and 638 nm lasers. A 50, NA 0.5 objective was used for all spectra acquisitions in SHIENRS and SERS experiments. TERS experiments were performed on a homebuilt TERS setup, the details about the TERS setup can be seen in our previous work. 1 A p-polarized, 633 nm laser was focused through a long distance, 50, NA 0.45 objective into the gap of the tip and sample. It was illuminated in a side illumination mode with an incident angle of 60. The laser power and the acquisition time can be adjusted to obtain TERS spectra with a high signal-to-noise ratio. In the TERS experiments, the tunneling current was 200 pa and the bias was 600 mv. Fluorescence measurements were performed on an Invia (Renishaw, UK) microscope. The excitation wavelength was 532 nm (the power is ~0.07 µw) and focused through a 50, NA 0.8 objective. The time-resolved fluorescence (lifetime of molecule in SHINs-enhanced fluorescence) was measured by a homebuilt time-correlated single photon counting (TCSPC) system. 2 A single-photon counting PMT detector (PMA-182, Picoquant) together with a standalone TCSPC electronic system (4 ps per channel and 6250 channels, PicoHarp 300 from Picoquant, Germany) were used to collect the signals, and a super-continuum pulsed fiber laser (SC400-pp-4; Fianium, UK) with a pulse duration of about 10 ps at a repetition rate of 40 MHz were used as the excitation source. An acousto-optic tunable filter (Fianium, UK) was used to select the excitation wavelength. The excitation is 532 nm and the signal is collected through 580 nm /70 band-pass and 552 nm long-pass filters to remove the excitation laser. In all of the simulations, the electric field amplitude of the excitation was 1 V/m, and the electric field vector was polarized perpendicular to the illumination. S1.3 Preparation of shell-isolated Ag nanostructures S2

3 The 96 nm spherical Ag NPs were prepared by a seed growth method in which 16 nm Au NPs act as seeds: 1.5 ml sodium citrate (38 mm) was added to 50 ml boiling HAuCl 4 (0. 24 mm) to get the Au seeds, after 1 h stirring the Au seeds were diluted 45 times. Next, the diluted Au seeds were mixed with ascorbic acid and sodium citrate, and then AgClO 4 was added to the mixture drop by drop. Ascorbic acid is a common reductant and each mole of ascorbic acid could reduce double the amount of Ag +. The concentration of reductant, sodium citrate and AgClO 4 in mixture was 1.84, 1.25 and 1.25 mm, respectively. To prepare the shell-isolated Ag NPs with various shell thicknesses, as-prepared 96 nm Ag sol was diluted two times with ultra-pure water. And then NaBH 4, APTMS, and sodium silicate solution were added to the diluted Ag sol under vigorous stirring. The amount of NaBH 4, APTMS, and sodium silicate in mixture was 5.5 mm, 0.22 mm, and 0.045%, respectively. During the preparation process, the ph value of growth solution was tuned by H 2 SO 4 to be ~9.7. The growth solution was immediately transferred to a 90 C bath and stirred for 60 min. Then the bath temperature was tuned down to 60 C and stirring times are changed from 0, 5, 35, and 150 min, thus the shell thicknesses are varying from 2, 4, 6, and to 10 nm, respectively. To get a more thicker silica shell of 20 nm, 4 ml of Ag sol was diluted to 30 ml, and other additions are kept the same except the ph value is tuned to ~7 to accelerate the hydrolysis of silicate. 4 h heating time results in 20 nm silica shell. Bare Ag tip was prepared with a slightly modified electrochemical etching method. 3 Typically, 2 cm Ag wire (0.25 mm diameter) and a Pt ring were used as the working and counter electrodes, respectively. Ag wire was immersed in a mixture of perchloric acid and methanol (1/2, v/v) and a potentiostat was used to fix the etching current at 0.06 A. After the electrochemical etching, the tip was rinsed thoroughly with ethanol. To prepare a shell-isolated Ag tip, the fresh Ag tip was immersed in the same growth solution as above-mentioned method to obtain shell-isolated Ag NPs. And the ph value was also kept the same of ~9.7. The growth solution was transferred to a 90 C bath and stirred for 30 min to get a silica shell with ~2 nm thickness. After the coating, the tip was rinsed by ultra-pure water for about 1 min. To examine the effect of the NaBH 4 treatment on smaller Ag NPs, we used 52 nm Ag NPs as the core of Ag SHINs. The 52 nm spherical Ag NPs was synthesized by a seed growth method in which 9 nm Ag NPs act as the seeds. 3.5 ml of sodium citrate (38 mm), 3 ml of AgClO 4 (20 mm) and 50 ml of S3

4 H 2 O were mixed together. To this mixture, 0.5 ml of NaBH 4 (110 mm) was injected under vigorous stirring. After 2 h stirring, the Ag seeds were obtained and diluted 214 times with ultra-pure water. Next, the diluted Ag seeds were mixed with AgClO 4 and sodium citrate, and then ascorbic acid was added to the mixture drop by drop. The concentration of ascorbic acid, sodium citrate, and AgClO 4 in mixture was 1.87, 1.39, and 1.07 mm, respectively. To coat 3 nm silica shell, 8 ml of 52 nm Ag sol was diluted to 30 ml, and the amount of NaBH 4, APTMS, and silicate solution are kept the same (ph value of the mixture was tuned to ~9.4). The mixture was immediately transferred to a 90 C bath and stirred for 60 min to get 3 nm silica shell. To coat 6 nm silica shell, 30 ml of the as-prepared 52 nm Ag sol was used without dilution, and the other condition was kept the same except ph value is tuned to ~9.0. After 60 min heating in 90 C bath, 6 nm silica shell was obtained. S1.4 Preparation of silica coated Ag NPs by the hydrolysis of TEOS To obtain 5 nm silica shell, 5 ml of the as-prepared 96 nm Ag sol, 20 ml of ethanol, 100 µl of NH 3 H 2 O (25%), and 10 µl of TEOS (98%) are mixed together, and then the mixture was stirred for 20 h at room temperature. S1.5 Substrate modification for fluorescence measurements Quartz substrate was cleaned thoroughly with the Piranha solution (H 2 SO 4 /H 2 O 2 = 3/1), and rinsed by ultra-pure water and ethanol sequentially. A solution of silane coupling agent (ethanol/water/acetic acid/apts/teps = 98/3/1.2/1/1, V/V/V/V/V) was prepared in which the substrates were immersed under vigorous rotation. RITC was functionalized to a quartz substrate using isothiocyanate-amino chemistry in a slightly basic ethanol solution adjusted by DIPEA (3 mm). The concentration of RITC was 0.25 mm and the reaction was kept for 24 h under vigorous rotation in darkness. After fluorescent probe modification, the substrate was washed thoroughly with ethanol and stored under a vacuum condition before conducting further experiments. S4

5 S2.Characterization of Ag NPs and Ag SHINs S2.1 Characterization of Ag NPs before/after NaBH 4 treatment by extinction spectra and SEM Figure S1. (a) Extinction spectra of 52 nm diameter Ag NPs before (black line) and after (red line) treatment of NaBH 4. SEM images of 52 nm diameter Ag NPs (b) before and (c) after treatment of NaBH 4. (d) Extinction spectra of 96 nm diameter Ag NPs before (black line) and after (red line) treatment of NaBH 4. SEM images of 96 nm diameter Ag NPs (e) before and (f) after treatment of NaBH 4. When diameter of Ag nanospheres is 96 nm, the relaeased Ag + is 1.09 µg/ml (0.8 wt%). Under identical conditions, the evolution of the silver ions increased for the smaller Ag NPs due to the higher surface free energy ([Ag + ] was 4.19 µg/ml (3.64 wt%) when the diameter was 52 nm). S5

6 S2.2 Characterization of SHINs by TEM and the role of NaBH4 treatment Figure S2. (a-e) TEM images of Ag SHINs with shell thickness of 2, 4, 6, 10 and 20 nm, respectively. (f-j) SEM images of Ag SHINs with shell thickness of 2, 4, 6, 10 and 20 nm, respectively. Figure S3. (a-b) TEM images of Ag SHINs with 52 nm diameter Ag core and shell thicknesses are 3 and 6 nm, respectively. (c-d) TEM images of Ag SHINs filled with pinholes, which are synthesized under the same condition as the ones shown in (a-b) except the treatment of NaBH4. S6

7 Figure S4. (a-e) TEM images of Ag SHINs filled with pinholes, which are synthesized under the same condition as the ones shown in Figure S2 (a-e) except the NaBH 4 treatment. When the NaBH 4 treatment was absent, the surface adsorbed-ag + transformed to AgOH with an eventual oxide layer. This is because of the alkaline environment after the addition of the silicate solution. Thus, the surface oxidation process impedes the interaction between the Ag surface and the silane molecules, which results in an uneven silica coating. S2.3 Characterization of pinhole-free property of ultra-thin silica shell of SHINs by SERS method Figure S5. Raman examination on the pinholes on the shell of the Ag SHINs at (a) 532 and (b) 638 nm. SHINERS spectra of 10 mm Py from pinhole-free Ag SHINs (96 nm core, 4 nm shell) on a smooth Ag substrate (i), Si wafer (ii), and pinhole-on Ag SHINs on Si wafer (iii). (c) The corresponding cartoon diagrams of Raman examinations. S7

8 To evaluate the pinhole-free character of the silica shell, a SERS measurement was conducted with pyridine (Py) molecules that are very sensitive to small pinholes. Figure S5 shows the pinhole tests for the Ag SHINs synthesized with/without NaBH 4. There was no observable Raman signal from the Py molecules when NaBH 4 was used in the coating process. To further investigate the plasmon-enhancement capability of Ag SHINs, we spread pinhole-free Ag SHINs on a smooth Ag substrate, and then, SHINERS spectra of 10 mm Py were obtained with 532 and 638 nm lasers (Figure S5). Because pyridine cannot adsorb at the silicon surface and the molecules would diffuse randomly during the measurement, it is reasonable to predict a poor and insufficient interaction of pyridine with electromagnetic field transmitted from SHINs particles. If the molecules have enough time to stay near SHINs particles during the measurement, the enhanced Raman signal of analyte would be observable on an arbitrary substrate. S2.4 Characterization of Ag@SiO 2 prepared with traditional method by SERS method and the chemical stability of Ag@SiO 2 in corrosive environment Figure S6. (a) Raman examination of pinholes on the shell of the Ag@SiO 2 NPs prepared with the traditional Stӧber method, excited at 532 (black line) and 638 nm (red line). Inset: The corresponding TEM image of Ag@SiO 2 NP. (b) Time-dependent extinction spectra of Ag@SiO 2 prepared with the traditional Stӧber method in H 2 O 2 (6 wt. %). Inset: The corresponding photographs at different times. S8

9 S2.5 Shell thickness dependence and the long-term performance of SHINs Figure S7. (a) SHINERS spectra of Py adsorbed on a smooth Ag surface coated with Ag SHINs (96 nm core) with different silica shell thickness. (b) SHINERS spectra of Py adsorbed on a smooth Ag surface coated with Ag SHINs (96 nm core, 4 nm shell) after different sample storage times. S3. Application of SHINERS/SITERS in plasmon-mediated reactions Figure S8. (a) SEM image of Au (111)/PATP/Ag SHINs junctions. (b) Raman spectra of Au(111)/PATP/Ag SHINs (red line, the power is 1.6 mw) and Au(111)/PATP/Ag NP junctions (black line, the power is 0.1 mw) under 532 nm illumination. S9

10 Figure S9. (a) TERS spectrum of the Au(111)/PATP/SITERS tip; (b) the far field spectrum when tip retracted from the surface; (c) TERS spectrum at a clean Au (111) surface recorded with the same SITERS tip used in black line. In SITERS experiments, the tunneling of electrons between tip and substrate were allowed when an optimum bias is applied. However, the transformation of PATP to DMAB in this photocatalysis reaction was not induced by tunneling electrons, but caused by the SPR hot electrons. 4 In plasmonic strong coupling regime, SPR-induced coherent quantum tunneling is possible when the gap between two coupling nanostructures is extremely small (< ~0.5 nm). 5 However, it could be neglected in our experiments because the shell thicknesses are 4 nm for SHINERS and ~2 nm for SITERS. Moreover, in charge transfer mechanism, SPR-induced hot electrons need to overcome the potential barrier at the interface. 6 And the SPR-induced hot electrons will be blocked by pinhole-free silica shell. In some reported SPR-induced photodegradations of organic molecules, silica shell could be also used to successfully block the hot electrons. 7 Therefore, in our experiments, this photocatalysis reaction was inhibited by the presence of silica shell. S10

11 S4. Lifetime measurements of SHINs-enhanced fluorescence Figure S10. The excitation and emission spectra of RITC ethanol solution. Figure S11. Fluorescence decay lifetime measurements of plasmon-enhanced fluorescence. The gray line shows the instrument response function (IRF). The lifetimes of RITC in the presence/absence of nanopariticles are fitted with deconvolution of the measured instrument response function (IRF). The average lifetime of reference sample in the absence of nanoparticles is mearsured to be 0.87 ns. Due to the interaction with strong near field, a fast decay component is observed in the SHINs-enhanced fluorescence. The fast decay lifetimes of SHINs-enhanced fluoresence are changed from 29 to 52, 35 and 118 ps when the shell thicknesses S11

12 are 2, 6, 10 and 20 nm, respectively. Because the measured lifetimes are close to the response limit of detector, it is resonable to anticipate a faster decay rate of lifetime in SHINs-enhanced fluorescence. S5. References 1. Liu, Z.; Ding, S. Y.; Chen, Z. B.; Wang, X.; Tian, J. H.; Anema, J. R.; Zhou, X. S.; Wu, D. Y.; Mao, B. W.; Xu, X.; Ren, B.; Tian, Z. Q., Nat. Commun. 2011, 2, Chen, Y.; Yang, T.; Pan, H.; Yuan, Y.; Chen, L.; Liu, M.; Zhang, K.; Zhang, S.; Wu, P.; Xu, J., J. Am. Chem. Soc. 2014, 136, Iwami, M.; Uehara, Y.; Ushioda, S., Rev. Sci. Instrum. 1998, 69, Huang, Y. F.; Zhang, M.; Zhao, L. B.; Feng, J. M.; Wu, D. Y.; Ren, B.; Tian, Z. Q., Angew. Chem. Int. Ed. 2014, 53, (a) Savage, K. J.; Hawkeye, M. M.; Esteban, R.; Borisov, A. G.; Aizpurua, J.; Baumberg, J. J., Nature 2012, 491, 574; (b) Zuloaga, J.; Prodan, E.; Nordlander, P., Nano Lett. 2009, 9, (a) Knight, M. W.; Sobhani, H.; Nordlander, P.; Halas, N. J., Science 2011, 332, 702; (b) Linic, S.; Christopher, P.; Ingram, D. B., Nat. Mater. 2011, 10, (a) Zhou, J.; Ren, F.; Zhang, S.; Wu, W.; Xiao, X.; Liu, Y.; Jiang, C., J. Mater. Chem. A 2013, 1, 13128; (b) Chen, J. J.; Wu, J. C. S.; Wu, P. C.; Tsai, D. P., J. Phys. Chem. C 2012, 116, S12