Synthesis, Characterization, and 3D-FDTD Simulation of Ag@SiO2

Apr 16, 2012 - We show that a SiO2 shell as thin as 2 nm can be synthesized pinhole-free on the Ag surface of a nanoparticle, which then becomes the c...
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Synthesis, Characterization, and 3D-FDTD Simulation of Ag@SiO2 Nanoparticles for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy Viviane Uzayisenga,†,§ Xiao-Dong Lin,†,§ Li-Mei Li,‡ Jason R. Anema,† Zhi-Lin Yang,‡ Yi-Fan Huang,† Hai-Xin Lin,† Song-Bo Li,† Jian-Feng Li,† and Zhong-Qun Tian*,† †

State Key Laboratory of Physical Chemistry of Solid Surfaces and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China ‡ Department of Physics, Xiamen University, Xiamen, 361005, China S Supporting Information *

ABSTRACT: Au-seed Ag-growth nanoparticles of controllable diameter (50−100 nm), and having an ultrathin SiO2 shell of controllable thickness (2−3 nm), were prepared for shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Their morphological, optical, and material properties were characterized; and their potential for use as a versatile Raman signal amplifier was investigated experimentally using pyridine as a probe molecule and theoretically by the three-dimensional finitedifference time-domain (3D-FDTD) method. We show that a SiO2 shell as thin as 2 nm can be synthesized pinhole-free on the Ag surface of a nanoparticle, which then becomes the core. The dielectric SiO2 shell serves to isolate the Raman-signal enhancing core and prevent it from interfering with the system under study. The SiO2 shell also hinders oxidation of the Ag surface and nanoparticle aggregation. It significantly improves the stability and reproducibility of surface-enhanced Raman scattering (SERS) signal intensity, which is essential for SERS applications. Our 3D-FDTD simulations show that Ag-core SHINERS nanoparticles yield at least 2 orders of magnitude greater enhancement than Au-core ones when excited with green light on a smooth Ag surface, and thus add to the versatility of our SHINERS method.

1. INTRODUCTION The principal goal of colloidal nanoplasmonics is to design and synthesize nanoparticles with unique physical properties. Since the optical and electronic properties of metal nanostructures are extremely sensitive to their morphology, methods for the preparation of monodisperse nanoparticles with controlled size and shape have become the subject of intense research.1 Progress made in this field during the past decade has indeed been significant, with the synthesis of well controlled nanostructures having different morphologies and compositions for various applications already achieved. In surface-enhanced Raman scattering (SERS),2−5 inelastic scattering of light by a molecule is enhanced tremendously because of the molecule’s proximity to a surface plasmon substrate. Surface plasmons are collective oscillations in electron density which occur at the surface of a nanostructured free-electron metal and result in localized regions of very high electromagnetic (EM) field strength.6 The synthesis and optimization of free-electron metal nanostructures for use as surface plasmon substrates is key, not only for increasing the intensity of Raman scattering (which is intrinsically weak) but also for understanding surface plasmon activity and controlling it for other applications.7−9 SERS may thus be considered a representative plasmonic phenomenon, and as such it gained a lot of attention during the rapid development of nanoscience in © XXXX American Chemical Society

the 1990s. The study of SERS is now considered an important branch of the ascendant nanotechnology. SERS has suffered from a lack of substrate versatility for decades. Most studies have been limited to Au, Ag and Cu nanostructures. Different methods have been developed to circumvent this problem, including optimization of transition metal roughening procedures10,11 and coating the more SERSactive metals with inactive or weakly active materials.12,13 The latter approach is sometimes called “borrowing SERS”, as enhanced EM field strength extends from the surface of the highly SERS-active metal to the surface of a weakly enhancing or even nonenhancing shell. Tip-enhanced Raman spectroscopy (TERS) was first reported in 2000.14−17 This was a breakthrough for SERS versatility in that the signal-enhancing surface plasmon substrate was completely separated from the surface under study for the first time. In principle, a sample of any material and any morphology can be examined by TERS. Practically, however, only a few different kinds of samples have been studied by TERS.18−20 The two most significant Special Issue: Colloidal Nanoplasmonics Received: February 6, 2012 Revised: April 13, 2012

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size and shape because it is more difficult to control the nucleation step with Ag than with Au and because Ag has a tendency to oxidize and form anisotropic nanoparticles during colloid preparation.29,30 To overcome the first of these two difficulties, we grow Ag on Au seeds. The optical properties of our Au-seed Ag-growth nanoparticles are similar to those of Ag-only nanoparticles, as the Au seed is a very small component at the center of the resulting structure. Au seeds of 12 nm were prepared by the Lee-Meisel method,31 and AgNO3 was subsequently reduced on the surface of the Au seeds by ascorbic acid with sodium citrate present as a stabilizing agent. First, 20 mL of water was put into each of three 100 mL round-bottom flasks: A, B, and C. Then 1 mL (flask A), 0.7 mL (flask B), and 0.5 mL (flask C) of Au seed solution, 1 mL of 1% by mass sodium citrate, and 3 mL of 10 mM ascorbic acid were added. The mixtures were stirred for 5 min at room temperature. Finally, 0.47 mL (flask A), 0.70 mL (flask B), and 2.4 mL (flask C) of 10 mM AgNO3 were dropped in at a rate of 0.15 mL/min using a syringe controlled by a step motor during vigorous stirring of the three mixtures. After synthesis, the sols were aged at 100 °C for 2 h. 2.3. Addition of the SiO2 Shell. Our method for addition of the SiO2 shell is based on that of Liz-Marzan et al.32 A total of 9 mL of the 100 nm Au-seed Ag-growth nanoparticle solution was put into a 100 mL round-bottom flask with 21 mL of water. The pH was decreased to 5 by adding 0.1 M H2SO4. Next, 0.4 mL of 1 mM APTMS was added drop-by-drop into the solution with vigorous stirring. After this addition, the solution was stirred for another 15 min. Then 3.2 mL of sodium silicate solution, diluted from the as-purchased concentration of 27% down to 0.54%, was added. After 3 more minutes of vigorous stirring, the pH was increased to 11.5 by adding 0.5 M NaOH. Finally, the mixture was placed in a 100 °C water bath and stirred for 30, 60, or 90 min to obtain a SiO2 shell thickness of 1, 2, or 3 nm, respectively. The influence of solution pH on the SiO2 shell is detailed in Supporting Information section 2.

limitations of TERS in this context are the weak signal obtained and the sophisticated equipment required. With our shellisolated nanoparticle-enhanced Raman spectroscopy (SHINERS) method,21,22 both of these weaknesses are overcome while the benefit of TERS for substrate versatility is retained. In SHINERS, a SERS-active nanoparticle is surrounded by a chemically inert dielectric shell. The resulting core−shell nanoparticles are then spread over a surface of any material and any morphology in order to obtain an enhanced Raman signal from an analyte or a probe molecule located there (Figure S1 in the Supporting Information). As in TERS, the signal-enhancing structure is separated from the surface of interest. In SHINERS, however, the equivalent of hundreds or thousands of tips are excited at the same time in a 2−5 μm laser spot and a SHINERS signal which is hundreds or thousands of times stronger than a TERS signal can be obtained using a standard Raman microscope (though of course the ∼10 nm spatial resolution of TERS, another of its great benefits, is lost in doing so). The synthesis of SHINERS nanoparticles having spherical and rod-shaped cores of Au, and having ultrathin shells of SiO2, Al2O3, and MnO2, are detailed in some of our previous works.23−25 Au-core SHINERS nanoparticles have already been used to obtain Raman spectra from pyridine and thiocyanate on Au(100), Au(111), and Au(110) surfaces, carbon monoxide on Pt(111) surfaces, hydrogen on Si(111), Pt(111), and Rh(111) surfaces, and even a parathion pesticide residue on an orange skin.21,26−28 It is widely known that surface plasmon efficiency is greater for Ag than it is for Au. Another advantage of Ag over Au is that the surface plasmon resonance (SPR) of Ag nanostructures can be tuned to any wavelength in the visible region of the spectrum.2 Ag is also much cheaper than Au. However, most SERS applications employ Au instead of Ag because Ag is more mobile and more easily oxidized, properties which adversely influence SERS signal stability and reproducibility. If Ag nanostructures can be stabilized by a chemically and electrically inert shell, these weaknesses can be managed. For all of the reasons just described, we have synthesized, characterized, tested, and modeled SHINERS nanoparticles with a mostly Agcore instead of a Au-core in the present study. Further exploration of the behavior and usefulness of our Ag@SiO2 nanoparticles will be published in a series of future works; this paper is intended to present only their synthesis and characterization, as well as some preliminary experimental testing and some field enhancement calculations which show their great promise.

3. RESULTS AND DISCUSSION 3.1. Au-Seed Ag-Growth Nanoparticles. 3.1.1. Characterization of Au-Seed Ag-Growth Nanoparticles. SEM images of the Au-seed Ag-growth nanoparticles are provided in Figure 1. The nanoparticles prepared in this study were in general

2. EXPERIMENTAL METHODS 2.1. Instruments and Reagents. The Au-seed Ag-growth nanoparticles and the Ag@SiO2 core−shell nanoparticles were imaged using a LEO1530 scanning electron microscope (SEM) and a Tecnai F30 transmission electron microscope (TEM). X-ray photoelectron spectroscopy (XPS) was carried out on a PHI Quantum 2000. Raman spectra were recorded using a Renishaw inVia Raman microscope. All reagents were of analytical grade and purchased from the Sinopharm Chemical Reagent Company Limited except chloroauric acid, (3-aminopropyl) trimethoxysilane (APTMS), and sodium silicate which were purchased from Johnson Matthey, Alfa Aesar, and Sigma Aldrich, respectively. Milli-Q water (18.2 MΩ cm) was used throughout the study, and all glassware was cleaned with aqua regia before use. 2.2. Synthesis of Au-Seed Ag-Growth Nanoparticles. It is quite challenging to prepare spherical Ag nanoparticles with a uniform

Figure 1. SEM images of the Au-seed Ag-growth nanoparticles: 50 ± 5 nm (a), 70 ± 5 nm (b), and 100 ± 10 nm (c).

quasi-spherical; however, some rodlike nanoparticles were produced as well. It was observed that both the distribution of shape and the distribution of size increased with nanoparticle size (Figure S3 in the Supporting Information). Figure 2A shows the extinction spectrum for the 12 nm Au seed solution. Figure 2B−D shows the extinction spectra for B

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size, and these increase with nanoparticle size. This lack of monodispersity prevented us from carrying out meaningful studies with even larger nanoparticles. We also used the three-dimensional finite-difference timedomain (3D-FDTD) method to examine EM field strength in the vicinity of our Au-seed Ag-growth nanoparticles. The result obtained for the 70 nm nanoparticle in air upon irradiation with a 532 nm excitation source is presented in Figure S4 of the Supporting Information. We note that the Au seed modeled was 24 nm instead of 12 nm to avoid distortions. The calculated data shows an EM field enhancement, averaged over the surface of the nanoparticle, of 13.8. EM field enhancements at the “hotspots” were calculated to be 2.0 × 103. Table 1 shows that the average and maximum EM field enhancements increase with increasing nanoparticle size, from 50 to 70 to 100 nm.

Figure 2. UV−vis extinction spectra for a solution of Au seeds of 12 nm (A, black), and for solutions of Au-seed Ag-growth nanoparticles of 50 ± 5 nm (B, red), 70 ± 5 nm (C, blue), and 100 ± 10 nm (D, green) after aging at 100 °C.

solutions of Au-seed Ag-growth nanoparticles having different sizes. A single peak occurs at about 520 nm in the former, but more complex spectra with a maximum in the 400−500 nm range are seen after Ag-growth to a size of 50, 70, and 100 nm. The absorption maximum in the 400−500 nm range can be attributed to the excitation of surface plasmons in the Ag. It is shifted to longer wavelength and it becomes broader as size increases. These changes in spectral features with nanoparticle size are consistent with what others have seen.29,33,34 The peak at about 520 nm in Figure 2A does not contribute much to the spectra in Figure 2B−D, and the plasmonic contribution from the Au seed is trivial. These nanoparticles have optical properties which are similar to those of Ag-only nanoparticles. Extinction decreases from Figure 2B−D as the volume of seed solution used in the synthesis decreases, and therefore, the concentration of nanoparticles in the solution also decreases. The effect of size on SERS intensity is shown in Figure 3. Auseed Ag-growth nanoparticles were cleaned three times by

Table 1. EM Field Enhancement Factors for Our Au-Seed Ag-Growth Nanoparticles nanoparticle diameter (nm)

Au diameter (nm)

Ag thickness (nm)

average field enhancement

field enhancement at the hotspots

50 70 100

24 24 24

19 29 44

8.6 13.8 23.8

1.9 × 103 2.0 × 103 3.3 × 103

3.2. Ag@SiO2 Core−Shell Nanoparticles. 3.2.1. Characterization of Ag@SiO2 Nanoparticles. TEM and high resolution TEM images of our SiO2-coated Au-seed Ag-growth (denoted Ag@SiO2) nanoparticles are shown in Figure 4. The Au seed can be seen as a dark spot in the center of the

Figure 3. (A) SERS spectra obtained from pyridine on Au-seed Aggrowth nanoparticles having different sizes dried on a Si wafer: 50 ± 5 nm (a, black), 70 ± 5 nm (b, red), and 100 ± 10 nm (c, blue). (B) Average ν1 mode (1009 cm−1) SERS intensity plotted as a function of nanoparticle size.

centrifuging and diluting with water, and then dried on a Si wafer. For each size of nanoparticle, 30 SERS spectra were acquired from different locations on the sample using pyridine as a probe molecule and 532 nm as the excitation wavelength. Typical spectra are presented in Figure 3A, and average ν1 mode (1009 cm−1) intensities are plotted against size in Figure 3B. The average SERS intensity for the ν1 mode of pyridine is around 5600, 14 700, and 27 800 counts per second (cps) for the 50, 70, and 100 nm nanoparticles, respectively. SERS intensity seems to increase in a linear way with nanoparticle size over the range investigated here. The error in SERS intensity increases with the distribution of shape and the distribution of

Figure 4. TEM images of Ag@SiO2 SHINERS nanoparticles with different shell thicknesses: 0.7 nm (a), 1 nm (b), 1.5 nm (c), and 3 nm (d). High resolution TEM images of a 2 nm SiO2 shell are also shown (e and f). C

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of 100 nm Au-seed Ag-growth nanoparticles (a in Figures 5 and 6) to the surface composition of 100 nm Ag@SiO2 SHINERS nanoparticles (b in Figures 5 and 6). Three component peaks underlie the experimentally observed peak shown in Figure 5a. We attribute the ones at 531.5 and 533.2 eV to oxygen atoms present in the sodium citrate stabilizing agent and oxygen atoms present in water. These assignments are in agreement with those of Tan et al.36 The component peak at 532 eV indicates that oxygen is present in other chemical states.37 Figure 5b reveals a 0.4 eV change in this peak, which may be due to O−Si bonds in the SiO2 shell.38 In Figure 6a, Ag 3d5/2 and Ag 3d3/2 peaks are located at 368.3 and 374.3 eV, respectively. These peak positions are similar to ones observed for bulk Ag.39 Another small peak is located at 367.9 eV, and it may be due to Ag−O bonding in Ag2O.40 Ag is easily oxidized when in contact with air, and Ag2O may have formed on the surface of the Au-seed Ag-growth nanoparticles after synthesis. In Figure 6b, the Ag 3d5/2 and Ag 3d3/2 peaks are shifted to lower energies and this could be due to the interaction between Ag and its environment.41 The absence of a Ag2O peak here indicates that the SiO2 shell protects the Auseed Ag-growth core from oxidation. One important requirement for successful SHINERS is that the ultrathin shell be pinhole-free. If it is not, its isolating function will be compromised and molecules from the surrounding environment will come into contact with the core. This may be a problem in studies where signal from the molecule on the core masks, or otherwise complicates, the signal of interest. It is therefore quite important that the ultrathin shell can be synthesized pinhole-free. It is difficult to identify pinholes in TEM or even high resolution TEM images of SHINERS nanoparticles. A more reliable test employs pyridine as a probe molecule (Figure 7). Here, some Ag@SiO2 nanoparticle solution is centrifuge-rinsed with water three times, concentrated, placed on a Si wafer, and dried under vacuum. Next, a drop of 10 mM pyridine is added and a quartz coverslip is placed over top. Pyridine is adsorbed on Ag, but it is not strongly adsorbed on the SiO2 shell or the underlying Si wafer. If pinholes are not present, pyridine will not be adsorbed and will not yield a SERS signal (Figure 7a). If pinholes are present, however, pyridine will pass through them and adsorb on the Ag. Then a SERS signal from pyridine on Ag will be observed (the symmetric ring breathing ν1 mode and the symmetric triangular ring deformation ν12 mode may be seen at 1008 and 1036 cm−1, respectively, in Figure 7b). We have found that the SiO2 shell must be ≥2 nm thick in order to be pinhole-free using the present synthetic method.

nanoparticle in Figure 4d, and this is consistent with observations made by Srnova-Sloufova et al.35 In all of Figure 4a−f, the SiO2 shell is seen as a bright region surrounding the mostly Ag-core. It is ultrathin and compact. The SiO2 shell thicknesses given in the figure caption are average values obtained by examining 30 to 50 nanoparticles on each sample. Extinction spectra of some Ag@SiO2 SHINERS nanoparticle solutions are compared to the extinction spectrum of the corresponding Au-seed Ag-growth nanoparticle solution in Figure S5 of the Supporting Information. The plasmon band shifts to the red slightly, from 469 nm when no shell is present to 472 nm when a 2 nm SiO2 shell is present. This shift is negligible, and Figure S5 shows that the SiO2 shell has essentially no effect on the optical properties of our Au-seed Ag-growth nanoparticles. Figures 5 and 6 show the XPS spectra of O 1s and Ag 3d, respectively. We used XPS to compare the surface composition

Figure 5. XPS spectra of O 1s in 100 ± 10 nm Au-seed Ag-growth nanoparticle (a) and 100 ± 10 nm Ag@SiO2 nanoparticle (b) samples.

Figure 6. XPS spectra of Ag 3d in 100 ± 10 nm Au-seed Ag-growth nanoparticle (a) and 100 ± 10 nm Ag@SiO2 nanoparticle (b) samples.

Figure 7. Pyridine test for a lack of pinholes in the shell. If pinholes are not present, as is the case with our 2 nm shell, pyridine will not be adsorbed on SiO2 and will not yield a SERS signal (a). If pinholes are present, however, as is the case with our 1 nm shell, pyridine will be adsorbed on Ag and will yield a SERS signal (b). Each of these spectra is an average of 30 which were acquired from randomly selected spots on the sample. D

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Figure 8. Spectra acquired from pyridine on smooth Au without (a) and with (b) Ag@SiO2 SHINERS nanoparticles. Each of these spectra is an average of 30 which were acquired from randomly selected spots on the sample.

Ag@SiO2 SHINERS of Pyridine on a Smooth Au Substrate. Raman spectra were collected from pyridine on a smooth Au surface without (Figure 8a) and with (Figure 8b) our 100 nm Ag @ 2 nm SiO2 nanoparticles. The experimental procedure used here (Figure 8) is similar to the one used when checking for a lack of pinholes in the shell (Figure 7). There are a couple of important differences between the two sets of experiments, however: pyridine is adsorbed on the underlying Au surface here whereas it was not adsorbed on the underlying Si surface there, and we used a 633 nm excitation wavelength to excite Ag-nanoparticle to Au-surface plasmon coupling here whereas we used a 532 nm excitation wavelength to excite Agnanoparticle to Ag-nanoparticle plasmon coupling there. No Raman signal was obtained from pyridine on smooth Au in the absence of our Ag@SiO2 nanoparticles (Figure 8a). This is because ordinary Raman scattering is not sensitive enough to detect a thin film of molecules, and a Au surface must be nanostructured in order to support the surface plasmon activity required for SERS. When Ag@SiO2 nanoparticles were added, however, the ring breathing ν1 mode, the ring deformation ν12 mode and the ring stretching ν8a mode were observed at 1012, 1039, and 1599 cm−1, respectively. We used Au instead of Ag for the underlying surface because the SERS spectrum of pyridine on Au differs from that of pyridine on Ag. The spectral differences seen in Figures 7 and 8 therefore allow us to confirm that pyridine is adsorbed on the smooth Au surface and not on the Ag surface of the SHINERS nanoparticle core through pinholes. 3D-FDTD Simulations of Ag@SiO2 and Au@SiO2 Nanoparticles on Ag and Au Substrates. We examined the distribution of EM field strength surrounding a 100 nm Ag @ 2 nm SiO2 nanoparticle on a smooth Au surface using the 3D-FDTD method (Figure 9). The incident light had a wavelength of 532 nm, and it was polarized in a direction parallel to the underlying Au. Since the contribution from the Au seed is trivial and the core has optical properties which are

similar to those of a Ag-only core, the Au seed was omitted to simplify our model. The region of greatest field strength was found to occur in the junction between the nanoparticle and the surface where an adsorbate would be located in a typical SHINERS experiment. The enhancement factor was calculated to be 6.4 × 103 for this “hotspot”. It is rather small but it is reasonable as the surface plasmon activity of Au is poor when the excitation wavelength is less than about 600 nm. We also calculated EM field strength surrounding a pair of 100 nm Ag @ 2 nm SiO2 nanoparticles on a perfectly smooth Ag substrate during excitation with green light (Figure 10). A 2

Figure 10. (a) Side view showing a pair of 100 nm Ag @ 2 nm SiO2 nanoparticles on a perfectly smooth Ag substrate. (b) Top view showing EM field distribution contours in the plane between the nanoparticles and the smooth Ag surface, where probe molecules would lie during a typical SHINERS experiment. The incident light had a wavelength of 532 nm and a polarization as shown.

nm gap was left between the nanoparticle shells for a total of 6 nm between the cores at their point of closest contact. The incident light had a wavelength of 532 nm, and it was polarized along the interparticle axis and parallel to the underlying Ag. The region of greatest EM field strength was found to occur in the nanoparticle-to-surface junction rather than in the nanoparticle-to-nanoparticle junction. It is generally believed that the “hotspot” would be located in the nanoparticle-tonanoparticle junction, but it can be transferred by introducing a Ag (or Au) substrate. This result is consistent with our previous work.26 Transfer of the “hotspot” to the nanoparticleto-surface junction is a tremendous advantage because analyte or probe molecules are adsorbed on the substrate during a typical SHINERS experiment. The maximum SHINERS enhancement factor was found to be 5.7 × 108. We then compared the two Ag@SiO2 nanoparticles on Ag system shown in Figure 10 to three other systems (Table 2). It may be seen in Table 2 that Ag@SiO2 nanoparticles yield a greater maximum enhancement factor than Au@SiO2 nanoparticles when irradiated with a 532 nm excitation source, and that this is the case no matter what the underlying substrate material as long as it remains the same for both pairs of

Figure 9. EM field surrounding a 100 nm Ag @ 2 nm SiO2 nanoparticle on a smooth Au surface. The incident light had a wavelength of 532 nm and a polarization as shown. E

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Table 2. Maximum Enhancement Factors for a Pair of Core−Shell Nanoparticles on a Substrate Using a 532 nm Excitation Wavelength situation modeled maximum enhancement factor

100 nm Ag @ 2 nm SiO2 on Ag 100 nm Au @ 2 nm SiO2 on Ag 100 nm Ag @ 2 nm SiO2 on Au substrate substrate substrate 5.7 × 108 1.0 × 106 1.1 × 105

Author Contributions

nanoparticles under consideration. It is also of interest that the maximum enhancement factor depends on the substrate material more so than it does on the nanoparticle core material. A Ag substrate is preferable to a Au one when a green excitation wavelength is employed. In comparison with the single Ag@SiO2 nanoparticle on Au system (Figure 9), the pair of Ag@SiO2 nanoparticles on Au system exhibits a greater maximum enhancement factor, 1.1 × 105 versus 6.4 × 103. The maximum enhancement factor obtained for Ag@SiO2 nanoparticles on Ag is roughly 2 orders of magnitude greater than that for Au@SiO2 nanoparticles on Ag, and roughly 5 orders of magnitude greater than that for Au@SiO2 nanoparticles on Au. Further work to investigate the benefits of our new Ag@SiO2 nanoparticles, using a probe molecule that undergoes resonance Raman scattering, are currently under way in our lab.

§

These two authors contributed equally to this work and are co-first authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the NSF of China (21021002 and 21033007) and the Innovation Method Fund of China (2010IM040100).

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ASSOCIATED CONTENT

S Supporting Information *

Overview of SHINERS, the influence of solution pH on the SiO2 shell, Au-seed Ag-growth nanoparticle size distribution, 3D-FDTD simulation of EM field surrounding a Au-seed Aggrowth nanoparticle, and the effect of SiO2 shell thickness on SPR. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Feldheim, D. L.; Foss, C. A., Jr. Metal Nanoparticles, Synthesis,Characterization and Applications; Marcel Dekker, Inc.: New York, 2002. (2) Le Ru, E. C.; Etchegoin, P. G. Principles of Surface-Enhanced Raman Spectroscopy and Related Plasmonic Effects; Elsevier: Amsterdam/Oxford, 2009. (3) Kneipp, K.; Moskovits, M.; Kneipp, H. Surface-Enhanced Raman Scattering Physics and Applications; Springer-Verlag: Berlin, Heidelberg, 2006; Vol. 103. (4) Aroca, R. Surface-Enhanced Vibrational Spectroscopy; John Wiley & Sons Ltd: Chichester, 2006. (5) Moskovits, M. Surface-enhanced Raman spectroscopy: a brief retrospective. J. Raman Spectrosc. 2005, 36, 485−496. (6) Raether, H. Surface plasmons on smooth and rough surfaces and on gratings; Springer-Verlag: Berlin, 1988. (7) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with plasmonic nanosensors. Nat. Mater. 2008, 7, 442−453. (8) Lee, K. S.; El-Sayed, M. A. Gold and silver nanoparticles in sensing and imaging: Sensitivity of plasmon response to size, shape, and metal composition. J. Phys. Chem. B 2006, 110, 19220−19225. (9) Wang, H.; Brandl, D. W.; Nordlander, P.; Halas, N. J. Plasmonic nanostructures: Artificial molecules. Acc. Chem. Res. 2007, 40, 53−62. (10) Ren, B.; Liu, G. K.; Lian, X. B.; Yang, Z. L.; Tian, Z. Q. Raman spectroscopy on transition metals. Anal. Bioanal. Chem. 2007, 388, 29−45. (11) Tian, Z. Q.; Ren, B.; Wu, D. Y. Surface-enhanced Raman scattering: from noble to transition metals and from rough surfaces to ordered nanostructures. J. Phys. Chem. B 2002, 106, 9463−94839483. (12) Park, S.; Yang, P.; Corredor, P.; Weaver, M. J. Transition MetalCoated Nanoparticle Films: Vibrational Characterization with SurfaceEnhanced Raman Scattering. J. Am. Chem. Soc. 2002, 124, 2428−2429. (13) Tian, Z. Q.; Ren, B.; Li, J. F.; Yang, Z. L. Expanding generality of surface-enhanced Raman spectroscopy with borrowing SERS activity strategy. Chem. Commun. 2007, 3514−3534. (14) Stöckle, R. M.; Suh, Y. D.; Deckert, V.; Zenobi, R. Nanoscale chemical analysis by tip-enhanced Raman spectroscopy. Chem. Phys. Lett. 2000, 318, 131−136. (15) Pettinger, B.; Picardi, G.; Schuster, R.; Ertl, G. Surface enhanced Raman spectroscopy: towards single molecule spectroscopy. Electrochemistry 2000, 68, 942. (16) Hayazawa, N.; Inouye, Y.; Sekkat, Z.; Kawata, S. Metallized tip amplification of near-field Raman scattering. Opt. Commun. 2000, 183, 333−336. (17) Anderson, M. S. Locally enhanced Raman spectroscopy with an atomic force microscope. Appl. Phys. Lett. 2000, 76, 3130−3132.

4. CONCLUSIONS It has been shown that 100 nm Ag nanoparticles have greater SERS activity than smaller Ag nanoparticles when a 532 nm excitation wavelength is employed. Therefore, they were selected for coating with 1, 2, and 3 nm of SiO2. Pinholes were present in the 1 nm shells; however, pinhole-free shells as thin as 2 nm could be prepared when due care was given to the solution pH during synthesis. The resulting Ag@SiO 2 SHINERS nanoparticles were characterized by TEM, XPS, UV−vis spectroscopy, Raman scattering measurements, and 3D-FDTD simulations. The 3D-FDTD calculations show that the maximum enhancement factor obtained for Ag@SiO2 nanoparticles on smooth Ag is roughly 2 orders of magnitude greater than that for Au@SiO2 nanoparticles on smooth Ag, and roughly 5 orders of magnitude greater than that for Au@ SiO2 nanoparticles on smooth Au, under 532 nm (green light) irradiation. Ag is more SERS-active than Au, it is more versatile with respect to excitation wavelength, and it is cheaper. Our approach helps overcome the stability and reproducibility challenges encountered when Ag is employed as a Raman signal amplifier, and it increases the breadth of potential applications for metal colloid nanoplasmonics.

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100 nm Au @ 2 nm SiO2 on Au substrate 2.8 × 103

AUTHOR INFORMATION

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*Telephone: +86-592-2186979. Fax: +86-592-2183407. E-mail: [email protected]. F

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dx.doi.org/10.1021/la3005536 | Langmuir XXXX, XXX, XXX−XXX