Modulation of Localized Surface Plasmons and SERS Response in

J. Phys. Chem. C 2010, 114, 10417–10423

10417

Modulation of Localized Surface Plasmons and SERS Response in Gold Dumbbells through Silver Coating M. Fernanda Cardinal,†,‡ Benito Rodrı´guez-Gonza´lez,† Ramo´n A. Alvarez-Puebla,† Jorge Pe´rez-Juste,† and Luis M. Liz-Marza´n*,† Departamento de Quı´mica Fı´sica and Unidad Asociada UniVersidade de Vigo-CSIC, Vigo, 36310, Spain, and International Iberian Nanotechnology Laboratory, Braga, 4710229, Portugal ReceiVed: March 19, 2010; ReVised Manuscript ReceiVed: May 1, 2010

We describe the modulation of localized surface plasmons in gold nanodumbbells through stepwise silver coating, along with a detailed discussion regarding the experimental parameters affecting the final core-shell morphology. Interestingly, whereas conformal growth was observed for thin coatings, for intermediate and high silver salt concentrations, the final nanoparticles end up with either rod-like or irregular faceted morphologies as a consequence of anisotropic silver growth. Upon silver reduction, pronounced changes in the optical properties were observed, which could be modeled using the boundary element method (BEM), which also allowed the assignment of different plasmon modes. Such core-shell Au@Ag nanoparticles are expected to serve as excellent SERS substrates, as significantly higher enhancement factors are expected for silver as compared to gold. Optical enhancing properties for SERS were tested with two laser lines, evidencing significantly larger enhancement factors for the bimetallic nanoparticles, as compared to those of gold. Research on metal nanoparticles has enormously progressed from various points of view, related for example to catalysis, drug delivery or sensing. Particular interest has been dedicated to their plasmonic optical response, associated with localized surface plasmon resonances, typically in the visible and near IR, for which a wide range of applications have been found. It has nowadays been well established that the localized surface plasmon frequency can be strongly affected by multiple parameters, but mainly by particle size, shape and composition, as well as the dielectric environment.1-5 Among a wide range of shapes,6-8 nanorods are still considered an extremely useful and flexible morphology, mainly because their synthesis can be routinely achieved at laboratory scale and because their optical response can be tuned over a wide spectral range, through variations in the aspect ratio, but also in small morphological details. In particular, a number of groups have found that, under certain experimental conditions, the obtained nanorods display dogbone- or dumbbell-like shapes. We have recently reported the controlled synthesis of such shapes, and analyzed in detail the effects of the corresponding tip deformations on the transverse and longitudinal plasmon modes. Interestingly, the UV-visible spectrum of a solution of small dumbbell-like gold nanoparticles is very similar to that for nanorods, with two bands that arise from two well-defined, dipolar modes, corresponding to coherent oscillation of conduction electrons, either parallel (longitudinal surface plasmon, LSP) or perpendicular (transverse surface plasmon, TSP) to the nanoparticle major axis. However, the relative band intensities and in particular the energy of the LSP are significantly affected by growth of the tips in the dumbbell, with a shift toward lower energies. Many applications of plasmonic nanoparticles require the optical analysis to be made in the visible, as well as narrow and intense plasmon bands. This can be obtained by using silver † Departamento de Quı´mica Fı´sica and Unidad Asociada Universidade de Vigo-CSIC. ‡ International Iberian Nanotechnology Laboratory.

rather than gold as the optical material, since the interband transitions for silver are restricted to the UV, so that plasmon resonance damping is minimized.9-11 As an example of a practical consequence, silver nanoparticles have been most often used as substrates for surface enhanced Raman scattering (SERS), because notably higher enhancement factors are obtained as compared to gold.12 Unfortunately, the synthesis of silver nanoparticles (nanorods in particular) with high size and shape monodispersity and tunability has been shown to be significantly harder than for gold. One possible approach to achieve nanoparticles with well-defined morphology and tailored optical response comprises the homogeneous coating of premade gold nanoparticles with silver. This approach has been demonstrated, with varied success, for different morphologies such as spheres,13,14 triangles15 and nanorods.16,17 Of particular interest is silver deposition on gold triangular prisms and nanorods since the combination of controlled coating and varying anisotropy leads to a narrow tuning of the surface plasmon resonance frequency along the visible and the near IR. These tunable optical properties, together with the presence of corners and edges make such coated particles ideal candidates for SERS studies, but their capabilities have not been studied in detail. So far, bimetallic rather than core-shell nanostructures have been proposed as good candidates as SERS media.10,18 Of particular relevance is the work by Tsukruk and co-workers,19 where bimetallic nanostructures based on the assembly of gold nanoparticles on 1D and 2D silver nanostructured surfaces were proposed as substrates for SERS due to the possibility to tune their respective plasmon resonances. Working along this line of thinking, we have developed and characterized a novel class of nanoparticles through gradual (but anisotropic, as discussed below) silver coating of gold nanodumbbells. Through this process, we have been able to achieve a fine modulation of the optical properties, mainly through variations in the LSP. The experimental UV-visible spectra were analyzed and compared with simulations based on the boundary element method (BEM),20,21 which allows numerical

10.1021/jp102519n  2010 American Chemical Society Published on Web 05/18/2010

10418

J. Phys. Chem. C, Vol. 114, No. 23, 2010

Fernanda Cardinal et al.

Figure 1. Time evolution of UV-visible-NIR extinction spectra during silver reduction over gold nanodumbbells.

resolution of Maxwell’s equations in frequency space. Since high resolution transmission electron microscopy (HRTEM) images show well-defined interfaces between the two metals, each metal was described in the simulations by tabulated dielectric functions that depend only on the frequency of light applying the local approximation. Calculations of the extinction cross section and near-field maps were carried out for gold dumbbells and Au@Ag core-shell nanoparticles of different morphologies, according to the experimental results. Additionally, assignment of transverse and longitudinal plasmon modes was achieved by simulating the extinction cross sections for incident light with different polarizations and comparing these data with experimental spectra from measurements on aligned nanorods. Finally, the near-field enhancement distributions were also analyzed and their relevance toward surface-enhanced Raman scattering was discussed. Experimental demonstration of the improved efficiency of these bimetallic nanoparticles as optically active substrates for SERS was demonstrated for two excitation laser lines in the visible. Results and Discussion The synthesis of gold dumbbells (AuDBs) was based on a recently reported method,22 comprising the seeded growth on preformed Au nanorods, through reduction of HAuCl4 with ascorbic acid, in the presence of a small amount of KI. The preferential adsorption of iodide ions on {111} tip facets was proposed as the main reason behind selective growth at the tips of the initial rods. Upon synthesis, AuDBs were used as new seeds for subsequent Ag growth. Although ascorbic acid was also used as reducing agent, it was necessary to increase the pH, as previously described for core-shell spheres.13 The success of the reduction was reflected on the color of the colloid, which gradually changed from pale reddish to green, as the principal (LSP) band blue-shifted. A representative UVvisible-NIR spectral evolution during silver growth on AuDBs is displayed in Figure 1 for [Ag+]/[Au0] ) 1. It is obvious from the figure that, as the reaction proceeds, the LSP band gets significantly blue-shifted, whereas a new band develops at low wavelengths and the intensity increases in the whole spectrum. In few minutes, the optical response of the AuDBs was strongly modified, presumably arising from deposition of a silver shell. Given the obvious tunability of the spectral features that can be achieved when AuDBs are coated with silver, we carried out a series of experiments involving a systematic variation of silver concentration in the growth solution for a constant amount of dumbbells as seeds, as well as constant [AA]/[Ag+] molar ratio, CTAB concentration and pH (8-9). In this series, the [Ag+]/[Au0] molar ratio was varied from 0 to 2.3. As shown in Figure 2, when increasing silver concentrations were used, the

Figure 2. Upper panel: UV-visible-NIR extinction spectra of gold dumbbells (a) and bimetallic nanoparticles grown with increasing [Ag+]/ [Au0] molar ratios (0.3, 0.7, 1, 1.7, 2.3 from b to f). Lower panel: Representative TEM images corresponding to samples a, d and f (scale bars: 100 nm).

resulting LSP band was progressively blue-shifted with regard to the spectrum of the original AuDB colloid. In addition, as the silver concentration was increased, the peaks arising at lower wavelengths became more evident, which is in agreement with the results discussed in Figure 1 and indicative of the growth of thicker Ag shells for higher [Ag]/[Au] ratios. Apart from the spectral changes, direct evidence of the growth of silver shells on the gold dumbbells was obtained through TEM (see Figure 2), with shell thickness increasing in good correlation with the [Ag+]/[Au0] molar ratio. From the appearance of the particles in these TEM images, we see that conformal growth was only obtained for rather low silver concentration, whereas for intermediate and high [Ag]/[Au] ratios, the obtained core-shell nanoparticles displayed either rodlike or irregular faceted morphologies, as a consequence of anisotropic silver growth. From a statistical analysis of additional TEM images for each sample, the dimensions were measured, both for the initial and all coated dumbbells with different [Ag+]/[Au0] ratios. A summary is provided in Table S1 (Supporting Information). Remarkably, these results indicate that the total length, as well as the thickness at the tips, of all coated rods did not change significantly. However, the thickness values measured at the center of the dumbbell did increase for higher silver concentrations, indicating anisotropic silver growth, which would initially cover the central (convex) part until a rod-like shape was reached and then growing into more irregular and faceted structures. A more detailed characterization of the nanoparticles was carried out by means of HRTEM and elemental mapping through scanning transmission electron microscopy X-ray energy dispersive spectra (STEM-XEDS). HRTEM images of AuDBs (not shown) revealed that the particles are single crystals in general, though often {111} twin planes were present in the vicinity of the tips. Given the similarity between the face centered cubic (fcc) lattice parameters of gold and silver (4.078 Å for fcc-Ag and 4.086 Å for fcc-Au), it can be expected that silver grows epitaxially on the AuDB surface, as previously reported for core-shell spheres.13 Shown in Figure 3 (upper

Modulating Localized Surface Plasmons in AuDBs

Figure 3. Upper panel: TEM and HRTEM images showing the core-shell contrast, the arrows in the TEM image point to twinning planes extending over the core-shell boundary. The HRTEM image was obtained in the [110] direction; its corresponding FFT analysis is shown in the inset. Lower panel: STEM image showing mass-thickness contrast and STEM-XEDS elemental map of AuDB@Ag nanoparticles prepared with [Ag+]/[Au0] ) 1.

panel) are two representative TEM images of silver coated AuDBs prepared with [Ag+]/[Au0] ) 1 (sample d in Figure 2). The presence of distinct areas with contrast difference in the image confirms that Ag indeed grows on top of the AuDB core, rather than forming an alloy. The contrast difference between the Au core and the Ag shell (Figure 3) is mainly due to electron scattering differences between both metals. Additionally, the fast Fourier transform (FFT) of the HRTEM image in Figure 3 (see inset) shows a spot pattern characteristic of an fcc single crystal oriented on the [110] zone axis. Note that in this FFT there are no extra spots, which confirms very high quality epitaxial growth of the silver shell over the whole surface of the Au core. From this FFT analysis we can define an epitaxial relationship as (110)[110]Ag//(110)[110]Au. It is interesting to realize that these HRTEM images clearly demonstrate the preferential growth of the silver shell at the central part of the dumbbell, where the particle thickness was initially lower. In addition, the silver coating was found to reproduce the crystalline defects occasionally found at AuDBs, such as twin planes, as indicated by the arrows in Figure 3. An example of HRTEM image of such twin planes extending over the core-shell boundary is shown in Figure S1 (Supporting Information). We additionally carried out STEM-XEDS analysis for confirmation of the chemical composition of the different areas within the nanoparticles. The atomic percentage of each element in the nanopaticle was quantified (see Figure S2 in the Supporting Information). We show in the lower panel of Figure 3 a STEMHAADF image showing mass-thickness contrast, together with a STEM-XEDS elemental map for several silver coated AuDBs. The elemental map was acquired using the Au MR and Ag LR lines, and plotting their emission intensity in red and green colors, respectively. In the central area of the map, yellow and green spots are observed as the signal intensities for gold and silver are similar. It is again evident from both the mass-thickness contrast image and the elemental mapping that silver was preferentially deposited at the middle of the gold dumbbell, while a much lower amount of silver was present at the tips.

J. Phys. Chem. C, Vol. 114, No. 23, 2010 10419 In order to explain the preferential silver reduction at the central area of the AuDBs over the tips, we start analyzing some of the proposed mechanisms for the formation of gold nanorods and dumbbells. Regarding single crystal gold nanorods, Wang et al.23 reported that short NRs prepared by the electrochemical method are enclosed within eight {110} and {100} alternating lateral facets, with their tips being terminated by {100}, {110}, and {111} facets. Additionally, Liu et al.24 proposed that the predominance of high energy {110} and {100} facets on single crystal AuNRs could be explained by underpotential deposition (UPD) of silver. According to their rationalization, more open surfaces as {110} favor Ag atom UPD as more neighbors surround the adsorbed atoms. Liu et al. suggested that the silver monolayer over Au {110} acts as a strongly binding surfactant that protects the facet from further growth, reducing the rate of gold growth on it. They also mentioned that the silver layer could be oxidized and replaced by gold ions from solution. Finally they proposed that nanorod tips are only partially covered by metallic silver, and therefore grow faster, leading to the onedimensional growth along the [100] direction. Furthermore, Grzelczak et al.22 proposed that the formation of dumbbell-like nanoparticles from single crystal nanorods in the presence of iodide ions was induced by catalytic reduction of gold at the tips where the surface redox potential is lower due to preferentially adsorbed AgI and AuI onto {111} facets and presumably bromide ions remain on the lateral faces. Considering these previous reports, we might argue that silver UPD occurred preferentially on the lateral facets of the initial rods and remained there during tip growth for formation of dumbbells. Therefore, it is plausible that metallic silver adsorbed on the central part of AuDBs acted as a catalyst for further silver reduction. Moreover, due to the shape of AuDBs, many surface defects are likely to be present at the curved areas, making them more reactive and amenable for silver reduction. Finally, the overall surface area reduction might also trigger silver deposition at such curved areas. In samples with high silver content (f), two kinds of particles were observed: whereas some particles still display rod-shaped morphology with the gold dumbbell located in the center, a second type comprised irregular particles appearing flatter and more triangular. Transition from rods to boatlike or irregular shapes through silver deposition has been previously reported,25,26 and explained on the basis of the preferential growth of some facets over others. Upon complete coverage of the gold dumbbell with silver, the bimetallic particle should behave as a pure silver rod, which would explain the transition from rod to plane-shaped silver particles, which has been reported even for silver rods with equivalent facet index.25 Because of the unusual distribution of gold and silver within these particles, assignment of the different bands in the UV-visible spectra required theoretical modeling using a numerical method. We used the boundary element method (BEM),2,20,21 which allows us to carry out fast computation of the extinction spectra and electric near field maps, in particular for nanoparticles with axial symmetry, so that parametrization of the external profile for each region is sufficient. We thus assumed axial symmetry for the different morphologies obtained in the experiments, but it should be realized that this is not strictly true for those samples grown with high silver concentration (samples e and f in Figure 2), where faceting was dominant. The average dimensions for each experimental sample were obtained from TEM images and used in the design of the simulated analogues. The various morphologies used for modeling are schematically depicted in Figure 4, as well as the obtained total extinction cross section spectra. Comparison of

10420

J. Phys. Chem. C, Vol. 114, No. 23, 2010

Fernanda Cardinal et al.

Figure 4. Left: Geometrical models used for simulation of the optical response of the nanoparticles shown in Figure 2. Right: Angle-averaged, calculated extinction spectra of gold dumbbells (a′) and Au@Ag nanoparticles grown with increasing [Ag]/[Au] ratios (b′-f′).

the calculated spectra with the experimental ones shown in Figure 2 reveals a good agreement, both in the trend and in the spectral features found for the different samples. The observed shift in the LSP band can be assigned to the reduction in the aspect ratio of the nanoparticles (because of faster growth at the sides than at the tips), as well as to the increasing influence of the optical properties of silver over those of gold. In order to try and separate these contributions, we compared the simulated extinction spectrum for a geometry of model c′, one with silver shell and another with gold shell (i.e., AuDB@Au); when exchanging silver with gold shell, the position of the longitudinal band is significantly less blue-shifted with respect to the starting gold dumbbell (see Figure S3 in the Supporting Information). Therefore the nature of the metal added was significant, and we assume that the longitudinal blue shift tendency when increasing silver coating thickness is attributed mainly to the decrease in aspect ratio but also to the composition of the shell. Additionally, the increase in intensity observed in Figure 4 is also in good agreement with the experimental results, which is related to the increasing amount of silver metal from model b′ to f′ in the shell and also to the intrinsic properties of silver. The dielectric function of silver has a larger imaginary part than that of gold, which means that it is more absorptive.24 Also, silver presents stronger plasmon resonances compared to gold as the TSP band of gold dumbbells is damped due to its proximity to gold interband transitions whereas silver interband transitions are located in the UV and thus do not affect silver plasmon modes.27,35 Another general feature is that the experimental spectra are broader than the calculated ones, which is mainly related to inhomogeneous broadening arising from polydispersity of the colloidal samples, which is not present in the calculations made for one single nanoparticle. Regarding the deviations in the precise location of the bands, these are likely to arise from the simplifications of the models used, in particular for the samples with thicker shells. However, the simulations do confirm the possibility of tuning the optical properties of AuDBs through silver coating. Prior to the assignment of transverse and longitudinal plasmon modes through the computational modeling, we carried out a simple experimental test, through which the AuDB@Ag core-shell nanorods could be aligned within PVA films, and in turn the UV-visible spectra could be registered at different polarization angles, with respect to the rods’ long axis. As earlier reported,28 alignment of nanorods within PVA films was easily achieved by simply mixing the corresponding colloid with a PVA aqueous solution, letting it dry, and subsequently heating and stretching the nanorod-doped PVA film. A representative example is described in what follows, for the particles prepared

Figure 5. Left panel: Photographs of a stretched PVA film loaded with AuDB@Ag nanoparticles, illuminated with parallel (a) and perpendicular (b) polarized light. (c) UV-visible spectra of the same films before and after stretching, at different polarization angles. (d) Calculated extinction cross section (ECS) spectra for one particle under polarized incident light.

with [Ag]/[Au] ) 1.7 (see sample e in Figure 2). Visual inspection of the stretched films under illumination with parallel and perpendicular polarization (Figure 5) reveals a strong color change, indicating that preferential alignment of the rods has occurred. This is further confirmed from the experimental spectra shown in Figure 5c, which allow us to assign the bands located at low wavelengths to transverse modes, while the band centered at ca. 700 nm would be the longitudinal mode. Calculation of the extinction cross section spectra for a model nanoparticle with the average dimensions of the experimental core-shell rods was carried out using the BEM for discrete polarizations, and the results are also shown in Figure 5, with good agreement. In the calculated spectra, the band close to 400 nm is less pronounced than in the experimental ones, which might be due to increased absorption at 420 nm in the experimental sample because of partial nucleation of silver spheres. A final analysis of the different plasmon modes was carried out through calculation of near field maps for a given morphology, using two different polarization directions and the maximum wavelengths obtained in the corresponding spectra. For comparison, we also calculated the spectra and near field maps for Au and Ag pure nanorods with the total dimensions of the AuDB@Ag rods. A selected set of data is shown in Figure 6, including the extinction cross section spectra for three modeled nanoparticles (AgNR, AuNR and AuDB@Ag), together with the near-field plots for AgNR and AuDB@Ag. In order to compare the optical response of the three different nanorods, the model for AuDB@Ag was chosen to match experimentally measured average dimensions, and for silver and gold nanorod models the same tip curvature and overall dimensions were

Modulating Localized Surface Plasmons in AuDBs

Figure 6. Calculated extinction spectra (upper panel) and near-field enhancement distributions (|E/Eincident|2) for AuDB@Ag and AgNR in water. The illumination wavelengths corresponding to equivalent plasmon modes, as well as the polarization and illumination directions, are indicated.

maintained, so that the observed differences cannot be due to variations in size or shape. The plasmon maxima were thus determined from the simulated spectra, and the near field distribution maps were calculated at 367 and 666 nm for AgNR, at 357 and 691 nm for AuDB@Ag, with both transverse and longitudinal polarization of the electric field of incoming light. Only maps for incident polarization that maximizes the electric field enhancement are displayed for each plasmon mode, and the results confirm our initial assignment. From the calculated electric near field distribution, we find that the electric field enhancement at the longitudinal plasmon band obtained for a AuDB@Ag nanoparticle is similar to that obtained at a silver nanorod. (Note that the intensity scale bar is shared for both maps.) This is a good indication that AuDB@Ag nanoparticles can be proposed as excellent SERS substrates in the visible. The final part of this study is related to the optical enhancing properties of the different AuDB@Ag colloids. This is an important section, since the use of anisotropic silver nanoparticles for SERS is still poorly studied, in particular in solution. Most examples deal with silver nanoprisms, mainly prepared by nanosphere lithography,29 but also marginally with colloidal nanoprisms, deposited on substrates.30,31 Regarding bimetallic nanoparticles, SERS experiments have been demonstrated on AuAg alloyed solid solutions of nanoshells32 and nanocages,10 as well as on heteroaggregates comprising Ag nanowires and Au nanoparticles.19 In contrast with these previous reports, we

J. Phys. Chem. C, Vol. 114, No. 23, 2010 10421 carried out average SERS (on the dilute colloids) and compared the efficiency between the new segregated bimetallic AuAg nanorods with those of the starting seeds (AuNRs and AuDBs). Average SERS (obtained from an ensemble of colloidal particles, characterized by a stable intensity pattern with welldefined and reproducible frequencies and bandwidths) was acquired directly from the various nanoparticle colloids, using 1-naphthalenethiol (1NAT) as a Raman active probe. The results for the different samples upon excitation with two visible lasers are summarized in Figure 7. In all cases, the characteristic SERS enhanced signals of 1NAT were registered: ring stretching (1553, 1503, and 1368 cm-1), CH bending (1197 cm-1), ring breathing (968 and 822 cm-1), ring deformation (792, 664, 539, and 517 cm-1), and CS stretching (389 cm-1). However, the results obtained using the different laser lines show different trends. Upon excitation with the green laser line (532 nm), the signal intensities consistently increase as a function of silver content; whereas pure gold samples (AuNRs and AuDBs) display very low enhancement due to SPR damping by interband transitions, interband transitions for silver are confined to the UV and therefore higher enhancement is observed, in agreement with the experimental trend.8-10,33 However, upon excitation with the red line, the enhancing behavior is affected by the LSP position (see Figure 2) as in a typical surface-enhanced Raman excitation spectroscopy (SERES) experiment.32,34 The signal intensity increases as a better resonance coupling with the longitudinal plasmon mode is achieved, in full agreement with the electromagnetic nature of SERS. The small decrease between samples e and f arises from an additional blue shift in the plasmon resonance, away from the excitation laser line. Interestingly, and considering that the silver content in the Ag-citrate colloids is 100% as compared to 60% in the AuDB@Ag rods with highest intensity, these results clearly point toward geometrical parameters, rather than compositional ones, as the major cause of SERS enhancement. Conclusions Modulation of surface plasmons in AuDBs was achieved by means of silver coating. The deposition of a silver shell over gold dumbbells originated a significant blue shift of the longitudinal plasmon band, and additionally a new plasmon band arose and its intensity increased with reaction time and silver salt concentration. Characterization of the bimetallic nanoparticles evidenced epitaxial growth of silver onto AuDBs, but conformal growth was only obtained for rather low silver concentration, whereas for intermediate and high silver concentration, the final nanoparticles display rodlike and/or irregular, faceted morphologies, as a consequence of anisotropic silver growth. Elemental mapping of gold and silver clearly depicted the core-shell structure. The optical properties of the prepared bimetallic nanoparticles were modeled with BEMAX, obtaining reasonably good agreement with the experimental data. Additionally, assignment of transverse and longitudinal plasmon modes was achieved by simulating the extinction cross sections for polarized incident light. From calculated electric near field distributions, the electric field enhancement at the longitudinal plasmon band obtained for a bimetallic nanoparticle was found to be similar to that for a silver nanorod of the same dimensions, suggesting that AuDB@Ag nanoparticles are efficient SERS substrates, which was demonstrated through average SERS experiments. Experimental Section Chemicals. Tetrachloroauric acid (HAuCl4 · 3H2O), cetyltrimethylammonium bromide 95% (CTAB), L-ascorbic acid +99%

10422

J. Phys. Chem. C, Vol. 114, No. 23, 2010

Fernanda Cardinal et al.

Figure 7. Left: Average SERS spectra of 1NAT on AuDB@Ag, acquired in solution with green and a red laser lines. Right: Intensities of the ring stretching (1368 cm-1) band as a function of silver molar percentage in the particles. AuNR and AuDB: 0%. AuDB@Ag b, c, d, e, f: 23, 41, 50, 63, 70%, respectively. Ag-citrate: 100%.

(AA), sodium borohydride 99% (NaBH4), trisodium citrate dihydrate, hydrochloric acid 36% (HCl), silver nitrate +99% (AgNO3), sodium hydroxide pellets +97% (NaOH), and poly(vinyl alcohol) MW 70000-100000 (PVA) were purchased from Aldrich and used as received. Potassium iodide (KI) and 1-naphthalenethiol were obtained from Scharlab and Acros Organics, respectively. Milli-Q water with a resistivity higher than 18.2 MΩ cm was used in all preparations. Synthesis. Dumbbell-like gold nanoparticles were prepared through seeded growth of preformed nanorods (average aspect ratio ∼4, see Table S1 in Supporting Information) by reduction of HAuCl4 with ascorbic acid, in the presence of CTAB and small amounts of KI.19 Briefly, 0.1 mL of 0.05 M HAuCl4 was allowed to complex in 18.6 mL of 0.1 M CTAB at 27 °C, then 23 µL of 5 mM KI and 80 µL of 0.1 M AA were added, followed by addition of 1.2 mL of 2.3 mM gold nanorod seed solution under stirring. This gives the following ratios: [KI]/ [Au0] ) 0.04, [AA]/[Au3+] ) 1.6, [Au3+]/[Au0] ) 1.7. The asprepared AuDB solution was cleaned from excess AA and concentrated by centrifugation (6000 rpm, 30 min), the pellets were redispersed in water, and the final gold concentration was estimated (from the absorbance at 400 nm) to be 1.6 mM. Core-shell nanoparticles were prepared by coating AuDBs with silver in an aqueous solution containing CTAB, AgNO3, NaOH and ascorbic acid. Growth solutions were prepared with constant molar ratio [AA]/[Ag+] ) 20 and 0.096 mM of Au0 in 0.05 M CTAB, to which different amounts of AgNO3 were added to yield [Ag+]/[Au0] molar ratios of 0, 0.3, 0.7, 1, 1.7, 2.3. Finally, NaOH was added to adjust the pH to 8-9 (under vigorous stirring but avoiding foam formation whenever possible).13 Upon synthesis, the core-shell nanoparticles were centrifuged and redispersed in water twice before further characterization. Characterization. UV-vis-NIR spectra were measured with either Agilent 8453 or Cary 5000 spectrophotometers, using 10 mm path length quartz cuvettes. Low magnification transmission electron microscopy (TEM) images were obtained with a JEOL JEM 1010 microscope operating at an acceleration voltage of 100 kV. Analysis of TEM images was performed with Digital Micrograph (C) and Image J software. For the statistical analysis of the particles, the dimensions of more than 150 nanoparticles for each sample were measured, and the results are summarized in Table S1 (Supporting Information). High resolution (HRTEM) and scanning transmission electron microscopy (STEM) images were obtained with a JEOL JEM

2010 FEG-TEM microscope operating at an acceleration voltage of 200 kV. X-ray energy dispersive spectra (XEDS) were acquired using an Inca Energy 200 TEM system from Oxford Instruments, and elemental mapping data was acquired by coupling the X-ray spectrometer to a STEM unit, equipped with a high angle annular dark field (HAADF) detector. Mapping was performed with 0.7 nm probe size, and the acquisition time was limited to 60 s to avoid sample drift. Background subtraction was carried out prior to mapping overlap. Modeling. Calculations of extinction cross section and nearfield distribution, based on the boundary element method for axial symmetry (BEMAX),2 were run for gold dumbbells and core-shell Au@Ag nanoparticles of different morphologies. In BEM, the electromagnetic field is determined by a distribution of charges and currents at the boundary of each homogeneous region constitutive of the nanoparticle and by imposition of the boundary conditions a set of self-consistent surface integral equations are obtained. Each surface integral is approximated by a sum over N representative points distributed at each boundary, leading to a linear set of equations that is solved by linear algebra techniques. In particular, for particles possessing axial symmetry, only the contour of the particle needs to be parametrized, which allows much fast computation compared to other numerical models. As input for the simulation, the refractive index was taken constant for water (1.333) and for PVA (1.5), while for gold and silver, published frequencydependent dielectric data were used.33 The shape and dimensions of model particles used for the simulations were based on TEM observations of experimental nanoparticles. The number of parametrization points used was enough to ensure convergence of the results obtained. For nonpolarized incident light, calculations were averaged for 50 different polarization angles. SERS Measurements. Samples for average SERS were prepared by adding 10 µL aliquots of analyte solution (10-4 M of 1-naphthalenethiol) per milliliter of colloidal dispersion (10-4 M in total metal, Au0 + Ag0). After 1 h, allowing for thermodynamic equilibrium to be reached, average SERS was directly recorded from these suspensions. Standard silver citrate colloids34 were also prepared as a blank for comparison. The inelastically scattered radiation was collected with a Renishaw Invia system, equipped with Peltier charge-coupled device (CCD) detectors and a Leica confocal microscope. The spectrograph has 1800 g/mm gratings with additional band-pass filter optics. Samples were excited with two different laser lines at 532 (Nd:YAG) and 633 nm (He-Ne). Spectra were collected

Modulating Localized Surface Plasmons in AuDBs in Renishaw extended mode with accumulation times of 10 s with a macrosampling 90° objective adaptor. Acknowledgment. The authors thank F.J. García de Abajo for providing access to the BEM code. M.F.C. acknowledges a PhD student scholarship from INL. R.A.A.-P. and J.P.-J. acknowledge the RyC (MEC, Spain) program. This work has been funded by the Spanish Ministerio de Ciencia e Innovacio´n (Grants MAT2007-62696, MAT2008-05755, and Consolider Ingenio 2010-CSD2006-12 NANOBIOMED). Supporting Information Available: Table with dimensions, TEM/STEM images, and simulated spectra. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (2) Myroshnychenko, V.; Rodrı´guez-Ferna´ndez, J.; Pastoriza-Santos, I.; Funston, A. M.; Novo, C.; Mulvaney, P.; Liz-Marza´n, L. M.; Garcı´a de Abajo, F. J. Modelling the Optical Response of Gold Nanoparticles. Chem. Soc. ReV. 2008, 37, 1792–1805. (3) Bohren, C. F.; Huffman, D. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (4) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668–677. (5) Liz-Marza´n, L. M. Tailoring Surface Plasmon Resonance through the Morphology and Assembly of Metal Nanoparticles. Langmuir 2006, 22, 32–41. (6) Pe´rez-Juste, J.; Pastoriza-Santos, I.; Liz-Marza´n, L. M.; Mulvaney, P. Gold Nanorods: Synthesis, Characterization and Applications. Coord. Chem. ReV. 2005, 249, 1870–1901. (7) Grzelczak, M.; Pe´rez-Juste, J.; Mulvaney, P.; Liz-Marza´n, L. M. Shape Control in Gold Nanoparticle Synthesis. Chem. Soc. ReV. 2008, 37, 1783–1791. (8) Pastoriza-Santos, I.; Liz-Marza´n, L. M. N. N-Dimethylformamide as a Reaction Medium for Metal Nanoparticle Synthesis. AdV. Funct. Mater. 2009, 19, 679–688. (9) Moskovits, M. Surface-enhanced Spectroscopy. ReV. Mod. Phys. 1985, 57, 783–826. (10) Rycenga, M.; Hou, K. K.; Cobley, C. M.; Schwartz, A. G.; Camargo, P. H. C.; Xia, Y. Probing the Surface-enhanced Raman Scattering Properties of Au-Ag Nanocages at Two Different Excitation Wavelengths. Phys. Chem. Chem. Phys. 2009, 11, 5903–5908. (11) Aikens, C. M.; Schatz, G. C. TDDFT Studies of Absorption and SERS Spectra of Pyridine Interacting with Au20. J. Phys. Chem. A 2006, 110, 13317–13324. (12) Zhao, J.; Pinchuk, A. O.; McMahon, J. M.; Li, S.; Ausman, L. K.; Atkinson, A. L.; Schatz, G. C. Methods for Describing the Electromagnetic Properties of Silver and Gold Nanoparticles. Acc. Chem. Res. 2008, 41, 1710–1720. (13) Rodrı´guez-Gonza´lez, B.; Burrows, A.; Watanabe, M.; Kiely, C. J.; Liz-Marza´n, L. M. Multishell Bimetallic AuAg Nanoparticles: Synthesis, Structure and Optical Properties. J. Mater. Chem. 2005, 15, 1755–1759. (14) Fan, F.-R.; Liu, D.-Y.; Wu, Y.-F.; Duan, S.; Xie, Z.-X.; Jiang, Z.Y.; Tian, Z.-Q. Epitaxial Growth of Heterogeneous Metal Nanocrystals: from Gold Nano-Octahedra to Palladium and Silver Nanocubes. J. Am. Chem. Soc. 2008, 130, 6949–6951.

J. Phys. Chem. C, Vol. 114, No. 23, 2010 10423 (15) Xue, C.; Millstone, J. E.; Li, S.; Mirkin, C. A. Plasmon-driven Synthesis of Triangular Core-Shell Nanoprisms from Gold Seeds. Angew. Chem., Int. Ed. 2007, 46, 8436–8439. (16) Liu, M.; Guyot-Sionnest, P. Synthesis and Optical Characterization of Au/Ag Core/Shell Nanorods. J. Phys. Chem. B 2004, 108, 5882–5888. (17) Ah, C. S.; Hong, S. D.; Jang, D.-J. Preparation of AucoreAgshell Nanorods and Characterization of Their Surface Plasmon Resonances. J. Phys. Chem. B 2001, 105, 7871–7873. (18) Gao, L.; Fan, L.; Zhang, J. Selective Growth of Ag Nanodewdrops on Au Nanostructures: A New type of Bimetallic Heterostructure. Langmuir 2009, 25, 11844–11848. (19) Ko, H.; Singamaneni, S.; Tsukruk, V. V. Nanostructured Surfaces and Assemblies as SERS Media. Small 2008, 4, 1576–1599. (20) Garcı´a de Abajo, F. J.; Howie, A. Relativistic Electron Energy Loss and Electron-Induced Photon Emission in Inhomogeneous Dielectrics. Phys. ReV. Lett. 1998, 80, 5180–5183. (21) Garcı´a de Abajo, F. J.; Howie, A. Retarded Field Calculation of Electron Energy Loss in Inhomogeneous Dielectrics. Phys. ReV. B 2002, 65, 115418/1–17. (22) Grzelczak, M.; Sa´nchez-Iglesias, A.; Rodrı´guez-Gonza´lez, B.; Alvarez-Puebla, R.; Pe´rez-Juste, J.; Liz-Marza´n, L. M. Influence of Iodide Ions on the Growth of Gold Nanorods: Tuning Tip Curvature and Surface Plasmon Resonance. AdV. Funct. Mater. 2008, 18, 3870–3876. (23) Wang, Z. L.; Mohamed, M. B.; Link, S.; El-Sayed, M. A. Crystallographic Facets and Shapes of Gold Nanorods of Different Aspect Ratios. Surf. Sci. 1999, 440, 809–814. (24) Liu, M.; Guyot-Sionnest, P. Mechanism of Silver(I)-assisted Growth of Gold Nanorods and Bipyramids. J. Phys. Chem. B 2005, 109, 22192– 22200. (25) Lofton, C.; Sigmund, W. Mechanisms Controlling Crystal Habits of Gold and Silver Colloids. AdV. Funct. Mater. 2005, 15, 1197–1208. (26) Xiang, Y.; Wu, X.; Liu, D.; Li, Z.; Chu, W.; Feng, L.; Zhang, K.; Zhou, W.; Xie, S. Gold Nanorod-Seeded Growth of Silver Nanostructures: From Homogeneous Coating to Anisotropic Coating. Langmuir 2008, 24, 3465–3470. (27) Wang, H.; Tam, F.; Grady, N. K.; Halas, N. J. Cu Nanoshells: Effects of Interband Transitions on the Nanoparticle Plasmon Resonance. J. Phys. Chem. B 2005, 109, 18218–18222. (28) Pe´rez-Juste, J.; Rodrı´guez-Gonza´lez, B.; Mulvaney, P.; Liz-Marza´n, L. M. Optical Control and Patterning of Gold Nanorod-PVA Nanocomposite Films. AdV. Funct. Mater. 2005, 15, 1065–1071. (29) Camden, J. P.; Dieringer, J. A.; Zhao, J.; Van Duyne, R. P. Controlled Plasmonic Nanostructures for Surface-enhanced Spectroscopy and Sensing. Acc. Chem. Res. 2008, 41, 1653–1661. (30) Bae, Y.; Kim, N. H.; Kim, M.; Lee, K. Y.; Han, S. W. Anisotropic Assembly of Ag Nanoprisms. J. Am. Chem. Soc. 2008, 130, 5432–5433. (31) Chen, Y.; Wang, C.; Ma, Z.; Su, Z. Controllable Colours and Shapes of Silver Nanostructures based on pH: Application to Surface-enhanced Raman Scattering. Nanotechnology 2007, 18 (1-5), 325602. (32) Alvarez-Puebla, R. A.; Ross, D. J.; Nazri, G.-A.; Aroca, R. F. Surface-enhanced Raman Scattering on Nanoshells with Tunable Surface Plasmon Resonance. Langmuir 2005, 21, 10504–10508. (33) Etchegoin, P. G.; Le Ru, E. C.; Meyer, M. An Analytic Model for the Optical Properties of Gold. J. Chem. Phys. 2006, 125, 164705/1–3. (34) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Van Duyne, R. P. Wavelength-scanned Surface-enhanced Raman Excitation Spectroscopy. J. Phys. Chem. B 2005, 109, 11279–11285. (35) Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. ReV. B 1972, 6, 4370–4379. (36) Aroca, R. F.; Alvarez-Puebla, R. A.; Pieczonka, N.; Sanchez-Cortes, S.; Garcia-Ramos, J. V. Surface-enhanced Raman Scattering on Colloidal Nanostructures. AdV. Colloid Interface Sci. 2005, 116, 45–61.

JP102519N