Correlating the Shape, Surface Plasmon Resonance, and Surface

May 19, 2009 - Seney et al. conducted a similar size-dependent SERS study for NaBH4-reduced Ag colloids.(19) To our knowledge, only Orendorff et al...
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J. Phys. Chem. C 2009, 113, 10459–10464

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Correlating the Shape, Surface Plasmon Resonance, and Surface-Enhanced Raman Scattering of Gold Nanorods Hongyan Guo,† Fangxiong Ruan,‡ Linghui Lu,† Jiawen Hu,*,† Jiangao Pan,† Zhilin Yang,*,‡ and Bin Ren§ State Key Laboratory for Chemo/Biosensing and Chemometrics, Biomedical Engineering Center, and College of Chemistry and Chemical Engineering, Hunan UniVersity, Changsha 410082, China, and Department of Physics, and State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, Xiamen UniVersity, Xiamen 361005, China ReceiVed: March 3, 2009; ReVised Manuscript ReceiVed: April 14, 2009

We report the study correlating the shape, surface plasmon resonance (SPR), and surface-enhanced Raman scattering (SERS) of gold nanorods (NRs) in dilute colloids. A series of gold NRs with various aspect ratios was prepared via an improved seed-mediated technique. Increasing the aspect ratio finely tunes the position of the longitudinal plasmon mode of the NRs in a wide spectral range. This shape-dependent SPR behavior was simulated by Gans theory and the discrete dipole approximation method. The subtle influence of SPR on SERS was then demonstrated by gradually tuning the SPR wavelength across a fixed excitation line. SERS experiments and theoretically predicted electromagnetic enhancement by the three-dimensional finite-difference time domain method clearly demonstrate that overlapping SPR and the excitation line maximizes the SERS enhancement. This correlation thus enables a quick diagnosis of SERS intensity by looking at the position of the SPR band. Introduction Although the physical origin of surface-enhanced Raman scattering (SERS) is still not completely understood, it is now widely accepted that chemical enhancement and electromagnetic (EM) enhancement make 0-102 and 104-1012 enhancements to the total enhancement, respectively.1-3 The chemical enhancement mechanism describes a resonance-Raman-like process resulting from the charge transfer between metal and adsorbate.4 In contrast, the EM mechanism results from the optical excitation of the surface plasmon (collective oscillations of conduction electrons) of metal nanoparticles. The surface plasmon excitation generates a gigantic EM field at the surfaces of the particles, which largely enhances the Raman signals of the surface-adsorbed molecules.5 When the nanoparticles are in aggregated form, their plasmons couple and much more enhanced EM fields are generated at the particle junctions.6 Great experimental and theoretical efforts have firmly established that both surface plasmon resonance (SPR) and SERS depend on the size and shape of the particles, interparticle spacing, and environment.7-9 Therefore, to optimize SERS signals and to test the EM theory, it is highly desirable to correlate the SPR and SERS. To this end, a vast majority of studies have utilized solid substrate-supported nanostructures9-12 and single particles.13-15 Colloidal particles, on the other hand, are advantageous for theoretical analysis. In dilute colloids, particle-particle and solid substrate-particle couplings can be avoided, which facilitates the investigation of the size- and shape-dependent SPR and SERS properties. Furthermore, recent developments in colloid * To whom correspondence should be sent. J.H.: fax, +86-731-8821740; e-mail, [email protected]. Z.Y.: e-mail: [email protected]. † Hunan University. ‡ Department of Physics, Xiamen University. § State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, Xiamen University.

optics allow simulation of the optical properties of arbitrarily shaped particle.7 Therefore, colloidal nanoparticle-based SERS studies provide an alternative for understanding the relationship between SPR and SERS. Although colloid SERS substrates are widely used, the size- and shape-dependent SERS studies on colloidal nanoparticles are relatively limited. One of the reasons is that SERS from metal colloids composed of only nanoparticle monomers is too weak to be detected because the SPR of monomers usually has a weak or little coupling to the excitation line.16 The detected strong SERS signals are mainly contributed by aggregated particles (e.g., dimers or trimers), the number of which can even be down to a very small fraction of monomers.17 Recently, Njoki et al. reported a size-dependent SERS study for Au colloids and revealed the existence of a critical size.18 Beyond the critical size of 50 nm, the particle-particle interaction is operative and responsible for the SERS effect. They ascribed their observations to (i) SPR of the particles >50 nm initiating effective coupling with the laser line and (ii) possible formation of small particle aggregates. Seney et al. conducted a similar size-dependent SERS study for NaBH4reduced Ag colloids.19 To our knowledge, only Orendorff et al. conducted a shape-dependent SERS study in dilute colloids for silver and gold NRs with two and three aspect ratios (defined by the length divided by the width), respectively,20 while most authors focused on aggregated and/or supported gold and silver NRs or wires, in which particle- and supported substrate-particle couplings largely interfered with the extraction of shapedependent SPR and SERS properties.21-25 Gold NRs are of great interest because their anisotropy in dimensions leads their SPR to split into a transverse mode and a longitudinal mode.26,27 The latter can easily be tuned across the visible to the near-infrared region via increasing the length of the rods. This shapedependent SPR property makes gold NRs ideal candidates for refining our understanding of the relationship between SPR band and SERS intensity. Orendorff et al. did pioneering work toward

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this goal and generally concluded that overlapping the excitation line with the longitudinal SPR contributed an additional 10-102 enhancment.20 However, they used NRs with relatively limited aspect ratios for their study and lacked theoretical simulations. In this work, our aim is to systematically correlate the shapedependent SPR and SERS properties in dilute colloids. We first synthesized a series of gold NRs, whose λmax position of longitudinal SPR was finely tuned to cover across a wide range of 570-700 nm. This subtle manipulation in SPR allows one to gradually tune the SPR of gold NRs to overlap with a 632.8 nm excitation line and to gradually optimize the EM enhancement. Then, Gans theory26 and the discrete dipole approximation (DDA) method7 were used to simulate the shape-dependent SPR behaviors, and the three-dimensional finite-difference time domain (3D-FDTD) method28 was used to help understanding the shape-dependent SERS behaviors. This correlation enables a quick diagnosis of SERS intensity by looking at the position of the SPR band and thus facilitates the design of Au nanorodbased, near-infrared SERS nanoprobes that are currently of great interest.29 Experimental Section HAuCl4, cetyltrimethylammonium bromide (CTAB), ascorbic acid, AgNO3, and absolute ethanol, all of AR grade, were purchased from Shanghai Reagents Co. 4-Mercaptobenzoic acid (MBA) was purchased from Aldrich and dissolved in absolute ethanol. CTAB was crystallized twice before use and other reagents were used as received. Ultrapure water purified with a Milli-Q system was used throughout the study. All glassware was cleaned with aqua regia, thoroughly rinsed with Milli-Q water, and dried prior to use. Gold NRs with different aspect ratios were prepared via the seed-mediated techniques developed by Murphy’s group with a slight modification.30 Briefly, CTAB-protected gold seeds were prepared by adding ice-cooled 0.01 M NaBH4 (0.6 mL) to reduce 20.37 mM HAuCl4 (0.122 mL) added in a 0.05 M CTAB solution (7.5 mL). The CTAB-protected seeds were cultivated in a 40 °C water bath for 2 h before use. Subsequently, nine sets of growth solutions were prepared by sequentially mixing 30 mL of 0.05 M CTAB, different amount (20, 25, 40, 50, 55, 65, 70, 75, and 85 µL) of 0.01 M AgNO3, 0.735 mL of 2.037 × 10-2 M HAuCl4, and 0.24 mL of 0.1 M ascorbic acid. Upon the addition of ascorbic acid, the color of the growth solution changed from dark yellow to colorless. Finally, 0.3 mL of the CTAB-protected gold seeds was added to each growth solution. The solution was gently mixed for 1 min and then left undisturbed for 3 h to complete rod growth. The prepared gold NRs were separated from spheres and excess surfactants by two successive centrifugations and were redispersed in Milli-Q water. Then, the samples were kept in the dark to avoid possible photoinduced shape transition. UV-vis spectra of the NRs were acquired with a UV2300 spectrophotometer (Tianmei Co.) using a minitype 1-cm quartz cell. TEM images were acquired on a JEOL-1320 instrument at 100 kV. SERS spectra were measured by an Advantage 200A Raman spectrometer with a 632.8 nm laser (DeltaNu). Gold NR dispersions were sampled in 8 mm (in diameter) vials for SERS measurements. The laser power at the sample position was 3 mW, and SERS spectra over 200-3400 cm-1 with a resolution of about 10 cm-1 were obtained. Results and Discussion Gold Nanorod Formation. The strategy used here for preparing gold NRs is basically according to the seeded synthesis

Figure 1. TEM images of gold NRs with varying aspect ratios: (A) 1.5, (B) 1.9, (C) 2.3, (D) 2.4, (E) 2.7, (F) 2.8, (G) 2.9, (H) 3.1, and (I) 3.5.

method, in which CTAB-protected seeds grew into short rods in a growth solution with the assistance of AgNO3 and soft template CTAB.30 Although the role of AgNO3 is not completely understood, it is essential to largely improve the rod yield and restrain formation of nonrod-shaped particles. Sau et al. speculated that a silver bromide layer adsorbed at the gold seed {111} surface and slowed the subsequent gold growth step, leading to gold deposition on the other faces to produce a rod with {111} facets on its longitudinal sides.30 Therefore, to prevent the possible formation of AgCl, we adopted Nikoobakht’s method to prepare the growth solutions. That is, solutions of CTAB, AgNO3, HAuCl4, and the mild reducing agent ascorbic acid were taken one by one and gently mixed in a sampling vial.31 The amounts of all other agents except AgNO3 are identical for each growth solution. When the same amount of gold seeds was added into the growth solutions, the increase in the AgNO3 amount led to the variation of the NR aspect ratio. That can be judged from the color changes; with the increase of AgNO3 amount, the color of the final products changed from purple, to blue, and finally to red. Figure 1 shows a typical set of TEM images of gold NRs with different aspect ratios, which were used for the following studies. The samples have only a small amount of cubes and spheres that were not able to be separated from the dispersions by centrifugation. Counting more than 200 gold NRs gives the aspect ratio of 1.5 (length ) 24.7 nm, width ) 16.8 nm), 1.9 (length ) 29.9 nm, width ) 16.0 nm), 2.3 (length ) 19.0 nm, width ) 8.2 nm), 2.4 (length ) 25.3 nm, width ) 10.4 nm), 2.7 (length ) 26.6 nm, width ) 9.8 nm), 2.8 (length ) 30.0 nm, width ) 10.5 nm), 2.9 (length ) 28.1 nm, width ) 9.7 nm), 3.1 (length ) 32.7 nm, width ) 10.53 nm), and 3.5 (length ) 33.0 nm, width ) 9.5 nm) for the nine rod samples, respectively. It is noted that the width distribution of the as-prepared rods is relatively narrower than that reported previously.20 Their tunability in SPR and high quality in rod shape makes gold NRs greatly attractive for studying the shape-dependent optical properties. Extinction Spectra of Gold Nanorods with Different Aspect Ratios. Figure 2 shows the measured UV-vis spectra for the gold NRs with different aspect ratios, which were

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Figure 2. UV-vis spectra of gold NRs with varying aspect ratios as that in Figure 1.

normalized at the SPR peaks at longer wavelengths. Typically, their UV-vis spectra show one transversal SPR peak at 520 nm and longitudinal ones at 570-700 nm.26,27 As expected, a notable red-shift of the longitudinal SPR mode was observed with increasing aspect ratio. When the aspect ratio was increased from 1.5 to 3.5, longitudinal λmaxwas finely tuned to cover a wide spectral range of 570-700 nm, which allows one to find an optimized aspect ratio to maximize the SERS signals at a fixed excitation line of 632.8 nm and thus facilitates the correlation of the shape-dependent SPR and SERS properties. To understand these shape-dependent SPR behaviors, we first used Gans theory to simulate the extinction spectra for gold NRs.26 Gans theory is extended from Mie’s theory and provides a simple, exact solution to Maxwell’s equations that is relevant to particles. Gans introduced a geometrical factor Pj to calculate the optical absorption spectrum of a collection of randomly orientated elongated ellipsoids (the model of gold NRs) in the dipole approximation. For details, some excellent references are highly recommended.32-35 With the known values for the complex dielectric constant of gold,36 panel A in Figure 3 shows the calculated absorption spectra for gold NRs with different aspect ratios dispersed in a water medium (n ) 1.33). It can be seen that the increase in aspect ratio leads to a linear, pronounced red-shift for the longitudinal mode (see the insert in panel A) and a minor blue-shift for the transversal mode. This simulation is consistent with the simulations made by other authors.33,34 Furthermore, the relative intensity of the longitudinal mode increases with the increase of aspect ratio, which is caused by the enhanced scattering due to the increased particle size.37 The only variable in Gans theory is the geometrical factor Pj, which is closely related to aspect ratio. Although Gans theory quantitatively reveals the observed optical characteristics, it does not allow the conclusion that aspect ratio is the only key parameter determining the optical properties of gold NRs. Because the real shape of gold NRs obviously deviates from the ideal ellipsoid model, the related Maxwell’s equations cannot be solved analytically. We therefore referred to the DDA method to simulate the extinction spectra for gold NRs. The DDA method allows solving Maxwell’s equations numerically and has been extensively developed in the past few years.7,38 The basic principles of the DDA method are dividing the studied target as a finite, cubic array of polarizable point dipoles. Each dipole of the system is subjected to an electromagnetic field constituting the incident radiation field and the field radiated by all other induced dipoles. With the complex-conjugate gradient (CCG) algorithm and fast-Fourier-transform (FFT)

Figure 3. Simulations of the extinction spectra based on Gans theory (A) and the DDA method (B) for gold NRs with varying aspect ratios. The insert shows experiment-measured and theory-predicted longitudinal λmax positions as a function of rod aspect ratio.

techniques, a self-consistent solution for the dipole polarizations is obtained,39 which can then be used to determine key optical properties such as the extinction efficiency and the electromagnetic field near the particle surface.40 There is no restriction on the location of the cubic lattice sites, so the DDA is particularly useful for describing a particle of arbitrary shape and composition. Because the width of the gold NRs from batch to batch is not uniform (see Figure 1), for simplification we used hemisphere-end-capped cylinders with rod radius 12 nm and varying length as the models to simulate the extinction spectra for gold NRs. This simplification, as discussed in the following, will cause some deviation from experimental data. For the application of the DDA method, we thank Draine and Flatau for their DDSCAT source code.41 In Figure 3, panel B shows the DDA simulated spectra for gold NRs. In this instance, the DDA-predicted optical trends are comparable to the Gans results and, in general, simulate experimentally observed results. We compare the results predicted by Gans theory and DDA method with experimental data in the inset in Figure 3, panel A. Even at the same aspect ratio, the longitudinal λmax predicted by the DDA method differs from that predicted by Gans theory and also deviates from experimental observations. It is easy to understand the deviation between the DDA and Gans simulations because of the difference in their geometrical models, i.e., hemisphere-end-capped cylinder vs nanoellipsoid. Another reason that should be responsible for this deviation is the extrinsic size effect, which, due to retardation of the electromagnetic field across the NR, was taken into consideration in the DDA calculation while it was neglected in Gans theory. Therefore, aspect ratio is one main parameter42 but not the sole parameter43 determining the extinction spectra. A more detailed DDA simulation by Prescott et al. has revealed that the rod width, rod end-cap geometry (flat, oblate spheroid, and sphere), and the rod size distribution all have a significant effect on the position of the peak absorbance.43 That may also account for the difference between DDA-predicted longitudinal λmax and experimental values because the hemisphere-end-capped cylinder model only approximates other than equates the real shape of gold NRs.

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Figure 5. Normalized SERS enhancement factor as a function of the longitudinal λmax position.

Figure 4. SERS spectra of MBA on isolated gold NRs with varying aspect ratios: (A) 1.5, (B) 1.9, (C) 2.3, (D) 2.4, (E) 2.7, (F) 2.8, (G) 2.9, (H) 3.1, and (I) 3.5. Each spectrum was an averaged result for 20 times 30-s acquisition time.

SERS on Gold Nanorods with Different Aspect Ratios. As has been discussed in the Introduction, to obtain shapedependent SERS behaviors on gold NRs, particle aggregation should be avoided. That was realized by conducting the SERS measurements in dilute dispersions and by timely monitoring the aggregation state. Upon the addition of the Raman-active molecule MBA into each gold rod sample, we first measured a UV-vis spectrum and then a SERS spectrum. To facilitate Raman molecule adsorption equilibrium on the gold rod surfaces, the UV-vis and SERS measurement steps were repeated, continued as long as possible, and were stopped once aggregation initiated (at that point, the longitudinal SPR absorption peak would broaden and a new weak peak would appear at the longer wavelength). In the end, SERS spectra before sample aggregation were taken for comparison and analysis. Furthermore, to fully maximize its enhancement ability, each gold rod sample adsorbed a monolayer analyte. The number of molecules needed to form a monolayer on the rod surfaces is calculated using the bonding area of each analyte,44 the number of NRs, and the surface area of a rod. We used inductively coupled plasma to determine the gold element content for all the centrifugation-purified gold NR samples. The number of NRs and the surface area of a NR can then be estimated from TEM images. The final concentration of MBA in each gold NR sample is on the order of about 10-6 M. Figure 4 shows a representative set of SERS spectra for MBA adsorbed on isolated gold NRs with different aspect ratios. The two strong SERS peaks appearing at 1074 and 1589 cm-1 are assigned to ν8a and ν12 aromatic ring vibrations, respectively.45 As can be seen from Figure 4, their SERS intensities gradually increase with the increase of aspect ratio. At R ) 2.7, the SERS intensity reaches the maximum and, thereafter, gradually falls. As a comparison, we also measured SERS spectra from aggregated gold NR samples (data not shown). All the aggregated samples show strong SERS activity but, as expected, demonstrate no shape-dependent SERS behaviors. While the SERS intensity in Figure 4 is shape-dependent, the particles in

the probed volume differ. To factor out this variation, the integrated SERS intensities of 1074 and 1589 cm-1 bands observed in Figure 4 are divided by particle number and molecule number on a particle (assuming a monolayer adsorption). This calculation obtains the relative SERS signal per molecule for each aspect ratio NRs. After normalization at the lowest relative SERS signal per molecule, normalized SERS enhancement factor (EF) was obtained for each aspect ratio of NRs. Figure 5 presents the normalized relative SERS EF as a function of longitudinal λmax of the NRs. It is clear that when longitudinal λmax overlaps with the 632.8 nm excitation line, the sample gives a stronger SERS signal. In contrast, the SERS intensity is weak when the excitation is off-resonance with the longitudinal λmax. Orendorff et al. concluded that enhancement factors are a factor of 10-102 greater for the NRs that have SPR band overlap with the excitation line than for the NRs whose SPR bands do not.20 SERS results obtained from gold nanoparticles supported on gold substrate suggested a similar conclusion.9 At first glance, our estimation in Figure 5 seems smaller than their estimation. It is because the lowest relative SERS signal per molecule that was used to normalize the relative SERS signal per molecule is obtained from gold NRs with an aspect ratio of 3.5, whose SPR has coupled to the excitation line to some extent (see Figure 2). If this fact was taken into consideration, our estimation agrees well with their estimation. To understand these shape-dependent SERS behaviors, we used the 3D-FDTD method to calculate the electromagnetic field distribution around the laser-illuminated gold NRs by numerically solving Maxwell’s equations.28 The 632.8 nm line was irradiated perpendicularly to the long axis of the gold NRs, with the light polarized in the long axis direction. The Yee cell size was set at 0.5 nm × 0.5 nm × 0.5 nm, and the total number of time steps was 40 000 to ensure convergence. Figure 6 presents the calculated electric field distribution around six gold NRs dispersed in water medium with aspect ratios of 1.5, 2.0, 2.5, 3.0, 4.0, and 5.0, respectively. One can see that the maximum field enhancement, defined as the ratio between the maximum local field (Eloc) and the incoming field (Ein) amplitude, M ) Eloc/Ein, appears at two ends of the gold NRs. The values of M are 6.7, 10.9, 31.3, 11.1, 3.5, and 2.5 for the six gold NRs, respectively. Because SERS is approximately proportional to the fourth power of the electric field enhancement at the position of the molecule,5 the maximum enhancements on the six gold NRs corresponds to 2.0 × 103, 1.4 × 104, 9.6 × 105, 1.5 × 104, 1.5 × 102, and 3.9 × 10 for the six gold NRs. These calculations clearly indicate that the enhancement has a strong

Correlating the Shape, SPR, and SERS of Gold Nanorods

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10463 the major absorption peaks of blood and water, these findings may serve as the basis for developing near-infrared SERS nanoprobes for bioanalytical applications. Acknowledgment. This study was financially supported by Natural Science Foundation of China (20603008 and 20703032), the 973 program (2009CB930703), Hunan Provincial Natural Science Foundation of China (06JJ3006), Natural Science Foundation of Fujian Province of China (No. E0710028), State Key Laboratory for Physical Chemistry of Solid Surfaces, and the “985” Foundation of Ministry of Education of China. References and Notes

Figure 6. 3D-FDTD simulated electric field amplitude patterns for gold NRs with varying aspect ratios: (a) 1.5, (b) 2.0, (c) 2.5, (d) 3.0, (e) 4.0, and (f) 5.0.

dependence on the coupling degree between the longitudinal SPR and the incident line, which can be easily tuned via changing the aspect ratio of the gold NRs. For example, for the gold rod with aspect ratio of 2.5, its SPR (λmax ) 630 nm) band is in resonance with the 632.8 nm incident line, and the enhancement is thus maximized. It should be pointed out that the calculated electromagnetic field is not uniform, and the SERS signal obtained in the experimental system is the averaged signal over the entire surface. However, since the effective SERS signals are mainly dominated by the hot site of the particle, the calculation results still agree well with experimental results and reveal the key role of EM enhancement played in the SERS on gold NRs. Conclusion Gold NRs with finely tunable SPR were prepared via a seedmediated growth strategy and were used to correlate the shapedependent SPR and SERS properties. Theoretical simulations by Gans theory and the DDA method reveal that rod aspect ratio is one main geometrical factor determining the SPR properties. Because the maximum absorption of the longitudinal SPR of the gold NRs can be finely tuned to be gradually overlapped with a fixed excitation source, for example, a 632.8 nm laser, these gold NRs may serve as ideal SERS substrates to test the EM theory. This can be accomplished by conducting SERS measurements on dilute gold NR dispersions, which largely deconvolutes particle coupling. It was corroborated that when the longitudinal SPR of the gold NRs overlaps with the excitation wavelength, the SERS signal experiences the strongest enhancement. Contrarily, the enhancement gradually falls off when the SPR is gradually shifted away from the excitation laser. This shape-dependent SERS closely agrees with the FDTD calculation and is ascribed mainly to the EM enhancement mechanism. Because it is easy to shift the longitudinal SPR of gold NRs to the spectral range of 650-900 nm, a “clear” window for in vivo optical imaging and sensing that separates

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