J. Phys. Chem. C 2009, 113, 11877–11883
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Wavelength-Dependent Surface-Enhanced Resonance Raman Scattering by Excitation of a Transverse Localized Surface Plasmon† Yasutaka Kitahama,*,‡ Yuhei Tanaka,‡ Tamitake Itoh,§ and Yukihiro Ozaki‡ Department of Chemistry, School of Science and Technology, Kwansei Gakuin UniVersity, Sanda, Hyogo 669-1337, Japan, and NanoBioanalysis Team, Health Technology Research Center, National Institute of AdVanced Industrial Science and Technology (AIST), Takamatsu, Kagawa 761-0395, Japan ReceiVed: March 10, 2009; ReVised Manuscript ReceiVed: May 02, 2009
We measured the polarization dependences of surface-enhanced (resonance) Raman scattering (SER(R)S) and localized surface plasmon (LSP) resonance Rayleigh scattering for the same single Ag nanoaggregate to explore whether SER(R)S is dominated by the resonance of incident light with LSPs. We excited the LSPs polarized parallel to the short axis of the Ag nanoaggregate (transverse LSPs) using laser lines from an Ar ion laser (λ ) 458, 488, and 514 nm). The polarization dependence of SERRS intensities is opposite to that of the transverse LSPs whose maximum wavelengths are below 500 nm. On the contrary, the polarization dependence of SERS intensities becomes similar to that of the transverse LSPs with the maximum wavelengths above 550 nm. The wavelength-dependent correlation between the transverse LSP and SER(R)S due to the absence and presence of just one junction on the short axis of the Ag aggregate will lead to an accurate understanding of the correlation between LSP and SER(R)S. Introduction Surface-enhanced resonance Raman scattering (SERRS) spectroscopy is sensitive enough to measure a Raman spectrum from a single molecule adsorbed on a noble metal nanoaggregate.1-3 The enormous enhancement factor of 1011-1014 for a Raman cross section is mainly explained using an electromagnetic (EM) field enhancement mechanism.4-9 A localized surface plasmon (LSP) in a noble metal nanoparticle is resonant with incident light, and then EM fields are enhanced enormously at the junctions of noble metal nanoparticles.4-6 The enhanced EM fields induce the enhancement of Raman scattering of molecules adsorbed at the junction.7,8 Furthermore, the resonance of the electronic transition in a molecule or a charge transfer complex consists of the metal surface and the chemisorbed molecule with incident light also contributes to the enhancement of Raman scattering(resonanceRamaneffectorCTmechanism,respectively).9,10 Thus, SERRS intensity is settled by the three factors: (1) adsorption of molecules at a nanoparticle junction, (2) resonance of incident light with an LSP, and (3) resonance of incident light with the electronic transition in a molecule or a chargetransfer complex. In other words, the efficient condition of SERRS excitation is described as a function of the three factors. The three factors of SERRS enhancement have been investigated individually by various experiments.6,8,9,11-16 For the first factor, atomic force microscope (AFM) images of SERRS-active nanoaggregates revealed that such nanoaggregates always have junctions,11 and the enhancement factor of SERRS increases by about 20-fold upon the dimerization of nanoparticles using optical tweezers.12 To study the second factor, we undertook a series of experiments showing that LSP resonance bands settle SERRS polarization, intensity, and spectral shapes.6,8,9,13,14 Furthermore, it has been reported that SERS depends on an †
Part of the “Hiroshi Masuhara Festschrift”. * Author for correspondence. E-mail address:
[email protected]. ‡ Kwansei Gakuin University. § National Institute of Advanced Industrial Science and Technology (AIST).
aspect ratio of noble metal nanorods due to the change of its longitudinal LSP resonance band.15 As for the third factor, we reported that the excitation wavelength dependence of SERRS is reproduced as the product of the LSP resonance band and the molecular absorption band.9 Moreover, the wavelengthscanned SERRS excitation profile in the 400-500 nm region is based on multiplication of the EM and resonance Raman effect.16 In these experiments, polarized SERRS and LSP resonance spectra of single SERRS-active Ag nanoaggregates have shown correlation between SERRS and LSP resonance.6,9,14 Wavelengths of LSP resonance are determined by the size and geometry of noble metal nanoparticles.17-19 Furthermore, aggregation of nanoparticles induces split of an LSP into two modes. In this paper, we call the LSPs polarized parallel to the short and long axes of the nanoaggregate as transverse and longitudinal LSPs, respectively. The transverse and longitudinal LSPs appear at shorter and longer wavelengths, respectively.18,19 The polarization dependence of SERRS peaks is the same as that of longitudinal LSP resonance bands and opposite to that of transverse LSP resonance bands.6,9,14 Thus, we have concluded that SERRS signals are enhanced by the longitudinal LSP resonance.6,9,14 In this study, we observed polarization dependences of SERRS and LSP resonance spectra of a single Ag nanoaggregate adsorbed by thiacyanine molecules. The single Ag nanoaggregates adsorbed by thiacyanine J-aggregates were excited with three wavelengths which are close to both the J-band and transverse LSP resonance band wavelengths. It was confirmed that the polarization dependence of SERRS is opposite to that of the transverse LSP resonance whose maxima are shorter than 500 nm, as reported in the previous papers.6,9,14 Interestingly, however, we found that the polarization dependence of SERRS becomes similar to that of the transverse LSP resonance whose maxima are longer than 550 nm. The comparison of the polarization experimental results with those of the finitedifference time-domain (FDTD) calculation indicates that the presence of just one junction in the direction of the short axis
10.1021/jp9021294 CCC: $40.75 2009 American Chemical Society Published on Web 05/19/2009
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Kitahama et al. against the sample surface using an objective lens (Olympus, UMPlanFl 5×, NA 0.15). The laser power of 20 mW in an elliptic spot produced a power density of ∼0.2 kW/cm2 on the sample surface. SERRS or LSP resonance of a single Ag nanoaggregate adsorbed with TC was collected by an objective lens (Olympus, LCPlanFl 60×, NA 0.7) and led to a polychromator (Acton, Pro-275) coupled with a thermoelectrically cooled CCD (Andor, DV434-FI) through a pinhole. The integration time per spectrum was 3 s. To measure the polarization dependence of SERRS and LSP resonance, a polarizer inserted before the polychromator was rotated. We used various Ag nanoaggregates with different sizes to measure SERRS in a wide range of LSP resonances. However, the simple shaped LSP resonance Rayleigh scattering spectra showing clear polarization dependences were used for this study. It has been confirmed that simple structures of Ag nanoaggregates coincide with the simple shaped Rayleigh scattering spectra by a scanning electron microscope (SEM) image, a FDTD calculation, and a Rayleigh scattering spectrum of the same nanoaggregate.22,23 We calculated polarized LSP resonance Rayleigh scattering spectra and near-field images of the EM field using an FDTD calculation. The software used was PLANC-FDTD (Information and Mathematical Science Laboratory Inc., Ver.6.2). For the calculation of the spectra, the mesh sizes were set to be 1 and 3 nm inside and outside an Ag nanoaggregate, respectively. They were halved for the calculation of the EM field. Results
Figure 1. (a) Microscope image of SERRS of TC molecules adsorbed on the Ag nanoaggregates excited at 458 nm. Inset: the structure of 5,5′-dichloro-3,3′-disulfopropyl thiacyanine (TC) sodium salt. (b) Microscope image of the corresponding LSP resonance Rayleigh scattering from the Ag nanoaggregates illuminated by white light through the dark-field condenser lens. The images cover an area of 102.25 × 76.81 µm2.
of an Ag nanoaggregate causes such correlation between the LSP resonance and SERRS different from those reported in the previous papers.6,8,9,14,15 Experiments 5,5′-Dichloro-3,3′-disulfopropyl thiacyanine (TC) sodium salt, whose structure is shown in Figure 1, was used as purchased from Hayashibara Biochemical Laboratories (Okayama, Japan). Colloidal Ag nanoparticles were prepared by the Lee-Meisel method.20 An aliquot of a mixture of 1 µM TC and 20 mM NaCl aqueous solutions was added to the colloidal solution. The Ag nanoaggregates adsorbed with TC were aged overnight at room temperature and spin-coated on a slide glass. A drop of 1 M NaCl solution was put onto the glass to stabilize the Ag nanoaggregates for the plate. This glass plate, on which the Ag nanoaggregates had been dispersed, was sandwiched by another glass plate. The details of the experimental setup were described elsewhere.21 Briefly, the sample set on an inverted microscope (Olympus, IX-70) was illuminated by a circular polarized beam from an Ar ion laser (λ ) 458, 488, or 514 nm) or white light from an Xe lamp through a dark-field condenser lens. An excitation laser beam was delivered through an angle of 30°
Parts a and b of Figure 1 show microscope images of SERRS and Rayleigh scattering due to LSP resonance of the Ag nanoaggregates on which TC was adsorbed from its 0.5 µM aqueous solution, respectively. One can see various colors of the Ag nanoaggregates shown by their individual LSP resonances. In the case of rhodamines and porphyrins, SERS-active Ag nanoaggregates usually exhibit yellow or red due to their LSP resonance.9,24 However, in the case of TC, SERRS-active Ag nanoaggregates yield various colors, not only red or yellow but also blue. We consider that the reason for showing various colors of the SERRS-active Ag nanoaggregates for TC lies in the absorption bands of TC which appear at short wavelengths (dimer, 408 nm; monomer, 430 nm; J-aggregate, 464 nm).25 Figure 2b and c show polarized SERRS spectra of TC molecules adsorbed on a single Ag nanoaggregate excited at 458 nm and the corresponding polarized LSP resonance Rayleigh scattering spectra of the same Ag nanoaggregate, respectively. The enlarged SERRS spectrum measured at the polarization angle of 210° is shown in Figure 2a. The peak positions of the SERRS spectra are almost the same as those of a conventional Raman spectrum of the J-aggregates of TC.26,27 The SERRS spectra are accompanied by background emission. We attribute the background emission to fluorescence of the J-aggregates, because the LSP-independent maximum and the bandwidth of the background emission is always similar to those of fluorescence of the J-aggregates.25-27 At the polarization angles of 120° and 210°, a LSP resonance band appeared at ∼470 and ∼580 nm, and the SERRS intensity became the minimum and maximum, respectively. The transverse and longitudinal LSP resonance bands are located at shorter and longer wavelengths, respectively.18,19 The transverse LSP resonance band (at the 120°) emerged at a similar wavelength (∼470 nm) to that of the excitation laser line (458 nm), but the intensity of the SERRS light was the weakest. Thus, the polarization dependence of the SERRS is the same as that of the longitudinal LSP resonance band, as shown in Figure 2d.
SERRS at Various LSP Resonances
Figure 2. (a) SERRS spectrum of TC molecules adsorbed on the single Ag nanoaggregate excited at 458 nm. (b) The polarized SERRS spectra and (c) polarized Rayleigh scattering spectra of the same Ag nanoaggregate due to its LSP resonance. The SERRS and Rayleigh scattering spectra at the polarization angles of 120° and 210° are shown as gray and black, respectively. (d) Polarization angle dependence of the intensities of the SERRS and LSP resonance maxima.
Figure 3b and c depict polarized anti-Stokes and Stokes SERRS spectra excited at 488 nm and the corresponding polarized LSP resonance Rayleigh scattering spectra, respectively. The SERRS spectrum in Figure 3a is almost the same as the one excited at 458 nm (Figure 2a), and their peak positions are again almost the same as those of the conventional Raman spectrum of the J-aggregates.26,27 The background emission due to the fluorescence of the J-aggregates is much weaker in this case because the excitation wavelength (488 nm) is apart from the J-band maximum (464 nm). Their anti-Stokes SERRS peaks are clearly seen up to -600 cm-1. At the polarization angles of 180° and 90°, a LSP resonance band appeared at ∼480 and ∼550 nm, and the SERRS spectrum vanishes and emerges, respectively. The transverse LSP resonance band (at the 180°)
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Figure 3. (a) SERRS spectrum of TC molecules adsorbed on the single Ag nanoaggregate excited at 488 nm. (b) The polarized SERRS spectra and (c) polarized Rayleigh scattering spectra of the same Ag nanoaggregate due to its LSP resonance. The SERRS and Rayleigh scattering spectra at the polarization angles of 90° and 180° are shown as black and gray, respectively. (d) Polarization angle dependence of the intensities of the SERRS and LSP resonance maxima.
is observed at a similar wavelength (∼480 nm) to that of the excitation laser (488 nm), but the SERRS light was not emitted. Thus, the polarization dependences of the anti-Stokes and Stokes SERRS are the same as those of the longitudinal LSP resonance band, as shown in Figure 3d. Figure 4b and c present polarized surface-enhanced Raman scattering (SERS) spectra excited at 514 nm and polarized LSP resonance Rayleigh scattering spectra, respectively. Note that we observe SERS not SERRS because the excitation wavelength of 514 nm is far from the molecular electronic resonance
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Figure 5. Difference between the polarization angles showing the maximum (or minimum) intensity of the transverse LSP resonance, θ(t-LSPR), and those of SERRS, θ(SERRS), plotted against transverse LSP resonance wavelengths.
accompanied by background emission attributable to fluorescence of the J-aggregates. Their SERS peaks are observed not in the low (the 1000-400 cm-1) but in the high wavenumber region (the 1800-1000 cm-1, around 550 nm). Raman peaks in the low wavenumber region are assigned to the out-of-plane vibration modes enhanced by the resonance Raman effect due to the formation of J-aggregates.28-32 The lack of fluorescence of the J-aggregates in background emission of SERS and the disappearance of Raman peaks in the low wavenumber region indicate that the SERS excited at 514 nm is not from the J-aggregates but from the monomers (or the dimer). The details of this attribution will be described elsewhere.26,27 At the polarization angles of 180°, a longitudinal LSP resonance band appeared at ∼680 nm, but the SERS intensity became zero. At the polarization angle of 270°, one of the transverse LSP resonance bands (∼550 nm) occurred at a similar wavelength to that of the excitation laser line (514 nm), and the SERS intensity was the strongest. Thus, the polarization dependence of the SERRS deviates from that of the longitudinal LSP resonance band, as shown in Figure 4d. This polarization dependence is clearly different from those of the SERRS spectra excited at 458 and 488 nm. Discussion
Figure 4. (a) SERS spectrum of TC molecules adsorbed on the single Ag nanoaggregate excited at 514 nm. (b) The polarized SERS spectra and (c) polarized Rayleigh scattering spectra of the same Ag nanoaggregate due to its LSP resonance. The SERS and Rayleigh scattering spectra at the polarization angles of 180° and 270° are shown as black and gray, respectively. (d) Polarization angle dependence of the intensities of the SERS and LSP resonance maxima. The scattering efficiency for the LSP resonance band at 550 nm is magnified 3 times.
maxima.25 The SERS spectrum in Figure 4a is not similar to the SERRS spectra excited at 458 and 488 nm (Figures 2a and 3a) and the conventional Raman spectrum of the J-aggregates, but it is similar to SERS spectra measured using the TC concentration, which is too low to form J-aggregates.26,27 The SERS spectra excited at 514 nm (Figure 4a and b) are not
We observed the two types of polarization dependence of SERRS and LSP resonance of the single Ag nanoaggregate on which TC molecules had been adsorbed. One was the polarization dependence observed with the excitation at 458 and 488 nm illustrated in Figures 2 and 3, while the other was that with the excitation at 514 nm shown in Figure 4. Figure 5 represents the difference between the polarization angles where the SERRS peak and the transverse LSP resonance band become the maximum plotted versus the transverse LSP resonance wavelength. It can be seen that the polarization dependences of the transverse LSP resonance band at 550 nm are opposite to each other and become similar to those of SERRS, respectively. First, we discuss the origin of the polarization dependence in Figures 2 and 3. The SERRS intensity in Figures 2 and 3 becomes the minimum at the polarization angle where the transverse LSP resonance band becomes the maximum. It is very likely that the minimum SERRS signal in Figure 2b is due to a resonance Raman effect without EM enhancement, because the minimum SERRS signals in Figures 3b and 4b, which do not get a resonance Raman effect,25 are zero. Besides, we previously confirmed that anti-Stokes SERRS of rhodamine 123 (R123) is enhanced by LSP resonance bands whose maxima
SERRS at Various LSP Resonances
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Figure 6. (a) Calculated Rayleigh scattering spectra of a nanodimer that consists of Ag spheres with a diameter of 20 nm. The 458 nm (b) transverse and (c) longitudinal polarization (along the short and long axes, respectively) excited near-field images of an electromagnetic field on the same Ag nanodimer by FDTD calculations. The color scales cover the same range: 29.8 (red) to 0.13 (blue) V/m.
are close to anti-Stokes SERRS maxima.8 However, the antiStokes SERRS in Figure 3b is not enhanced by the LSP resonance band, even though the anti-Stokes SERRS maximum (
