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Synthesis and Spectroscopic Studies of Composite Gold Nanorods with a Double-Shell Structure Composed of Spacer and Cyanine Dye J-Aggregate Layers Akihito Yoshida,*,† Naoko Uchida,‡ and Noritsugu Kometani† †

Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, Osaka 558-8585, Japan, and ‡Department of Research Support, Division of University Management, Osaka City University, Osaka 558-8585, Japan Received April 22, 2009. Revised Manuscript Received June 25, 2009

The composite gold nanorods (Au NRs) having a double-shell structure composed of Au NR (core), spacer layer (inner shell), and J-aggregate (JA) layer (outer shell) have been synthesized to examine the spectroscopic properties of the hybrid system in which the localized surface plasmon is coupled with the molecular exciton of JA. The spacer layer consisting of N,N,N-trimethyl(11-mercaptoundecyl)ammonium chloride plays a significant role in the formation of JA shell for several kinds of cyanine dyes. The absorption spectra of composite NRs are characterized by a distinct dip near the J-band when the plasmon energy of Au core is close to the exciton energy of JA shell, whereas a normal J-band peak appears when two energies are widely different from each other. The gradual change from the dip type to peak type absorption was observed when the plasmon energy was modulated by varying the aspect ratio of Au NR. Furthermore, composite NRs with thicker spacer layers have been fabricated by inserting the multilayer shell of polyelectrolytes between TMA and JA layers. They exhibited an alteration of the spectral line shape from the dip type to peak type with increase in the thickness of spacer layer. These observations have been interpreted in terms of the strength of the exciton-plasmon coupling, which is sensitive to the configuration of composite NRs as well as the relative difference between plasmon and exciton energies.

Introduction The noble metal nanoparticle (NP) is one of the most attractive materials because of many unique properties. Among them, much attention has been paid to the optical and spectroscopic properties arising from the excitation of a surface electromagnetic mode of metal NPs, called a localized surface plasmon resonance (LSPR).1-3 The LSPR can be used to confine electromagnetic waves of the visible or near-IR light to a nanoscale region below the diffraction limit and induce a large extinction of light as well as the enhanced electromagnetic field in the vicinity of metal surface. Thus, manipulating the LSPR is considered to be a key issue leading to the development of technologies in the areas of photonics, optoelectronics, and plasmonics. Furthermore, the enhanced electromagnetic field induced by the LSPR can dramatically alter the situation in molecules near noble metal surfaces, causing many intriguing phenomena such as surface-enhanced Raman scattering (SERS),4,5 enhanced luminescence,6,7 enhanced F€orster resonance energy transfer (FRET),8,9 and so on. *To whom correspondence should be addressed: Fax +81-66605-2984; email [email protected]. (1) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677. (2) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410–8426. (3) Maier, S. A.; Atwater, H. A. J. Appl. Phys. 2005, 98, 011101. (4) Moskovits, M. J. Raman Spectrosc. 2005, 36, 485–496. (5) Talley, C. E.; Jackson, J. B.; Oubre, C.; Grady, N. K.; Hollars, C. W.; Lane, S. M.; Huser, T. R.; Nordlander, P.; Halas, N. J. Nano. Lett. 2005, 5, 1569–1574. (6) Tam, F.; Goodrich, G. P.; Johnson, B. R.; Halas, N. J. Nano Lett. 2007, 7, 496–501. (7) K€uhn, S.; Ha˚kanson, U.; Rogobete, L.; Sandoghdar, V. Phys. Rev. Lett. 2006, 97, 017402. (8) Zhang, J.; Fu, Y.; Chowdhury, M. H.; Lakowicz, J. R. J. Phys. Chem. C 2007, 111, 11784–11792. (9) Komarala, V. K.; Bradley, A. L.; Rakovich, Y. P.; Byrne, S. J.; Gun’ko, Y. K.; Rogach, A. L. Appl. Phys. Lett. 2008, 93, 123102.

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These surface-enhanced phenomena suggest that the hybrid system of noble metal NPs and organic molecules has a large potential for the applications in novel molecular plasmonic devices. The J-aggregate (JA) of cyanine dye, a kind of molecular assembly, has been known to readily form on the solid surfaces and extensively studied for many years due to the practical application to a photographic sensitizer. The photoexcitation of JA produces a molecular exciton inside it owing to the strong coupling between transition dipoles of dye molecules. The coherent delocalization of the exciton brings about several remarkable optical properties such as a narrow absorption band (J-band) redshifted with respect to the monomer band, resonance fluorescence with a short radiative lifetime, and so on.10,11 In view of such properties, the JA is a promising material for constructing the hybrid system with noble metal NPs because one can expect a strong interaction between molecular exciton and surface plasmon. Recently, we have devised the synthesis method of the doubleshell-type NP consisting of three components;spherical Au core, spacer layer (inner shell), and JA layer (outer shell);and undertaken a systematic study on the spectroscopic properties of the hybrid system in which the surface plasmon is coupled with the exciton of JA shell.12 The spacer layer was made of the selfassembled monolayer (SAM) of N,N,N-trimethyl(11-mercaptoundecyl)ammonium chloride (TMA), which has a cationic trimethylammonium group at the ω-terminal. It was found that the TMA-capped Au NPs (Au/TMA NPs) effectively promotes the formation of JA shell for several kinds of anionic cyanine dyes (10) M€obius, D. Adv. Mater. 1995, 7, 437–444. (11) Kobayashi, T., Ed. J-Aggregates; World Scientific: Singapore, 1996. (12) Yoshida, A.; Yonezawa, Y.; Kometani, N. Langmuir 2009, 25, 6683–6689.

Published on Web 08/05/2009

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Figure 1. (a) Schematic illustration of an Au/TMA/dye composite NR. Molecular structures of (b) materials for the spacer layer and (c) the cyanine dyes used in this study.

as supported by the transmission electron microscopy (TEM) observations of the double-shell structure and the detection of resonance fluorescence from JA shell. The absorption spectra of the composite Au/TMA/JA NPs were characterized by an absorption dip or peak around the J-band of cyanine dye JA, depending on the spectral overlap between the J-band and the plasmon band, the size of Au core, and the presence of spacer layer. These results have suggested a significance of the interaction between exciton and surface plasmon (EP coupling) for tuning the spectroscopic properties of composite NPs. This paper presents an additional possibility of controlling the EP coupling in the double-shell-type composite NPs by employing a gold nanorod (Au NR) as a core material. Unlike the spherical Au NP, the Au NR has two LSPRs, i.e., transverse plasmon (T-plasmon) and longitudinal plasmon (L-plasmon), and the resonance frequency of the latter can be considerably modulated by controlling the aspect ratio. In recent years, there has been a great deal of progress in the synthetic methodology of Au NR. Especially, the seed-mediated growth method13-15 has been widely used because of its easiness and high reproducibility. In this study, Au NRs with different aspect ratios were prepared by this method and utilized as cores of the double-shell-type composite NRs (Figure 1a). The SAM of TMA formed on the surface of Au NR successfully facilitated the formation of JA shell for several kinds of anionic cyanine dyes. On the basis of measurements of absorption and photoluminescence (PL) spectra, it is demonstrated that the spectroscopic properties of composite NRs can be tuned by changing the aspect ratio of core Au NR (the control of L-plasmon energy) and the kind of cyanine dye in the JA shell (the control of exciton energy). We also prepared the composite NRs with thicker spacer layers by means of the alternate adsorption of oppositely charged polyelectrolytes, and the effect of separation between the metal surface and the JA shell on the spectroscopic properties was examined. On the basis of the observations, the strength of EP coupling in the composite NRs is qualitatively discussed. (13) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065–4067. (b) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414–6420. (c) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857–13870. (14) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957–1962. (15) (a) Niidome, Y.; Nishioka, K.; Kawasaki, H.; Yamada, S. Chem. Commun. 2003, 2376–2377. (b) Nishioka, K.; Niidome, Y.; Yamada, S. Langmuir 2007, 23, 10353–10356.

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Experimental Section Materials. Hydrogen tetrachloroaurate tetrahydrate, trisodium citrate dihydrate, L-ascorbic acid, and sodium chloride were purchased from Kishida Chemical Co., Ltd. Sodium borohydride was purchased from Wako Pure Chemical Industries, Ltd. Cetyltrimethylammonium bromide (CTAB), poly(diallyldimethylammonium) chloride (PDDA, Mw = 100 000-200 000 g/ mol), and poly(sodium 4-styrenesulfonate) (PSS, Mw ∼ 70 000 g/ mol) were purchased from Sigma-Aldrich Co. 3,30 -Disulfopropyl5,50 -dichlorothiacyanine sodium salt (TC), 3,30 -disulfopropyl5,50 -dichloro-9-ethylthiacarbocyanine potassium salt [Thia(Et)], 3,30 -disulfopropyl-5,50 -dichloro-9-phenylthiacarbocyanine triethylammonium salt [Thia(Ph)], and 1,10 -disulfopropyl-2,20 -cyanine triethylammonium salt (PIC) were received from Hayashibara Biochemical Laboratories, Inc. The chemical structures of cyanine dyes are illustrated in Figure 1c. All materials were used without further purification. TMA was synthesized according to the procedure in ref 16. All aqueous solutions were prepared with distilled water. Preparation Method of Au/TMA/JA Composite NRs. First, CTAB-capped Au NRs with various aspect ratios were synthesized according to the procedure developed by Sau and Murphy13 with slight modifications. A 0.25 mL of 10 mM HAuCl4 aqueous solution was mixed with a 7.5 mL of 0.10 M CTAB solution, and then 0.60 mL of ice-cooled 10 mM NaBH4 aqueous solution was added all at once, yielding pale brownyellow seed solution. This seed solution was stayed in a thermostated water bath and kept at ∼30 °C for at least 2 h before next use. The growth solution was the mixture of 9.5 mL of 0.10 M CTAB solution, 0.40 mL of 10 mM HAuCl4 solution, a certain amount of 10 mM of AgNO3 solution, and 64 μL of 0.10 M Lascorbic acid solution. The aspect ratio of NR was varied by adjusting the additive amount of 10 mM AgNO3 solution between 10 and 90 μL.13-15 Here, nine kinds of growth solutions were prepared with different amounts of AgNO3 solution. In several minutes after adding 20 μL of the seed solution to the growth solution, it exhibited a color development indicative of the production of CTAB-capped Au NRs. These solutions were kept in the water bath for at least 3 h at ∼30 °C. To perform the ligand exchange of CTAB-capped Au NRs by TMA, the excess CTAB was first removed by the centrifugation (13 000 rpm, 20 min) and decantation. The precipitate was then dispersed in a 10 mL of 2 mM TMA aqueous solution under stirring for 2 h. The excess TMA was removed by the centrifugation (13 000 rpm, 20 min) and decantation. The resulting Au/ (16) Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 5349–5355.

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Figure 2. TEM images of Au/TMA NRs with different aspect ratios, (a) Au_1/TMA, (b) Au_2/TMA, and (c) Au_3/TMA NRs. Scale bars in the TEM images correspond to 50 nm. TMA NRs were redispersed in distilled water so that the optical density at 350 nm became 0.25. Finally, Au/TMA/JA composite NRs were prepared by mixing 4 mL of the Au/TMA colloidal solution with 1 mL of 10 μM dye solutions.

Preparation Method of Au/TMA/(PSS/PDDA)n/JA NRs. Coating of Au/TMA NR with polyelectrolyte (PE) multilayer was carried out by means of the alternate adsorption technique analogous to the report by Gole and Murphy.17 To form the PSS layer on the surface of Au/TMA NR, a few milliliters of Au/TMA NR solution was added into a 0.17 wt % PSS aqueous solution including 8.33 mM NaCl under stirring. After being stirred for 2 h, the colloidal solution was centrifuged for 15 min at 10 000 rpm, and the supernatant was decanted off to remove the excess PSS. The precipitate NRs were redispersed in a few milliliters of 10 mM NaCl solution. Subsequently, this colloidal solution was poured into 0.17 wt % PDDA solution including 8.33 mM NaCl and stirred for 2 h, yielding the PDDA layer on the PSS surface. It was again subjected to centrifugation, decantation, and redispersion in water. Successive repetitions of above steps gave rise to the PE multilayer of (PSS/PDDA)n on Au/TMA NR. After a desired number of multilayer was coated (n = 1, 2, and 3), the colloidal solution was centrifuged and decanted twice. The resulting Au/TMA/(PSS/PDDA)n NRs were again dispersed in a distilled water so that the optical density at 350 nm became 0.25. Finally, Au/TMA/(PSS/PDDA)n/JA composite NRs were prepared by mixing 4 mL of the colloidal solution with 1 mL of 10 μM dye solutions. Instrumentation and Measurements. UV/vis absorption spectra of the colloidal solutions in a quartz cuvette (10 mm optical path length) were measured using a V-530 spectrophotometer (JASCO Co.). PL measurements were carried out with a FP-750 fluorimeter (JASCO Co.). All measurements were performed at room temperature in air. TEM images were acquired with a JEOL JEM-2100 or a Hitachi H-7000 electron microscope operated at 200 kV or 75 kV, respectively. The samples for TEM observations were prepared by putting a small amount of the colloidal solution on the carbon-supported copper grids and subsequently drying in air. The samples for negative staining were prepared by putting a very small amount of the colloidal solution on the hydrophilically treated carbon-supported copper grid and then placing a small drop of 2% uranyl acetate solution on it, followed by the desiccation in air.

Results and Discussion Au NRs capped with TMA (Au/TMA NRs) caused no agglomeration and could be well dispersed in water, suggesting the formation of stable SAM of TMA on the Au NR surface. Figure 2 shows typical TEM images of Au/TMA NRs. Here, (a) Au_1, (b) Au_2, and (c) Au_3 correspond to Au NRs prepared by the growth solutions containing 30, 50, and 80 μL of 10 mM AgNO3 solution, respectively. The widths of Au/TMA NRs are (17) Gole, A.; Murphy, C. J. Chem. Mater. 2005, 17, 1325–1330.

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13 ( 3 nm and almost constant for all kinds of NRs. The average value of aspect ratio increases with increasing the amount of AgNO3 solution in the growth solution and was estimated to be 2.7, 3.0, and 3.5 for Au_1, Au_2, and Au_3/TMA NRs, respectively. Absorption spectra of colloidal solutions containing Au/ TMA NRs are shown in Figure 3a (black line). These spectra are characterized by two absorption peaks attributable to the surface plasmon resonance, i.e., T-plasmon band at shorter wavelength and L-plasmon band at longer wavelength. It is also evident that the peak wavelength of L-plasmon band (λL) shows the bathochromic shift with the increase in aspect ratio (λL = 667, 703, and 741 nm for Au_1, Au_2, and Au_3/TMA NRs, respectively). According to the theoretical simulation reported by Pileni and co-workers, the relationship between the aspect ratio (AR) and the wavelength of L-plasmon peak (λL) of Au NRs can be expressed as λL = 96AR þ 418.18 This equation gives λL = 677, 706, and 754 nm for AR = 2.7, 3.0, and 3.5, respectively, which are roughly in accordance with the present observations. In a previous study, we have pointed out that positively charged trimethylammonium moiety at the ω-terminal of TMA facilitates the spontaneous formation of JA shell on the surface of Au/TMA NPs because of a strong electrostatic interaction with anionic cyanine dyes.12 Indeed, it was confirmed by the zetapotential measurement (Zetasizer Nano ZS, Malvern Instruments Ltd.) that Au/TMA NRs synthesized in this study are positively charged. Thus, Au/TMA/JA composite NRs were synthesized in a similar fashion by simply mixing an Au/TMA NR colloidal solution with a cyanine dye solution. In Figure 3a (red line) are shown absorption spectra of colloidal solutions containing Au/ TMA/Thia(Ph) NRs with different aspect ratios. It is evident that an anomalous “dip” appears near the J-band of Thia(Ph) JA (λJ = 671 nm19) in each spectrum (λdip = 671, 673, and 677 nm for Au_1, Au_2, and Au_3/TMA/Thia(Ph) NRs, respectively), suggesting the formation of Thia(Ph) JA shell on Au/TMA NRs. Furthermore, the spectral line shape near the J-band gradually changes with changing aspect ratio. Similar observations, the appearance of dip type absorption in metal/JA nanocomposite system, have been reported by many researchers including our group12,20-27 and interpreted as indicating the strong coupling between molecular exciton of JA and surface plasmon of metal (18) Brioude, A.; Jiang, X. C.; Pileni, M. P. J. Phys. Chem. B 2005, 109, 13138– 13142. (19) The reference peak position of Thia(Ph) J-band was estimated from the absorption spectra of mixed solution dissolving a small amount of PDDA in the Thia(Ph) aqueous solution (see Figure S1a). Here, the PDDA was used to accelerate the J-aggregation of Thia(Ph) dyes in solutions. (20) (a) Kometani, N.; Tsubonishi, M.; Fujita, T.; Asami, K.; Yonezawa, Y. Langmuir 2001, 17, 578–580. (b) Yoshida, A.; Kometani, N.; Yonezawa, Y. Colloids Surf., A 2008, 313/314, 581–584. (21) Sato, T.; Tsugawa, F.; Tomita, T.; Kawasaki, M. Chem. Lett. 2001, 402– 403.

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Figure 3. (a) Absorption spectra of colloidal solutions containing Au/TMA NRs (black line) and Au/TMA/Thia(Ph) NRs (red line). The upper, middle, and bottom panels correspond to Au_1, Au_2, and Au_3 NRs, respectively. (b) PL spectra of colloidal solutions containing the Au/TMA/Thia(Ph) NRs. The excitation wavelength for PL measurements is 600 nm. (c) Energy diagram plotting the exciton-plasmon mixing states estimated from the two peaks in the absorption spectra of the Au/TMA/Thia(Ph) NRs with different aspect ratios. The dashed green and solid gray lines show the uncoupled exciton (1.86 eV) and plasmon energies, respectively. (22) (a) Hranisavljevic, J.; Dimitrijevic, N. M.; Wurtz, G. A.; Wiederrecht, G. P. J. Am. Chem. Soc. 2002, 124, 4536–4537. (b) Wiederrecht, G. P.; Wurtz, G. A.; Hranisavljevic, J. Nano Lett. 2004, 4, 2121–2125. (c) Wiederrecht, G. P.; Wurtz, G. A.; Bouhelier, A. Chem. Phys. Lett. 2008, 461, 171–179. (23) Wurtz, G. A.; Evans, P. R.; Hendren, W.; Atkinson, R.; Dickson, W.; Pollard, R. J.; Zayats, A. V. Nano Lett. 2007, 7, 1297–1303. (24) Uwada, T.; Toyota, R.; Masuhara, H.; Asahi, T. J. Phys. Chem. C 2007, 111, 1549–1552. (25) Wiederrecht, G. P.; Hall, J. E.; Bouhelier, A. Phys. Rev. Lett. 2007, 98, 083001.

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core (EP coupling). Our group has demonstrated that the spectral line shape of the composite NPs near the J-band changes from the peak type absorption, in which two bands, i.e., J-band and plasmon band, independently appear in the absorption spectra of composite NPs, to the dip type absorption with increase in the spectral overlap between the J-band and the surface plasmon band, decrease in the distance between the metal core and the JA shell, and increase in size of metal core. These factors are considered to have a crucial impact on the strength of EP coupling, resulting in the occurrence of the dip type absorption for the strong coupling and the peak type absorption for the weak coupling. It is therefore reasonable to assume that the spectral line shape near the J-band can be used as a kind of measure for estimating the strength of EP coupling. The observations in Figure 3a thus indicate that the strong EP coupling takes place in all Au/TMA/Thia(Ph) NRs. The gradual change of spectral line shape with aspect ratio may be attributed to the change of spectral overlap between the J-band and the L-plasmon band, which leads to the modulation of EP coupling. On the other hand, it should be noted that some researchers have recently discussed the spectral line shape of composite NPs in a different way.23,26 They have regarded the dip type absorption typical of strong EP coupling as the appearance of new two peaks stemmed from the exciton-plasmon state mixing. On the basis of this interpretation, we attempted to estimate the energies of exciton-plasmon mixed states from the peak positions in absorption spectra of Au/TMA/Thia(Ph) NRs in Figure 3a along with the ones with different aspect ratios given in Figure S2. Then, those energies were plotted as a function of uncoupled plasmon energy evaluated from the position of L-plasmon band for corresponding Au/TMA NRs without JA shell (Figure 3c). The dashed green and solid gray lines show the uncoupled exciton and plasmon energies, respectively. As seen in this diagram, the positions of two peaks are gradually shifted with changing aspect ratio of Au NRs. This indicates that the energies of mixed states can be tuned by controlling the resonance energy of L-plasmon band. It is also apparent that the mixed sates exhibit a marked anticrossing behavior, supporting the presence of a strong EP coupling in Au/TMA/Thia(Ph) NRs.23,26,28 Figure 3b shows the PL spectra of colloidal solutions containing Au/TMA/Thia(Ph) NRs. The maximum PL intensity is located at λPL = 666, 665, and 664 nm for Au_1, Au_2, and Au_3/TMA/Thia(Ph) NRs, respectively. Interestingly, the peak positions are nearly independent of aspect ratio. Those PL peaks are assigned to the resonance fluorescence of Thia(Ph) JA, which may be another evidence of the formation of Thia(Ph) JA shell (Figure S1b). It is somewhat striking that Au/TMA/Thia(Ph) NRs exhibit the significant PL emission despite the fact that fluorophores adjacent to the metal surfaces often undergo a strong quenching due to the energy or electron transfer to metal. In fact, the composite Au NPs directly covered with the JA shell did not show any detectable emissions.12,20a The insertion of a TMA layer between the Au core and the JA shell is therefore considered to effectively suppress the quenching by such processes. This finding offers attractive opportunities to investigate the exciton-plasmon state mixing by means of the fluorescence lifetime measurement and so on, which will be a subject of our future study. (26) Fofang, N. T.; Park, T. H.; Neumann, O.; Mirin, N. A.; Nordlander, P.; Halas, N. J. Nano Lett. 2008, 8, 3481–3487. (27) Ni, W. H.; Yang, Z.; Chen, H. J.; Li, L.; Wang, J. F. J. Am. Chem. Soc. 2008, 130, 6692–6693. (28) (a) Bellessa, J.; Bonnand, C.; Plenet, J. C. Phys. Rev. Lett. 2004, 93, 036404. (b) Bonnand, C.; Bellessa, J.; Plenet, J. C. Phys. Rev. B 2006, 73, 245330. (c) Bonnand, C.; Bellessa, J.; Symonds, C.; Plenet, J. C. Appl. Phys. Lett. 2006, 89, 231119.

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Figure 5. (a) Absorption and (b) PL spectra of colloidal solutions containing Au_2/TMA/Thia(Et) (blue), Au_2/TMA/PIC (green), and Au_2/TMA/TC NRs (red) along with the absorption spectrum of Au_2/TMA NRs (black). The excitation wavelengths for PL measurements are 550 nm for Thia(Et), 520 nm for PIC, and 430 nm for TC, respectively.

Figure 4. (a-f) TEM images of negatively stained samples for Au/ TMA NRs (without the JA shell; left column) and Au/TMA/ Thia(Ph) NRs (with the JA shell; right column). The upper, middle, and bottom rows correspond to Au_1, Au_2, and Au_3 NRs, respectively. Scale bars in the TEM images correspond to 50 nm.

The formation of the double-shell structure can be confirmed in more straightforward way by TEM observations with the help of a negative staining method. The obtained TEM images of Au/TMA and Au/TMA/Thia(Ph) NRs are shown in left and right panels of Figure 4. It can bee seen in these images that Au NRs are surrounded by relatively bright layers which are thicker for Au/ TMA/Thia(Ph) NRs than for Au/TMA NRs. Average thicknesses of these layers were estimated to be 1.4 ( 0.3 nm for Au_1/TMA NRs (Figure 4a) and 4.1 ( 1.0 nm for Au_1/TMA/Thia(Ph) NRs (Figure 4b), which can be attributed to the thicknesses of TMA shell and TMA/Thia(Ph) double shell, respectively. It is therefore concluded that Au/TMA/Thia(Ph) NRs definitely possess the double-shell structure composed of TMA and Thia(Ph) JA layers. As aforementioned, the EP coupling is highly sensitive to the spectral overlap between the J-band of JA shell and the plasmon band of metal core. Larger overlap should lead to a stronger coupling. This may be examined by using the dyes having different exciton energies for JA shell. Figure 5 shows the absorption and PL spectra of Au_2/TMA/JA NRs with three kinds of cyanine dyes (TC, PIC, and Thia(Et), see Figure 1c). These results indicate that the TMA layer on Au surface successfully promotes the formation of JA shell for these dyes, as supported by the occurrence of peak or dip type absorption near the J-band of respective JAs and the observation of resonance fluorescence in PL spectra. The absorption spectrum of the colloidal solution containing Au_2/TMA/Thia(Et) NRs (blue line) shows a distinct dip at 650 nm, which reflects a strong 11806 DOI: 10.1021/la901431r

coupling in this system. This result is consistent with the fact that the J-band of Thia(Et) JA (λJ = 652 nm29) sufficiently overlaps with the L-plasmon band of Au_2/TMA NR (λL = 703 nm). Similar results were observed in the case of Au_1 and Au_3/ TMA/Thia(Et) NRs, too (see Supporting Information Figure S3). On the other hand, the absorption spectrum of Au_2/TMA/ PIC NR (green line) is characterized by a sharp peak at λpeak = 577 nm and a broad peak at λpeak = 529 nm. The former peak is assigned to the J-band of PIC JA and the latter to the monomer band of PIC. The appearance of only peaks implies weak EP coupling in this composite NR, which agrees with an extremely small overlap between the PIC J-band and the L-plasmon band. It is also noticed that the L-plasmon band considerably shifts to longer wavelength and broadens, which would not be understood only by the interaction between Au NR core and PIC JA shell. Interestingly, both peaks and dip are observed for the absorption spectrum of Au_2/TMA/TC NR (λpeak = 431, 465 nm and λdip = 477 nm; red line). The absorption band peaked at 431 nm is attributed to the monomer band of TC dye. The appearances of a peak at 465 nm and a dip at 477 nm may be understood by considering the intermediate strength of EP coupling because of the modest overlap between the J-band of TC JA and the T-plasmon band of Au_2/TMA NR. The strength of EP coupling is also affected by the spatial configuration of metal core and JA shell, particularly the distance between them. For instance, Kelley has carried out the theoretical simulations for the optical absorption of dye-coated Au NPs and demonstrated that the spectral shape is altered by the separation between the dye and the Au core.30 To examine the effect of the separation between JA shell and Au NR surface on the EP coupling strength, we attempted to insert the multilayer of PDDA and PSS with different thickness between the TMA layer and the JA shell by means of an alternate adsorption technique. In this method, two kinds of polyelectrolytes, anionic PSS and cationic PDDA, were alternately stacked on the surface of Au/TMA NRs, yielding Au/TMA/(PSS/PDDA)n NRs (n = 1, 2, and 3). Because the surface of a TMA layer is positively charged in aqueous solutions, the PSS layer was first deposited on Au/TMA NRs. It is noted that the JA shells could not be formed on the surface of Au/ TMA/PSS NRs with any kind of cyanine dyes examined here, implying that negatively charged PSS layer hindered the adsorption of anionic cyanine dyes (Figure S4). Figure 6 shows the (29) The reference peak position of Thia(Et) J-band was estimated from the absorption spectra of LB film incorporating the Thia(Et) JA layers supported on a dimethyldioctadecylammonium bromide matrix. (30) Kelley, A. M. Nano Lett. 2007, 7, 3235–3240.

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thickness of spacer layer for Au_1/TMA/(PSS/PDDA)2 NR is about 13 nm and much larger than that of the TMA layer, which would result in a weak EP coupling and therefore the occurrence of a peak type absorption. Similar peak type absorption was observed for Au_1/TMA/(PSS/PDDA)3 NR (Figure S5). These results are considered to demonstrate the qualitative dependence of EP coupling strength on the separation between the JA shell and the Au NR.

Conclusion Figure 6. Absorption spectra of colloidal solutions containing Au_1/TMA/(PSS/PDDA)n/Thia(Ph) NRs (n = 0, 1, and 2) along with that of Au_1/TMA NR (dashed line).

Figure 7. TEM images of negatively stained samples for (a) Au_1/ TMA/(PSS/PDDA) NRs and (b) Au_1/TMA/(PSS/PDDA)2 NRs. Scale bars in the TEM images correspond to 50 nm.

absorption spectra of Au_1/TMA(PDDA/PSS)n/Thia(Ph) NRs (n = 0, 1, and 2) along with that of Au_1/TMA NR (dashed line). It is evident that positively charged PDDA layer promotes the formation of Thia(Ph) JA shell in a similar way to the TMA layer. What is important here is that the spectral dip becomes shallower and even turns into a peak as the number of n is increased. This means that the EP coupling becomes weaker with increasing the separation between the JA shell and the Au NR surface. Figure 7 displays the TEM images of negatively stained samples for Au_1/TMA/(PSS/PDDA)n NRs (n = 1 and 2). The estimated thickness of spacer layer for Au_1/TMA/(PSS/PDDA) NR is about 4 nm, which is somewhat thicker than the TMA layer. This thickening of the spacer layer should be responsible to the shallow spectral dip as seen in Figure 6 because of slight weakening of the EP coupling. On the other hand, the average

Langmuir 2009, 25(19), 11802–11807

We have described the synthesis and spectroscopic properties of composite NRs composed of Au NR core, inner shell of spacer layer, and outer shell of cyanine dye JA layer. The successful preparation of such double-shell type composite NRs allowed us to control the resonant energies of surface plasmon and molecular exciton as well as a separation length between Au surface and JA shell by changing the aspect ratio of Au NR core, kind of cyanine dye, and thickness of the spacer layer. A series of spectroscopic measurements have revealed that the strength of exciton-plasmon coupling is largely modulated by the relative difference between plasmon and exciton energies and the spatial configuration of Au NR core and JA shell. The tunability of the excitonplasmon coupling is of great significance for the implementation of new plasmonic or optoelectronic devices. The double-shell type composite NRs prepared in this study certainly possess such capabilities and therefore appear promising in future applications. Acknowledgment. This work is a part of the Osaka Central Area Industry-Government-Academia Collaboration Project on “City Area Program” sponsored by MEXT (Ministry of Education, Culture, Sports, Science & Technology, Japan), 2007-2009. This work is also partially supported by KAKENHI (20510079), MEXT. Supporting Information Available: Absorption and PL spectra of Thia(Ph) JA aqueous solution (S1); absorption spectra of colloidal solutions containing the Au/TMA/Thia(Ph) composite NRs with different aspect ratio (S2), Au/ TMA/Thia(Et) NRs (S3), mixed solution of Au_1/TMA/ PSS NRs and Thia(Ph) dye (S4), Au_1/TMA/(PSS/PDDA)3 NRs and Au_1/TMA/(PSS/PDDA)3/Thia(Ph) NRs along with the TEM image of Au_1/TMA/(PSS/PDDA)3 NRs (S5); absorption spectra of cyanine dye solutions used in this study (S6). This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la901431r

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