Tuning of the Spectroscopic Properties of Composite Nanoparticles by

Apr 16, 2009 - ... De Giorgi , Dario Ballarini , Lorenzo Dominici , Iolena Tarantini , Massimo Cuscuná , Fabio Della Sala , Giuseppe Gigli , and Dani...
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Tuning of the Spectroscopic Properties of Composite Nanoparticles by the Insertion of a Spacer Layer: Effect of Exciton-Plasmon Coupling Akihito Yoshida,* Yoshiro Yonezawa, and Noritsugu Kometani Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, Osaka 558-8585, Japan Received January 14, 2009; Revised Manuscript Received March 13, 2009 Composite nanoparticles (NPs) having a double-shell structure, Au core, spacer layer (inner shell), and J-aggregate (JA) layer (outer shell) (Au/spacer/JA) have been synthesized. The spacer layer composed of N,N,N-trimethyl(11mercaptoundecyl)ammonium chloride played an important role in promoting the J-aggregation of anionic cyanine dyes on the surface, as evidenced by the successful formation of the JA layers with four kinds of anionic cyanine dyes. It was found that the presence of a spacer layer causes a significant change in the line shape of the absorption spectrum, particularly near the J-band; there is the appearance of a peak type absorption for the composite NPs with the doubleshell structure, while there is a dip type absorption for the ones without the spacer layer. The change from the peak type absorption to the dip type absorption in the Au/spacer/JA NPs occurs when the size of the Au core is varied from 5 to 15 nm. These observations would indicate that the strength of exciton-plasmon coupling between the Au core and the JA layer is enhanced with the increase in the core size or the decrease in the separation between the Au core and the JA shell. The photoluminescence arising from the JA can be detected for the composite NPs with the double-shell structure, showing that the quenching by the Au core is effectively suppressed by the spacer layer.

Introduction Molecular assemblies called the J-aggregate (JA) composed of a certain type of cyanine dyes have been easily formed in various systems such as concentrated solutions with inorganic salts, adsorbed layers at the solid surfaces, LB films, etc.1-3 The optical properties of JA have been extensively studied for a long time because of the use as an excellent photosensitizer in silver-halide photography4,5 as well as the potential application to novel optoelectronic materials.6,7 The spectroscopic properties of JA are characterized by the sharp absorption band (Jband) red-shifted with respect to the monomer band, resonance fluorescence with a small Stokes shift, ultrashort radiative lifetime, and large nonlinear optical susceptibility. These unique properties have been explained in terms of the delocalization of Frenkel exciton, resulting from the strong coupling between transition dipole moments of dye molecules.8 On the other hand, it has been well-known that colloidal solutions of gold or silver nanoparticles (NPs) show a vivid red or yellow color because of the particular interaction between noble metal NPs and incident electromagnetic waves of light, called localized surface plasmon (LSP) resonance.9-11 The LSP resonance *To whom correspondence should be addressed. Fax: +81-66605-2984. E-mail: [email protected]. (1) Jelley, E. E. Nature (London) 1936, 138, 1009–1010. (2) Scheibe, G. Angew. Chem. 1936, 49, 563. :: (3) Mobius, D. Adv. Mater. 1995, 7, 437–444. (4) Rubtsov, I. V.; Ebina, K.; Satou, F.; Oh, J. W.; Kumazaki, S.; Suzumoto, T.; Tani, T.; Yoshihara, K. J. Phys. Chem. A 2002, 106, 2795–2802. (5) Tani, T. Imaging Sci. J. 2007, 55, 65–79. (6) Tischler, J. R.; Bradley, M. S.; Zhang, Q.; Atay, T.; Nurmikko, A.; Bulovic, V. Org. Electron. 2007, 8, 94–113. (7) Scheblykin, I. G.; Lepnev, L. S.; Vitukhnovsky, A. G.; Van der Auweraer, M. J. Lumin. 2001, 94, 461–464. (8) Kobayashi, T., Ed. J-Aggregates; World Scientific: Singapore, 1996. (9) Kerker, M. The Scattering of Light and Other Electromagnetic Radiation; Academic Press: New York, 1969. (10) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677. (11) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410–8426.

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induces quite a large extinction in the visible and near-infrared region and, more importantly, the enhancement of the electromagnetic field in the vicinity of the metal surface leading to various intriguing phenomena such as surface-enhanced Raman scattering (SERS),12-14 enhanced fluorescence,15-17 and so on. Furthermore, these features can be controlled or adjusted by modifying the size, morphology, and composition of NPs, which are of great significance for future applications in biological sensors18,19 and photonics devices.20,21 For these reasons, considerable efforts have been devoted to research on noble metal NPs over the past decade. In view of the fascinating nature of JA and noble metal NPs, it is reasonable to expect that the composite systems of these two materials exhibit certain novel properties useful for optical or optoelectronic materials. Such systems may also offer a good opportunity to study the interaction between Frenkel exciton and LSP. In a previous study, our group reported the fabrication method and the spectroscopic properties of the noble metal/JA composite NPs (Figure 1a), in which Au, Ag, or Au/Ag cores are directly coated with JA of an anionic cyanine dye, 3,30 -disulfopropyl-5,50 -dichloro-thiacyanine (TC).22 It was demonstrated (12) Kerker, M. Acc. Chem. Res. 1984, 17, 271–277. (13) Schatz, G. C. Acc. Chem. Res. 1984, 17, 370–376. (14) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Chem. Rev. 1999, 99, 2957–2975. (15) Lakowicz, J. R. Anal. Biochem. 2005, 337, 171–194. (16) Zhang, J.; Fu, Y.; Chowdhury, M. H.; Lakowicz, J. R. Nano Lett. 2007, 7, 2101–2107. :: (17) Mackowski, S.; Wormke, S.; Maier, A. J.; Brotosudarmo, T. H. P.; :: Harutyunyan, H.; Hartschuh, A.; Govorov, A. O.; Scheer, H.; Brauchle, C. Nano Lett. 2008, 8, 558–564. (18) Haes, A. J.; Van Duyne, R. P. Anal. Bioanal. Chem. 2004, 379, 920–930. (19) Mayer, K. M.; Lee, S. G.; Liao, H. W.; Rostro, B. C.; Fuentes, A.; Scully, P. T.; Nehl, C. L.; Hafner, J. H. ACS Nano 2008, 2, 687–692. (20) Hutter, E.; Fendler, J. H. Adv. Mater. 2004, 16, 1685–1706. (21) Zhang, X. P.; Sun, B. Q.; Friend, R. H.; Guo, H. C.; Nau, D.; Giessen, H. Nano Lett. 2006, 6, 651–655. (22) Kometani, N.; Tsubonishi, M.; Fujita, T.; Asami, K.; Yonezawa, Y. Langmuir 2001, 17, 578–580.

Published on Web 04/16/2009

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Figure 1. Schemes of (a) Au/dye and (b) Au/TMA/dye composite NPs along with the molecular structure of TMA. (c) Molecular structures of cyanine dyes.

that the absorption spectrum of a colloidal solution containing TC-coated Au NPs, hereafter referred to as Au/TC composite NPs, is not a simple sum of the contributions of colloidal absorption of the Au NP and the J-band of TC but is characterized by an evident absorption “dip” at the position of the J-band. In contrast, an absorption “peak” ascribed to the J-band of TC appeared in the absorption spectrum of the Ag/TC composite NP suspension. These spectral features could be reproduced by the Maxwell-Garnett type treatment. Since then, a number of research groups have reported on the spectroscopic properties of noble metal/JA nanocomposite systems.23-27 For example, Hranisavljevic et al. measured the transient absorption spectra of colloidal solutions containing Ag/TC or Au/TC composite NPs.23 For the Ag/TC composite NP, they reported a relatively long-lived photoinduced charge-separate state as a consequence of coherent coupling (constructive interference) between molecular exciton and LSP, which is not observed for the JA on the surface of bulk metals. For the Au/ TC composite NP, it was shown that the excitonic state interferes destructively with bound electronic transition dipoles in the Au core, reducing the exciton lifetime by 2 orders of magnitude. Wurtz et al. fabricated an assembly of oriented Au nanorods covered with a shell of cyanine dye JA and demonstrated that the strength of exciton-plasmon coupling could be easily turned by engineering the geometry of the nanorods assembly.24 We recently examined the spectroscopic properties of the self-assembled (SA) films incorporating the Ag/TC composite NPs.28 It was found that the electromagnetic interactions between (23) (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. (24) Wurtz, G. A.; Evans, P. R.; Hendren, W.; Atkinson, R.; Dickson, W.; Pollard, R. J.; Zayats, A. V. Nano Lett. 2007, 7, 1297–1303. (25) Uwada, T.; Toyota, R.; Masuhara, H.; Asahi, T. J. Phys. Chem. C 2007, 111, 1549–1552. (26) Wiederrecht, G. P.; Hall, J. E.; Bouhelier, A. Phys. Rev. Lett. 2007, 98, 083001. (27) Fofang, N. T.; Park, T. H.; Neumann, O.; Mirin, N. A.; Nordlander, P.; Halas, N. J. Nano Lett. 2008, 8, 3481–3487. (28) Yoshida, A.; Kometani, N.; Yonezawa, Y. Colloids Surf., A 2008, 313/314, 581–584.

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composite NPs in the single layer SA film and in the multilayer SA film cause profound changes in the spectral line shape of the composite NPs, especially near the J-band of TC. In this study, we have prepared double-shell type composite NPs composed of the Au core and the spacer/JA double-shell as shown in Figure 1b. The composite NPs, Au/spacer/JA NPs, have some advantages over the previous Au/JA type NPs. For instance, as pointed out by Hranisavljevic et al., the fluorescence of metal/JA NPs is almost completely quenched in spite of the high fluorescence quantum yield of the original JAs due to the excitation/charge transfer between metal and excited dyes.23 However, it is expected that the fluorescence quenching will be effectively suppressed when the spacer layer having an adequate thickness and properties is inserted between the Au core and the JA shell. Moreover, we may realize the optimum conditions for various kinds of dyes to form the JA shell by using the long-chain compounds bearing functional groups as the spacer layer. It has been noted that only a few kinds of dyes are capable of forming the JA shell on the bare Au or Ag NP surface in the solution, making the systematic study quite difficult. In this sense, the properties of the long-chain compounds available for the spacer layer are crucial for the synthesis of metal/spacer/JA composite NPs. From such a point of view, the following three criteria have to be taken into account: It must be tightly fixed on the surface of the Au core, agglomeration between NPs in solution must be avoided after the modification of metal surfaces, and various kinds of cyanine dyes must readily form the JA shell on the surface. Here, we have adopted the self-assembled monolayer (SAM) of an alkanethiol compound with a cationic trimethylammonium group at the ω-terminal, N,N,N-trimethyl(11-mercaptoundecyl)ammonium chloride (TMA, Figure 1). As shown later, the Au NPs coated by this compound not only undergo no agglomeration in aqueous solutions but effectively facilitate the J-aggregation of four kinds of anionic cyanine dyes. We also prepared the Au/spacer/JA composite NPs having different core diameters to examine the core-size dependence of the spectroscopic properties of the composite NPs. It was found that the spectral features of the composite NPs are highly dependent on the presence of the spacer layer, the size of the Au core, and the Langmuir 2009, 25(12), 6683–6689

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relative position of the J-band with respect to the LSP resonance band. Furthermore, resonance fluorescence of the JA is evident in the Au/spacer/JA composite NPs, which is hardly detected in the metal/JA NPs.

Experimental Section Materials. Hydrogen tetrachloroaurate tetrahydrate, dodecylamine (DDA), trisodium citrate dihydrate, and L -ascorbic acid were purchased from Kishida Chemical Co., Ltd. Didodecyldimethylammonium bromide (DDAB), tetrabutylammonium borohydride (TBAB), and anhydrous hydrazine were purchased from Tokyo Chemical Industry Co., Ltd. Oleylamine (OLA) and sodium borohydride were purchased from Wako Pure Chemical Industries, Ltd. Cetyltrimethylammonium bromide (CTAB) was purchased from Sigma-Aldrich Co. TC, 3,30 -disulfopropyl-5,50 -dichloro-9-ethyl-thiacarbocyanine potassium salt [Thia(Et)], 3,30 -disulfopropyl-5,50 -dichloro-9-phenyl-thiacarbocyanine triethylammonium salt [Thia(Ph)], and 1,10 -disulfopropyl2,20 -cyanine triethylammonium salt (PIC) were received from Hayashibara Biochemical Laboratories, Inc. The chemical structures of cyanine dyes are illustrated in Figure 1c. Those materials were used without further purification. TMA was synthesized according to the procedure in ref 29. All aqueous solutions were prepared with distilled water. Preparation Method of Au/TMA/Cyanine Dye JA Composite NPs. Synthesis of Au_S/TMA NPs (a = 5.2 nm). DDA-coated Au NPs (Au/DDA NPs) were synthesized according to the procedure developed by Jana et al.30 with slight modifications. A 10.3 mg amount of HAuCl4 3 4H2O (0.025 mmol) and 90 mg of DDA were dissolved together in 2.5 mL of DDAB solution (0.1 M in toluene). Twenty-five milligrams of TBAB was dissolved in 1 mL of the DDAB solution in another vessel. Then, these two solutions were mixed under stirring. The resultant solution was used as the seed solution to grow larger particles. Next, the seed solution (∼3.5 mL) was added into 25 mL of the growth solution, which was prepared in advance by dissolving 103 mg of HAuCl4 3 4H2O (0.25 mmol), 500 mg of DDAB, and 925 mg of DDA in 25 mL of toluene with sonication until the appearance of a clear yellow color. Finally, 10 mL of the DDAB solution containing anhydrous hydrazine (0.2 M) was added in dropwise under stirring. The average diameter of the Au/DDA NPs was 5.2 nm as determined by the TEM observation, which was consistent with the published data.31 The ligand exchange from DDA to TMA was performed according to the procedure analogous to that reported by Kalsin et al.31 First, 5 mL of as-prepared Au/DDA suspension was poured into 25 mL of methanol, yielding the turbid suspension. This suspension was centrifuged at 6000 rpm for 10 min, and subsequently, the supernatant was decanted to remove excess DDA. Next, the precipitation was redispersed in 15 mL of toluene, and subsequently, 5 mL of methanol containing 40 mg of TMA was added into the suspension, yielding the turbid suspension. Again, centrifugation at 6000 rpm for 10 min and decantation were carried out. The precipitate was washed with ethyl acetate (15 mL) three times. Finally, dried solid product was dissolved in deionized water. The Au NP covered with TMA is hereafter referred to as Au_S/TMA NP. Synthesis of Au_M/TMA NPs (a = 9.1 nm). OLA-coated Au NPs (Au/OLA NPs) were synthesized by the procedure reported by Hiramatsu et al.32 First, a mixture of OLA (3 mL) and toluene (15 mL) was heated at ∼140 °C. Then, a solution of HAuCl4 3 4H2O (50 mg) and OLA (1.2 mL) dissolved in 1.0 mL (29) Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 5349–5355. (30) Jana, N. R.; Peng, X. G. J. Am. Chem. Soc. 2003, 125, 14280–14281. (31) Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A. Science 2006, 312, 420–424. (32) Hiramatsu, H.; Osterloh, F. E. Chem. Mater. 2004, 16, 2509–2511.

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of toluene was injected quickly into the boiling solution. Heating was stopped after 2 h, and the solution was cooled to room temperature. Next, 25 mL of methanol was added into 9 mL of the colloidal solution, yielding the turbid suspension. This was centrifuged at 6500 rpm for 15 min, and supernatant was decanted to remove excess OLA. Subsequent ligand exchange of Au/OLA with TMA was done in almost the same manner as that for the Au_S/TMA NPs. Synthesis of Au_L/TMA NPs (a = 15.2 nm). Au_L/TMA NPs were prepared in almost the same manner as reported by Jana et al.33 First, the seed solution was prepared. Ten milliliters of 0.5 mM HAuCl4 and 10 mL of 0.5 mM trisodium citrate were mixed in a flask. Then, 0.6 mL of ice-cooled, freshly prepared 0.1 M NaBH4 aqueous solution was added into the solution and kept on stirring for 2-5 h. Second, the growth solution was prepared. In another vessel, 1.2 g of CTAB was dissolved in 40 mL of 2.5  10-4 M HAuCl4 aqueous solution. Subsequently, 0.2 mL of 0.1 M L-ascorbic acid in water was poured into the CTAB/HAuCl4 solution under stirring, yielding the colorless solution. Third, 8 mL of the seed solution was added into 32 mL of the growth solution under stirring. After it was stirred for 30 min, the solution was colored wine red. Ten milliliters of this solution was again poured into 30 mL of freshly prepared growth solution, yielding CTAB-capped Au NPs with an average diameter of 15.2 nm. The excess CTAB dissolved in the solution was removed by centrifugation and decantation. Finally, the ligand exchange from CTAB to TMA was performed by adding the CTAB-capped Au colloid solution into 20 mL of 2 mM TMA aqueous solution under stirring for 2 h. Then, excess TMA was removed by centrifugation. Preparation of Au/TMA/JA Composite NPs. Au_S/TMA/ JA composite NPs were prepared by mixing 4 mL of the Au_S/ TMA colloidal solution (∼0.2 mM in terms of gold atoms) and 1 mL of aqueous solution of various cyanine dyes (5  10-5 M). In the case of Au_M/TMA/JA or Au_L/TMA/JA composite NPs, the concentration of colloidal solutions was adjusted so that the optical density at 400 nm became 0.625 before mixing with the dye solution. Then, 4 mL of the Au_M/TMA or the Au_L/TMA colloidal solution was mixed with 1 mL of the 5  10-5 M dye solution. 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.). Photoluminescence (PL) measurements were carried out with a FP-750 fluorimeter (JASCO Co.). All measurements were performed at room temperature in air. Transmission electron microscopy (TEM) images were acquired with a JEOL JEM-2100 or a Hitachi H-7000 electron microscope operated at 200 or 75 kV, respectively. The sample for the TEM observation was prepared by putting a small amount of the colloidal solution on the carbon-supported copper grids and drying.

Results and Discussion The insertion of a spacer layer between the Au core and the JA shell plays a key role in this study. The substance serving as the spacer layer, TMA, is an alkanethiol compound having a trimethylammonium group at the ω-terminal of the long alkyl chain. It has been well-known that thiol compounds with a long alkyl chain possess a substantial capability to sheathe Au, Ag, or other noble metal NPs to form the stable SAM, as evidenced by their long-term stability in solutions or solid matrices.30,34,35 Because the TMA molecules are fixed on the Au surface by a strong S-Au bond, it is expected that Au NPs covered with the (33) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782–6786. (34) Brust, M.; Walker, M; Bethell, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 801–802. (35) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27–36.

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Figure 2. TEM images of (a) Au_S/TMA/TC NPs and (b) Au/TC NPs. Scale bars in these TEM images correspond to 100 nm. The insets show the size distributions.

Figure 3. (a) Absorption spectra of colloidal solutions containing Au_S/TMA (broken line) and Au_S/TMA/TC NPs (solid line). (b) PL spectrum of Au_S/TMA/TC colloidal solution. The excitation wavelength is 430 nm. (c) Absorption spectra of colloidal solutions containing Au (broken line) and Au/TC composite NPs (solid line).

SAM of TMA (Au/TMA NPs) expose the trimethylammonium moiety to the outside and form the positively charged surface. Such a situation renders the Au/TMA NPs to be easily dispersed in aqueous solution and more importantly promotes the J-aggregation of anionic cyanine dyes on the surface of TMA layer. Figure 2a shows the TEM image and size distribution of Au_S/ TMA NPs. Most of the Au_S/TMA NPs have a spherical shape with a relatively narrow size distribution. The average diameter, a, is estimated to be a = 5.2 nm, which is consistent with the published data of 5.1 nm. No agglomeration between NPs was observed, implying the effective stabilization by TMA. Figure 3a, b displays the absorption and PL spectra of the Au_S/TMA/TC colloidal solution, along with the absorption spectrum of Au_S/ TMA colloidal solution (dashed line). As seen in Figure 3a, the absorption spectrum of the Au_S/TMA/TC composite NPs is characterized by a sharp peak at λ = 459 nm and a broader peak at λ = 526 nm. The former peak is attributed to the J-band of TC, suggesting the formation of the JA shell in the Au_S/TMA/TC colloidal solution. The latter peak arises from the LSP resonance of the Au core, which is red-shifted with respect to the one for the Au_S/TMA NPs (λ = 516 nm). These results indicate that the TMA layer on the surface of Au core possesses a promoting effect 6686 DOI: 10.1021/la900169e

on the J-aggregation of TC molecules because the TC concentration (in the present case, [TC] = 1  10-5 M) in the colloidal solution is too low for TC molecules to spontaneously form JA in aqueous solution. In conjunction with the fact that the LSP resonance frequency is significantly modulated by the addition of TC, it is reasonable that the outermost surface of the Au_S/ TMA NPs is covered with TC JAs, generating the double-shell structure as depicted in Figure 1b. Fukumoto and Yonezawa previously reported that anionic TC molecules could be easily organized into JAs on the SA thin film of cationic polyelectrolyte, poly(diallyldimetylammonium chloride) (PDDA), by virtue of the strong electrostatic interaction between negatively charged TC and positively charged quaternary ammonium moiety of PDDA.36 Because the TMA molecule has a quaternary ammonium moiety at the ω-terminal, a similar mechanism would work on the TMA layer of Au/TMA NP as well. As shown later, the formation of the double-shell structure in the Au/TMA/TC NPs has been more definitely confirmed by TEM observations. It is notable that the Au_S/TMA/TC NPs show a significant emission peaked at λ = 469 nm (Figure 3b; the excitation wavelength λex was 430 nm). Considering the appearance of the TC J-band at λ = 459 nm in the absorption spectrum, this (36) Fukumoto, H.; Yonezawa, Y. Thin Solid Films 1998, 327-329, 748–751.

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emission is assigned to the resonance fluorescence of TC JA. Note that no fluorescence associated with the TC JA could be detected for the Au/TC composite NPs, in which the Au cores are directly coated with TC JAs, probably due to the strong quenching by the excitation energy transfer or the charge transfer from the JA shell to the Au core. However, such a quenching process could be effectively suppressed in the case of the Au_S/TMA/TC NPs because of the adequate separation between the TC JA shell and the Au core by the TMA layer. For a comparison, we prepared the Au/TC composite NPs by the same procedure as previously reported in ref 22. Figure 2b shows the TEM image and size distribution of the Au/TC composite NPs. The average diameter of the Au/TC composite NPs (a = 5.2 nm) is almost the same as that of the Au_S/TMA/ TC NPs, while the distribution is somewhat broader. Figure 3c shows the absorption spectra of the colloidal solutions containing bare Au NPs (dashed line) or Au/TC composite NPs (solid line). It is noticed that there is a major difference in the spectral line shape near the J-band of TC (λJ = 465 nm for TC JA in the PDDA film36). Whereas the spectrum of the Au_S/TMA/TC composite NPs shows a sharp “peak” at λ = 459 nm, an anomalous “dip” is observed at λ = 475 nm for the Au/TC NPs. Here a dip means the spectral line shape of composite NPs near the J-band in which the absorbance is reduced relative to that of NPs without JA shell. The presence of the dip in the Au/TC NPs is in accordance with the previous publication, which has been interpreted as a result of the strong exciton-plasmon coupling between Au core and TC JA shell.22 Hence, the appearance of the peak instead of the dip for the Au_S/TMA/ TC NPs indicates that such coupling is weakened due to the presence of the TMA layer. To investigate the possibility of JA shell formation with other kinds of cyanine dyes, the following dyes, PIC, Thia(Et) and Thia (Ph), were tested. The chemical structures of these dyes are shown in Figure 1c. Absorption and PL spectra of the Au_S/TMA NP solutions including these dyes are summarized in Figure 4. The development of a sharp J-band at λJ = 566 nm for PIC, λJ = 621 nm for Thia(Et), and λJ = 641 nm for Thia(Ph) in the absorption spectra as well as the observation of corresponding resonance fluorescence peaked at λ = 579 nm for PIC, λ = 638 nm for Thia(Et), and λ = 658 nm for Thia(Ph) demonstrate that those dyes really form the JA shell on the surface of the Au_S/ TMA NPs, too. Considering that these dyes are hardly organized into the JA shell on the surface of bare Au NPs in the solution, the TMA layer is crucial for the successful formation of JA shell. The fabrication of the Au/TMA/JA composite NPs with various kinds of cyanine dyes may open up the way for the systematic modulation of spectroscopic properties with varying the relative peak position of the J-band to the LSP band. Next, we examined the spectroscopic properties of the Au/ TMA/TC NPs with different core diameters. TEM images and size distributions of the Au_M/TMA and the Au_L/TMA NPs are shown in Figure 5. The average diameters of core particles are estimated to be a = 9.1 nm for the Au_M/TMA NPs and a = 15.2 nm for the Au_L/TMA NPs, respectively. Figure 6 shows absorption and PL spectra of the Au_M/TMA/TC and the Au_L/ TMA/TC NPs. It is notable that not only the peak but also the dip similar to the one observed for the Au/TC NPs (Figure 3c) appear in these absorption spectra (λpeak = 456 nm, λdip = 473 nm for Au_M/TMA/TC NP and λpeak = 458 nm, λdip = 472 nm for Au_L/TMA/TC NPs), which is in contrast with the observation of only a peak for the Au_S/TMA/TC NPs (λpeak = 459 nm). Moreover, the dip becomes deeper with an increase in the core diameter from a = 9.1 nm (Au_M/TMA/TC) to a = 15.2 nm Langmuir 2009, 25(12), 6683–6689

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Figure 4. (a) Absorption and (b) PL spectra of colloidal solutions containing Au_S/TMA/PIC (red), Au_S/TMA/Thia(Et) (blue), and Au_S/TMA/Thia(Ph) NPs (green) along with Au/TMA NPs without dye. The excitation wavelengths for PL measurements are 520 nm for PIC, 550 nm for Thia (Et), and 600 nm for Thia (Ph), respectively.

(Au_L/TMA/TC). These results should be related to the enhancement of exciton-plasmon coupling with an increase in the core diameter. We also observed the emission from the Au_M/TMA/ TC (λem = 473 nm) and the Au_L/TMA/TC NPs (λem = 483 nm) in the solution (Figure 6b,d), which is assigned to the resonance fluorescence of TC JA and partially to the emission from TC monomer. Similar alternations in the absorption spectra from the peak type to the dip type near the J-band with an increase in the core diameter and occurrence of resonance fluorescence were observed for the Au/TMA/PIC NPs (see the Supporting Information, Figure S1.) We have presumed so far that the Au/TMA/JA NPs have the double-shell structure as depicted in Figure 1b solely from the spectroscopic measurements. To obtain further evidence, we have attempted to visualize the shell layer of Au/TMA and Au/TMA/ TC NPs by the TEM observations with the help of negative staining method with uranyl acetate solution. The samples for the negative staining were prepared by putting a very small amount of the colloidal solution of Au/TMA NPs or Au/TMA/TC NPs 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. Because the NPs in the sample were then surrounded by uranium, the contrast of TMA and TC JA shell mainly composed of light elements like carbon, nitrogen, etc. was enhanced in the TEM image. Figure 7a,b shows TEM images for the Au_L/TMA NPs and the Au_L/TMA/TC NPs, respectively. It is obvious that relatively bright rings encompass the Au cores and that they become thicker for the Au_L/TMA/TC NPs than the Au_L/TMA NPs. These observations clearly indicate that the bright rings are nothing but the TMA shell or the TMA + TC JA double-shell. Therefore, we can conclude that the Au/TMA/TC NPs actually have the double-shell structure. The thickness of the ring is estimated to be 1.1 ( 0.3 nm for the Au_L/TMA NPs and 2.8 ( 0.7 nm for the Au_L/TMA/TC NPs, from which the thickness of TC JA shell amounts to ∼1.7 nm. The thickness of the TMA layer (1.1 nm) obtained here is somewhat smaller than the length of TMA molecule (∼1.9 nm) estimated by the simple molecular model, implying that TMA molecules adsorbed on the Au surface would be tilted to the surface normal.37 The thickness of TC JA shell is slightly larger than the nominal size of TC molecule (chromophore size ∼1.6  0.6  0.4 nm3). Similar results obtained for the Au_S/TMA/TC and the Au_M/TMA/TC NPs are summarized in the Supporting Information (Figure S2). (37) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558–569.

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Figure 5. TEM images of (a) Au_M/TMA/TC NPs and (b) Au_L/TMA/TC NPs. The insets show the size distributions.

Figure 7. TEM images of negatively stained samples with uranyl acetate aqueous solution for (a) Au_L/TMA NPs and (b) Au_L/ TMA/TC NPs. The bright rings around Au cores represent TMA shells in panel a and the sum of TMA + TC JA shells in panel b.

Figure 6. Absorption and PL spectra of colloidal solutions containing (a, b) Au_M/TMA/TC and (c, d) Au_L/TMA/TC NPs. The excitation wavelength is 430 nm for both solutions.

It is reasonable that the line shape of the absorption spectrum for Au/JA composite NPs is highly sensitive to the strength of exciton-plasmon coupling.22-24 In the previous paper, we reported that with an increase in the overlap between the J-band and the LSP bands, the line shape of the absorption spectrum of Ag/TC composite NPs in the PDDA films changes from the peak type to the dip type near the J-band of TC JA.28 Furthermore, experimental results in this study strongly suggest that the distance between the Au core and the JA shell as well as the core size of Au NP have a crucial influence on the spectral line shape, too. To understand the above experimental results, we have assumed that the enhanced electromagnetic field (E-field) generated around the Au NP is linked to the strength of excitonplasmon coupling in the Au/spacer/JA NPs. The E-fields localized in the vicinity of the metal surface are dependent on the size and shape of NPs, the wavelength of incident light, and the kind of surrounding medium.10 The magnitude of the E-field also 6688 DOI: 10.1021/la900169e

attenuates with the distance, r, from the metal surface. One can expect that a larger E-field at the position of JA shell should induce a stronger interaction between JA and Au NP. We therefore carried out a simple calculation of the E-field for the Au NP and estimated an electromagnetic field enhancement factor (EFE) as a function of r.38 The EFE is 3.0 at the surface of the Au sphere (a = 5.0 nm, r = 0.0 nm) and 1.5 at the position of the JA shell (r = 1.1 nm). The separation of r = 1.1 nm from the Au surface is responsible for the thickness of the TMA layer. In other words, the enhanced E-field where the TC JA of the Au/TC NP feels is twice as large as that of the Au_S/TMA/TC NP, resulting in the difference in the spectral features between the Au/TC NP (dip type) and the Au_S/TMA/TC NP (peak type). Similarly, the EFE values of Au NPs having diameters a = 5.0, 10.0, and 15.0 nm at r = 1.1 nm were estimated to be 1.5, 2.0, and 2.2, respectively. The increase in the value of EFE with increasing core diameter is consistent with the alteration of spectral features from the peak (Au_S/TMA/TC NP) to the dip (Au_M/TMA/TC, Au_L/TMA/ TC NPs). However, to establish the detailed relation between the magnitude of the E-field and the development of the peak/dip

(38) The electric fields outside the Au NP (Eout) were calculated by using the simple equations described in ref 10 assuming that the Au core is spherical and much smaller than the wavelength of light. The EFE value was defined as the magnitude of Eout divided by that of the E-field of incident electromagnetic wave. The incident wavelength was selected to be 471 nm (near the J-band of TC). The EFE values for the Au sphere with a = 5 nm were estimated to be 3.0 at the Au surface (r = 0.0 nm) and 1.5 at the position of JA shell (r = 1.1 nm).

Langmuir 2009, 25(12), 6683–6689

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Article

type spectra, more elaborate theoretical works are necessary. These are the subject of our further study. We would finally remark that the concept of double-shell type composite NPs developed in this study is applicable to various types of noble metal NPs such as Ag, Au, and Au/Ag alloyed NPs. Especially, it is interesting to extend the current method to the nonspherical NPs like nanorods and nanoplates because those NPs are characterized by relatively large E-field near the surface edge, and the LSP resonance frequency may be modulated to a considerable extent. This will enable us to control a wider range of exciton-plasmon coupling and, thus, the spectroscopic properties of composite NPs. Such kinds of works are closely related to the development of novel optical and optoelectronic materials.

Conclusion Novel double-shell-type composite NPs composed of an Au core, inner shell of the TMA layer, and outer shell of cyanine dye JA have been synthesized. With the help of a negative staining method, the TEM observations have substantiated the actual formation of the double-shell structure in the synthesized NPs. Absorption and PL spectra have demonstrated that the TMA layer facilitates the J-aggregation of four kinds of anionic cyanine dye at the Au NPs surface. On the basis of a series of spectroscopic measurements, it has been suggested that the spectral line shape of the composite NPs near the J-band depends on the strength of the exciton-plasmon coupling between the Au core and the JA shell.

Langmuir 2009, 25(12), 6683–6689

It would be expected that the exciton-plasmon coupling is substantially affected not only by the overlap between the J-band and the LSP band but also by the magnitude of the electromagnetic field enhancement at the position of the JA shell. These considerations afford the possibility of tuning the spectroscopic properties of composite NPs by controlling the size of the metal core as well as the distance between the JA layer and the metal surface, which will pave the way for new applications in the optical and optoelectronic materials. 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 (No. 20510079), MEXT. We greatly appreciate Dr. Tatsuya Minami and Hiroshi Takayama (Osaka City University) for the kind assistance with the TMA synthesis and also thank Naoko Uchida (OCU) for the technical support with the TEM measurements and fruitful discussions. Supporting Information Available: Absorption and PL spectra of the Au/TMA_M/PIC and the Au/TMA_L/PIC composite NPs (S1) and other TEM images of negatively stained Au/TMA NPs (S2). This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la900169e

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