Light-Controllable Surface Plasmon Resonance Absorption of Gold

Sep 14, 2009 - ... Nag , Evan R. MacQuarrie , Anthony B. Kaye , and Richard F. Haglund , Jr. ... Yue Bing Zheng , Brian Kiraly , Sarawut Cheunkar , To...
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J. Phys. Chem. C 2009, 113, 17359–17366

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Light-Controllable Surface Plasmon Resonance Absorption of Gold Nanoparticles Covered with Photochromic Diarylethene Polymers Hiroyasu Nishi,† Tsuyoshi Asahi,‡ and Seiya Kobatake*,†,§ Department of Applied Chemistry, Graduate School of Engineering, Osaka City UniVersity, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan, Department of Applied Physics, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan, and Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Kawaguchi Center Building, 4-1-8 Honcho, Kawaguchi 332-0012, Japan ReceiVed: July 6, 2009; ReVised Manuscript ReceiVed: August 24, 2009

Various sizes of gold nanoparticles covered with photochromic diarylethene polymers (Au-poly(DE)s) were prepared by Brust’s method, citrate reduction, or a seeding growth method. The diarylethene polymers on the particles exhibited reversible photochromism upon alternating irradiation with ultraviolet and visible light, both in solution and in the solid state. Au-poly(DE) of larger size, prepared by the seeding growth method, showed a significant shift in the local surface plasmon resonance (LSPR) band photoreversibly as a result of photochromic reaction of the diarylethene polymers around the gold nanoparticles. This change is due to a change in the refractive index of the photochromic chromophore and is remarkably enhanced in the solid state. However, the smaller sized Au-poly(DE) prepared by Brust’s method hardly showed any such spectral shift. These results are qualitatively reproduced by a theoretical simulation, which indicates that the sensitivity of the LSPR band strongly depends on the particle size of the gold nanoparticle. The Au-poly(DE)s can be potentially used as new types of plasmonic materials with a light-controllable LSPR band. Introduction Gold nanoparticles (AuNPs) have attracted much attention because of their specific properties derived from their local surface plasmon resonance (LSPR).1 The LSPR band of the AuNP has a very strong absorption in the visible region, which gives the nanoparticles a brilliant red color. Since the optical property depends on their particle size, interparticle distance, and ambient surrounding, AuNPs have been widely utilized in a variety of fields, including physicochemical investigations into their optical properties1-6 and various analytical applications as sensing probes.1,7-11 In addition, AuNPs have been widely researched as a specific photoreaction field12-15 in terms of the enhancement in light due to the LSPR. Fluorescent enhancement and surface-enhanced Raman scattering near AuNPs have been reported by numerous researchers.1,16-18 These enhancement effects are derived from the photoelectric field enhancement and are related closely to the LSPR. Recently, a variety of methods have been investigated to control the optical properties of AuNPs. In one report, AuNPs were covered with the thermoresponsive poly(N-isopropylacrylamide) so they could be made to aggregate through a thermoresponsive phase transition with the aggregation leading to variation in the LSPR band.19 The LSPR band could be changed as a result of thermal trigger by a lower critical solution temperature. Similarly, covering the AuNPs with a pH-sensitive poly(4-vinylpyridine) gives rise to a two stage red shift in the LSPR band upon polymer collapse and particle agglomeration.20 In another example, an azobenzene derivative was attached to the AuNPs, which allowed a network of AuNPs to form through * To whom correspondence should be addressed. E-mail: kobatake@ a-chem.eng.osaka-cu.ac.jp. † Osaka City University. ‡ Osaka University. § PRESTO, JST.

interparticle connection. The LSPR band of the network of AuNPs could then be changed by photoirradiation, because the interparticle distance can be controlled by trans-cis isomerization of the derivatives.21 These effects arise due to aggregation of AuNPs in solution upon application of external stimuli. In the solid state, however, it is difficult to control the interparticle distance of AuNPs by such methods. In many situations, heat and pH are commonly used as the external stimuli.22-29 However, in order to use the LSPR-controlled AuNPs as a device or sensor it would be desirable for the material to be able to respond to external stimuli in the solid state and for the external stimuli to easily operate without any direct contact unlike thermal stimulus or addition of acid and base. Most recently, some new approaches for controlling the LSPR band have been reported. Zheng and co-workers controlled the LSPR band by manipulating the redox states of the rotaxane adsorbates.30 van der Molen and co-workers changed the LSPR band by photochromic reaction of a dithienylcyclopentene derivative.31 These LSPR band shifts were derived from change in refractive index around gold nanodisk or AuNP. Although these are better methods to readily change the LSPR band through an external stimulus, the spectral shifts in above papers are less than 10 nm. In order to accomplish large change in the LSPR band, it is important to give rise to large changes in the refractive index at wavelength near the LSPR band30 with molecules possessing a number of chromophores with absorption in the visible region. Furthermore, the diameter of the AuNPs should be controlled because the sensitivity of the LSPR band of the AuNPs to the ambient environment is strongly influenced by the particle size.2,3,6 Along these lines, we have reported on the synthesis and optical properties of AuNPs (5-7 nm in size) covered with diarylethene polymers (Au-poly(DE)s) and on the interaction between the diarylethene chromophore and the AuNPs both in

10.1021/jp906371k CCC: $40.75  2009 American Chemical Society Published on Web 09/14/2009

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SCHEME 1: Synthetic Routes to Prepare Various Sized Au-poly(DE)s by (a) Brust’s Method, (b) Citrate Reduction, and (c) a Seeding Growth Method

solution and in the solid state.32 Diarylethenes are photochromic compounds that can reversibly isomerize between colorless and colored forms upon alternating irradiation with ultraviolet (UV) and visible light. Since the physical and chemical properties of these compounds, such as absorption spectra, refractive indices, and dielectric constants, can be changed reversibly by photoisomerization, they have been used for various switching devices.33-41 Therefore, AuNPs surrounded by a poly(DE) shell are expected to exhibit a change in the LSPR band upon photochromic reaction. However, in our previous work, no switching behavior in the LSPR band of the Au-poly(DE)s could be observed as a result of photochromic reactions.32 Since the sensitivity of the LSPR to a change in dielectric constant or refractive index in the material around the AuNP largely depends on the AuNP diameter,1-3,5-7 control of the optical property may be possible for Au-poly(DE) particle of larger size. The preparation of AuNPs with water-soluble covering agents of size larger than 20 nm has been intensively studied,6,7,42-44 but there are limited reports on AuNPs covered with water-insoluble agents. It is thus necessary to establish a new synthetic method to make Au-poly(DE) particles of larger size. In the present work, we initially synthesized larger watersoluble AuNPs by citrate reduction45 and a seeding growth method42-44,46 before exchanging the water-soluble covering agent with poly(DE). The resultant Au-poly(DE)s exhibited a reversible spectral change in the LSPR band upon photoirradiation as a result of photoisomerization of the poly(DE). The spectral shift corresponds qualitatively to a theoretical simulation based on a core-shell model5 and is significantly enhanced in the solid state. Finally, we accomplished ca. 45 nm shift of the LSPR band with the largest sized Au-poly(DE). The larger sized Au-poly(DE)s have potential as new types of plasmonic materials in which control of the LSPR band is possible by photoirradiation. Results and Discussion Synthesis of Various Sizes of Au-poly(DE)s. The synthetic scheme to form the Au-poly(DE)s is shown in Scheme 1. The Au-poly(DE)s were synthesized by Brust’s method, citrate

reduction, or a seeding growth method to obtain various sizes of particles. Figure 1 shows the TEM images, core size histograms, and schematic illustrations of the Au-poly(DE)s prepared by the three different methods. As seen in Figure 1, the Au-poly(DE)s are not aggregated and the particles are well separated on the copper grid. The Au-poly(DE) synthesized by Brust’s method has a smaller mean diameter than that synthesized by citrate reduction. The Au-poly(DE) prepared by the seeding growth method has the largest mean diameter among the different preparations. The relative standard deviations (σ/d) for the different sized Au-poly(DE)s are 39% (d ) 5.9 nm), 14% (d ) 13 nm), and 20% (d ) 22 nm), respectively. The largest sized Au-poly(DE) was obtained within a σ/d of 20%. These TEM images indicate that the size of the Aupoly(DE)s can be controlled by the use of different synthetic methods and that the size of the Au core could be varied. Photochromism of the Au-poly(DE)s. Figure 2 shows the absorption spectral changes of the different Au-poly(DE)s in toluene upon irradiation at 313 nm. Before UV irradiation, a strong absorption band due to the LSPR band was observed in the visible region (around 530 nm) for the different Aupoly(DE)s. After irradiation of the solutions at 313 nm, a new absorption band could be observed as a shoulder on the LSPR band corresponding to the existence of the diarylethene closedring form. Upon exposure to visible light, the absorption band in the visible region then disappeared and the initial absorption band was observed, indicating the Au-poly(DE)s were undergoing reversible photochromism in solution. The photocyclization conversion of Au-free poly(DE) in the photostationary state reached 97% in toluene upon irradiation at 313 nm.47 The conversion of the Au-poly(DE) (d ) 5.9 nm) reached only 81% in toluene32 because of quenching of the excited state openring form of the diarylethene near the AuNP surface. The conversion of the other Au-poly(DE)s (d ) 13 nm and d ) 22 nm) were also estimated to be around 80%. This result indicates that the cyclization conversion of the Au-poly(DE)s does not depend on the diameter of Au core, because the quenching of the excited state occurs effectively only in a distance of 1 nm order from the AuNP surface.32,48,49

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Figure 1. TEM images, core size histograms, and schematic illustrations of (a) Au-poly(DE) (d ) 5.9 nm), (b) Au-poly(DE) (d ) 13 nm), and (c) Au-poly(DE) (d ) 22 nm). The polymer chain length in the right illustration shows the theoretical fully stretched length of the polymer chain. The parameter σ indicates the standard deviation of the AuNP diameter.

Change of Local Surface Plasmon Resonance Band. Figure 3 shows the difference absorption spectra of Au-free poly(DE) (n ) 42) and Au-poly(DE)s in toluene upon irradiation at 313 nm. These spectra were obtained by subtracting the absorption spectrum before UV irradiation from that after UV irradiation. The spectral shape in UV region far from the LSPR peak of the AuNPs is the same as that of the Au-free poly(DE). On the other hand, in the visible region a significant difference from the Au-free poly(DE) was observed. When the AuNPs of the larger size (d ) 13 and 22 nm) were used, the spectral peak was shifted to longer wavelength and a deeper dip was observed at the LSPR band peak (around 520 nm) of the AuNPs covered with open-ring from of poly(DE) shown in Figure 2. These spectral modifications can be ascribed to a change in the refractive index upon photochromic reaction of the poly(DE) around the AuNPs, as discussed later. Upon exposure to visible light, these spectra returned to the initial state, namely using photoirradiation as an external stimulus we have demonstrated reversible control of the LSPR band of AuNPs in solution. Simulation of LSPR Band Alteration by Mie Theory. In order to reproduce the spectral changes of the LSPR band of the Au-poly(DE)s, it is essential to calculate the extinction spectra of the core-shell structure in which a Au core coated homogeneously with a poly(DE) layer based on Mie theory using the algorithm described in ref 50. Figure 4 shows the complex refractive index (n* ) np + ik) of poly(DE) shell in the open- and closed-ring forms used for the present numerical calculation. Here, the real part, np, is refractive index and the

imaginary part k corresponds to optical absorption. It can be seen that the refractive index of the open-ring form hardly depends on wavelength in the visible region, while in the closedring form the refractive index is dependent on wavelength due to the absorption in the visible region. Figure 5a illustrates the core-shell structure used for the calculation of the extinction spectra of Au-poly(DE)s. The detail parameters used for the calculation are described in the experimental section. Figure 5b shows the extinction spectrum of a Au core (22 nm diameter)-poly(DE) shell (10 nm thickness) as an example of the calculation. After photocoloration of poly(DE) shell, the spectrum becomes a broad shape and the absorbance at the peak wavelength decreases. The difference spectra between before and after photocoloration of several sizes of Au cores are presented in Figure 5c. The difference spectra are due to the changes in the absorption band of the photochromic chromophore and the LSPR band. As can be seen in Figure 3, the spectral dip in the region of 500-550 nm was reproduced by the theoretical simulation. Furthermore, the simulation also replicated the experimental result in which the larger sized Au-poly(DE)s gave rise to a larger spectral dip. This size dependence is mainly due to different proportion of the extinction of the AuNP to the absorption of poly(DE) around the particle among the Au-poly(DE)s. The extinction coefficient of the particle is proportional to the volume,1,2 which means that the extinction is proportional to the cube of the radius. On the other hand, the number of poly(DE) per particle is considered to be proportional to the square of the radius. Consequently,

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Figure 2. Absorption spectral changes of (a) Au-poly(DE) (d ) 5.9 nm), (b) Au-poly(DE) (d ) 13 nm), and (c) Au-poly(DE) (d ) 22 nm) in toluene upon irradiation at 313 nm.

the larger spectral dip corresponding to the change in the LSPR band can be observed in the larger sized Au-poly(DE)s that has higher proportion of the extinction of the AuNP to the absorption of poly(DE). The theoretical simulation was able to qualitatively reproduce the change in the LSPR band of the Au-poly(DE)s upon photochromic reaction as well as the dependence on the AuNP diameter. The insignificant difference between the simulation and the experimental spectra may be ascribed to the core size distribution, the polymer shell thickness distribution, and the heterogeneity of the open- and closed-ring forms by the quenching of the photocyclization near the AuNP. Enhanced Change in the LSPR Band in the Solid State. Because of the shorter interparticle distance as seen in Figure 1, the initial LSPR band of the Au-poly(DE)s before UV irradiation should be significantly influenced by not only the poly(DE) shell but also adjacent AuNPs. In addition, since the Au core in the solid state is surrounded by the diarylethene chromophores at a higher density than that in solution, the LSPR band is expected to be greatly affected by the large refractive index change in poly(DE) upon photochromic reaction. Thus,

Nishi et al. it is expected that a large LSPR band shift would be observed for Au-poly(DE)s in the solid state upon photochromic reaction. Figure 6 shows the changes in the absorption spectra of the different Au-poly(DE)s on a quartz glass substrate upon irradiation at 313 nm. The samples were prepared by drop casting a toluene solution of the Au-poly(DE)s onto the substrate. The solid Au-poly(DE)s before UV irradiation have a strong absorption band in the visible region due to the LSPR band. The peak intensity and wavelength of the LSPR band depend not only on the refractive index of surrounding but also on the particle size and interparticle distance.1-3,6,21-25,29 Therefore, the LSPR band for the Au-poly(DE)s on the quartz glass is broader and appears at longer wavelength than that in solution. When the sample was irradiated at 313 nm, a similar change could be observed for the absorption spectrum of the Aupoly(DE) (d ) 5.9 nm) on quartz glass as to that observed in toluene. However, the spectral changes for the Au-poly(DE)s (d ) 13 nm and d ) 22 nm) were clearly different for the quartz samples as compared to those in toluene as shown in Figure 6b,c. The intensity change in the absorption spectra for the LSPR band is much larger than that of the diarylethene chromophore. From these changes in absorption spectra, we could readily observe a spectral shift in the LSPR band. Surprisingly, the Aupoly(DE) (d ) 22 nm) showed ca. 45 nm shift in the LSPR band upon UV irradiation as shown in Figures 6c. This is a significantly large shift among the previous papers in which the LSPR band was controlled by changing refractive index around AuNP not interparticle distance. The absorption spectra returned to the initial state upon exposure to visible light, demonstrating that photoreversible changes in the LSPR band of Au-poly(DE)s could be accomplished in the solid state. The changes in difference spectra of the Au-free poly(DE) (n ) 42) and Au-poly(DE)s on quartz glass upon irradiation at 313 nm are shown in Figure 7. The shape of the spectrum of the Au-free poly(DE) on quartz glass is very similar to that seen in toluene. However, the positive and negative peak intensities of the Au-poly(DE)s are significantly different on quartz from those in solution. A large negative peak was observed in the visible region (500-600 nm), especially for the larger Aupoly(DE) (d ) 22 nm). The photocyclization conversion of Aupoly(DE)s on quartz glass is known to be slightly lower than that in the solution.32 Nevertheless, a significant shift in the LSPR band of the Au-poly(DE) can be observed upon photochromic reaction of the poly(DE) around the AuNPs on quartz glass compared with that in solution. The considerable change in the LSPR band may be attributed to the shorter interparticle distance of the AuNPs and the higher density of the diarylethene chromophores around the AuNPs. The polymer chains around the AuNPs on quartz glass are in more coiled state compared to that in solution.32 Thus, the higher density of the diarylethene chromophores in more coiled state by the shorter interparticle distance of AuNPs gives rise to the large and extensive change in refractive index by photoswitching. Conclusion We have succeeded in preparing new types of AuNP materials that have a light-controllable LSPR band and have shown that the sensitivity of the LSPR band depends on the diameter of the AuNPs. By using various synthetic methods, such as Brust’s method, citrate reduction, and the seeding growth method, the different sized AuNPs could be prepared. These nanoparticles were subsequently covered with narrow-polydispersity diarylethene polymers. All the Au-poly(DE)s exhibited reversible photochromism upon alternating irradiation with UV and visible

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Figure 3. Change in difference spectra of (a) poly(DE) (n ) 42), (b) Au-poly(DE) (d ) 5.9 nm), (c) Au-poly(DE) (d ) 13 nm), and (d) Aupoly(DE) (d ) 22 nm) in toluene upon irradiation at 313 nm.

observed. Furthermore, the spectral shifts in the solid state were significantly enhanced due to the short interparticle distance and high-density of the poly(DE) around the Au core. These Aupoly(DE)s can be used as new types of plasmonic materials in which control of the LSPR band is possible simply by photoirradiation. Experimental Section

Figure 4. Complex refractive indices of the (a) open- and (b) closedring forms of poly(DE) in toluene calculated based on the absorption spectra of poly(DE).

light, both in solution and in the solid state. The LSPR bands of the Au-poly(DE)s (13 and 22 nm in mean diameter) were reversibly shifted as a result of a change in the refractive index of the poly(DE) upon photochromic reaction. When the larger sized particles were used, a larger LSPR band shift was

General. The 1H NMR spectra were recorded using a Jeol A-400 NMR spectrometer at 400 MHz. The absorption spectra were measured with a Jasco V-560 spectrophotometer. Photoirradiation was carried out using a 200 W mercury-xenon lamp (Moritex MUV-202) as the light source. Monochromic light was obtained by passing the light through a monochromator and a UV filter. The transmission electron microscope (TEM) images were performed on a Hitachi H-7000 at 75 kV. The TEM samples were prepared by dropping a toluene solution of AuNPs on a carbon-coated copper grid. The mean diameter of the AuNPs (d) was determined from the TEM image using the image analysis program package ImageJ (http://rsb.info.nih.gov/ ij/index.html). The refractive index of the poly(DE) in toluene was measured using an Atago Abbe refractometer at 20 °C. Elemental analysis was performed at the Microanalytical Center of Osaka City University. Materials. All reagents, 2,2′-azobis(2,4,4-trimethylpentane) (ATMP), tetraoctylammonium bromide (TOAB), sodium borohydride, hydrogen tetrachloroaurate tetrahydrate, trisodium citrate, cetyltrimethylammonium bromide (CTAB), and ascorbic acid were commercially obtained from Wako Chemicals. ATMP was recrystallized from hexane. All other reagents were used as received. 1-Phenylethyl dithiobenzoate (PEDB) and the diarylethene monomer (DE) were synthesized according to a method described in our previous paper.32

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Figure 5. (a) Illustration of the core-shell structure used for calculating the extinction spectra of Au-poly(DE)s, (b) the calculated extinction spectra of Au-poly(DE) (d ) 22 nm) in the open-ring form (red line) and closed-ring form (blue line), and (c) the calculated difference spectra of Au-poly(DE) (d ) 5.9, red line), Au-poly(DE) (d ) 13 nm, orange line), and Au-poly(DE) (d ) 22 nm, blue line) in toluene.

Synthesis of Poly(DE)-SH. A low-polydispersity diarylethene polymer with a dithiobenzoate-end group was synthesized by a reversible addition-fragmentation chain transfer (RAFT) radical polymerization as described in our previous paper32 ([DE] ) 2.9 mol dm-3, [ATMP] ) 1.0 × 10-3 mol dm-3, [PEDB] ) 0.045 mol dm-3 in toluene for 60 h at 100 °C). The numberaverage molecular weight (Mn) and polymerization degree (n) of poly(DE) were determined by 1H NMR spectroscopic analysis of the end group of the polymer32,51 (Mn ) 26100, n ) 38). Poly(DE) with a thiol-end group (poly(DE)-SH) was obtained by reduction of the dithiobenzoate-end group.32,52 Synthesis of Various Sized Au-poly(DE)s. The Au-poly(DE) (d ) 5.9 nm) had been previously prepared by Brust’s method as described in our previous paper.32,53 The Au-poly(DE) (d ) 13 nm) was synthesized by citrate reduction, followed by an exchange reaction of the citrate with the water-insoluble covering agent. For the citrate reduction, an aqueous solution (75 mL) of trisodium citrate (180 mg; 0.70 mmol) was quickly added to a boiling aqueous solution (150 mL) of hydrogen tetrachloroaurate tetrahydrate (60 mg; 0.15 mmol), and the mixture was refluxed for 30 min. The solution was then cooled to room temperature and mixed with a tetrahydrofuran (THF) solution (100 mL) of poly(DE)-SH (n ) 38) (477 mg; 0.018

Figure 6. Change in the absorption spectra of (a) Au-poly(DE) (d ) 5.9 nm), (b) Au-poly(DE) (d ) 13 nm), and (c) Au-poly(DE) (d ) 22 nm) on quartz glass upon irradiation at 313 nm.

mmol) for several minutes. Toluene (100 mL) was added to the solution to separate the mixture into water and organic phases. The red color derived from the AuNPs was completely transferred into the organic phase. The colorless water phase was eliminated and the colored organic phase was washed with distilled water several times. The organic solution was concentrated in a rotary evaporator and dried in a vacuum. The resulting residue was mixed with a few tens of milliliters of diethyl ether and purified according to our previous paper.32 The Au-poly(DE) (d ) 22 nm) was synthesized by a seeding growth method, followed by exchange with the desired covering agent. The growth solution was prepared by adding an aqueous solution (7.8 mL) of L(+)-ascorbic acid (137 mg; 0.78 mmol) to an aqueous solution (150 mL) of hydrogen tetrachloroaurate tetrahydrate (120 mg; 0.29 mmol) and CTAB (2.9 g; 8.0 mmol). Then an aqueous solution of citrate-capped AuNPs prepared by the above procedure (150 mL) was added to the growth solution and stirred overnight. An aliquot of the solution (120 mL) was taken in a flask and mixed with a THF solution (100 mL) of

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Figure 7. Change in the difference spectra of (a) poly(DE) (n ) 42), (b) Au-poly(DE) (d ) 5.9 nm), (c) Au-poly(DE) (d ) 13 nm), and (d) Au-poly(DE) (d ) 22 nm) on quartz glass upon irradiation at 313 nm.

poly(DE)-SH (n ) 38) (330 mg; 0.013 mmol) for several minutes. Toluene (120 mL) and trisodium citrate (700 mg) were added to the solution to separate the mixture into water and organic phases, and worked up as described above. All of the Au-poly(DE)s were soluble in toluene, chloroform, and THF. Theoretical Simulation of the LSPR band of the Aupoly(DE)s. Theoretical calculations of absorption spectra of the different Au-poly(DE)s in toluene were carried out using a core-shell model.50 The scattering spectra of the core-shell spheres were calculated based on Mie theory, using the algorithm described in ref 50.5 The dielectric constant of the metal core is assumed to be the same as that of bulk gold.54 The shell thickness of poly(DE) was set to 10 nm in all simulations, and the shell was regarded as uniform. The refractive index of the colorless poly(DE) shell at 600 nm was set to 1.545 estimated from relationship between the density of the toluene solution of Au-free poly(DE) and the refractive index using the same concentration as that calculated for the poly(DE) shell (ca. 0.4 g cm-3 in toluene) based on the coverage of the poly(DE) chains relative to the Au core. The relationship was obtained by measuring the refractive indices of various concentrations of the toluene solutions using an Abbe refractometer. The coverage was determined from the elemental analysis of the Au-poly(DE)s, 4.0 nm2/molecule for the d ) 13 nm particles and 4.8 nm2/ molecule for the d ) 22 nm particles. The density of the polymer in the poly(DE) shell corresponds to 42% of that of a bulk poly(DE) film (0.96 g cm-3). By taking into account the polymer density and assuming that the poly(DE) shell has the same absorption spectral shape to the toluene solution, we estimated the absorption coefficient k of the shell from the absorption spectrum of colored poly(DE) in toluene. The refractive index np was estimated from the obtained k using

the Kramers-Kronig relations. Poly(DE) was assumed to undergo photochromic reaction homogeneously within the shell. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Area “Strong Photon-Molecule Coupling Fields” (470) (Nos. 19049011 and 21020032) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, and by PRESTO, Japan Science and Technology Agency. H.N. thanks Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. References and Notes (1) Ghosh, S. K.; Pal, T. Chem. ReV. 2007, 107, 4797–4862. (2) Link, S.; El-Sayed, A. M. J. Phys. Chem. B 1999, 103, 8410–8426. (3) Miller, M. M.; Lazarides, A. A. J. Phys. Chem. B 2005, 109, 21556– 21565. (4) Cheng, P. P. H.; Silvester, D.; Wang, G.; Kalyuzhny, G.; Douglas, A.; Murray, R. W. J. Phys. Chem. B 2006, 110, 4637–4644. (5) Uwada, T.; Toyota, R.; Masuhara, H.; Asahi, T. J. Phys. Chem. C 2007, 111, 1549–1552. (6) Njoki, P. N.; Lim, I-I. S.; Mott, D.; Park, H.-Y.; Khan, B.; Mishra, S.; Sujakumar, R.; Luo, J.; Zhong, C.-J. J. Phys. Chem. C 2007, 111, 14664– 14669. (7) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293–346. (8) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547–1562. (9) Kim, Y.; Johnson, R. C.; Hupp, J. T. Nano Lett. 2001, 1, 165–167. (10) Joseph, Y.; Besnard, I.; Rosenberger, M.; Guse, B.; Nothofer, H.G.; Wessels, T. V.; Wind, U.; Knop-Gericke, A.; Su, D.; Schlgl, R.; Yasuda, A.; Vossmeyaer, T. J. Phys. Chem. B 2003, 107, 7406–7413. (11) Ahn, H.; Chandekar, A.; Kang, B.; Sung, C.; Whitten, J. E. Chem. Mater. 2004, 16, 3274–3278. (12) Ueno, K.; Juodkazis, S.; Shibuya, T.; Yokota, Y.; Mizeikis, V.; Sasaki, K.; Misawa, H. J. Am. Chem. Soc. 2008, 130, 6928–6929. (13) Ueno, K.; Juodkazis, S.; Mizeikis, V.; Sasaki, K.; Misawa, H. AdV. Mater. 2008, 20, 26–30. (14) Ikeda, A.; Hatano, T.; Shinkai, S.; Akiyama, T.; Yamada, S. J. Am. Chem. Soc. 2001, 123, 4855–4856.

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