Gold Nanoparticle Localization at the Core Surface by Using

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Gold Nanoparticle Localization at the Core Surface by Using Thermosensitive Core-Shell Particles as a Template Daisuke Suzuki and Haruma Kawaguchi* Faculty of Science & Technology, Keio University, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received June 23, 2005. In Final Form: September 21, 2005 We report novel thermosensitive hybrid core-shell particles via in situ gold nanoparticle formation using thermosensitive core-shell particles as a template. This method for the in situ synthesis of gold nanoparticles with microgel interiors offers the advantage of eliminating or significantly reducing particle aggregation. In addition, by using thermosensitive microgel structures in which the shell has thermosensitive and gel properties in water-whereas the core itself is a water-insoluble polymer-we were able to synthesize the gold nanoparticles only at the surface of the core, which had reactive sites to bind metal ions. After the gold nanoparticles were synthesized, electroless gold plating was carried out to control the thickness of the gold nanoshells. The dispersions of the obtained hybrid particles were characterized by dynamic light scattering and UV-vis absorption spectroscopy, and the dried particles were also observed by electron microscopy. Adaptation of the technique shown here will create a number of applications as optical, electronic, and biomedical functional materials.

Introduction Over the past decade, immense effort has been focused on fabricating nanostructures. Especially, the organization of inorganic nanoparticles into diverse structures is essential for their application in optical, electronic, and magnetic devices and catalysis, for instance.1 Inorganic nanoparticles have been assembled into one-, two-, and three-dimensional architectures,1-5 and colloidal aggregates have been formed for applications such as biological sensing.6-9 Among inorganic nanoparticles, gold ones have a variety of applications based on their optical and electronic properties.10 In particular, their most attractive property is their strong surface plasmon resonance absorption due to the high electronic polarizability of nanoparticles. The resonance frequency depends on a particle’s size, shape, material properties, surrounding medium, and state of aggregation.10 However, the tunability of the resonance of pure inorganic nanoparticles is relatively limited. Moreover, it is difficult to synthesize large (over 100 nm) monodisperse inorganic particles. Recently, on the other hand, inorganic nanoparticles have also been used as building blocks to construct core-shell or hollow particles.11-26 The inorganic shells can have * To whom correspondence should be addressed. Phone: +8145-566-1563. Fax: +81-45-564-5095. E-mail: haruma@ applc.keio.ac.jp. (1) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (2) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499. (3) Gittins, D. I.; Bethell, D.; Nichols, R. J.; Schiffrin, D. J. Adv. Mater. 1999, 11, 737. (4) Wang, Z. L. Adv. Mater. 1998, 10, 13. (5) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (6) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (7) Shenton, W.; Davis, S. A.; Mann, S. Adv. Mater. 1999, 11, 449. (8) Boal, A. K.; Lihan, F.; Derouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (9) Maye, M. M.; Chun, S. C.; Han, L.; Rabinovich, D.; Zhong, C.-J. J. Am. Chem. Soc. 2002, 124, 4958. (10) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (11) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (12) Westcott, S. L.; Oldenburg, S. J.; Lee, T, R.; Halas, N. J. Langmuir 1998, 14, 5396.

resonances that can be tuned over a wide range as a function of the core/shell ratio. Halas and co-workers have developed gold nanoshells on silica particles11-13 and have designed such particles for biomedical applications.14-16 Caruso and co-workers have also fabricated a variety of metal nanoshells on silica or polystyrene particles using the layer-by-layer assembly method.17-21 Other groups have reported several modified synthesis methods.22-26 Another approach to building nanostructures using inorganic particles employs hybrids with hydrogels.27-32 Kumacheva and co-workers have reported that the synthesis of inorganic particles with microgel interiors was effective, and they showed several examples of the use of hybrid microgels in optical and biomedical applications.29-31 (13) Pham, T.; Jackson, J. B.; Halas, N. J.; Lee, T. R. Langmuir 2002, 18, 4915. (14) Sershen, S. R.; Westcott, S. L.; Halas, N. J.; West, J. L. J. Biomed. Mater. Res. 2000, 51, 293. (15) Hirsch, L. R.; Jackson, J. B.; Lee, A.; Halas, N. J.; West, J. L. Anal. Chem. 2003, 75, 2377. (16) West, J. L.; Halas, N. J. Annu. Rev. Biomed. Eng. 2003, 5, 285. (17) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (18) Caruso, F. Adv. Mater. 2001, 13, 11. (19) Gittins, D. I.; Susha, A. S.; Schoeler, B.; Caruso, F. Adv. Mater. 2002, 14, 508. (20) Liang, Z.; Susha, A.; Caruso, F. Chem. Mater. 2003, 15, 3176. (21) Salgueirinõ-Maceria, V.; Caruso, F.; Liz-Marzań, L. M. J. Phys. Chem. B 2003, 107, 10990. (22) Dokoutchaev, A.; James, J. T.; Koene, S. C.; Pathak, S.; Prakash, G. K. S.; Thompson, M. E. Chem. Mater. 1999, 11, 2389. (23) Ji, T.; Lirtsman, V. G.; Avny, Y.; Davidov, D. Adv. Mater. 2001, 13, 1253. (24) Graf, C.; van Blaaderen, A. Langmuir 2002, 18, 524. (25) Dong, A. G.; Wang, Y. J.; Tang, Y.; Ren, N.; Yang, W. L.; Gao, Z. Chem. Commun. 2002, 350. (26) Shi, W.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N. Langmuir 2005, 21, 1610. (27) Kroll, E.; Winnik, F. M.; Ziolo, R. F. Chem. Mater. 1996, 8, 1594. (28) Breulmann, M.; Co¨lfen, H.; Hentze, H.-P.; Antonietti, M.; Walsh, D.; Mann, S. Adv. Mater. 1998, 10, 237. (29) Xu, S.; Zhang, J.; Paquet, C.; Lin, Y.; Kumacheva, E. Adv. Funct. Mater. 2003, 13, 468. (30) Zhang, J.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 7908. (31) Gorelikov, I.; Field, L. M.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 15938. (32) Suzuki, D.; Kawaguchi, H. Langmuir 2005, 21, 8175.

10.1021/la0516882 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/02/2005

Gold Nanoparticle Localization at the Core Surface Scheme 1. Synthesis of Thermosensitive Hybrid Multi-Core-Shell Particles with Gold Nanoshells Localized at the Surface of the Core

Langmuir, Vol. 21, No. 25, 2005 12017 Table 1. Preparation Conditions and Weight-Averaged Hydrodynamic Diameters (Dw) of the NGc-N Particles code NGc-N1 NGc-N2 NGc-N3 a

In this study, we developed thermosensitive hybrid core-shell particles via in situ gold nanoparticle formation using thermosensitive core-shell particles as a template (Scheme 1). We have also reported a paper about thermosensitive hybrid core-shell particles that reversibly change color as the temperature changes.32 This is due to the interparticle distance change of gold nanoparticles in the thermosensitive hydrogel phase by the temperature change. In contrast, in the present paper we show that gold nanoparticles were fixed between the rigid core and thermosensitive shell by using thermosensitive core-shell particles as templates. The proposed method for the in situ synthesis of gold nanoparticles with microgel interiors offers the advantage of eliminating or significantly reducing particle aggregation. In addition, by using thermosensitive microgel structures in which the shell has thermosensitive and gel properties in waterswhereas the core itself is a water-insoluble polymerswe were able to synthesize gold nanoparticles only at the surface of the core, which had reactive sites to bind metal ions. After the gold nanoparticles were synthesized, electroless gold plating was carried out to control the thickness of the gold nanoshells. The dispersions of the obtained hybrid particles were characterized by dynamic light scattering (DLS) and UV-vis absorption spectroscopy, and the dried particles were also observed by electron microscopy. Experimental Section Materials. N-Isopropylacrylamide (NIPAM) was kindly given by Kojin Co. and recrystallized from hexane-toluene (1:1 on a volume basis). Glycidyl methacrylate (GMA) was purified by distillation under reduced pressure to remove inhibitors. N,N′Methylenebisacrylamide (MBAAm) was used without further purification. Azobis(amidinopropane) dihydrochloride (V-50) was used without further purification. 2-Aminoethanethiol (2-AET; Tokyo Kasei Kogyo Co., Ltd.) was used without further purification. Chloroauric acid (HAuCl4), sodium borohydrate (NaBH4), and hydroxylamine hydrochloride (NH2OH‚HCl; Junsei Chemical Co., Ltd.) were used as received. The water used in all experiments was from a Milli-Q reagent water system (Millipore). All reagents were purchased from Wako Pure Chemical Industries, Ltd., unless otherwise noted. Preparation and Modification of the NIPAM-co-GMA Core-Shell Particles. To obtain the template particles, NIPAM, GMA, and MBAAm were copolymerized in a soap-free aqueous medium using V-50 as an initiator. First, a mixture of 0.2 g of NIPAM, 2.2 g of GMA, 0.04 g of MBAAm, and 90 g of water was put into a 200 mL three-neck, round-bottom flask equipped with a stirrer, a nitrogen gas inlet, and a condenser. Nitrogen gas was bubbled into the mixture to

NGc core NIPAM MBAAm V-50 Dw at mass (g) mass (g) mass (mg) mass (mg) 20 °Ca (nm) 0.5 0.5 0.5

0.1 0.25 0.38

2 5 10

10 10 20

356 413 749

Dw at 20 °C of the NGc particle is 302 nm.

purge it of oxygen for 30 min. The system was kept at 70 °C in a water bath. A total of 10 g of water containing 0.06 g of V-50 was added to the flask for the initiation of polymerization, which then continued for 6 h. The obtained particles (NGc particles) were purified by centrifugation and then washed four times with water. Second, NGc particles were coated with PNIPAM by seeded polymerization. A mixture of 0.5 g of NGc particles, 0.10-0.38 g of NIPAM, 0.002-0.01 g of MBAAm, and 90 g of water was put into a 200 mL flask equipped in the same way as the flask described above. The system was kept at 70 °C in a water bath. A total of 10 g of water containing 0.01-0.02 g of V-50 was added to the flask for the initiation of polymerization, which was then continued for 2 h. The obtained particles (NGc-N particles) were purified by centrifugation and then washed four times with water. Third, the epoxy groups of the inner NGc-N particles were allowed to react with 2-AET to introduce the amino groups to the particles (NGc-N-NH2 particles). A mixture of 0.4 g of NGc or NGc-N particles, 0.78 g of 2-AET, and 70 g of water was put into a 100 mL glass vial with stirring under room temperature, and the pH was adjusted to 11.0. The reaction was continued for 24 h. The obtained particles were purified by centrifugation using the same method described above and by one week of dialysis. In Situ Synthesis of Gold Nanoparticles inside CoreShell Particles. A mixture of 10 mg of template particles and 1.0 mg of HAuCl4 was stirred in 10 mL of aqueous medium (pH 1.5-2) for 24 h. After that, excess HAuCl4 was removed by centrifugation and then washed with aqueous medium (pH 3) for generating no gold nanoparticles except around the core. A total of 10 mL of water containing the particles was poured into a 30 mL glass vial at 4 °C. A 1 mL aqueous solution of 0.25 mg of NaBH4 was added dropwise to the vial. After the addition of NaBH4, the mixture stood for 30 min. Then the particles were purified by centrifugation and washed four times with water. Electroless Gold Plating of the Core-Shell Hybrid Particles. The adsorbed gold nanoparticles can serve as seeds for subsequent growth by electroless gold plating. A total of 20 mL of water containing 2 mg of the hybrid particles, 10-500 µL of 1 wt % HAuCl4, and 50-2500 µL of 40 mM NH2OH‚HCl was put into a 30 mL glass vial with stirred at 4 °C. The reaction continued for 15 min, after which the particles were purified by centrifugation and then washed four times with water. Characterization. Approximately 2 µL of the diluted particle suspension was dried on a carbon-coated copper grid (Okenshoji Co., Ltd.) and observed by field emission transmission electron microscopy (TEM; TECNAI F20, Philips Electron Optics Co., operated at 200 kV). Also, approximately 5 µL of the diluted particle suspension was dried on a polystyrene substrate and observed by field emission scanning electron microscopy (SEM; S-4700, Hitachi Ltd.). The samples were sputter-coated with platinum/palladium before examination. The hydrodynamic diameters of the particles were determined by DLS using a laser particle analyzer system (PAR-3, Otsuka Electronics Co.). The incident wavelength was 632 nm of a HeNe laser, and the measurement angle was 90°. UV-vis absorption data were recorded on a Hitachi U-2001 spectrophotometer. The samples for DLS and UV-vis absorption experiments were allowed to equilibrate at each temperature for 10 min before measurement. X-ray photoelectron spectra were obtained on a spectrometer with a Mg KR X-ray source (JPS-9000MX, Nippon Electronics Co.). The atomic ratios (N1s/C1s, S2s/C1s) were calculated from the areas of C1s, N1s, and S2s peaks in the spectra. Relative sensitivity factors of 4.079042, 3.977336, and 4.745428 were used for C1s, N1s, and S2s, respectively. An elemental analysis of dried particles

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Figure 1. SEM observation of thermosensitive core-shell particles deposited on a polystyrene substrate dried at room temperature: (A) NGc, (B) NGc-N1, (C) NGc-N2, and (D) NGc-N3 particles. for carbon, hydrogen, nitrogen, oxygen, and sulfur was conducted using Vario EL (Elementar Analysensysteme GmbH).

Results and Discussion Preparation and Characterization of the Template Particles. Monodisperse NGc particles were prepared in a soap-free aqueous medium. Given the different reactivity ratios of NIPAM and GMA (0.39 and 2.69, respectively),33 GMA tends to be consumed faster than NIPAM. Therefore, the interior of the core particle was thought to be rich in PGMA, while the exterior was rich in PNIPAM chains; these expectations were confirmed by X-ray photoelectron spectroscopy (XPS) analysis (shown below). In the subsequent seeded polymerization for growing the shell layer, the collapsed NGc particles served as seeds for further polymerization, thereby resulting in preferential growth of existing particles over the nucleation of new ones. The preparation conditions and hydrodynamic diameters at 20 °C of the core-shell particles are shown in Table 1. Figure 1 shows SEM views of the particles before (part A) and after (parts B-D) seeded polymerization. Because the shells were composed of water-soluble polymer, apparent diameters from SEM images were different from those from DLS measurements. In SEM observations, the water in the shell layer evaporated, which resulted in the particles shrinking. You can see that the particles settled down with retention of their own excluded volume as seeded polymerization was carried out. The two-dimensional microgel array has already been reported in detail.34,35 Because the interparticle distance was determined by the concentration and by the hydrodynamic diameters of the particles,35 we can confirm that different shell sizes of NG-N particles were obtained by changing the NIPAM feed ratio in seeded polymerization. The particles to which amino groups were introduced were also arranged, as were particles having no amino groups (not shown). We also confirmed from SEM observations that the PNIPAM particles were not generated independently in these experiments. (33) Virtanen, J.; Tenhu, H. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 3716. (34) Tsuji, S.; Kawaguchi, H. Langmuir 2004, 20, 2449. (35) Tsuji, S.; Kawaguchi, H. Langmuir 2005, 21, 2434.

Figure 2 shows the temperature dependence of the hydrodynamic diameters of NGc, NGc-NH2, NGc-N, and NGc-N-NH2 particles in aqueous dispersion. The dispersity, the ratio of the weight-averaged diameter to the number-averaged diameter, was less than 1.03 in all cases. The diameter of the NGc-N particles in the swollen state is larger than that of NGc core particles, indicating that seeded polymerization proceeded successfully. We have already confirmed from Figure 1 that the thickness of the shell was controllable by the additional feed of NIPAM in seeded polymerization. It is widely known that PNIPAM exhibits a lower critical solution temperature (LCST) in water around 32 °C, so the particles obtained in this study showed the volume phase transition of PNIPAM around 32 °C, but the response was dull, especially in NGc, NGcN1, and NGc-N2 particles. This is because the network structure of the shell restricted the chain motion and retarded the response.36,37 Recently, our group overcame this problem to create particles whose shells have a minimum of cross-linked points.34 Then we introduced a positive charge using 2-AET to the NGc and each NGc-N particle by the reaction between thiol groups and epoxy groups at pH 11. The introduction of amino groups to NGc and to each NGc-N particle was successful, and one of the effects of amino group incorporation was that the particle diameters in swollen states were slightly larger than those before amino group incorporation in the temperature range between 20 and 37 °C. Researchers studying the effect of ionic monomer incorporation into PNIPAM on the swelling behavior attributed it mainly to osmotic pressure or the Donnan potential due to counterions38 and to repulsion between amino groups.39 In this case, the increments of the particle size are small, because amino groups were localized at the surface of rigid core particles. Additionally, NGc-NH2 and NGc-N1-NH2 particles showed higher transition temperatures of PNIPAM, which were attributed to the protonated amino groups that make the (36) Ohshima, H.; Makino, K.; Kato, T.; Fujimoto, K.; Kondo, T.; Kawaguchi, H. J. Colloid Interface Sci. 1993, 159, 512. (37) Kato, T.; Fujimoto, K.; Kawaguchi, H. Polym. Gels Networks 1994, 2, 307. (38) Fernańdez-Nieves, A.; Fernańdez-Barbero, A.; Vincent, B.; de las Nieves, F. J. Macromolecules 2000, 33, 2114. (39) Jones, C. D.; Lyon, L. A. Macromolecules 2003, 36, 1988.

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Figure 2. Temperature dependence of the weight-averaged hydrodynamic diameters of NGc and NGc-NH2 (A), NGc-N1 and NGc-N1-NH2 (B), NGc-N2 and NGc-N2-NH2 (C), and NGc-N3 and NGc-N3-NH2 (D) particles in aqueous dispersions measured by dynamic light scattering (temperature-raising process).

Figure 3. N/C values by XPS and elemental analysis: (A) NGc and NGc-N particles and (B) NGc-N-NH2 particles.

polymer chains more hydrophilic and repulsive to prevent collapsing up to higher temperatures. This is a common phenomenon observed for ionic-group-carrying PNIPAM.40 The control experiments using the thermosensitive coreshell particles with the PGMA-co-PNIPAM shell showed a shift of the transition temperature after the reaction (40) Maeda, Y.; Yamamoto, H.; Ikeda, I. Langmuir 2001, 17, 6855.

with 2-AET,32 which suggested that the shells of NGc-N2 and NGc-N3 particles shown in this paper were composed solely of PNIPAM and were segregated from the PGMAco-PNIPAM region. Figure 3 shows the N/C values measured by both XPS and elemental analysis. Before the amino groups were introduced to the particles (see part A of Figure 3), the

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Figure 4. S/C values for the NGc-NH2 and NGc-N-NH2 particles by XPS and elemental analysis.

thicker the shells were, the larger the N/C values measured by elemental analysis. This tendency suggested that the shell is composed of PNIPAM (NIPAM has one nitrogen atom per unit). On the other hand, the N/C values by XPS almost plateaued (between 0.2 and 0.25) from NGc-N1 to NGc-N3 particles. In general, XPS analysis provides information on only the chemical elements localizing at the outermost surface to a depth of 5-10 nm.41,42 In this study, the plateaued values by XPS are attributable to the constant composition of the polymer in the top-surface layer (to a depth of 5-10 nm) for NGc-N particles. After the reactions of amino groups to the particles (see part B of Figure 3), the N/C values by elemental analysis were larger than those before the reactions, which indicated


that 2-AET was introduced. On the other hand, the values by XPS were not changed (between 0.2 and 0.25) except for the NGc-NH2 particles. Because NGc particles are exposed to epoxy groups at their outermost surface, the amino groups introduced to the particles were reflected in the XPS data. From the unchanged N/C values before and after the incorporation of amino groups for NGc-N particles, we confirmed that the reactions proceeded only inside the particles. Figure 4 shows the S/C values by both XPS and elemental analysis. 2-AET has one sulfur atom per unit. Note that the S/C values for NGc-N-NH2 particles measured by XPS were almost all zero (in fact, the peaks originated from S2s of NGc-N1-NH2, NGc-N2-NH2, and NGc-N3-NH2 particles were not detected, and the calculated values were from noise). This fact also indicated that the introduced amino groups were localized inside the particles. In Situ Synthesis of Gold Nanoparticles at the Core Surfaces. We synthesized gold nanoparticles using a variety of amino-functionalized NGc and NGc-N particles as templates. Those particles had large shell volumes, since the shells were composed of polymer chain networks and a lot of water. However, the AuCl- must be localized at the core particles, where amino groups were localized, at low pH. We have already synthesized hybrid particles using the void space of the shell for the nucleation and growth of gold nanoparticles, and the particles we obtained changed color reversibly as the temperature changed.32 In this experiment, first the dispersion of the template particles and precursor anions of AuCl4- was mixed for 24 h at 4 °C, where the shells were well hydrated. The particles were then collected by centrifugation to remove excess anions in the dispersion medium. The sedimented particles were yellow, which indicated the particles

Figure 5. TEM views of thermosensitive hybrid core-shell particles with gold nanoparticles deposited on a carbon-coated copper grid dried at 20 °C: (A-C) hybrid particles using NGc-NH2 as templates (NGc-NH2-Au), (D-F) hybrid particles using NGc-N1-NH2 as templates (NGc-N1-NH2-Au), (G-I) hybrid particles using NGc-N2-NH2 as templates (NGc-N2-NH2-Au), (J-L) hybrid particles using NGc-N3-NH2 as templates (NGc-N3-NH2-Au). Each triplet was taken at different magnifications.


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Figure 6. TEM view of thermosensitive hybrid core-shell particles with gold nanoparticles deposited on a carbon-coated copper grid dried at 20 °C. The template particles were not modified by 2-AET. The conditions for the synthesis of the gold nanoparticles were the same as for the particles shown in Figure 5J-L.

contained precursor anions. By contrast, none of the amino-group-carrying particles showed a yellow color. After the NaBH4 was added, the dispersion turned red immediately. In the control experiments without template particles, gold nanoparticles were not synthesized and only large black aggregates were sedimented. These experiments revealed that NGc-NH2 and each NGc-NNH2 particle behaved as stabilizers for synthesizing gold nanoparticles. Figure 5 shows TEM views of the hybrid particles using a variety of templates (NGc-NH2 and each NGc-N-NH2 particle). All dispersions were dried at room temperature on the carbon-coated copper grid. Each triplet (parts A-C, D-F, G-I, and J-L) was taken at different magnifications. From these views, we can confirm that gold nanoparticles are localized at each core. The hybrid particles using templates of NGc-NH2 and NGc-N1-NH2 particles did not show a uniformly distributed arrangement on the solid substrate, although the template particles of NGc-N1-NH2 were arranged with retention of their own excluded volume (see part B of Figure 1). This is because of the physical cross-linking between the polymers and the gold nanoparticles, which results in a reduction in the steric repelling effect. Although the dried particles shown in parts A-F look agglomerated, they are well dispersed in an aqueous medium, so the agglomeration is due to the drying process. On the other hand, the hybrid particles using NGc-N2-NH2 and NGcN3-NH2 particles as templates remained in their regular arrangements on the substrate (see parts G and J of Figure 5). Unlike the case in our previous paper,32 in this case close-up views show that the gold nanoparticles are localized at the core particles. TEM views did not reveal any large aggregates of gold or independent gold nanoparticles. The mean diameters of gold nanoparticles prepared under these preparation conditions were about 4-10 nm. Control experiments using particles that did not carry amino groups (NGc-N3) as templates were also carried out, and the obtained particles are shown in Figure 6. Almost no gold nanoparticles are localized at the cores, because AuCl- was released from the particles during the centrifugations. Figure 7 shows the temperature response of the hydrodynamic diameters of the hybrid particles. We confirmed this response did not change after the gold nanoparticles were hybridized, but the hybrid particles using the NGc-NH2 template were aggregated in a heating process at 34 °C. The dehydrated state of these particles (41) Holm, R.; Storp, S. Surf. Interface Anal. 1980, 2, 96. (42) Yamamoto, F.; Yamakawa, S. J. Polym. Sci., Polym. Phys. Ed. 1979, 17, 1581.

Figure 7. Temperature dependence of the hydrodynamic weight-averaged diameters of each NGc-N-NH2 and each hybrid (NGc-N-NH2-Au) particle in aqueous dispersions measured by dynamic light scattering (temperature-raising process). Table 2. Preparation Conditions of the Electroless-Plated Particles

code

template material

NGc-N2-NH2-Au-EP1 NGc-N2-NH2-Au-EP2 NGc-N2-NH2-Au-EP3 NGc-N2-NH2-Au-EP4 NGc-N3-NH2-Au-EP1 NGc-N3-NH2-Au-EP2 NGc-N3-NH2-Au-EP3 NGc-N3-NH2-Au-EP4 NGc-N3-NH2-Au-EP5 NGc-N3-NH2-Au-EP6

NGc-N2-NH2-Au NGc-N2-NH2-Au NGc-N2-NH2-Au NGc-N2-NH2-Au NGc-N3-NH2-Au NGc-N3-NH2-Au NGc-N3-NH2-Au NGc-N3-NH2-Au NGc-N3-NH2-Au NGc-N3-NH2-Au

1 wt % 40 mM HAuCl4 NH2OH‚HCl vol (µL) vol (µL) 10 50 100 150 10 50 100 150 300 500

50 250 500 750 50 250 500 750 1500 2500

might expose the gold nanoparticles, resulting in aggregation. The diameters of the hybrid particles overall were almost the same as those of each type of nonhybrid particle. We concluded that gold nanoparticles can be localized at the cores by in situ synthesis using aminogroup-localized particles as templates and that the hybrid particles also showed a thermosensitive property. Electroless Gold Plating of Hybrid Particles. The adsorbed gold nanoparticles can serve as seeds for subsequent growth by electroless gold plating. Here we conducted electroless gold plating using a previously reported method,20 with the aim of controlling the optical properties of a particle by controlling the thickness of the gold nanoshell over the core. The preparation conditions of each electroless-plated particle are listed in Table 2. Some researchers have reported that, in reactions using this method, the obtained particles tend to have less colloidal stability.20,26 In fact, the particles obtained by using NGc-N1-NH2-Au particles were aggregated. On the other hand, the electroless-plated particles obtained by using NGc-N2-NH2-Au or NGc-N3-NH2-Au particles were stable until the critical thickness of the gold nanoshell was reached. Stable electroless-plated particles were observed by TEM and are shown in Figures 8 and 9. Figure 8 shows electroless-plated particles obtained by using NGcN3-NH2-Au particles as templates. We confirmed that this treatment enlarged the gold nanoparticles. Compared to the samples shown in parts A-C, the larger increases in the gold nanoparticles in the samples in parts D-F and G-I are attributable to the use of more HAuCl4 for their electroless plating than was used for the plating of the

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Figure 8. TEM views of thermosensitive hybrid core-shell particles treated with electroless gold plating by using NGc-N3-NH2-Au particles deposited on a carbon-coated copper grid dried at 20 °C: (A-C) NGc-N3-NH2-Au-EP1 particles, (D-F) NGc-N3-NH2Au-EP3 particles, (G-I) NGc-N3-NH2-Au-EP5 particles. Each triplet was taken at different magnifications.

Figure 9. TEM view of NGc-N3-NH2-Au-EP3 particles deposited on a carbon-coated copper grid dried at 50 °C.

samples shown in parts A-C. In addition, the distribution of dried electroless-plated particles on the substrate in part G was disordered compared to those shown in parts A and D. This might be attributable to the reduced effect of steric hindrances of those particles and to the reduced ability of heavier particles to adjust their own position on the substrate. After these treatments, we found unfortunately that the electroless-plated particles had several gold nanoparticles dispersed in the PNIPAM shells (we can find the gold nanoparticles in the shell layer from part K of Figure 5). The reason gold nanoparticles were generated when thicker shell templates (NGc-N3-NH2

particles) were used was that it was difficult to remove the precursor anions from the shell completely. Indeed, when we tried to remove them, the anions localized at the core surfaces were also released, thus causing a decrease in the number of gold nanoparticles. These gold nanoparticles were surely generated in the PNIPAM shells, as we confirmed by TEM (Figure 9). These electroless-plated hybrid particles were deposited on the copper grid at 50 °C, at which temperature they shrank and did not exhibit steric hindrance. We can confirm that the gold nanoparticles in the PNIPAM shell, as shown in part D of Figure 8, are taken into the template particles. We concluded that these gold nanoparticles are parts of the hybrid particles and do not disperse independently. By contrast, using thinner shell templates did not cause the formation of these byproducts. Figure 10 shows electroless-plated particles obtained by NGc-N2-NH2-Au particle templates. Although the distribution of the dried electroless-plated particles on the substrate was disordered, as shown in parts D and G, gold nanoparticles were not generated in the PNIPAM shells. As a result, to control the template particles, we obtained a variety of hybrid particles having different gold nanoshell thicknesses by using NGc-N2-NH2-Au templates. Figure 11 shows UV-vis absorption spectra of the electroless-plated particles. For a series of electrolessplated particles obtained using NGc-N2-NH2-Au particle templates (part A), first a weak absorption peak at 532 nm appeared (NGc-N2-NH2-Au-EP1). This is due to the surface plasmon resonance of gold nanoparticles. In general, gold nanoparticles show surface plasmon resonance absorption of around 520 nm in an aqueous medium. At this stage, although gold nanoshells are not necessarily


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Figure 10. TEM views of thermosensitive hybrid core-shell particles treated with electroless gold plating by using NGc-N2NH2-Au particles deposited on a carbon-coated copper grid dried at 20 °C: (A-C) NGc-N2-NH2-Au-EP1 particles, (D-F) NGcN2-NH2-Au-EP2 particles, (G-I) NGc-N2-NH2-Au-EP4 particles. Each triplet was taken at different magnifications.

Figure 11. UV-vis absorption spectra of electroless-plated particles measured at 20 °C. (A) NGc-N2-NH2-Au particles were used as templates for electroless plating: 0, NGc-N2-NH2-Au particles; 9, NGc-N2-NH2-Au-EP1 particles; 2, NGc-N2-NH2-Au-EP2 particles; b, NGc-N2-NH2-Au-EP3 particles; [, NGc-N2-NH2-Au-EP4 particles. (B) NGc-N3-NH2-Au particles were used as templates for electroless plating: [, NGc-N3-NH2-Au particles; 9, NGc-N3-NH2-Au-EP1 particles; 2, NGc-N3-NH2-Au-EP2 particles; b, NGc-N3-NH2-Au-EP3 particles; 0, NGc-N3-NH2-Au-EP4 particles; 4, NGc-N3-NH2-Au-EP5 particles; O, NGc-N3-NH2-Au-EP6 particles.

continuous, the red-shifted peak for NGc-N2-NH2-Au-EP1 particles is due to the interactions between neighboring gold nanoparticles and to their nonspherical morphology. Next, the intensity of the peak of NGc-N2-NH2-Au-EP2 particles became stronger than that of NGc-N2-NH2-Au-

EP1 particles because of the increase in the gold nanoparticle size, from 10 ( 1 to 15 ( 2 nm, and the red-shifted peak was observed at 555 nm. Furthermore, by using more HAuCl4, the resultant particles showed more red-shifted absorption peaks (NGc-N2-NH2-Au-EP3 particles at 587

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nm and NGc-N2-NH2-Au-EP4 particles at 606 nm) that, in addition, became broader in the infrared region. The nanoshell thickness was increased with subsequent electroless plating, but we found that the resultant particles did not disperse stably and that the aggregates were detected by DLS measurement. For a series of electrolessplated particles obtained from NGc-N3-NH2-Au particle templates (part B), a feature similar to that mentioned above appeared. In this case, however, the electrolessplated particles were more stable than those obtained from NGc-N2-NH2-Au particle templates, and we obtained stable thicker gold nanoshell hybrid particles. As electroless plating continued, the particles showed broader peaks from 500 to 800 nm, which suggested that the gold nanoparticles were coalescent (see the spectra of NGcN3-NH2-Au-EP3 to NGc-N3-NH2-Au-EP6 particles). By contrast, the peak absorbance shifted to shorter wavelengths (for NGc-N3-NH2-Au-EP4 to NGc-N3-NH2-AuEP6). These events happened when the gold nanoshell completely covered the spherical substrate.11 Halas and co-workers reported that gold shells can have resonances that can be tuned over a wide range as a function of the core/shell ratio.11-13,43 In our case, although gold nanoshells are fused with polymer networks, properties similar to those reported by Halas and co-workers emerged. In addition, these particles have the potential to show absorption bands across the near-infrared region (800 nm to 2.2 µm),43 although we did not investigate this. Figure 12 shows the temperature dependence of the electroless-plated particles as mentioned above. Even after several modifications, the thermosensitive property remained, indicating that PNIPAM shells were not fully fused with gold nanoshells. These hybrid particles have potential biomedical14-16 and optical44-50 applications. For example, theoretical calculations indicate that a colloidal crystal, composed of metallodielectric spheres, can have a full photonic band gap.44-46 In addition, although it is difficult to assemble the soft particles presented in this paper into threedimensional particles in a dry state, construction of additional shell layers composed of water-insoluble materials over the shrunken hybrid particles enables their assembly into a colloid crystal.29 Without that treatment, soft particles can be assembled in water, and some groups have explored this extensively.47-49 Furthermore, met(43) Oldenburg, S. J.; Jackson, J. B.; Westcott, S. L.; Halas, N. J. Appl. Phys. Lett. 1999, 75, 2897. (44) Moroz, A. Europhys. Lett. 2000, 50, 466. (45) Moroz, A. Phys. Rev. Lett. 1999, 83, 5274. (46) Zhang, W. Y.; Lei, X. Y.; Wang, Z. L.; Zheng, D. G.; Tam, W. Y.; Chan, C. T.; Sheng, P. Phys. Rev. Lett. 2000, 84, 2853. (47) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959. (48) Hu, Z.; Lu, X.; Gao, J. Adv. Mater. 2001, 13, 1708. (49) Lyon, L. A.; Debord, J. D.; Debord, S. B.; Jones, C. D.; McGrath, J. G.; Serpe, M. J. J. Phys. Chem. B 2004, 108, 19099. (50) Proan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. Science 2003, 302, 419.


Figure 12. Temperature dependence of the hydrodynamic weight-averaged diameters of electroless-plated particles in aqueous dispersions measured by dynamic light scattering (temperature-raising process).

allodielectric spheres with more complex nanostructures showed unique optical properties.50 We believe that the adaptation of the technique shown in this paper will create a number of applications as mentioned above. Conclusions We developed thermosensitive hybrid core-shell particles via in situ gold nanoparticle formation using thermosensitive core-shell particles as templates. By using core-shell microgel structures in which the shell has gel properties in water whereas the core consists of a water-insoluble polymer, we synthesized gold nanoparticles at the surface of the core, which had reactive sites to bind metal ions. In addition, gold nanoparticles were localized completely at the core surface by adjusting the PNIPAM shell thickness. The absorption peak originating from gold nanoparticles and nanoshells appeared by subsequent electroless gold plating in the range of 500800 nm. The electroless-plated particles also showed high colloidal stabilities and temperature-sensitive hydrodynamic diameters. These hybrid particles have potential biomedical and optical applications. Acknowledgment. D.S. thanks the Japan Society for the Promotion of Science for Young Scientists for research fellowships. This work was supported by a Grant-in-Aid for the 21st Century COE program “KEIO Life-Conjugated Chemistry” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. LA0516882

Gold Nanoparticle Localization at the Core Surface by Using

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