Star-Block Copolymers as Templates for the Preparation of Stable

A five-arm star-shaped macroinitiator (PEO) was utilized for the automated parallel controlled ring-opening polymerization of ε-caprolactone to prepa...
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Langmuir 2005, 21, 7995-8000

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Star-Block Copolymers as Templates for the Preparation of Stable Gold Nanoparticles Mariam Filali,† Michael A. R. Meier,‡ Ulrich S. Schubert,*,‡ and Jean-Franc¸ ois Gohy*,† Unite´ de Chimie des Mate´ riaux Inorganiques et Organiques (CMAT) and Research Center in Micro- and Nano-Materials and Electronic Devices (CERMIN), Universite´ catholique de Louvain, Place Pasteur 1, 1348 Louvain-la-Neuve, Belgium, and Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology and Dutch Polymer Institute (DPI), Post Office Box 513, 5600 MB Eindhoven, The Netherlands Received February 10, 2005. In Final Form: June 16, 2005 Gold nanoparticles of improved stability against aggregation were prepared using poly(ethylene oxide)block-poly(-caprolactone) (PEO-b-PCL) star-block copolymers. A five-arm star-shaped macroinitiator (PEO) was utilized for the automated parallel controlled ring-opening polymerization of -caprolactone to prepare a series of PEO-b-PCL star-block copolymers with a constant PEO core linked to PCL blocks of variable length. The PEO core was swelled with KAuCl4 in N,N-dimethylformamide (DMF), and gold nanoparticles were subsequently obtained by reduction with NaBH4. Since the process was always templated by the same PEO core for all investigated polymers, the average dimension of the formed gold nanoparticles was in the same range for all star-block copolymers. In sharp contrast, the size distribution and long-term stability against aggregation of the gold nanoparticles dispersed in DMF were strongly dependent on the PCL block length, confirming the role of PCL blocks as stabilizing blocks for these nanoparticles.

Introduction Conducting and semiconducting nanoparticles are currently a topic of intense research due to their novel properties, which are significantly different from those of the corresponding bulk materials.1 Since the unique properties of these nanoparticles are directly related to their size, a precise control over the size, size polydispersity, shape, and internal structure is required. As a typical example, the band gap of a semiconductor nanoparticle is reported to depend on the particle size.2 Because of their numerous applications ranging from optoelectronics to catalysis, gold nanoparticles have been widely studied. Low-molecular-weight (MW) surfactants have been added during the synthesis of gold nanoparticles to obtain these materials with controlled sizes, a narrow size distribution, and long-term stability.3 This approach has been referred to as “templating” since the characteristic features of the accordingly formed gold particles are in principle fixed by the corresponding features of the “nanoreactors” formed by the self-associating surfactants. Moreover, block copolymer micelles have been considered in this approach because they offer some advantages compared to classical surfactants. First, the critical micelle concentration of block copolymers is much smaller, and their kinetic stability is larger than that of low-MW surfactants.4 Second, the size and shape of block copolymer * To whom correspondence should be addressed. E-mail: [email protected] (U.S.S.); [email protected] (J.-F.G.). † Universite ´ catholique de Louvain. ‡ Eindhoven University of Technology and DPI. (1) See for example: (a) Service, R. F. Science 1996, 271, 920. (b) Schmid, G.; Corain, B. Eur. J. Inorg. Chem. 2003, 3081. (c) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem.sEur. J. 2002, 8, 29. (2) Alivisatos, A. P. Science 1996, 271, 933. (3) See for example: (a) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (b) Esumi, K.; Suzuki, A.; Yamahira, A.; Torigoe, K. Langmuir 2000, 16, 2604. (c) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (4) Riess, G. Prog. Polym. Sci. 2003, 28, 1107.

micelles can be easily tuned by varying the composition of the copolymer, the length of the constituent blocks, and the architecture of the copolymer.4 Third, the stability of the accordingly formed nanoparticles can be enhanced by increasing the length of the coronal blocks.4 Poly(styrene)-block-poly(4(or 2)-vinylpyridine) (PS-bP4(2)VP) block copolymer micelles have been widely used as templates for the synthesis of gold nanoparticles, as demonstrated in ref 5. To achieve this goal, HAuCl4 has been added to the micellar solution, which results in the protonation of the P2VP or P4VP blocks, and therefore, AuCl4- is bound as the counterion. In a further step, the metal precursor has been reduced by NaBH4 or H2NNH2 to form the metal. The reduction initially leads to the formation of primary metal atoms that further aggregate to form larger clusters by nucleation and growth processes, as previously reviewed by Fo¨rster and Antonietti.6 This process can lead either to one single colloid per micellar core or to several small colloids within a micellar core. Furthermore, the addition of HAuCl4 has been used to trigger the micellization of double hydrophilic PEO-b-P4(2)VP (PEO ) poly(ethylene oxide)) block copolymers in aqeuous solution, as demonstrated in ref 7. Many other related examples can be found in the scientific literature.4 HAuCl4 has been shown to interact with PEO as previously demonstrated by Mo¨ller et al.8 for PS-b-PEO micelles in toluene with a PEO core and more recently by Alexandridis et al., who prepared gold nanoparticles in aqueous Pluronic micelles without the need of a reductant.9 (5) (a) Spatz, J. P.; Ro¨scher, A.; Sheiko, S.; Krausch, G.; Mo¨ller, M. Adv. Mater. 1995, 7, 73. (b) Spatz, J. P.; Sheiko, S.; Mo¨ller, M. Macromolecules 1996, 29, 3220. (c) Roescher, A.; Mo¨ller, M. Adv. Mater. 1995, 7, 151. (d) Antonietti, M.; Wenz, E.; Bronstein, L. M.; Seregina, M. S. Adv. Mater. 1995, 7, 1000. (6) Fo¨rster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195. (7) (a) Bronstein, L. M.; Sidorov, S. N.; Valetsky, P. M.; Hartmann, J.; Co¨lfen, H.; Antonietti, M. Langmuir 1999, 15, 6256. (b) Sidorov, S. N.; Bronstein, L. M.; Kabachii, Y. A.; Valetsky, P. M.; Soo, P. L.; Maysinger, D.; Eisenberg, A. Langmuir 2004, 20, 3543. (8) Spatz, J. P.; Roescher, A.; Mo¨ller, M. Adv. Mater. 1996, 8, 337. (9) Sakai, T.; Alexandridis, P. Langmuir 2004, 20, 8426.

10.1021/la050377o CCC: $30.25 © 2005 American Chemical Society Published on Web 07/15/2005

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Other metal nanoparticles such as platinum and palladium were prepared in PS-b-PEO micelles as reported by Bronstein and co-workers.10 Several drawbacks are associated with block copolymer micelles as templates for the synthesis of gold nanoparticles. For example, the relatively large size of the templating core (generally on the order of 10 nm) impedes the formation of small gold nanoparticles that are required for several applications; e.g., gold nanoparticles exhibit intense luminescence only when they are smaller than 5 nm.1 The stability of the micelles containing gold nanoparticles is also a critical factor as stressed by Mo¨ller et al. for PS-b-P2VP-based systems.11,12 In this respect, as the concentration of the copolymer decreased, the polymer chains were molecularly dispersed and gold nanoparticles started to aggregate to form clusters with various shapes and sizes. Unimolecular core-shell micelles from star-block copolymers could circumvent these drawbacks as reported by Mays et al. for gold nanoparticles prepared in PS-bP2VP star-block copolymers.13 Compared to micelles formed from classical amphiphilic block copolymers, polymeric unimolecular micelles offer a higher stability in solution since the core-forming blocks contain covalently fixed branching points. Since all the polymer chains are fixed to a cross-linked core, no exchange between free chains and micelles is observed in unimolecular micelles. Therefore, these objects are not characterized by a critical micellar concentration. In this paper, we report the preparation of gold nanoparticles templated into the PEO core of unimolecular micelles from PEO-b-PCL (PCL ) poly(-caprolactone)) star-block copolymers dissolved in organic solvents. A series of samples have been investigated in which the PEO core has been kept constant while the length of the PCL coronal blocks has been varied. In a previous study, we have reported the synthesis of these PEO-b-PCL starblock copolymers and have demonstrated that these block copolymers form unimolecular micelles in organic solvents.14 Since a low-MW five-arm PEO core has been selected, small gold nanoparticles are expected to be formed. The long-term stability of these gold nanoparticles is studied as a function of the length of the coronal PCL blocks. Experimental Section Materials. All reagents were used without further purification unless stated otherwise. Solvents were purchased from Biosolve Ltd. (Valkenswaard, The Netherlands) and from Sigma-Aldrich (Bornem, Belgium). Stannous octoate and -caprolactone were purchased from Aldrich (Oakville, ON, Canada). The five-arm star poly(ethylene glycol) prepolymer was donated by BASF AG (Ludwigshafen, Germany) and purified by column chromatography prior to use (Al2O3, CH2Cl2). KAuCl4 and NaBH4 were purchased from Sigma-Aldrich (Bornem, Belgium). Synthesis and Characterization of the Copolymers. The synthesis of the PEO-b-PCL star-block copolymers was performed in parallel in a fully automated way utilizing a Chemspeed ASW2000 robot, as described in detail elsewhere.14 Briefly, a five-arm star-shaped PEO macroinitiator (Mn ) 2150) was used (10) Bronstein, L. M.; Chernyshov, D. M.; Timofeeva, G. I.; Dubrovina, L. V.; Valetsky, P. M.; Obolonkova, E. S.; Khokhlov, A. R. Langmuir 2000, 16, 3626. (11) Mo¨ssmer, S.; Spatz, J. S.; Mo¨ller, M.; Aberle, T.; Schmidt, J.; Burchard, W. Macromolecules 2000, 33, 4791. (12) Spatz, J. P.; Mo¨ssmer, S.; Hartmann, C.; Mo¨ller, M. Langmuir 2000, 16, 407. (13) Youk, J. H.; Park, M. K.; Locklin, J.; Advincula, R.; Yang, J.; Mays, J. Langmuir 2002, 18, 2455. (14) Meier, M. A. R.; Gohy,J.-F.; Fustin, C.-A.; Schubert, U. S. J. Am. Chem. Soc. 2004, 126, 11517.

Filali et al. Table 1. Characteristic Features of the PEG-b-PCL Star-Block Copolymers sample 1

sample 2

sample 3

sample 4

sample 5

sample 6

Mna 4200 6400 8200 9600 11200 13100 DPPCL 3 6 9 12 15 18 b Dh (nm) 3.2 ( 0.5 3.4 ( 0.8 4.5 ( 1.0 5.1 ( 0.7 6.4 ( 0.9 7.4 ( 0.8 a Calculated by 1H NMR. b D was measured by DLS in DMF h from data extrapolated to zero concentration.

for the controlled ring-opening polymerization of -caprolactone. The copolymers were characterized by 1H NMR in CDCl3 with a Bruker Mercury 400 NMR spectrometer. Complete information on the characterization of star-block copolymers is found in ref 14. Dynamic light scattering (DLS) was performed on the copolymers dissolved in N,N-dimethylformamide (DMF) with a Malvern CGS-3 apparatus correlator equipped with an ALV 5000/ EPP digital correlator and a 22 mW He-Ne laser with a wavelength of 633 nm. The scattering angle used for the measurements was 90°. The experimental correlation function was analyzed by the method of the cumulants, as described elsewhere.15 The Stokes-Einstein approximation was used to convert the diffusion coefficient into hydrodynamic diameter (Dh). The molecular characteristic features of the synthesized copolymers are listed in Table 1. Synthesis of Gold Nanoparticles. The copolymers were dissolved in DMF at a concentration of 1 g/L. A known amount of a KAuCl4 solution in DMF (c ) 10 g/L) was then added to the copolymer solution, and the mixture was stirred for 24 h. Since KAuCl4 molecules are expected to be essentially located within the PEO core of the star-block copolymer, the critical parameter to control is the KAuCl4/EO molar ratio. In preliminary experiments, we have varied this ratio from 1/1 to 1/2, 1/4, and 1/10. The best results were obtained with a KAuCl4/EO molar ratio of 1/4. The excess of KAuCl4 not interacting with the PEO core was removed by dialysis for 30 min against pure DMF. A known amount of a NaBH4 solution in DMF was finally added to the copolymer loaded with KAuCl4. The KAuCl4/NaBH4 molar ratio was changed. The best results were obtained with a 1/2 KAuCl4/ NaBH4 molar ratio. The color of the gold precursor-loaded micelles then immediately turned from yellow to red-purple. Moreover, gold nanoparticles were prepared without any added copolymer. For this sample, the amounts of KAuCl4 and NaBH4 were identical to the ones used for the metallization of sample 1 with a KAuCl4/ EO molar ratio of 1/4 and a KAuCl4/NaBH4 molar ratio of 1/2. Gold nanoparticles were finally prepared in the presence of the PEO macroinitiator at 1 g/L without any PCL outer blocks with a KAuCl4/EO molar ratio of 1/4 and a KAuCl4/NaBH4 molar ratio of 1/2. Characterization of Gold Nanoparticles. Transmission electron microscopy (TEM) was performed on a Leo 922 microscope, operating at a 200 kV accelerating voltage in bright field mode. Samples for TEM experiments were prepared by dropcasting the gold-loaded micelles before and after reduction with NaBH4 on a carbon-coated TEM grid. UV-vis spectra of the gold-loaded micelles were recorded with a Cary 50 spectrometer.

Results and Discussion The chemical structure of the PEO-b-PCL star-block copolymers is shown in Figure 1, while the molecular characteristic features of the investigated samples are listed in Table 1. Since all these samples have been prepared from the same PEO five-arm macroinitiator, they all contain an identical PEO core. This macroiniator has been used to initiate the ring-opening polymerization of -CL in the presence of a tin catalyst. A linear correlation between the targeted MW as fixed by the -CL/initiator ratio and the experimental one was observed, as previously reported.14 This indicates that the ring-opening polym(15) Berne, B. J.; Pecora, R. J. Dynamic Light Scattering; John Wiley and Sons: Toronto, 1976.

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Figure 1. Chemical structure of the PEG-b-PCL star-block copolymers (PEG chains in gray, PCL chains in black) and schematic representation of the process leading to stabilized gold nanoparticles (gray spheres).

erization of -CL was carried out in a controlled manner. The polydispersity indexes were measured by GPC to be around 1.4 for all the investigated copolymers. Six samples were prepared in parallel, and the use of an automatic synthesizer allowed the reproducibility of the synthetic procedure to be checked, as detailed elsewhere.14 The average degree of polymerization (DP) of the PCL outer blocks varied from 3 to 18, as listed in Table 1. These samples were readily soluble in a variety of organic solvents. In the following, experiments were carried out in DMF because the reactants (KAuCl4 and NaBH4) further required for the formation of gold nanoparticles were readily soluble in this solvent. The hydrodynamic diameter of the star-block copolymers was measured by DLS (Table 1). Dh was found to be small but increased with the length of the outer PCL blocks as expected. KAuCl4 was subsequently added to solutions of samples 1-6 in DMF at a concentration of 1 g/L. The loaded solutions were stirred for 24 h to ensure an effective incorporation of the gold precursor into the micellar core. The nonincorporated precursor was eliminated by dialysis, and the reduction of the loaded micelles was performed

by adding a known amount of a NaBH4 solution in DMF. The originally yellow colored solution turned to a deep purple-red color upon the addition of the reductant. The UV-vis characteristics of the prepared gold nanoparticles are presented in Figure 2. The absorption band at ∼530 nm is assigned to the surface plasmon resonance of small gold nanoparticles.1 The absorption bands showed only spectral shifts of less than 5 nm for all prepared nanoparticles, irrespective of the KAuCl4 concentration. However, their intensity increased with increasing concentration. At the same concentration, the intensity of the absorption band was found to decrease with the copolymer MW (see Figure 2). Because the star-block copolymer concentration has been fixed to 1 g/L for all experiments, less star-block copolymer molecules are present and less gold nanoparticles are present as the copolymer MW increases. The UV-vis spectrum of a blank sample in which no star-block copolymers have been added for the preparation of the gold nanoparticles is also included in Figure 2. The surface plasmon resonance peak is now shifted to ∼560 nm, indicating that larger gold nanoparticles have been formed in this case.16

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Filali et al. Table 2. Characteristic Diameters of the Gold-Loaded PEG-b-PCL Star-Block Copolymers As Measured by TEM (DTEM, nm) sample 1

sample 2

sample 3

sample 4

sample 5

sample 6

DTEMa 5.2 ( 0.5 3.6 ( 0.1 3.7 ( 0.1 3.5 ( 0.1 3.5 ( 0.1 2.9 ( 0.1 DTEMb 5.9 ( 0.5 6.8 ( 0.6 3.4 ( 0.2 3.2 ( 0.8 4.0 ( 0.2 3.6 ( 0.2 a Measured after reduction with NaBH . b Measured before 4 reduction with NaBH4.

Figure 2. UV-vis absorption spectra of gold nanoparticles prepared without any copolymer (a) and prepared within the core of the star-block copolymers 3 (b) and 6 (c).

Figure 4. TEM micrographs of gold nanoparticles prepared by electron-beam irradiation during observation of star-block copolymers 1-6 (with a KAuCl4/EO loading molar ratio of 1/4).

Figure 3. TEM micrographs of gold nanoparticles prepared with star-block copolymers 1-6 with a KAuCl4/EO loading molar ratio of 1/4 and after reduction with NaBH4 (KAuCl4/ NaBH4 molar ratio of 1/2).

Figure 3 shows TEM micrographs of gold nanoparticles templated by samples 1-6 with a KAuCl4/EO loading molar ratio of 1/4 and a KAuCl4/NaBH4 molar ratio of 1/2. Rather monodisperse small gold nanoparticles with a spherical shape and an average size of approximately 3 nm were formed in the core of all the investigated starblock copolymers. The data obtained after image analysis of the TEM pictures are listed in Table 2. The dimensions of at least 50 gold nanoparticles recorded on different (16) (a) Garbar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353. (b) Kreibig, U.; Genzel, L. Surf. Sci. 1985, 156, 678. (c) Quinten, M.; Kreibig, U. Surf. Sci. 1986, 172, 557.

locations of the TEM grid have been compiled to obtain these data. Different TEM grids were also analyzed for each sample, to preclude any preparation effect on the particle dimensions. Number-averaged diameters are shown in Table 2. Furthermore, gold nanoparticle formation has been directly investigated during TEM observations. In this case, the reduction of the gold precursor is triggered by electron irradiation. The data obtained for the “electron-beam” (e-beam)-reduced samples are also listed in Table 2 and shown in Figure 4. These results are in agreement with the results obtained on the “chemically” reduced samples, although the latter seem to be more uniform. However, a closer look at these TEM pictures revealed a broader polydispersity for the gold nanoparticles templated by sample 1, i.e., the star-block copolymer with the shorter PCL outer blocks. In some cases, fused gold nanoparticles were observed. The same observation holds for gold nanoparticles prepared by e-beam irradiation in samples 1 and 2. Whenever PS-b-P4VP diblock copolymers were used for the preparation of gold nanoparticles, some of the micelles contained two or more gold nanoparticles because of a collision-induced aggregation of particles, resulting in a broad size distribution of gold nanoparticles.11,17 This drawback was circumvented by slightly cross-linking the micellar cores with p-xylene dibromide to increase the stability of the micelles and by adding

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Figure 5. TEM micrographs of gold nanoparticles prepared without any added copolymer (a) and in the presence of the PEO macroinitiator (KAuCl4/EO loading molar ratio of 1/4 and KAuCl4/NaBH4 molar ratio of 1/2) (b).

small amounts of methanol to provide a higher mobility of the micellar cores.17 The unimolecular micelles formed by the PEO-b-PCL star-shaped block copolymers are generally able to prevent such unwanted coalescence events. Coalescence was only observed if the outer PCL blocks were very short (DP ) 3 for the PCL blocks of sample 1). This observation highlights the crucial role of the PCL outer block as stabilizing entities for the accordingly formed nanoparticles. A critical PCL block length has to be reached to prevent contact between two adjacent unimolecular micelles and further coalescence of the embedded gold nanoparticles. To give credit to this hypothesis, additional experiments were performed. First, the templating role of the PEO core has been ascertained by examining the gold nanoparticles prepared without any added polymer. Large gold crystals with various shapes such as triangles are observed as shown in Figure 5a. These nanoparticles are quite different from those obtained in the presence of star-block copolymers. Second, gold nanoparticles have been prepared in the presence of the five-arm PEO macroinitiator with a KAuCl4/EO loading molar ratio of 1/4 and a KAuCl4/NaBH4 molar ratio of 1/2. Small gold nanoparticles are expected to be formed in this case, but the lack of a stabilizing PCL outer block resulted in clustering of the initial gold nanoparticles. Highly polydisperse small gold nanoparticles were finally observed as shown in Figure 5b. Third, the long-term stability of the gold nanoparticles prepared in the star-block copolymer 1-6 has been measured. The gold nanoparticles prepared in samples 3-6 with a KAuCl4/EO loading molar ratio of 1/4 and a KAuCl4/NaBH4 molar ratio of 1/2 are unaffected after six months, while some clustering is observed for the other samples. Clustering has been followed by TEM. Figure 6 shows large spherical aggregates of gold nanoparticles observed after one month in samples 1 and 2. These three observations clearly demonstrate that the PCL outer blocks can provide an effective steric barrier against gold nanoparticle coalescence provided that they have a sufficient length. It is clear that the long-term stability of gold nanoparticles against aggregation is very important for their applications. Therefore, control of particle-particle interaction is critical for the preparation of a stable particle dispersion, resulting in quantum confinement effects. As mentioned earlier, both the KAuCl4/EO loading and KAuCl4/NaBH4 molar ratios have been varied. The best results have been obtained with the conditions described above, i.e., with a KAuCl4/EO loading molar ratio of 1/4 and a KAuCl4/NaBH4 molar ratio of 1/2. Larger amounts of KAuCl4 resulted in gold precursor molecules not incorporated into the PEO core and therefore nucleation outside the micelles. In this case, large gold nanoparticles (17) Fendler, J. H. Nanoparticles and Nanostructured Films; WileyVCH: Weinheim, Germany, 1998.

Figure 6. TEM microcraphs of aggregated gold nanoparticles prepared in samples 1 and 2 and observed one month after preparation. A magnification of a spherical aggregate in sample 2 shows that it results from the aggregation of several individual gold nanoparticles.

outside the micelles were observed to coexist with the smaller nanoparticles templated by the star-block copolymers. Dialysis of the excess of noninteracting KAuCl4 was able to improve the situation. The amount of added reductant also has an effect, as previously reported by Mo¨ller and co-workers.11 Indeed, aggregation of the primary gold nanoparticles templated by PS-b-P2VP copolymers was observed whenever an excess of hydrazine was used for the reduction of HAuCl4loaded micelles. This clustering could be suppressed by removing the excess of hydrazine due to the addition of HCl directly after the reduction. In our case, the addition of an excess of NaBH4 also resulted in an increased clustering of the initial gold nanoparticles and the longterm stability of the solutions accordingly decreased from months to days. This was experimentally evidenced by TEM observations in which the size and size polydispersity of the spherical gold nanoparticles were found to increase and by UV-vis spectroscopy since a red shift of the surfaceplasmon absorption band was noted. Once again, the stability strongly depended on the length of the PCL stabilizing blocks. The worst situation was observed whenever gold nanoparticles were prepared with a KAuCl4/EO loading molar ratio of 1/1 and reduced by a large excess of NaBH4. In this case, macroscopic precipitation was observed after a few seconds for the gold nanoparticles templated by copolymer 1 while about 1 h was needed to observe precipitation from gold nanoparticles in copolymer 6. Conclusions In this paper, we have demonstrated that star-block copolymers are promising precursors for the preparation of well-defined gold nanoparticles. Star-block copolymers based on a PEO five-arm core and containing PCL outer blocks of variable length have been utilized for this study. Whenever these block copolymers are dissolved in DMF, they can be seen as unimolecular objects consisting of a PCL shell and a PEO core, which is however still swelled by the solvent. KAuCl4 can specifically interact with the PEO core, resulting in gold-precursor-loaded micelles. Gold nanoparticles can be generated either by electron irradiation during TEM observation of the samples or by chemical reduction in solution by addition of NaBH4. TEM investigations have revealed the formation of spherical, mono-

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disperse gold nanoparticles with an average size of 3-4 nm for both methods. The amount of gold precursor has been optimized to ensure its complete incorporation inside the PEO core and hence to avoid nucleation outside the micellar core. A KAuCl4/EO molar ratio of 1/4 has been found to be optimal. The long-term stability of the accordingly formed gold nanoparticles has also been investigated and found to be dependent on the length of the PCL outer blocks and on the amount of NaBH4 added for reduction. In this respect, the long-term stability of the gold nanoparticles decreases with the length of the PCL blocks, which act as steric stabilizers against gold nanoparticle clustering. An optimal molar amount of 1/2 KAuCl4/NaBH4 has been determined. Larger amounts resulted in a decreased stability of the gold nanoparticles, in agreement with previous results.11 Gold nanoparticles stable over six months could be obtained whenever the star-block copolymers with long PCL blocks (DP > 9) and optimal KAuCl4 and NaBH4 amounts were used. Compared to previously reported star-block copolymer templates,13 our system provides the advantage of being prepared from a five-arm initiator, leading to a welldefined small templating core. In contrast, star-block copolymers resulting from the cross-linking of diblocks are characterized by a much less defined core. Furthermore, the small size of the PEO core in the PEO-b-PCL star-block copolymer resulted in the formation of gold

Filali et al.

nanoparticles with a suitable size for applications in optoelectronics. In addition to its beneficial effect on the steric stabilization of the gold nanoparticles in solution, the PCL outer shell is also quite interesting for the further preparation of polymer-based nanocomposites.18,19 Such nanocomposite materials are usually formed by blending of nanoparticles into the polymer matrix. Since PCL is miscible with a number of different polymer materials, the well-defined gold nanoparticles embedded in a PCL shell might display a wide versatility in such applications. Last but not least, the biocompatible and biodegradable features of PCL could be of advantage for biomedical imaging applications. Acknowledgment. M.F. and J.-F.G. thank the Communaute´ franc¸ aise de Belgique for financing this project in the frame of the Action de Recherches Concerte´es NANOMOL 03/08-300. M.A.R.M. and U.S.S. thank the Dutch Polymer Institute and the Fonds der Chemischen Industrie for financial support. LA050377O (18) Corbierre, M. K.; Cameron, N. S.; Sutton, M.; Mochrie, S. G. J.; Lurio, L. B.; Ru¨hm, A.; Lennox, R. B. J. Am. Chem. Soc. 2001, 123, 10411. (19) Zehner, R. W.; Lopes, W. A.; Morkved, T. L.; Jaeger, H.; Sita, L. R. Langmuir 1998, 14, 241.