Tuning the Hydrophilicity of Gold Nanoparticles Templated in Star

The work of M.A.R.M. and U.S.S. is part of the research program of the Dutch ..... See, for example, (a) Leff, D. V.; Brandt, L.; Heath, J. R. Langmui...
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6690

Langmuir 2006, 22, 6690-6695

Tuning the Hydrophilicity of Gold Nanoparticles Templated in Star Block Copolymers Charles-Andre´ Fustin,† Catheline Colard,† Mariam Filali,† Pierre Guillet,† Anne-Sophie Duwez,‡ 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, Unite´ de Chimie et de Physique des Hauts Polyme` res (POLY) and CeRMiN, UniVersite´ catholique de LouVain, Place Croix du Sud 1, 1348 LouVain-la-NeuVe, Belgium, Laboratory of Macromolecular Chemistry and Nanoscience, EindhoVen UniVersity of Technology and Dutch Polymer Institute, P.O. Box 513, 5600 MB EindhoVen, The Netherlands ReceiVed March 21, 2006. In Final Form: May 10, 2006 We report on a simple procedure to tune the hydrophilicity of hybrid gold nanoparticles. The nanoparticles have been prepared in the core of a poly(ethylene glycol)-block-poly(-caprolactone) (PEG-b-PCL) five-arm star block copolymer. A hydrophilic corona was then added to these hybrid gold nanoparticles by direct chemisorption of trithiocarbonate-containing poly(acrylic acid) chains. These polymers were synthesized by RAFT polymerization with a trithiocarbonate as the chain-transfer agent. The efficiency of the grafting was evidenced by TEM, AFM, and DLS and by the successful transfer of these nanoparticles from organic solvent to water.

Introduction Nanoparticles, and in particular gold nanoparticles (AuNP’s), are a rapidly growing area of material science because of their numerous applications in various fields ranging from chemical separations1 and sensing,2-7 to applications in the medical community such as the diagnosis and treatment of some cancers.8-11 The unique properties of AuNP’s are directly related to their size and are significantly different from those of the corresponding bulk materials.12 Besides precise control over the size, size polydispersity, and shape, the colloidal stability of AuNP’s is an extremely important issue. AuNP’s have been synthesized using a variety of methods including citrate reduction,13 two-phase synthesis,14 and one* Corresponding author. E-mail: [email protected]. † 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. ‡ Unite ´ de Chimie et de Physique des Hauts Polyme`res (POLY) and CeRMiN, Universite´ catholique de Louvain. § Eindhoven University of Technology and Dutch Polymer Institute. (1) Gross, G. M.; Nelson, D. A.; Grate, J. W.; Synovec, R. E. Anal. Chem. 2003, 75, 4558. (2) Dos Santos, D. S., Jr.; Goulet, P. J. G.; Pieczonka, N. P. W.; Oliveira, O. N., Jr.; Aroca, R. F. Langmuir 2004, 20, 10273. (3) Faulds, K.; Littleford, R. E.; Graham, D.; Dent, G.; Smith, W. E. Anal. Chem. 2004, 76, 592. (4) Grate, J. W.; Nelson, D. A.; Skaggs, R. Anal. Chem. 2003, 75, 1868. (5) Krasteva, N.; Besnard, I.; Guse, B.; Bauer, R. E.; Muellen, K.; Yasuda, A.; Vossmeyer, T. Nano Lett. 2002, 2, 551. (6) Matsui, J.; Akamatsu, K.; Nishiguchi, S.; Miyoshi, D.; Nawafune, H.; Tamaki, K.; Sugimoto, N. Anal. Chem. 2004, 76, 1310. (7) Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc. 2004, 126, 15950. (8) Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 13549. (9) Loo, C.; Lin, A.; Hirsch, L.; Lee, M.-H.; Barton, J.; Halas, N.; West, J.; Drezek, R. Technol. Cancer Res. Treat. 2004, 3, 33. (10) Loo, C.; Lowery, A.; Halas, N.; West, J.; Drezek, R. Nano Lett. 2005, 5, 709. (11) O’Neal, D. P.; Hirsch Leon, R.; Halas Naomi, J.; Payne, J. D.; West Jennifer, L. Cancer Lett. 2004, 209, 171. (12) 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.

phase synthesis in organic solvents.15 Low molecular weight (MW) surfactants and thiol end-functionalized molecules have also been used in the synthesis of AuNP’s to promote steric stabilization of the accordingly formed particles.16 Because low MW surfactants are self-organized in micellar nanocontainers, a templating effect could occur provided that the nucleation and growth processes of AuNP’s are limited to one specific locus, most often the core of the micelles. Such a templating effect has been widely investigated because it allows the simultaneous tuning of the size and shape of the resulting AuNP’s as well as their stabilization.16 The use of polymers has been considered to be a valuable approach to the synthesis of gold nanoparticles because polymers can act as reducing and/or stabilizing agents.17 For example, gold nanoparticles have been prepared in one step by mixing (in water or organic solvents) a gold precursor, a reducing agent, and a polymer made by reversible addition-fragmentation chain transfer (RAFT) polymerization. By virtue of the polymerization mechanism, this polymer bears a dithioester moiety at one end of the chain. When the three components are mixed together, the reducing agent turns the gold salt into metallic gold and at the same time reduces the dithioester into thiol, which chemisorbs onto the nanoparticles, stabilizing them.18-20 Block copolymers can also be used as templates because of their ability to self-assemble on the nanoscale.21 Block copolymer micelles indeed appear to be a valuable alternative to low MW (13) Turkevich, J.; Stevenson, P. C.; Hillier, J. Faraday Discuss. 1951, 11, 55. (14) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 801. (15) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. Chem. Commun. 1995, 1655. (16) 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. (17) Sakai, T.; Alexandridis, P. Langmuir 2004, 20, 8426. (18) Lowe, A. B.; Sumerlin, B. S.; Donovan, M. S.; McCormick, C. L. J. Am. Chem. Soc. 2002, 124, 11562. (19) Zhu, M. Q.; Wang, L. Q.; Exarhos, G. J.; Li, A. D. Q. J. Am. Chem. Soc. 2004, 126, 2656.

10.1021/la060758h CCC: $33.50 © 2006 American Chemical Society Published on Web 06/14/2006

Au Nanoparticles Templated in Star Block Copolymers

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Scheme 1. Structure of the Five-Arm PEG-b-PCL Star Block Copolymers Used in This Study and a Representation of the Templating Process Leading to AuNP’s

surfactants. First, the critical micelle concentration of block copolymers is much smaller and the kinetic stability of the formed micelles is larger than that of low MW surfactants.22 Second, the size and shape of block copolymer micelles can easily be tuned by varying the composition of the copolymer, the length of the constituent blocks, and the architecture of the copolymer.23 Third, the stability of the accordingly formed nanoparticles can be enhanced by increasing the length of the coronal blocks.24 The synthesis of AuNP’s in block copolymer micelles is widely documented in the scientific literature.25 The strategy toward AuNP’s generally involves the noncovalent interaction of a gold precursor (e.g., HAuCl4) with one specific compartment of the micelle that contains functional groups (e.g., vinylpyridine), followed by reduction of the precursor by, for example, NaBH4 or hydrazine to form Au0. 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.26 This process can lead either to one single colloid per micellar core or to several small colloids within a micellar core. Gold salts have been shown to interact not only with nitrogen-containing polymers but also with other polymer blocks including poly(ethylene oxide) as demonstrated by Mo¨ller et al.27 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.17 Other metal nanoparticles such as Pt and Pd were prepared in PS-b-PEO micelles as reported by Bronstein and co-workers.28 In these examples, the formation of metal nanoparticles is considered either in the core or in the shell of the PEO-containing copolymers, depending on the solvent used for the micellization process. In this respect, AuNP’s templated in (20) (a) Shan, J.; Nuopponen, M.; Jiang, H.; Kauppinen, E.; Tenhu, H. Macromolecules 2003, 36, 4526. (b) Shan, J.; Nuopponen, M.; Jiang, H.; Viitala, T.; Kauppinen, E.; Kontturi, K.; Tenhu, H. Macromolecules 2005, 38, 2918. (21) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, England, 1998. (22) Riess, G. Prog. Polym. Sci. 2003, 28, 1107. (23) Gohy, J.-F. AdV. Polym. Sci. 2005, 190, 65. (24) Filali, M.; Meier, M. A. R.; Schubert, U. S.; Gohy, J.-F. Langmuir 2005, 21, 7995. (25) See, for example, (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. (26) Fo¨rster, S.; Antonietti, M. AdV. Mater. 1998, 10, 195. (27) Spatz, J. P.; Roescher, A.; Mo¨ller, M. AdV. Mater. 1996, 8, 337. (28) 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.

the core of PEO-containing copolymers are observed whenever micellization is conducted in an organic solvent. In a recent paper, we reported on the formation of quite monodisperse AuNP’s of improved stability against aggregation, prepared using poly(ethylene glycol)-b-poly(-caprolactone) (PEG-b-PCL) star block copolymers (Scheme 1).24 Those AuNP’s were templated by the PEG core of the star block copolymers, and their stability was directly related to the length of the PCL chains acting as stabilizing groups against aggregation. These AuNP’s were reported in organic solvents such as DMF and THF. However, most of the applications related to AuNP’s require an aqueous environment. In this article, we report on a strategy that allows us to tune the hydrophilicity of the outer shell of AuNP’s templated in PEG-b-PCL star block copolymers and further enables us to transfer these AuNP’s from an organic solvent to an aqueous medium. Experimental Section Materials. All reagents were used without further purification unless otherwise stated. Solvents were purchased from Biosolve Ltd. (Valkenswaard, The Netherlands) and from Sigma-Aldrich (Bornem, Belgium). Acrylic acid, V501 initiator, A-26 Amberlyst resin, carbon disulfide, benzyl bromide, KAuCl4, and NaBH4 were purchased from Sigma-Aldrich (Bornem, Belgium). PAA (Mn ) 20 000, Mw/Mn ) 1.09) was purchased from Polymer Source Inc. (Dorval, Canada). Synthesis of the Five-Arm Star Block Copolymer. The synthesis of the PEG-b-PCL star block copolymers was performed in parallel in a fully automated way utilizing a Chemspeed ASW2000 robot, as described in details elsewhere.29 Briefly, a five-arm-star-shaped PEG macroinitiator (Mn ) 2150) was used to control the ring-opening polymerization of -caprolactone. Complete information on the characterization of star block copolymers can be found elsewhere.29 Synthesis of the RAFT Chain-Transfer Agent (CTA). Dibenzyl trithiocarbonate was synthesized following a reported procedure.30 In brief, Amberlyst A-26 (OH-) (40 g, 20 ( 50 mesh) was added to carbon disulfide (200 mL) and stirred at room temperature for about 3 min. To this suspension was added benzyl bromide (3.42 g, 20 mmol), and the reaction mixture was stirred under reflux for about 5 h. The mixture was then filtered and washed with THF. The filtrate was dried over anhydrous magnesium sulfate and condensed under vacuum to afford dibenzyl trithiocarbonate as a yellow oil (90%).1H NMR (CDCl3): 4.71 (s, 4H, CH2-Ph), 7.36-7.41 (m, 10H, ArH). (29) Meier, M. A. R.; Gohy,J.-F.; Fustin, C. A.; Schubert, U. S. J. Am. Chem. Soc. 2004, 126, 11517. (30) Tamani, B.; Kiasat, A. R. J. Chem. Res. 1998, 454.

6692 Langmuir, Vol. 22, No. 15, 2006 Synthesis of Poly(acrylic acid) with a Trithiocarbonate Central Group (PAA-S-CS-S-PAA). Acrylic acid (AA) was dried over CaH2 and distilled under vacuum. AA was polymerized in n-butanol using V501 as the initiator and dibenzyl trithiocarbonate as the CTA.31 A CTA/V501 10/1 molar ratio was used. Polymerizations were conducted in Schlenk tubes at 90 °C for 2 h. Three freeze-pumpthaw cycles were performed before polymerization. The polymer was recovered by precipitation in toluene followed by drying in a vacuum oven. In a typical experiment, 4 mL of AA (5.8 × 10-2 mol), 34 mg of CTA (1.1 × 10-4 mol), and 3 mg of V501 (1.1 × 10-5 mol) were dissolved in 21 mL of n-butanol. Synthesis of Gold Nanoparticles. The particles were prepared following our previously reported procedure.24 In brief, a solution of KAuCl4 in DMF was added to a solution of the PEG-b-PCL star block copolymers in DMF, and the mixture was stirred for 24 h (KAuCl4/EO molar ratio of 1/4). Excess KAuCl4 not interacting with the PEG core was removed by dialysis for 30 min against pure DMF. A solution of NaBH4 in DMF was finally added to the copolymer loaded with KAuCl4 (1/2 KAuCl4/NaBH4 molar ratio). The color of the gold precursor-loaded micelles immediately turned from yellow to red-purple during the addition of the reductant. Transfer of AuNP’s Templated in the PEG-b-PCL Star Block Copolymers from DMF to Water. One milliliter of a 1% w/v solution of PAA or PAA-S-CS-S-PAA in DMF was added to 1 mL of a AuNP’s solution (5 g/L) in DMF. The mixture was stirred for 24 h and then dialyzed against water at pH 9 (pH adjusted by the addition of NaOH) for 24 h. Instrumentation. Gel permeation chromatograms were measured on a Waters system consisting of an isocratic pump, solvent degasser, column oven, 2414 refractive index detector, 717plus autosampler, and Styragel HT 4 GPC column with the precolumn installed. The eluent was N,N-dimethyl formamide (DMF) with 5 mM NH4PF6 at a flow speed of 0.5 mL/min. The column temperature was 50 °C. Dynamic light scattering (DLS) was performed in DMF with a Malvern CGS-3 system equipped with an ALV 5000/EPP digital correlator and a 22 mW He-Ne laser with a wavelength of 633 nm. The measurements were performed at an angle of 90° and at 25 °C. The experimental correlation function was analyzed by the CONTIN routine, a method that is based on a constraint inverse Laplace transformation of the data and that gives access to a size distribution histogram for the aggregates.32 Transmission electron microscopy (TEM) was performed on a Leo 922 microscope, operating at 200 kV accelerating voltage in bright field mode. Samples for TEM experiments were prepared by spincoating the gold-loaded micelles after reduction with NaBH4 on a carbon-coated TEM grid. Atomic force microscopy (AFM) measurements were performed in tapping mode with a Veeco Nanoscope IV Multimode microscope operated in air. Cantilevers (NCL type, Nanosensors) with a resonance frequency of approximately 190 kHz and a spring constant of 42 N m-1 were used. Samples were prepared by spin-coating AuNP solutions onto silicon wafers. AFMbased force spectroscopy experiments were carried out in water (pH 9, 10 mM NaCl) with a PicoSPM equipped with a fluid cell (Molecular Imaging) and controlled by Nanoscope III electronics (Digital Instruments). Silicon nitride cantilevers with a spring constant of 0.05 N m-1 were used.

Results and Discussion In a previous paper, we reported on the synthesis of AuNP’s templated in PEG-b-PCL star block copolymers.21 These copolymers were prepared by using a five-arm-star-shaped (PEG) macroinitiator containing diethylene triamine as the core molecule (Scheme 1) for the automated parallel controlled ring-opening polymerization of -caprolactone. A series of PEG-b-PCL star block copolymers with a constant PEG core linked to PCL blocks (31) Khousakoun, E.; Gohy, J.-F.; Je´roˆme, R. Polymer 2004, 45, 8303. (32) Berne, B. J.; Pecora, R. J. Dynamic Light Scattering; John Wiley and Sons: Toronto, 1976.

Fustin et al. Scheme 2. Representation of the RAFT Polymerization Process Leading to PAA-S-CS-S-PAA

Scheme 3. Representation of the Chemisorption Process of PAA-S-CS-S-PAA Chains onto AuNP’s Prepared in a Five-Arm PEG-b-PCL Star Block Copolymera

a The trithiocarbonate moieties are schematized by the white rectangles.

of variable length were obtained. The PEG core was swelled with KAuCl4 in N,N-dimethylformamide (DMF), and AuNP’s were subsequently obtained by reduction with NaBH4. Because the process was always templated by the same PEG core for all investigated polymers, the size of the formed AuNP’s 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. Whenever the DP of the PCL block exceeded 9, these AuNP’s were stable in organic solvents (DMF, THF) for more than 6 months.24 However, these AuNP’s could not be dispersed in water because of the hydrophobic PCL shell. To circumvent this problem and thus broaden the range of application, we propose a strategy to transform the initial hydrophobic outer shell into a hydrophilic one. Our approach consists in adding a new corona, made of hydrophilic polymer chains that can overrule the hydrophobicity of the PCL blocks, to the nanoparticles by the “grafting-to” method. To this end, we have used PAA made by RAFT with a trithiocarbonate as CTA (Scheme 2). The PAA thus prepared bears a trithiocarbonate moiety in the middle of the chain that will act as an anchoring group on the gold nanoparticle. These chains are expected to anchor on the AuNP’s as schematically depicted in Scheme 3. We have indeed recently shown that such a moiety is able to chemisorb onto gold.33 PEG-b-PCL copolymers with a Mn of 8200 g mol-1 and a polydispersity index of 1.35 was selected as a template for AuNP synthesis. Because the Mn of the central PEG core is 2150 g mol-1, each PCL arm has an average DP of 9. This copolymer was dissolved in DMF and loaded with KAuCl4, which was subsequently reduced by NaBH4. Rather monodisperse AuNP’s (33) Duwez, A. S.; Guillet, P.; Colard, C.; Gohy, J.-F.; Fustin, C. A. Macromolecules 2006, 39, 2729.

Au Nanoparticles Templated in Star Block Copolymers

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Figure 2. AuNP’s transferred from DMF to water at pH 9 in the presence of the nonfunctional PAA (left) and of the PAA-S-CSS-PAA polymer (right).

Figure 1. TEM picture of AuNP’s templated in a PEG-b-PCL star block copolymer in DMF. The average DP of each PCL arm is 9.

with an average diameter of 3.7 nm ((0.1) were obtained at the end of this process. A typical TEM picture of these AuNP’s is shown in Figure 1. Because the size of these AuNP’s is similar to the size of the star block copolymer used as a template, it is clear that the PEG-b-PCL copolymer actually controls the nucleation and growth process of the AuNP. Moreover, much larger and polydisperse AuNP’s were obtained in blank experiments with only the metal salt and no copolymer. However, at this point, we cannot conclude that a single AuNP is formed in each star block copolymer. PAA-S-CS-S-PAA was synthesized using RAFT polymerization with dibenzyltrithiocarbonate as the CTA (Scheme 2). Because a difunctional CTA has been used, two different polymer chains are bridged by the trithiocarbonate group.31 In the present study, we have considered that this trithiocarbonate group is essentially located in the center of a PAA chain. A PAA-SCS-S-PAA polymer with a total Mn of 15 000 g mol-1 (PMMA equivalent) and a polydispersity index of 1.3 was used in this study. The PAA-S-CS-S-PAA polymer was added to the AuNP’s in DMF, and the mixture was stirred for 24 h. DMF was then gradually replaced via a dialysis process by water at pH 9. The AuNP’s covered by PAA-S-CS-S-PAA chains were successfully transferred from DMF to basic water. Indeed, the characteristic red color of the AuNP’s was not changed after the transfer process (Figure 2). Moreover, this characteristic red color remained for more than 3 months, evidencing the good stability of these nanoparticles. To prove that the stabilization is linked to a brush effect due to the grafting of a new corona via chemisorption of the trithiocarbonate group onto the gold nanoparticles and not to the nonspecific adsorption of PAA onto the nanoparticles (carboxylic acid groups are indeed known to have a high affinity for gold), we have performed a “blank” experiment in which a nonfunctional PAA chain of a similar Mn was used instead of PAA-S-CS-S-PAA. In this case, the AuNP’s could not survive the transfer from DMF to basic water as indicated by the bluegray color of the water solution and the observation of a black precipitate (Figure 2). The solvent transfer process was also not successful when tested on the hybrid AuNP’s alone.

Figure 3. Approach (grey)-retraction (black) profiles of a force curve obtained in water between an AFM tip and a gold substrate immersed for 24 h in a 1 g/L solution of PAA-S-CS-S-PAA (top) and a gold substrate immersed for 24 h in a 1 g/L solution of the nonfunctional PAA (bottom).

To prove further that the stabilization of the nanoparticles is linked to a brush effect, we have performed force spectroscopy experiments on model systems. Gold substrates on silicon were immersed for 24 h in 1 g/L solutions of PAA-S-CS-S-PAA and the nonfunctional PAA in DMF, followed by copious rinsing with pure DMF and water. These samples were then characterized by AFM-based force spectroscopy in aqueous solutions. Figure 3 (top) shows the approach and retraction profiles obtained in water between a bare silicon nitride tip and a layer of the PAAS-CS-S-PAA. The profile clearly shows monotonically increasing repulsive forces, which are typical of a polymer brush under compression in a good solvent.34 They are caused by the reduced configurational entropy of the polymer chains, which increases the osmotic pressure upon approach of the surface. These repulsive forces are at the origin of the steric stabilization of colloidal dispersions.35 Moreover, the profile is completely reversible, which is another characteristic of terminally attached brushes. Physisorbed polymers often display hysteresis between compression and decompression because of nonequilibrium relaxation effects.34 For the sample prepared with the nonfunctional PAA (Figure 3, bottom), no repulsive forces are observed upon tip approach, which indicates that no brush was formed. On the retraction profile, bridging interactions are observed in about (34) Taunton, H. J.; Toprakcioglu, C.; Fetters, L. J.; Klein, J. Nature 1988, 332, 712. (35) Zhang, W.; Cui, S.; Fu, Y.; Zhang, X. J. Phys. Chem. B 2002, 106, 12705.

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Fustin et al.

Figure 4. TEM picture of AuNP’s templated in a PEG-b-PCL star block copolymer and transferred into water after grafting of PAAS-CS-S-PAA.

50% of the cases (as in the profile shown in lower panel of Figure 3). These saw-tooth-like profiles are characteristic of polymer chains physisorbed in a “trains and loops” conformation.36 In the other cases, a contact adhesion peak due to the van der Waals forces between the silicon nitride tip and the gold substrate is observed, evidencing the inhomogeneity of the surface coverage. All of these experiments indicate that only PAA-SCS-S-PAA is able to form a brush and stabilize the nanoparticles. The AuNP’s coated with PAA-S-CS-S-PAA and transferred into an aqueous medium were visualized by TEM and by AFM. The TEM pictures (Figure 4) reveal nicely dispersed nanoparticles very similar to the initial AuNP’s in DMF. The mean diameter obtained from the TEM images is 4.6 nm ((0.74), in very good agreement with the size of the initial AuNP’s (3.7 nm). The slightly larger size and polydispersity could result from the clustering of a few AuNP’s during the solvent transfer process, although the structure of these clusters is not clearly visualized in Figure 4. The presence of the additional corona around AuNP’s covered by PAA-S-CS-S-PAA chains has been evidenced by AFM. Indeed, the height of the hybrid AuNP’s increases from 2 nm ((0.24) before grafting to 4 nm ((0.85) after chemisorption of the PAA. Only the dimension in the Z direction has been considered because X and Y dimensions are affected by tip convolution effects. This difference may appear to be rather small considering the length of the PAA-S-CS-S-PAA chains (if we assume a central location for the trithiocarbonate group, each PAA arm has a DP of 104) compared to the original PCL corona (each PCL arm has a DP of 9), but we have to keep in mind that the AFM experiments have been performed in the dry state and therefore on a collapsed corona. To evidence the size difference before and after grafting better, DLS has been performed on the original AuNP’s in DMF and on the PAA-modified particles in water. The size distributions of these two samples were obtained by the CONTIN routine and are shown in Figure 5. This analysis gives a mean Rh of 3.1 nm for the initial AuNP’s stabilized in the PEG-b-PCL star block and a mean Rh of 21 nm after the grafting of the PAA-S-CS-S-PAA and the transfer into water, (36) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: San Diego, CA, 1991.

Figure 5. CONTIN size distribution (number-average data) obtained by DLS measurements on the initial AuNP’s in DMF (a) and after grafting of PAA-S-CS-S-PAA and transfer into water at pH 9 (b). The raw DLS data (correlation function) are shown in the inset.

confirming that the grafting of PAA-S-CS-S-PAA results in a significant increase in the stabilizing corona of the hybrid AuNP’s. The data have been plotted as number-averaged to show the peak corresponding to the AuNP’s better. When unweighted data are plotted, two populations are seen for both samples: the first one corresponding to the AuNP’s and the second one, around 100 nm, probably corresponding to a few larger gold particles formed outside the templating five-arm copolymer. These large gold particles, however, represent a very minor fraction of the sample because only the AuNP’s population is seen in the CONTIN histogram when number-averaged data are plotted (Figure 5) and because TEM pictures show mainly well-dispersed AuNP’s and no large particles. Therefore, we can conclude that the most numerous objects are AuNP’s and that the grafting of the new corona increases the Rh of these AuNP’s but does not change their size distribution.

Conclusions In this article, we have demonstrated that hybrid AuNP’s synthesized in a five-arm PEG-b-PCL star block copolymer, and thus containing hydrophobic PCL stabilizing chains, could be turned into hydrophilic particles by grafting a new corona in a one-step approach using PAA containing a trithiocarbonate group. Such a functional group is easily introduced into macromolecular chains by a RAFT polymerization process and can chemisorb onto gold. The modified hybrid AuNP’s were successfully transferred into water at pH 9. The presence of the trithiocarbonate group is mandatory because the transfer process failed when nonfunctional PAA was used. Finally, the presence of an additional PAA-S-CS-S-PAA-based stabilizing layer on the AuNP’s transferred in water was evidenced by both AFM and

Au Nanoparticles Templated in Star Block Copolymers

DLS techniques. The obtained water-soluble AuNPs might be interesting materials for medical applications as addressed in the Introduction. The proof of concept reported in this article is but one example of what could be achieved with this simple procedure. Considering the broad range of (co)polymers accessible by RAFT polymerization, this procedure could be applied to graft new coronas of varied polarity to metallic nanoparticles prepared in polar or nonpolar media, allowing their transfer to any kind of solvent. Moreover, functional groups could also be grafted onto nanoparticles because functional polymers such as poly(N-succin-

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imidyl acrylate) or poly(pentafluorophenyl acrylate) can also be prepared by RAFT polymerization. Acknowledgment. C.-A.F. is Charge´ de Recherches FNRS. M.F., P.G., and J.-F.G. thank the Communaute´ franc¸ aise de Belgique for an Action de Recherches Concerte´es grant (ARC NANOMOL 03/08-300). The ESF Program STIPOMAT is also acknowledged. The work of M.A.R.M. and U.S.S. is part of the research program of the Dutch Polymer Institute (DPI), project no. 360. LA060758H