Water-Soluble Surface-Anchored Gold and Palladium Nanoparticles

Nov 28, 2007 - Stabilized by Exchange of Low Molecular Weight Ligands with ... Russian Academy of Sciences, 28 VaViloV Street, Moscow 119991, Russia...
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Langmuir 2008, 24, 6521-6529

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Water-Soluble Surface-Anchored Gold and Palladium Nanoparticles Stabilized by Exchange of Low Molecular Weight Ligands with Biamphiphilic Triblock Copolymers Tony Azzam,† Lyudmila Bronstein,‡,§ and Adi Eisenberg*,† Department of Chemistry and Centre for Self Assembled Chemical Structures (CSACS), McGill UniVersity, 801 Sherbrooke Street West, Montreal, Quebec H3A 2K6, Canada, Department of Chemistry, Indiana UniVersity, 800 East Kirkwood AVenue, Bloomington, Indiana 47405-7012, and NesmeyanoV Institute of Organoelement Compounds, Russian Academy of Sciences, 28 VaViloV Street, Moscow 119991, Russia ReceiVed NoVember 28, 2007. ReVised Manuscript ReceiVed February 22, 2008 A study is presented of the stabilization of gold and palladium nanoparticles (NPs) via a place-exchange reaction. Au and Pd NPs of ∼3.5 nm were prepared by a conventional method using tetraoctylammonium bromide (TOAB) as the stabilizing agent. The resulting nanoparticles, referred to as Au-TOAB or Pd-TOAB, were later used as templates for the replacement of TOAB ligand with poly(ethylene oxide)-b-polystyrene-b-poly(4-vinylpyridine) (PEO-b-PSb-P4VP) triblock copolymer. This biamphiphilic triblock copolymer was synthesized by atom transfer radical polymerization (ATRP) with control over the molecular weight and polydispersity. The place-exchange reaction was mediated through strong coordination forces between the 4-vinylpyridine copolymer and the metal species located on the surface of the nanoparticles. In addition, the displacement of the outgoing low molecular weight TOAB ligands by high molecular weight polymers is an entropy-assisted process and is believed to contribute to stabilization. The prepared complex, polymer-NP, exhibits greatly improved stability over the metal-NP complex in common organic solvents for the triblock copolymer. Self-assembly in water after ligand exchange resulted in micellar structures of about ∼20 nm (electron microscopy) with the metal NP found located on the surface of the micelles. The stability of the nanoparticles in water was shown to depend greatly on the grafting density of the copolymer, with high stability (more than 6 months) at high grafting density and low stability, accompanied with irreversible agglomeration, at relatively low grafting densities. The surprising location of the metal NP (for both Au and Pd) on the surface of the micelles in water is explained by the fact that, upon self-assembly in THF/water system, the most hydrophobic chains (i.e., PS) undergo self-assembly first at low water content forming the core, followed by the P4VP (whether or not associated with the metal) forming a shell, and finally the PEO forming the corona. In lower metal content assemblies, the P4VP chains located in the shell undergo swelling in an acidic medium causing a substantial increase in micellar corona size, as confirmed by dynamic light scattering measurements. The present study offers a simple approach for the stabilization of various metal nanoparticles of catalytic interest, using a unique polymeric support that can be dispersed in organic solvents as well as aqueous solutions.

Introduction Gold nanoparticles (Au NPs) have been the subject of increasing attention for the past decade, particularly since Brust et al. introduced a simple preparation of Au NPs stabilized with various thiols.1 In this preparation, Au NPs of 1-3 nm diameter were prepared by the reduction of AuCl4- in a two-phase system (water/toluene) in the presence of an alkanethiol. Although the gold core can interact with different groups (amines, phosphines, thiols, etc.), thiols provide a stronger interaction with and coverage of the gold surface and are more commonly used in stabilization/ functionalization of Au NPs.2 These gold clusters are characterized both by the properties of the ligands which form the monolayer and by the properties of the metal cluster core.3 When the size of the gold cluster is larger than ca. 2.5-3 nm, it presents a * Corresponding author: Tel: (+1)-514-3986934. Fax: (+1)-5143983797. E-mail: [email protected]. † McGill University. ‡ Indiana University. § Russian Academy of Sciences. (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, (7), 801–802. (2) Pasquato, L.; Pengo, P.; Scrimin, P. J. Mater. Chem. 2004, 14(24), 3481– 3487. (3) Daniel, M.-C.; Astruc, D. Chem. ReV. (Washington, DC, US) 2004, 104(1), 293–346. (4) Lucarini, M.; Franchi, P.; Pedulli, G. F.; Pengo, P.; Scrimin, P.; Pasquato, L. J. Am. Chem. Soc. 2004, 126(30), 9326–9329.

characteristic surface plasmon band with a maximum at 510-530 nm.4 Au NPs are of special interest due to their potential applications in catalysis,5 biosensors,6 biolabeling,7 biomedical,8 electronics,9 and optics.10 In biomedical applications, Au NPs are often used to enhance the sensitivity of diagnostic assays11 in radiotherapy12 as well as in drug and gene delivery.13 In biolabeling, gold nanoparticles are used as an alternative for fluorophores in cellular internalization studies, particularly due to their high contrast which can be visualized with nanometer resolution by electron microscopy.14–16 (5) Haruta, M. Encycl. Nanosci. Nanotechnol. 2004, 1, 655–664. (6) Haes, A. J.; Stuart, D. A.; Nie, S.; Van Duyne, R. P. J. Fluoresc. 2004, 14(4), 355–367. (7) Selvakannan, P. R.; Mandal, S.; Phadtare, S.; Pasricha, R.; Sastry, M. Langmuir 2003, 19(8), 3545–3549. (8) Hu, M.; Chen, J.; Li, Z.-Y.; Au, L.; Hartland, G. V.; Li, X.; Marquez, M.; Xia, Y. Chem. Soc. ReV. 2006, 35(11), 1084–1094. (9) Schmid, G.; Corain, B. Eur. J. Inorg. Chem. 2003, (17), 3081–3098. (10) Norman, T. J., Jr.; Grant, C. D.; Zhang, J. Z. Nanoparticle Assemblies and Superstructures 2006, 193–206. (11) Goodman, C. M.; McCusker, C. D.; Yilmaz, T.; Rotello, V. M. Bioconjugate Chem. 2004, 15(4), 897–900. (12) Hainfeld, J. F.; Slatkin, D.; Smilowitz, H. M. Phys. Med. Biol. 2004, 49(18), N309-N315. (13) Thomas, M.; Klibanov Alexander, M. Proc. Natl. Acad. Sci. U.S.A. 2003, 100(16), 9138–43. (14) Shukla, R.; Bansal, V.; Chaudhary, M.; Basu, A.; Bhonde, R. R.; Sastry, M. Langmuir 2005, 21(23), 10644–10654.

10.1021/la703719f CCC: $40.75  2008 American Chemical Society Published on Web 05/17/2008

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In recent years, catalysis employing metal nanoparticles has become a subject of increasing attention due, in major part, to the enhanced activity and selectivity of nanostructured catalysts.17 It is believed that small metal nanoparticles dispersed in an inert polymer matrix which stabilizes the nanoparticles showed improved catalytic activity as the external metal atoms of the NP can be considered as coordinatively unsaturated species. On the other hand, the high efficiencies of the nanodispersion allow one to provide the polymer matrix with unique optical properties without altering the thermoplastic behavior of the matrix.18 However, when catalytic nanoparticles are formed on the surface of an inorganic or carbon substrate, the nanoparticle size and morphology cannot be precisely regulated.19 The concept of a “nanoreactor” (restricted environment) for controlling the growth and stabilization of nanoparticles has been be quite successful, as numerous researchers have been pursuing it for a number of years.20–23 Depending on the requirements for the particular system, the nanoreactor could be developed either in solution or in the solid state. The idea of nanoreactors in solutions was first explored when nanoparticles were synthesized in surfactant micelles in organic media.24 By use of block copolymer micelles with a functionalized micelle core, better controlled and stabilized nanoparticles can be obtained.20,21,25 This permits a rich variation of nanoparticle morphology with good control over particle size coupled with the positioning of the nanoparticles in the filmforming polymers.26 Cross-linking of either the micelle cores27 or the coronas28 leads to the preservation of the intact micelles after metal compound uptake and yields a narrower particle size distribution. The place-exchange reactions of various thiols described by Murray and co-workers offer an easy approach in multifunctionalization of Au NPs under relatively mild conditions.29 However, this process requires the use of large excess of incoming thiol which, in part, leads to mixed monolayers.30,31 In addition, thiol ligands are known to poison the catalytic activity of metal NPs.32 In a similar approach, place-exchange reaction was successfully applied in the functionalization of Au NPs starting (15) Soo, P. L.; Sidorov, S. N.; Mui, J.; Bronstein, L. M.; Vali, H.; Eisenberg, A.; Maysinger, D. Langmuir 2007, 23(9), 4830–4836. (16) Azzam, T.; Eisenberg, A. Langmuir 2007, 23(4), 2126–2132. (17) Brayner, R.; Viau, G.; Bozon-Verduraz, F. J. Mol. Catal. A 2002, 182-183, 227–238. (18) Ciardelli, F.; Pertici, P.; Vitulli, G.; Giaiacopi, S.; Ruggeri, G.; Pucci, A. Macromol. Symp. 2006, 231(Polymers for Africa), 125–133. (19) Horvath, A.; Beck, A.; Sarkany, A.; Guczi, L. Solid State Ionics 2002, 148, 219–225. (20) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. AdV. Mater. 1995, 7, 1000–1005. (21) Moffitt, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 7, 1185–1192. (22) Zhao, M.; Crooks, R. M. AdV. Mater. 1999, 11, 217–220. (23) Bronstein, L. M. Top. Curr. Chem. 2003, 226, 55–89. (24) Tanori, J.; Duxin, N.; Petit, C.; Lisiecki, I.; Veillet, P.; Pileni, M. P. Colloid Polym. Sci. 1995, 273, 886. (25) Spatz, J. P.; Roescher, A.; Mo¨ller, M. AdV. Mater. 1996, 8, 337–340. (26) Corbierre, M. K.; Cameron, N. S.; Sutton, M.; Mochrie, S. G. J.; Lurio, L. B.; Ruehm, A.; Lennox, R. B. J. Am. Chem. Soc. 2001, 123(42), 10411–10412. (27) Bronstein, L.; Antonietti, M.; Valetsky, P. Metal colloids in block copolymer micelles: formation and material properties. In Nanoparticles and Nanostructured Films; Fendler, J. H., Ed.; Wiley-VCH Verlag: Weinheim, 1998; pp 145-171. (28) Cao, L.; Manners, I.; Winnik, M. A. Macromolecules 2001, 34, 3353– 3360. (29) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33(1), 27–36. (30) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15(11), 3782–3789. (31) Montalti, M.; Prodi, L.; Zaccheroni, N.; Baxter, R.; Teobaldi, G.; Zerbetto, F. Langmuir 2003, 19(12), 5172–5174. (32) Cornils, B.; Herrmann, W. A.; Schloegl, R.; Wong, C.-H. Catalysis From A to Z: A Concise Encyclopedia; Wiley-VCH: Weinheim, 2000.

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from labile capping agents such as citrate33,34 and triphenylphosphine.35,36 Such capping agents can be displaced easily by thiols with no residual contamination from citrate or phosphines. The place-exchange reaction was also shown to be an effective strategy in replacing hindered quaternary ammonium stabilizers with lowmolecular weight amines.37,38 In these studies, Au NPs were prestabilized with tetraoctylammonium bromide (TOAB) or triphenylphosphine in organic solvents following the classical Brust-Schiffirin method.39 Next, the organic phase which contains the Au NPs was mixed with an aqueous solution containing an excess of aliphatic/aromatic amines, leading to full and rapid transfer of the Au NP to the water phase via a ligand-exchange reaction.37,40 Micelles of coordinating diblock copolymer such as polystyreneb-poly(4-vinylpyridine) (PS-b-P4VP) have been successfully applied in the stabilization of many colloidal metal nanoparticles.41–43 The size, size distribution, and shape (in several cases) of metal colloids were found to be determined by the rates of nucleation and growth of nanoparticles which, in turn, are controlled by the type of reducing agent, metal compound loading, and characteristics of block copolymer micelles. In these systems, the hydrophobic block forms the corona, which provides stabilization, while the polar block forms the core in which the metal species are incorporated due to coordination with vinylpyridine units.44–46 In addition, triblock copolymers of various architectures were used in the stabilization of Au NPs. Examples of such copolymers are poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide),47 poly(ethylene oxide)-b-poly(4-vinylpyridine)b-poly(N-isopropylacrylamide),48 poly(2-(dimethylamino)ethyl methacrylate)-b-poly(methyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate),49 polystyrene-b-polyisoprene-bpolystyrene,50 poly(ethylene oxide)-b-poly(2-hydroxyethyl methacrylate)-b-poly(2-(diethylamino)ethyl methacrylate),51 among others.52–54 In all these systems, the gold salt (normally AuCl4or AuCl3) is allowed to coordinate with the block copolymers (33) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55–75. (34) Levy, R.; Thanh, N. T. K.; Doty, R. C.; Hussain, I.; Nichols, R. J.; Schiffrin, D. J.; Brust, M.; Fernig, D. G. J. Am. Chem. Soc. 2004, 126(32), 10076–10084. (35) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121(4), 882–883. (36) Petroski, J.; Chou, M. H.; Creutz, C. Inorg. Chem. 2004, 43(5), 1597– 1599. (37) Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40(16), 3001– 3004. (38) Gandubert, V. J.; Lennox, R. B. Langmuir 2005, 21(14), 6532–6539. (39) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, (16), 1655–1656. (40) Mayya, K. S.; Caruso, F. Langmuir 2003, 19(17), 6987–6993. (41) Bronstein, L.; Chernyshov, D.; Valetsky, P.; Tkachenko, N.; Lemmetyinen, H.; Hartmann, J.; Foerster, S. Langmuir 1999, 15(1), 83–91. (42) Bronstein, L. M.; Chernyshov, D. M.; Volkov, I. O.; Ezernitskaya, M. G.; Valetsky, P. M.; Matveeva, V. G.; Sulman, E. M. J. Catal. 2000, 196, 302–314. (43) Sulman, E.; Bronstein, L.; Matveeva, V.; Sulman, M.; Lakina, N.; Doluda, V.; Valetsky, P. Nanocatalysis 2006, 51–98. (44) Agnew, N. H. J. Polym. Sci., Polym. Chem. Ed. 1976, 14(11), 2819–2830. (45) Klingelhoefer, S.; Heitz, W.; Greiner, A.; Oestreich, S.; Foerster, S.; Antonietti, M. J. Am. Chem. Soc. 1997, 119(42), 10116–10120. (46) Sidorov, S. N.; Bronstein, L. M.; Kabachii, Y. A.; Valetsky, P. M.; Soo, P. L.; Maysinger, D.; Eisenberg, A. Langmuir 2004, 20(9), 3543–3550. (47) Bakshi Mandeep, S.; Kaura, A.; Bhandari, P.; Kaur, G.; Torigoe, K.; Esumi, K. J. Nanosci. Nanotechnol. 2006, 6(5), 1405–1410. (48) Zheng, P.; Jiang, X.; Zhang, X.; Zhang, W.; Shi, L. Langmuir 2006, 22(22), 9393–9396. (49) Jewrajka, S. K.; Chatterjee, U. J. Polym. Sci., Part A: Polym. Chem. 2006, 44(6), 1841–1854. (50) Ansari, I. A.; Hamley, I. W. J. Mater. Chem. 2003, 13(10), 2412–2413. (51) Liu, S.; Weaver, J. V. M.; Save, M.; Armes, S. P. Langmuir 2002, 18(22), 8350–8357. (52) Even, M.; Haddleton, D. M.; Kukulj, D. Eur. Polym. J. 2003, 39(4), 633–639. (53) Song, Q.; Ai, X.; Wang, D.; Hong, X.; Wei, L.; Yang, W.; Liu, F.; Bai, Y.; Li, T.; Tang, X. J. Nanopart. Res. 2000, 2(4), 381–385. (54) Zhou, Z.; Yang, Y.-W.; Booth, C.; Chu, B. Macromolecules 1996, 29(26), 8357–8361.

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via metal-ligand bonding or simply by hydrophobic-hydrophobic interactions, followed by reduction of the salt to obtain metal nanoparticles.44,55,56 These loading techniques, in part, lead to broad distribution of the metal nanoparticle size and, in many cases, to low-stability accompanied by irreversible agglomeration. In addition, most of these methods require repetitive purifications which are time-consuming and often lead to low yields. To our knowledge, the use of biamphiphilic triblock copolymers in the stabilization of metal nanoparticles, where the amphiphiles have different properties and functions, has not been reported. We refer to this polymer as biamphiphilic because it can be considered as a chain consisting of the amphiphile PEO-b-PS connected to the amphiphile PS-b-P4VP through the PS segment. Alternatively, the polymer could be considered bihydrophilic triblock copolymer except that the P4VP is not hydrophilic under all conditions. Herein we report on the synthesis and characterization of poly(ethylene oxide)-block-polystyrene-block-poly(4-vinylpyridine) (PEO-b-PS-b-P4VP) triblock copolymer of various molecular weights and its use as capping agent for Au and Pd NP via the place-exchange reaction. The use of P4VPcontaining polymers, in the displacement of the small molecule cationic surfactant TOAB, as in the present study, leads to an increase in stability due to strong coordination forces between the vinylpyridine units and the metal surface. In addition, the replacement of low-molecular-weight ligands with high-molecular-weight polymer chains is an entropy-assisted process and is believed to contribute further to the stabilization of the colloids. Finally, the PEO segment of the triblock copolymer provides steric stabilization of the colloid. This study offers a simple approach for the stabilization of various metal nanoparticles, of catalytic interest, using a unique polymeric support that can be dispersed in both organic solvents and aqueous solutions. Specifically, the aggregates in water consist of a spherical PS core sterically stabilized by the PEO chains, with the metal NP enveloped by the P4VP chains located on the surface of the styrene core. The advantage of this structure over a typical surface absorption of the metal NP is in the fact that the metal NPs are being held tightly to the surface of the styrene core through strong interactions. This interaction between the NP and the PS surface is due to the strong covalent anchoring of the P4VP block to the PS core. Furthermore, the P4VP envelope surrounding the NP on the surface is very thin, thus allowing easy access of the solution to the metal surface.

Experimental Section Materials. Syntheses of PEO macroinitiator and block and triblock copolymers are described in the Supporting Information. Characterization. Transmission electron microscopy (TEM) studies were conducted using a JEOL 2000FX instrument operating at an accelerating voltage of 80 kV. Stabilized metal nanoparticles (Au-TOAB or Pd-TOAB) from a dilute THF solution were deposited on copper grids coated with carbon (Electron Microscopy Science (EMS), Hatfield, PA). Excess solvent was removed by touching the edge of the grids with a small piece of filter paper (Whatman-1). The grids were allowed to dry at ambient temperature for 24 h before measurement. Polymer-gold micelles in dilute aqueous solutions were deposited on copper grids that had been precoated with a thin film of Formvar (Poly(vinylformal), EMS) and then coated with carbon. The grids were also allowed to dry at room temperature overnight. TEM images were analyzed using SigmaScan Pro 4.0 software. The average diameter of the gold nanoparticles was obtained from measurements of at least 300 nanoparticles per (55) Bronstein, L. M.; Sidorov, S. N.; Valetsky, P. M.; Hartmann, J.; Coelfen, H.; Antonietti, M. Langmuir 1999, 15(19), 6256–6262. (56) Astruc, D.; Blais, J.-C.; Daniel, M.-C.; Gatard, S.; Nlate, S.; Ruiz, J. C. R. Chim. 2003, 6(8-10), 1117–1127.

Langmuir, Vol. 24, No. 13, 2008 6523 sample. UV-vis spectra were recorded on a Varian Cary 50 spectrophotometer, between 300 and 800 nm. Dilute solutions of the gold nanoparticles (in organic or aqueous solution) were measured in quartz cuvettes, using pure solvent as a reference. Dynamic light scattering (DLS) measurements were performed on a Brookhaven photon correlation spectrometer with a BI9000 AT digital correlator. The instrument is equipped with a compass 315M150 laser (Coherent Technologies), which was used at a wavelength of 532 nm. Dust-free vials were used for the aqueous solutions, and measurements were made at 22 °C at a scattering angle of 90°. Average particle sizes were obtained from a Gaussian fit of the CONTIN analysis mode from five averaged measurements of the dilute dispersion. With this technique, the probabilities for a series of relaxation decays (Γ) can be obtained. Assuming the measured particles are spherical, the effective diameter of a particle can be calculated, and a probability plot can be made from these data for a certain particle size.57 Preparation of Au-TOAB and Pd-TOAB Nanoparticles. AuTOAB and Pd-TOAB NP were prepared according to the literature.37 Briefly, 30 mM aqueous solution of either HAuCl4 or Na2PdCl4 (30 mL) was added to a 25 mM solution of TOAB in toluene (80 mL). The organic phase, which now contains the metal salt, was separated and added in one portion under stirring to a freshly prepared aqueous solution of NaBH4 (0.4 M, 25 mL). After 60 min the two phases were separated, and the organic phase was subsequently washed with 0.1 M H2SO4, 0.1 M NaOH, and water (three times each). Finally, the organic phase was dried over anhydrous MgSO4, filtered (Whatman-1), and stored at 4 °C under a nitrogen atmosphere. A solution of the NP in THF was prepared by evaporating the toluene under reduced pressure and resuspending it in THF. Typical Place-Exchange Reaction of TOAB with PEO-b-PSb-P4VP in Au NP. A freshly prepared solution of PEO45-b-PS75b-P4VP70 triblock copolymer (TB) in THF (1 wt %) was stirred overnight at room temperature to dissolve the copolymer. To this, under continuous stirring, a calculated amount of Au-TOAB solution in toluene (1 wt %) was added dropwise and the mixture was allowed to stir at room temperature for 24 h. The weight ratio of the TB to Au-TOAB was varied between 1/5 and 20/1 w/w (TB/Au-TOAB). For purification, the mixture was transferred to a PVDF dialysis membrane (500 kDa MWCO, Spectra/Por) and dialyzed against THF for 3 days. The exterior THF was replaced at least twice a day and was collected and evaporated in a separate flask for further analysis. Finally, the pure TB/Au-TOAB NP solution in THF was transferred to clean and dried vials and solvent removed under a stream of nitrogen to give a uniform film. The film was redissolved in benzene and freeze-dried overnight to give a deep violet solid (high Au content) or light pink powder (low Au content). All polymer/ Au NP systems in the powder form were stored at -20 °C under nitrogen until use. TB/Pd-TOAB NP of various weight ratios were prepared using a similar protocol as that for TB/Au-TOAB NPs. Preparation of Micelles in Water. A solution of the purified TB/Au-TOAB in THF (1 wt %) was prepared and allowed to stir overnight at room temperature. Milli-Q water (acidified to pH ∼3) was added dropwise (25 µL/min) to 1 mL of this solution, under continuous stirring, until a final water content of 90 wt % was achieved. The aggregate solution was then dialyzed against Milli-Q (pH ∼3) for 2 days and stored at room temperature protected from light until further analysis. A similar protocol was applied for the formation of TB/Pd-TOAB micelles.

Results and Discussion Synthesis of Triblock Copolymer. The PEO-b-PS-b-P4VP triblock copolymer was prepared by atom transfer radical polymerization (ATRP) as depicted in Figure 1A. Briefly, poly(ethylene oxide) monomethyl ether (PEO) terminated with a halogen atom (PEO-Cl) was prepared by the reaction of PEO with R-chlorophenyl acetyl chloride in refluxing toluene. Next, (57) Zackrisson, M.; Stradner, A.; Schurtenberger, P.; Bergenholtz, J. Langmuir 2005, 21(23), 10835–10845.

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Figure 1. (A) Synthesis of PEO-b-PS-b-P4VP triblock copolymer. (B) SEC traces of PEO45-b-PS110-Cl diblock and PEO45-b-PS107-b-P4VP56 triblock copolymers.

PEO-b-PS-Cl was obtained by the polymerization of styrene in bulk using PEO-Cl as the macroinitiator and CuCl/bpy complex as the catalyst. Finally, the PEO-b-PS-b-P4VP triblock copolymer was obtained by the polymerization of 4-vinylpyridine in DMF using PEO-b-PS-Cl as the macroinitiator and CuCl/Me6TREN as the catalyst. Typical SEC traces of PEO-b-PS-b-P4VP triblock vs the starting PEO-b-PS-Cl macroinitiator diblock copolymer are shown in Figure 1B. The shift of the diblock chromatogram toward lower elution volume provides evidence for the formation of the triblock copolymer. The number average molecular weights (Mn) and polydispersity indexes (PDI) of the copolymers were calculated to be 13600 (PDI ) 1.17) and 20400 (PDI ) 1.26) for the diblock and triblock copolymers, respectively (B3 and T3, Table S1 in the Supporting Information). The degree of polymerization (DP) values of the copolymers were estimated by 1H NMR (based on the DP ) 45 of the PEO chain) and found to be PEO45-b-PS110-Cl for the diblock macroinitiator and PEO45b-PS107-b-P4VP56 for the triblock copolymer (Figures S2 and S3 in the Supporting Information). Triblock copolymers of lower Mn (T1 and T2, Table S1 in the Supporting Information) were prepared in a similar fashion starting from their corresponding diblock copolymers with shorter PS blocks (B1 and B2, Table S1 in the Supporting Information). The short diblock copolymers were prepared under similar conditions to B3 applying shorter reaction times (1 h for B1 and 2 h for B2, Table S1 in the Supporting Information). The slight differences in the DP of the PS blocks between the diblock and the triblock copolymers are probably due to chain fractionation during the purification step. It is worth noting, however, that the above DP values are only an estimate and are based on the assumption that the DP of PEO in the diblock and triblock copolymers remains unchanged. Synthesis of Au/Pd NP and a Ligand Exchange Reaction. For the stabilization of Au/Pd NP using the above triblock copolymers, Au or Pd nanoparticles were initially prepared under controlled environments in a two-phase system using TOAB as the phase-transfer catalyst (PTC).39 HAuCl4 (or Na2PdCl4) was initially dissolved in water and mixed with toluene containing TOAB in 2-fold molar excess. This procedure leads to a complete transfer of the AuCl4-/PdCl42- ions to the organic phase where it undergoes reduction in the presence of TOAB-BH4-. Figure 2A shows a typical TEM image and size distribution histogram

of the Au-TOAB NPs prepared from a dilute THF solution (1 mg/mL). The average diameter of the Au-TOAB was calculated to be 3.6 ( 1.4 nm based on statistics accumulated from 400 nanoparticles. A similar diameter of 3.5 ( 1.5 nm (n ) 300) was calculated for Pd-TOAB (Figure S4 in the Supporting Information). Au-TOAB and Pd-TOAB solutions in toluene (∼10 mg/ mL) were found to be stable for several weeks when kept at 4 °C. However, low stability accompanied by irreversible precipitation was observed within a few days when samples were left at room temperature (data not shown). Prior to the stabilization of the Au/Pd NP with the copolymer, the self-assembly of the PEO45-b-PS75-b-P4VP70 triblock copolymer in THF/water was investigated in the absence of metal. Water was added dropwise under stirring to a dilute copolymer solution in THF to induce self-assembly (Experimental Section). The rate of water addition was maintained at 0.5 wt % per minute to minimize precipitation and to allow equilibrium to be reached between additions. Finally, after reaching 90 wt % water content, the residual organic solvent was removed by successive dialysis against water. The TEM image obtained from PEO45-b-PS75b-P4VP70 triblock copolymer revealed micelle-like structures, as shown in Figure 2B. The average size of these micelles was calculated to be 18 ( 2 nm based on statistics accumulated from 300 micelles (histogram in Figure 2B). Spherical-shaped structures were the predominant morphology seen; however, pentagonal and hexagonal structures were occasionally observed. The average diameter from DLS was calculated to be 104 ( 10. The reason for the large difference in average size diameter between TEM and DLS is most likely due to existence of a swollen P4VP/PEO corona and will be discussed later in more detail. A typical ligand-exchange experiment was conducted by mixing a solution of the triblock copolymer in THF with a predetermined amount of Au-TOAB in toluene. It was reported in the literature that upon addition of an aqueous solution containing 4-(dimethylamino)pyridine (DMAP) to Au-TOAB in toluene, a phase transfer of the Au NP was initiated instantaneously with transfer across the phase boundary completed within 1 h (no stirring or agitation was required).37,40 On the basis of this information, an agitation time of 24 h was assumed to be sufficient for complete ligand exchange between the

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Figure 2. Typical TEM images and size distribution histograms of Au-TOAB in THF (A) and micelles in water made from the PEO45-b-PS75-b-P4VP70 triblock copolymer (B) (Experimental Section).

incoming PEO-b-PS-b-P4VP triblock copolymer and the outgoing TOAB. In addition, the ligand-exchange reaction between incoming high molecular weight polymers and outgoing low molecular weight molecules is an entropy-assisted process, which contributes to the greater stability of the polymer/Au complex. The complex obtained by the above procedure was purified from the outgoing TOAB by dialysis against THF using PVDF dialysis membranes (see Experimental Section). 1H NMR analysis on the exterior THF dialysate (after removal of the solvent) suggests, from the integrated peak areas, that only a negligible amount of the polymer had managed to escape through the PVDF membrane during the purification. Indeed, most of the integrated peak area corresponded to the TOAB molecules as depicted in Figure S5 of the Supporting Information. Assessment of TOAB content in the complexes after ligand displacement was impossible to achieve by NMR, due in part to low S/N (signal-to-noise) of the TOAB chemical shifts. It is assumed, based on this observation, that the content of TOAB is indeed negligible. TGA of the complexes (Figure S6 (B) in the Supporting Information) suggest that complexes grafted with low amounts of the copolymer, as in TB/Au (1/5), are indeed “contaminated” with TOAB, as clearly seen from its TGA profile. However, complexes grafted with relatively large amounts of copolymers, as in TB/ Au (1/2) to TB/Au (20/1), showed nearly similar TGA profiles to the copolymer control without the metal, which prove that TOAB content is below the limit of detection by this technique. TEM images of polymeric micelles in water with different amounts of Au NP are shown in Figure 3. Each individual preparation is referred to as TB/Au-TOAB (x/y) w/w, where x refers to the starting relative weight of the PEO45-b-PS75-b-P4VP70 triblock copolymer (TB) and y refers to the relative weight of the Au-TOAB. x/y thus refers to the weight ratio of TB (x) to

Au-TOAB (y). It should be noted, however, that the TOAB content after ligand-exchange and purification (i.e., removal of the liberated TOAB by dialysis) is probably negligible in the Au/ polymer complex, as judged by NMR. Figure 3A shows a typical TEM image of the micelles obtained from TB/Au-TOAB (1/2) w/w. The average diameter of the micelles determined by TEM was found to be 16 ( 3 nm (n ) 200) compared with DLS which gave an average size of 42 ( 5 nm. The incorporation efficiency in this particular preparation was shown to be as high as ∼80%, with the vast majority of micelles loaded with three to six Au NPs per micelle. The incorporation efficiency was determined from statistics accumulated from an average of at least 10 TEM images. The incorporation efficiency is defined as the number of Au-loaded micelles (at least one Au NP per micelle) divided by the total number of micelles. It is worth noting that this number, expressed as a percentage, is an approximation and is given only to indicate the relative Au content to starting feed weight ratio. Thermogravimetric analysis (TGA) of the TB/Au-TOAB (1/2) preparation showed an average Au content of 30 wt % (TGA of all preparations are given in Figure S6 of the Supporting Information). The TEM image of the micelles made from TB/Au-TOAB (1/1) w/w is shown in Figure 3B. The average diameters measured by TEM and DLS were 15 ( 3 and 60 ( 5 nm, respectively. The incorporation efficiency was estimated to be 50% with the majority of labeled micelles containing two Au NPs per micelle. TGA of this preparation showed a ∼45% decrease in the Au content (down to 17 wt % Au) compared to the previous sample (TB/ Au-TOAB (1/2), which held 30 wt % Au). Micelles prepared from TB/Au-TOAB (2/1) resulted, as expected, in a lower Au content (∼10 wt % Au) and also showed a slightly lower incorporation efficiency (∼45%); however most of the loaded

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Figure 3. Typical TEM images of polymeric micelles in water made from TB/Au-TOAB (1/2) (A), TB/Au-TOAB (2/1) (B), TB/Au-TOAB (5/1) (C), and TB/Au-TOAB (20/1) (D).

micelles in this preparation were found to contain one Au NP per micelle (Figure S7 in the Supporting Information). The average diameters from TEM and DLS were 20 ( 3 and 60 ( 8 nm, respectively. A TEM image of the micelles made from TB/Au-TOAB (5/1) is shown in Figure 3C. It is clearly seen that the incorporation efficiency for these micelles is very low (∼10%) and almost all loaded micelles contain only a single Au NP per micelle. The average diameters are 14 ( 3 and 77 ( 5 nm by TEM and DLS, respectively. As expected, when a large excess of the polymer was introduced, as in TB/Au-TOAB (10/1) and in TB/Au-TOAB (20/1), a low to negligible Au content was obtained (3.2 wt % Au for TB/Au-TOAB (10/1) and 1.5 wt % Au for TB/Au-TOAB (20/1)). Figure 3D shows a typical TEM image of the micelles in water made from TB/Au-TOAB (20/1). The incorporation efficiency in this preparation was very low (