Metal Nanocrystals Incorporated within pH-Responsive Microgel

P.O. Box 1527, 711 10 Heraklion Crete, Greece, Department of Chemistry, ... of Materials Science and Technology, UniVersity of Crete, 710 03 Heraklion...
0 downloads 0 Views 257KB Size
Download PDF
Langmuir 2007, 23, 5761-5768

5761

Metal Nanocrystals Incorporated within pH-Responsive Microgel Particles D. Palioura,†,‡ S. P. Armes,⊥ S. H. Anastasiadis,†,§,# and M. Vamvakaki*,†,| Institute of Electronic Structure and Laser, Foundation for Research and TechnologysHellas, P.O. Box 1527, 711 10 Heraklion Crete, Greece, Department of Chemistry, UniVersity of Crete, 714 09 Heraklion Crete, Greece, Department of Physics, UniVersity of Crete, 710 03 Heraklion Crete, Greece, Department of Materials Science and Technology, UniVersity of Crete, 710 03 Heraklion Crete, Greece, Department of Chemistry, UniVersity of Sheffield, Sheffield S3 7HF, U.K., and Department of Chemical Engineering, Aristotle UniVersity of Thessaloniki, 541 24 Thessaloniki, Greece ReceiVed NoVember 17, 2006. In Final Form: February 15, 2007 Cross-linked sterically stabilized latexes of approximately 250 nm diameter were synthesized by emulsion polymerization of 2-(diethylamino)ethyl methacrylate using a bifunctional oligo(propylene oxide)-based diacrylate cross-linker and a poly(ethylene oxide)-based macromonomer as the stabilizer at pH 9. These particles exhibit reversible swelling properties in water by adjusting the solution pH. At low pH, they exist as swollen microgels as a result of protonation of the tertiary amine units. Deswelling occurs above pH 7 [the effective pKa of poly(2-(diethylamino)ethyl methacrylate)], leading to the formation of the original compact latex particles. The swollen microgels can be used as nanoreactors: efficient impregnation with Pt nanoparticles can be achieved by incorporating precursor platinum compounds, followed by metal reduction. Dynamic light scattering was used to compare two methods of Pt nanoparticle impregnation with respect to the size and stability of the final Pt-loaded microgel particles. In the first method, the H2PtCl6 precursor was added to hydrophobic latex particles at high pH, followed by metal reduction. In the second method, H2PtCl6 was added to hydrophilic swollen microgel particles at low pH, and then this metal salt was reduced in situ using NaBH4 and the pH was raised by the addition of base. Both the Pt salt-loaded (metalated) microgels and the final Pt nanoparticle-loaded microgels had well-defined structures that were independent of the synthesis route. Polymer-metal interactions were investigated by UV-visible absorption spectroscopy, which confirmed that the Pt salt was completely reduced to zero-valent Pt. Transmission electron microscopy and X-ray diffraction studies verified the formation of nanometer-sized Pt nanoparticles within these microgels, which can be used as recoverable colloidal catalyst supports for various organic reactions.

I. Introduction Responsive microgels typically comprise lightly cross-linked latex particles of submicrometer dimensions that can become highly swollen in response to certain external stimuli. These materials have attracted particular attention because of their potential applications in various fields such as drug delivery,1 the development of protein-based vaccines,2 sensor technology,3 bioseparation,4 membrane filtration,5 and catalysis.6 Both thermoresponsive and pH-responsive microgels have been reported in the literature.7 The former are usually based on poly(N-isopropylacrylamide) (PNIPAM), which exhibits a lower * Corresponding author. E-mail: [email protected]. † Foundation for Research and TechnologysHellas. ‡ Department of Chemistry, University of Crete. § Department of Physics, University of Crete. | Department of Materials Science and Technology, University of Crete. ⊥ University of Sheffield. # Aristotle University of Thessaloniki. (1) (a) Bromberg, L.; Temchenko, M.; Hatton, T. A. Langmuir 2002, 18, 4944. (b) Bromberg, L.; Temchenko, M.; Hatton, T. A. Langmuir 2003, 19, 8675. (c) Nolan, C. M.; Serpe, M. J.; Lyon, L. A. Biomacromolecules 2004, 5, 1940. (d) Serpe, M. J.; Yarmey, K. A.; Nolan, C. M.; Lyon, L. A. Biomacromolecules 2005, 6, 408. (e) Lopez, V. C.; Hadgraft, J.; Snowden, M. J. Int. J. Pharm. 2005, 292, 137. (2) (a) Murthy, N.; Xu, M. C.; Schuck, S.; Kunisawa, J.; Shastri, N.; Frechet, J. M. J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 4995. (b) Kwon, Y.J.; James, E.; Shastri, N.; Frechet, J. M. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 18264. (3) Mori, T.; Maeda, M. Langmuir 2004, 20, 313. (4) Elaissari, A.; Ganachaud, F.; Pichot, C. Top. Curr. Chem. 2003, 227, 169. (5) (a) Peng, S.; Wu, C. Macromolecules 2001, 34, 6795. (b) Kraft, M. L.; Moore, J. S. Langmuir 2003, 19, 910. (6) (a) Biffis, A.; Sperotto, E. Langmuir 2003, 19, 9548. (b) Cao, R.; Gu, Z.; Patterson, G. D.; Armitage, B. A. J. Am. Chem. Soc. 2004, 126, 726.

critical solution temperature (LCST) in neutral aqueous solutions of ∼32 °C.8 Above the LCST, the particles exist in their nonsolvated latex form, whereas below the LCST they become hydrophilic and water-swollen microgels are obtained.9 Similar temperature-sensitive microgels based on poly(N-vinylcaprolactam) (LCST ≈ 32 °C) and hydroxypropyl cellulose (LCST ≈ 44 °C) have also been prepared.10-12 pH-responsive microgels usually comprise either acidic or basic monomers, with hydrophobic comonomers incorporated in some cases. For example, Loxley and Vincent synthesized poly(2-vinylpyridine-co-styrene) microgels with a mean diameter of ∼200 nm.13 In another study, Saunders et al. examined the swelling properties of poly(methyl methacrylate-co-methacrylic acid), P(MMA-co-MAA), particles dispersed in water as a function of pH.14 The extent of pH-induced swelling is these systems depended on the proportion of the hydrophobic comonomer. (7) (a) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher S. A. Science 1996, 274, 959. (b) Daly, E.; Saunders, B. R. Langmuir 2000, 16, 5546. (c) Gan, D.; Lyon, L. A. J. Am. Chem. Soc. 2001, 123, 7511. (d) Gan, D.; Lyon, L. A. Macromolecules 2002, 35, 9634. (e) Plunkett, K. N.; Moore, J. S. Langmuir 2004, 20, 6535. (8) (a) Pelton, R. H.; Pelton, H. M.; Morphesis, A.; Rowell, R. L. Langmuir 1989, 5, 816. (b) Wu, C.; Zhou, S. Q.; Auyeung, S. C. F.; Jiang, S. H. Angew. Macromol. Chem. 1996, 240, 123. (c) Gan, D.; Lyon, L. A. J. Am. Chem. Soc. 2001, 123, 8203. (d) Gao, J.; Hu Z. Langmuir 2002, 18, 1360. (9) Wu, C.; Zhou, S. Q. Macromolecules 1997, 30, 574. (10) Gao, Y.; Au-Yeung, S. C. F.; Wu, C. Macromolecules 1999, 32, 3674. (11) Laukkanen, A.; Hietala, S.; Maunu, S. L.; Tenhu H. Macromolecules 2000, 33, 8703-8708. (12) Lu, X.; Hu, Z.; Gao, J. Macromolecules 2000, 33, 8698. (13) Loxley, A.; Vincent, B. Colloid Polym. Sci. 1997, 275, 1108.

10.1021/la063359v CCC: $37.00 © 2007 American Chemical Society Published on Web 04/05/2007

5762 Langmuir, Vol. 23, No. 10, 2007

More complex microgels that exhibit both thermoresponsive and pH-responsive character have been also synthesized by incorporating basic and/or acidic comonomers within PNIPAMbased cross-linked particles.15,16 The swelling behavior of these particles was strongly influenced by the distribution of the pHsensitive comonomer. Finally, polyampholytic thermosensitive PNIPAM nanogels were synthesized by the copolymerization of acrylic acid and 1-vinylimidazole as anionic and cationic monomers, respectively. The pH- and temperature-induced volume collapse of these particles occurred at their isoelectric point and the LCST of PNIPAM, respectively.17 In the above-mentioned studies, the fraction of pH-sensitive moieties within the gel phase was limited to a few percent, resulting in weak pH-responsive behavior. Only recently has the preparation of pH-responsive microgels based solely on pHsensitive monomers been reported.18-21 Their use as recoverable catalyst supports in organic chemistry has been suggested.22 The binding capacity and accessibility of the microgel active sites for metal ions are important properties for such applications.23 During the last two decades, various methods of metal nanoparticle synthesis within soft matter nanostructures have been developed. Pileni et al.24,25 prepared metal nanoparticles within the cores of reverse micelles formed by anionic surfactants in organic media, and Brust et al.26 employed the transfer of metal ions from a polar to a nonpolar micellar phase using a surfactant as the transferring agent. Recently, functional polymeric materials (i.e., diblock copolymers,27 dendrimers,28 and polymer microgels29) have attracted particular attention as nanoscopic reaction vessels for growing inorganic metal nanocrystals. The use of polymers allows the potential control of nanoparticle characteristics and properties by manipulating the polymer molecular structure, size, and composition, whereas it permits the utilization of both organic and aqueous dispersing media. Microgel-metal nanoparticle hybrid materials have been prepared using two general strategies. The first route involves metal loading of the microgels using preformed inorganic particles, whereas (14) Saunders, B. R.; Crowther, H. M.; Vincent, B. Macromolecules 1997, 30, 482. (15) (a) Jones, C. D.; Lyon, L. A. Macromolecules 2000, 33, 8301. (b) Jones, C. D.; Lyon, L. A. Langmuir 2003, 19, 4544. (c) Jones, C. D.; Lyon, L. A. Macromolecules 2003, 36, 1988. (d) Debord, S. B.; Lyon, L. A. J. Phys. Chem. B 2003, 107, 2927. (e) Serpe, M. J.; Jones, C. D.; Lyon, L. A. Langmuir 2003, 19, 8759. (16) Pinkrah, V. T.; Snowden, M. J.; Mitchell, J. C.; Seidel, J.; Chowdhry, B. Z.; Fern, G. R. Langmuir 2003, 19, 585. (17) Ogawa, K.; Nakayama, A.; Kokufuta, E. Langmuir 2003, 19, 3178. (18) Ma, G. H.; Fukutomi, T. J. Appl. Polym. Sci. 1991, 43, 1451. (19) Amalvy, J. I.; Wanless, E. J.; Li, Y.; Michailidou, V.; Armes, S. P.; Duccini, Y. Langmuir 2004, 20, 8992. (20) Dupin, D.; Fujii, S.; Armes, S. P.; Reeve, P.; Baxter, S. M. Langmuir 2006, 22, 3381. (21) Bradley, M.; Vincent, B.; Warren, N.; Eastoe, J.; Vesperinas, A. Langmuir 2006, 22, 101. (22) Dickerson, T. J.; Reed, N. N.; Janda, K. D. Chem. ReV. 2002, 102, 3325. (23) (a) Eichenbaum, G. M.; Kiser, P. F.; Shah, D.; Meuer, W. P.; Needham, D.; Simon, S. A. Macromolecules 2000, 33, 4087. (b) Peng, S.; Wu, C. J. Phys. Chem. B 2001, 105, 2331. (c) Amigoni-Gerbier, S.; Desert, S.; Gulik-Kryswicki, T.; Larpent, C. Macromolecules 2002, 35, 1644. (24) Petit, C.; Pileni M. P. J. Phys. Chem. 1988, 92, 2282. (25) (a) Lisiecki, I.; Pileni, M.-P. Langmuir 2003, 19, 9486. (b) Wikander, K.; Petit, C.; Holmberg, K.; Pileni, M. P. Langmuir 2006, 22, 4863. (26) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 7, 801. (27) (a) Chernyshov, D. M.; Bronstein, L. M.; Borner, H.; Berton, B.; Antonietti, M. Chem. Mater. 2000, 12, 114. (b) Spatz, J. P.; Mossmer, S.; Hartmann, C.; Moller, M.; Herzog, T.; Krieger, M.; Boyen, H.-G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16, 407. (28) (a) Gro¨hn, F.; Bauer, B. J.; Amis, E. J. Polym. Prepr. 2000, 41, 560. (b) Gro¨hn, F.; Bauer, B. J.; Akpalu, Y. A.; Jackson, C. L.; Amis, E. J. Macromolecules 2000, 33, 6042. (29) (a) Biffis, A.; Sperotto, E. Langmuir 2003, 19, 9548. (b) Pich, A.; Bhattacharya, S.; Lu, Y.; Boyko, V.; Adler, H. A. P. Langmuir 2004, 20, 10706. (c) Mei, Y.; Lu, Y.; Polzer, F.; Ballauff, M.; Drechsler, M. Chem. Mater. 2007, 19, 1062.

Palioura et al.

in the second route, the nanoparticles are grown in situ within the microgels. The former method usually suffers from low particle loading and aggregation of the nanoparticles. Moreover, the polymer matrix exerts no control over the growth and size of the nanoparticles.30 To avoid such problems, the synthesis of metal nanoparticles in the presence of the polymeric stabilizer has been explored. Antonietti et al. was the first to report the in situ preparation of noble metal colloids within microgel nanoreactors.31 The effects of the microgel cross-link density and the concentration of functional groups were investigated and related to the fraction of metal-nanoparticle-containing microgels. Subsequent studies involved the metalation of microgels comprising a major fraction of nonfunctional groups in addition to a minor fraction of the metal-binding functional moieties.32-36 However, the relatively low fraction of functional groups that can bind the metal compound reduced the effective metal loading, which was limited to a maximum of 20 wt % relative to the polymer.33,35,37 Moreover, in some cases the presence of a high fraction of hydrophobic comonomers required the use of organic solvents for efficient metal incorporation.34 Recently, Kumacheva et al. have reported using a single class of P(NIPAM-co-AA) microgel template for the in situ production of three different types of nanoparticles (semiconductor, metal, and magnetic), leading to a range of composites with unique properties and high structural hierarchy.38 However, the metal loading did not exceed 20%, whereas a 3-fold increase in microgel diameter was observed. Such an increase in size could lead to diffusion limitations when using these hybrid microgels in catalysis or sensor technology applications. In the present work, we describe the use of pH-responsive microgels based solely on 2-(diethylamino)ethyl methacrylate (DEA) as colloidal templates for the in situ synthesis of Pt nanoparticles at high metal loadings. This polymer was selected because it is soluble in a remarkably wide range of organic solvents, which augurs well for its widespread use as a catalyst support. Dynamic light scattering was used to characterize the microgel swelling that occurred on lowering the solution pH and on complexation with H2PtCl6 and also during the in situ metal reduction. UV-visible absorption spectroscopy was utilized to investigate the interactions in the metalated microgels and determine their overall metal loading. Finally, the size and crystallinity of the Pt nanoparticles formed within the microgels after metal reduction were investigated by transmission electron microscopy and X-ray diffraction. II. Experimental Section Materials. All chemicals used were commercially available and were purchased from Sigma-Aldrich (Germany). The microgel particles were synthesized in their latex form at pH 9 by emulsion copolymerization of DEA at 70 °C using an oligo(propylene glycol) diacrylate cross-linker in the presence of a monomethoxy-capped (30) (a) Xu, X.; Majetich, S. A.; Asher, S. A. J. Am. Chem. Soc. 2002, 124, 13864. (b) Kim, J. H.; Lee, T. R. Chem. Mater. 2004, 16, 3647. (c) Gorelikov, I.; Field, L. M.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 15938. (d) Gong, Y. J.; Gao, M. Y.; Wang, D. Y.; Mohwald, H. Chem. Mater. 2005, 17, 2648. (31) Antonietti, M.; Grohn, F.; Hartmann, J.; Bronstein, L. Angew. Chem., Int. Ed. Engl. 1997, 36, 2080. (32) Martinez-Rubio, M. I.; Ireland, T. G.; Fern, G. R.; Silver, J.; Snowden, M. J. Langmuir 2001, 17, 7145. (33) Zhang, J. G.; Coombs, N.; Kumacheva, E.; Lin, Y. K.; Sargent, E. H. AdV. Mater. 2002, 14, 1756. (34) Biffis, A.; Orlandi, N.; Corain, B. AdV. Mater. 2003, 15, 1551. (35) Pich, A.; Hain, J.; Lu, Y.; Boyko, V.; Prots, Y.; Adler, H. J. Macromolecules 2005, 38, 6610. (36) Suzuki, D.; Kawaguchi, H. Langmuir 2005, 21, 8175. (37) Xu, S.; Zhang, J.; Paquet, C.; Lin, Y.; Kumacheva, E. AdV. Mater. 2003, 13, 468. (38) Zhang, J. G.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 7908.

Metal Nanocrystals in Microgel Particles poly(ethylene glycol) methacrylate stabilizer, as described elsewhere.19 The microgel particles were purified by extensive ultrafiltration, which was used to eliminate excess stabilizer as well as traces of monomer and initiator. Sample Preparation. Aqueous dispersions of PDEA latex were prepared by serial dilution of a 0.92 wt % stock solution to the required concentration using Milli-Q water prefiltered through a 0.20 µm syringe filter. The solution pH was then adjusted to the required value using 0.1 M HCl or 0.1 M NaOH and was stirred for 24 h before measurement to ensure equilibrium swelling of the microgel particles. After being stirred, the solution was filtered through a 1.2 µm pore size filter to eliminate any dust and was allowed to equilibrate for about 1 h before being measured. Metal Incorporation. Two different methods were evaluated for the introduction of the metal compound into the microgel particles. In the first method (method A), the 0.92 wt % aqueous latex suspension was first diluted with water without pH adjustment in order to obtain a 0.05 wt % latex concentration at pH 7.3. After being stirred overnight, the metal precursor (H2PtCl6‚6H2O) was added to this dilute aqueous latex at the required N/Pt molar ratio. The solution was stirred for 24 h, and the excess platinic acid was eliminated by ultrafiltration using a 3 nm filter. Finally, the metal reduction was carried out using NaBH4, and the sample was analyzed by DLS after being stirred for 24 h. In the second method (method B), the initial 0.92 wt % aqueous latex suspension was first diluted to 0.05 wt % by adding the appropriate amount of water at pH 2, as described above. The resulting microgel solution was stirred overnight to ensure complete protonation of the tertiary amine groups, and H2PtCl6‚6H2O was added at the appropriate N/Pt molar ratio. The solution was stirred for 24 h to allow for metal complexation to occur, followed by ultrafiltration. Finally, in situ metal reduction was carried out using NaBH4, and the pH was increased to around 10 with the addition of 0.1 M NaOH. This sample was stirred overnight before measurement. Dynamic Light Scattering (DLS). The autocorrelation function of the polarized light scattering intensity, GVV(q, t) ) 〈I(q, t)I(q, 0)〉/〈I(q, 0)〉2, was measured at different scattering angles, θ, using an ALV spectrophotometer and an ALV-5000 full digital correlator over the time range of 10-7-103 s; I(q, 0) is the mean scattering intensity. Generally, both the incident and the scattered beams were polarized perpendicular to the scattering plane (VV geometry). An Adlas diode-pumped Nd:YAG laser was used as the light source with a wavelength λ of 532 nm and a single-mode intensity of 100 mW. The magnitude of the scattering wavevector is q ) (4πn/λ)sin(θ/2), where n is the refractive index of the medium. Under homodyne conditions, GVV(q, t) is related to the desired scattering field autocorrelation function C(q, t) by C(q, t) ) {[GVV(q, t) - 1]/f*}1/2, where f* is an experimental factor calculated by means of a standard. The experimental correlation functions C(q, t) are analyzed by performing the inverse Laplace transform (ILT) using the routine CONTIN, assuming a superposition of exponentials for the distribution of relaxation times L(ln τ) (i.e., ∞ C(q, t) ) ∫-∞ L(ln τ) exp[-t/τ] d(ln τ). The rate Γ of each process is calculated as the inverse of its relaxation time, 1/τ, whereas the dynamic intensities are calculated from the areas under the peaks (determined by integration) of L(ln τ) multiplied by I(q, 0). To facilitate comparisons of the intensities, they are always shown normalized to the respective intensity of toluene. In the case of a diffusive process, its diffusion coefficient D is obtained from the slope of Γ versus q2 as Γ ) Dq2. The latter is related to the hydrodynamic radius, Rh, of the diffusing species by the StokesEinstein equation, Rh ) kBT/(6πηD), where η is the viscosity of the solvent, kB is the Boltzmann constant, and T is the absolute temperature. (At the low concentrations used in these studies, it is assumed that D corresponds to its limiting value at zero concentration.) All measurements were performed at 20 °C. Transmission Electron Microscopy (TEM). Samples for TEM were prepared following a procedure similar to that described above for the DLS experiments. All samples were previously diluted with water adjusted to the appropriate pH. A drop of the diluted sample was then placed on a carbon-coated copper grid and dried in air

Langmuir, Vol. 23, No. 10, 2007 5763

Figure 1. Intensity autocorrelation functions of a 0.05 wt % PDEA microgel solution at a 45° scattering angle for pH 2.1 (0) and pH 9.0 (O). (Inset) Distributions of relaxation times multiplied by the total scattering intensity (normalized to that of toluene) for the respective pH values. overnight. A JEOL JEM-100C instrument at an electron accelerating voltage of 80 kV was employed for the measurements. UV-Visible Absorption Spectroscopy. All solutions were prepared as described above. Dilutions to the required concentration were performed using water titrated to the pH of the sample, and the microgel solutions were placed in a 1 cm quartz cell. The platinum contents of the microgels were determined for the two different metalation methods by recording UV-visible spectra from 190 to 1100 nm (Perkin-Elmer Lambda 45 spectrometer) of the filtrate solutions, (obtained after ultrafiltration of the platinic acid-containing microgels and dilution to an appropriate concentration). A calibration curve was constructed using nine H2PtCl6 solutions of predetermined concentration ((0-2.075) × 10-3 wt %). X-ray Diffraction (XRD) Studies. Specimens for XRD were prepared as described above for the DLS measurements. The reduced samples were purified by ultrafiltration to eliminate excess NaBH4 and the NaCl salt that was formed and were dried under vacuum at room temperature. A Rigaku RINT 2000 series wide-angle diffractometer was used. Measurements were made from 2θ ) 2 to 110° in steps of 2θ ) 0.006° using the following operating conditions: I ) 40 mA and V ) 178 kV. Thermogravimetric Analysis (TGA). Dried microgels were prepared as described above for the XRD measurements. TGA was performed on 5-10 mg samples using a Perkin-Elmer Diamond Pyris model at a scan rate of 10 °C/min under nitrogen. The metal nanoparticle loadings of the microgels were determined by the measured weight loss over the 150-500 °C range due to the pyrolysis of the polymer.

III. Results and Discussion Swelling Behavior of the Microgel Particles as a Function of Solution pH. The swelling behavior of the pH-responsive PDEA microgel particles in water was investigated first. The change in the hydrodynamic diameter of the microgel particles on adjusting the solution pH was followed by dynamic light scattering. Figure 1 shows the autocorrelation functions for a 0.05 wt % aqueous dispersion of PDEA particles at a scattering angle θ of 45ο at pH 2.1 and 9, which are below and above the effective pKa of around 7 for PDEA,39 respectively. The inset shows the distribution of relaxation times from the inverse Laplace transformations of the correlation functions. A single process with very strong intensity and diffusive dynamics dominates the autocorrelation functions at pH 9. From the distribution of relaxation times (inset), the diffusion coefficient D ) lim(Γ/q2) for this process is 1.9 × 10-8 cm2/s, and the qf0 (39) Lee, A. S.; Gast, A. P.; Butun, V.; Armes, S. P. Macromolecules 1999, 32, 4302.

5764 Langmuir, Vol. 23, No. 10, 2007

Palioura et al.

Figure 2. Schematic representation of the acid-induced latex-tomicrogel transition observed for the PDEA latex particles. This transition is reversible on addition of base.

corresponding hydrodynamic radius, Rh, is 112 ( 2 nm.40 Moreover, the intensity of this process has a strong q dependence that, when fitted with the Guinier expression I(q) ) I(0) exp(-q2Rg2/3), results in a radius of gyration of 89 ( 4 nm. This results in an Rg/Rh ratio of 0.79 ( 0.04, which signifies a sphere of uniform density for which a value of 0.774 is predicted.41 At pH 9, the DEA repeat units are deprotonated and thus hydrophobic, which results in the formation of compact, strongly scattering uniform PDEA latex particles. In contrast, a substantial reduction in the scattering intensity is observed at pH 2.1, which is accompanied by an increase in the hydrodynamic radius to Rh ) 170 ( 2 nm (D ) 1.3 × 10-8 cm2/s). The latter process possesses a q-dependent intensity that, when fitted with the Guinier expression, results in a radius of gyration of 98 ( 4 nm. Thus, at pH 2.1 Rg/Rh ) 0.58 ( 0.03, which is less than the value for uniform spheres. This signifies particles with a periphery having a lower density than the center region,42 which is due to the swelling of the microgels.43 From the above results, the volumetric swelling factor, calculated as the cube of the ratio of the microgel diameter at pH 2 to the latex diameter at pH 9, was found to be approximately 3.5. This particle swelling is due to protonation of the tertiary amine groups below their pKa and allows the ingress of water within the microgels, resulting in a large reduction in the refractive index of the particles and much lower scattering intensities. This latex-to-microgel transition, which is schematically illustrated in Figure 2, is fully reversible: the original nonswollen PDEA latex particles are reformed above pH 7 on addition of a sufficient amount of NaOH. Visual inspection confirmed that this acid-induced swelling process is accompanied by a change in turbidity from a milkywhite latex in alkaline media to a clear, transparent microgel solution at low pH. Because of the relatively high latex turbidity at high pH, all DLS measurements were performed at very low particle concentration (typically 0.05 wt %) in order to achieve light transmission greater than 90% during the DLS experiments. Metalated Microgel Particles. Two methods for the formation of Pt nanoparticles within these PDEA particles were evaluated. Figure 3 shows a schematic representation of these two protocols. The first method involves the incorporation of a metal salt precursor, H2PtCl6, into the hydrophobic latex particles at high pH, followed by in situ metal reduction (Figure 3a). In contrast, (40) The relaxation rates in this and all other systems investigated exhibit a q2 dependence, which signifies Fickian diffusion of a Brownian particle in a noninteracting system. The number density of the microgel particles is very low, thus the solution is very dilute and the particles can be considered to be noninteracting. (41) Burchard, W.; Schmidt, M.; Stockmayer, W. H. Macromolecules 1980, 13, 1265. (42) Wu, C.; Zhou, S. Macromolecules 1995, 28, 8381. (43) It is noted that the large size of the microgel particles and, thus, their high scattering intensities mask all contributions to the correlation functions as a result of the so-called polyelectrolyte effects in charged systems. Actually, when the scattering from charged diblock copolymers is investigated (in the homogeneous dilute-solution regime), these effects complicate the correlation functions, which then exhibit slow modes.44

Figure 3. Schematic representation of the two synthetic protocols adopted for metal nanoparticle formation within the PDEA microgel: (a) method A at pH 7 and (b) method B at pH 2. The illustrations are based on the DLS results described in the text. Table 1. Hydrodynamic Radii (Rh), Radii of Gyration (Rg), and the Rg/Rh Ratio for the Two Methods of Nanoparticle Synthesis methoda

sample description

A A

nonsolvated latex pH 7.3 H2PtCl6-loaded microgel, pH 4.7b Pt nanoparticle-loaded latex, pH 7.2b microgel at pH 2.1 H2PtCl6-loaded microgel, pH 2.0b Pt nanoparticle-loaded latex, pH 8.7b

A B B B

Rh (nm)c Rg (nm)c

Rg/Rh

111 ( 2 90 ( 4 0.81 ( 0.04 124 ( 2 97 ( 4 0.79 ( 0.04 113 ( 2 91 ( 4 0.80 ( 0.04 170 ( 2 98 ( 4 0.58 ( 0.03 123 ( 2 95 ( 4 0.77 ( 0.04 113 ( 2 90 ( 4 0.80 ( 0.04

a See Experimental Section. b At a 3:1 N/Pt molar ratio. c Radii determined by DLS.

the second method involves the metalation of the hydrophilic microgel particles at low pH, followed by in situ reduction using NaBH4 and then raising the solution pH by the addition of NaOH (Figure 3b). The schematic illustration in Figure 3 is based on the DLS results discussed below. Dynamic Light Scattering Studies. DLS was used to examine the behavior of the microgel at every step of the metalincorporation procedure. In the first method (method A), the as-synthesized PDEA latex at pH 7.3 was diluted to 0.05 wt %, and H2PtCl6‚6H2O was added in a 3:1 N/Pt molar ratio, which lowered the pH to 4.7. This acidic solution was stirred for 24 h in order to reach equilibrium. Finally, metal reduction took place on addition of excess NaBH4, and the pH increased to 7.2, which is similar to the solution pH of the original microgel particles before the addition of H2PtCl6 (Figure 3a). The above protocol was monitored by DLS after both metal incorporation and metal reduction. For the H2PtCl6-loaded microgel, a 10-fold dilution to 0.005 wt % concentration was conducted prior to DLS studies to avoid a significant light absorbance due to the yellow color of the solution. The NaBH4-treated microgel was further diluted to 0.001 wt % before being analyzed because its light absorbance at 0.005 wt % was judged to be too high. Nevertheless, some absorbance (∼10%) was observed even at this low concentration, but this did not affect the DLS measurements significantly. Table 1 summarizes the hydrodynamic radii (Rh), radii of gyration (Rg), and Rg/Rh ratios determined by DLS for microgels prior to metal incorporation, after the addition of H2PtCl6 and following metal reduction using the two synthesis protocols. These light-scattering data are discussed in detail below. Figure 4 shows the inverse Laplace transformations at a scattering angle of 30° for a microgel at 0.05 wt % and pH 7.3,

Metal Nanocrystals in Microgel Particles

Figure 4. Distribution of relaxation times obtained from the inverse Laplace transform of the experimental intensity autocorrelation functions of a dilute aqueous solution of lightly cross-linked PDEA particles at a 30° scattering angle multiplied by the total scattering intensity (normalized to that of toluene): as a nonsolvated latex at 0.05 wt % and pH 7.3 (0), as a H2PtCl6-loaded microgel at 0.005 wt % and pH 4.7 (O), and for the Pt nanoparticle-loaded latex at 0.001 wt % and pH 7.2 after in situ reduction using NaBH4 (4) at a 3:1 N/Pt molar ratio.

the H2PtCl6-loaded microgel at 0.005 wt % and pH 4.7, and the Pt-loaded microgel at 0.001 wt % and pH 7.2. In all cases, the scattering intensity is normalized over the concentration. From these distributions of relaxation time, the respective hydrodynamic sizes were calculated. For the microgel at 0.05 wt % and pH 7.3, a single process dominates the autocorrelation functions. This process has a diffusion coefficient of 1.9 × 10-8 cm2/s, which corresponds to a hydrodynamic radius Rh of 111 ( 2 nm, and is attributed to the nonswollen latex particles, as discussed above. Moreover, this process has a q-dependent intensity that, when fitted with the Guinier expression, results in a radius of gyration, Rg, of approximately 90 ( 4 nm and Rg/Rh ) 0.81 ( 0.04; these values are similar to those reported above at pH 9. After adding H2PtCl6 to the aqueous microgel solution (which results in a reduction of the pH to 4.7), the scattering intensity increases sharply. A single process is again identified, with a diffusion coefficient of 1.7 × 10-8 cm2/s and a corresponding Rh of 124 ( 2 nm, which is attributed to the diffusion of the metalated microgel particles. Guinier fitting of the q-dependent intensity of this process gives a radius of gyration, Rg, of approximately 97 ( 4 nm. To explain this relatively small increase in size for these metal compound-containing particles, two opposing effects that take place on addition of the metal precursor to the microgel should be considered. First, adding H2PtCl6 to the PDEA particles results in partial protonation of the tertiary amine groups, causing particle swelling analogous to that observed on addition of HCl. However, one would then expect the size of the metalated microgel particles to increase up to 170 nm, as found above for the HCl-treated particles. To explain the lower degree of swelling of the metalated microgel particles, which at first glance seems anomalous, a second effect should be considered. In earlier work,45 we reported that an acidic solution of a PDEA-based double-hydrophilic diblock copolymer formed micelles on addition of H2PtCl6. This was attributed to ionic complexation between the divalent PtCl62- anions and the (44) Vamvakaki, M.; Papoutsakis, L.; Katsamanis, V.; Afchoudia, T.; Anastasiadis, S. H.; Fragouli, P. G.; Iatrou, H.; Hadjichristidis, N.; Armes, S. P.; Sidorov, S.; Zhirov, D.; Zhirov, V.; Kostylev M.; Bronstein, L. M. Faraday Discuss. 2005, 128, 129. (45) Bronstein, L. M.; Vamvakaki, M.; Kostylev, M.; Katsamanis, V.; Stein, B.; Anastasiadis, S. H. Langmuir 2005, 21, 9747.

Langmuir, Vol. 23, No. 10, 2007 5765

Figure 5. Intensity autocorrelation functions at a 30° scattering angle for the 0.05 wt % microgel at pH 2.1 (0), the H2PtCl6-loaded microgel solution at 0.005 wt % and pH 2.0 (O), and the Pt nanoparticle-loaded latex at 0.001 wt % and pH 8.7 after in situ reduction using NaBH4 (4) at a 3:1 N/Pt molar ratio. (Inset) Distributions of relaxation times multiplied by the total scattering intensity (normalized to that of toluene) for the respective samples.

protonated tertiary amine groups on the copolymer chains. Similar ionic “cross-linking” probably occurs within the microgel particles investigated in the present study. This additional cross-linking prevents more extensive swelling of the H2PtCl6-loaded microgels, as indicated by the DLS data. Moreover, the Rg/Rh ratio is 0.79 ( 0.04, which signifies the formation of uniform spherical metalcontaining microgel particles. Finally, reduction of the PtCl62- anions with NaBH4 causes the solution pH to increase to 7.2, thus the hydrodynamic radius of the Pt nanoparticle-loaded PDEA particles decreases to approximately 113 ( 2 nm (D ) 1.9 × 10-8 cm2/s) as a result of microgel deswelling. This size is similar to that of the original PDEA latex at high pH, suggesting that the incorporation of the metal nanoparticles has a negligible effect on the latex dimensions. This is in good agreement with our previous observations of the metalation of pH-sensitive block copolymer micelles, where no significant change in the micelle diameter was observed after metal nanoparticle formation.45 The latter is also verified by calculating the expected change in latex volume due to the presence of Pt nanoparticles. From UV-visible studies and the thermogravimetric analysis of the metal-nanoparticle-containing microgels, a metal mass loading of ∼39 wt % was found. (See below.) From the above weight fraction and the density of platinum metal (21.45 g/cm3), the volume fraction of the metal nanoparticles within the microgels was found to be around 2.67 v/v%. Such an increase in latex volume would result in an increase in the latex radius from 111 to 112 nm, which is beyond the resolution limit of our technique and thus would not be detectable by DLS. In the second method (method B), a 0.05 wt % aqueous microgel solution was first adjusted to pH 2.1. Next, H2PtCl6‚ 6H2O was added at a 3:1 N/Pt molar ratio, and the solution pH decreased slightly to 2. After stirring for 24 h to ensure equilibration, in situ reduction was carried out with NaBH4. The reaction solution was stirred for another 0.5 h before adjusting to pH 8.7, which is well above the pKa of the protonated tertiary amine groups, using NaOH, followed by stirring for 24 h (Figure 3b). Figure 5 shows the intensity autocorrelation functions and the corresponding inverse Laplace transformations (normalized over the microgel concentration) at a 30° scattering angle for a 0.05 wt % aqueous solution of the original PDEA microgel at pH 2.1, a 0.005 wt % solution of the H2PtCl6-loaded microgel at pH 2.0

5766 Langmuir, Vol. 23, No. 10, 2007

(ultrafiltration with a 3 nm filter), and a 0.001 wt % solution of the Pt nanoparticle-loaded microgel at pH 8.7. From the distributions of relaxation times shown in the inset, the respective hydrodynamic radii were calculated. At pH 2.1, swollen hydrophilic microgel particles with a hydrodynamic radius of 170 ( 2 nm were obtained. On addition of H2PtCl6, the scattering intensity increases sharply, and a single process is identified with a diffusion coefficient of 1.7 × 10-8 cm2/s, which corresponds to a hydrodynamic radius of 123 ( 2 nm. The latter process possesses a q-dependent intensity that, when fitted with the Guinier expression, results in a radius of gyration of 95 ( 4 nm. This gives an Rg/Rh ratio of 0.77 ( 0.04, which signifies the formation of uniform metalated spheres similar to those obtained above with the first method. Again, the decrease in hydrodynamic size for the H2PtCl6-loaded microgel particles is believed to be due to ionic cross-linking between the protonated cationic tertiary amine groups and the hexachloroplatinate divalent anions. It is noteworthy that the size of the H2PtCl6-loaded microgel particles is very similar for the two methods. This suggests that the H2PtCl6-loaded microgels are equilibrium structures whose swelling characteristics are not significantly influenced by the method of preparation used. Instead, the overall microgel dimensions are dictated by two competitive processes: (i) acid-induced swelling due to protonation of the DEA repeat units and (ii) additional ionic cross-linking by the divalent PtCl62anions. This hypothesis was confirmed by investigating the influence of PtCl42- concentration on the size of the microgel particles. To a 0.05 wt % aqueous microgel dispersion at pH 2.2, H2PtCl6‚6H2O was added in an 8:1 N/Pt molar ratio (i.e., a lower H2PtCl6‚6H2O concentration compared to the 3:1 N/Pt molar ratio discussed above). After being stirred for 24 h to ensure equilibration, this aqueous dispersion was analyzed by DLS. A single process with very strong intensity and diffusive dynamics dominated the autocorrelation function. From the distribution of relaxation times (not shown), the mean diffusion coefficient was calculated to be 1.4 × 10-8 cm2/s, and the corresponding hydrodynamic radius, Rh, was 152 ( 2 nm. Moreover, the lightscattering intensity has a strong q dependence that, when fitted to the Guinier expression, resulted in a radius of gyration of 100 ( 4 nm. This leads to an Rg/Rh ratio of 0.66 ( 0.04, which is significantly less than that expected for a uniform sphere (Rg/Rh ) 0.774). At the lower platinic acid loading, the hydrodynamic radius of the metalated microgel is smaller than that of the original microgels at pH 2 (Rh ) 170 nm) but larger than that of the metalated microgel obtained at a 3:1 N/Pt molar ratio (Rh ) 123 nm). Thus, these DLS data indicate a gradual decrease in size for the swollen microgel as a result of ionic cross-linking by the divalent metal anions, which is consistent with our hypothesis of two opposing effects dictating the overall microgel dimensions. The Rg/Rh ratio increases from 0.58 to 0.79 as the degree of ionic cross-linking increases, indicating a transition from swollen particles with a periphery of relatively low density to more uniform spheres. Finally, after NaBH4 reduction and on adjusting the solution pH to 8.7, a sharp increase in light-scattering intensity is observed while the hydrodynamic radius of the microgel particles decreases to 113 ( 2 nm, which is similar to that obtained with the first method. This is attributed to the formation of the Pt nanoparticle-loaded PDEA latex with a well-defined equilibrium structure that is independent of the synthetic protocol employed. The presence of the Pt nanoparticles has no significant effect on the dimensions of the deswollen latex, as discussed above for the first method. Transmission Electron Microscopy. The nature and morphology of both the H2PtCl6-loaded and Pt nanoparticle-loaded

Palioura et al.

Figure 6. Representative TEM images of (a) the H2PtCl6-loaded PDEA microgel and (b) the Pt nanoparticle-loaded PDEA microgel obtained after metal reduction at a 1:1 N/Pt molar ratio.

microgels were investigated by TEM. Figure 6 shows the TEM images obtained for the H2PtCl6-loaded microgel at a 1:1 N/Pt molar ratio (Figure 6a) and the same sample after in situ NaBH4 reduction (Figure 6b). The metal species incorporated within the microgels provide the necessary contrast for TEM imaging. In fact, after NaBH4 reduction, the contrast between the Pt nanoparticles formed within the microgels and the PDEA chains is so high that the organic component becomes invisible. The mean TEM diameter of the H2PtCl6-loaded microgels before reduction was estimated to be around 100 nm, although a few smaller structures and some larger aggregates were also visible. The latter can be attributed to the low Tg of PDEAEMA that results in the formation of a polymer film on the TEM grid even for the metal-loaded microgels. Such particle coalescence would make it very difficult to image the individual PDEAEMA particles as reported earlier by Amalvy et al.19 The diameter estimated by TEM is much smaller than that found by DLS. This difference is attributed to the fact that DLS estimates a hydrodynamic size of the microgel in solution whereas TEM deals with a dried sample. The mean diameter of the platinum nanoparticles obtained after in situ NaBH4 reduction was approximately 5 nm. The small size of these nanoparticles provides a very high surface area and renders them ideal candidates for use in catalysis.

Metal Nanocrystals in Microgel Particles

Figure 7. UV-visible absorption spectra of a dilute aqueous dispersion of PDEA particles at pH 2 (-) and pH 10 (‚‚‚), a PDEA microgel in the presence of H2PtCl6 introduced at pH 2 (---), and the Pt nanoparticle-loaded PDEA latex particles at pH 10 (-‚-) obtained at a 1:1 N/Pt molar ratio.

UV-Visible Absorption Spectroscopy Studies. UV-visible absorption spectroscopy was used to probe the polymer-metal interactions that occur both on addition of H2PtCl6 to the microgel and also after in situ reduction using NaBH4. This technique was also used to assess the platinum content of the metalated microgel particles. Figure 7 shows the UV-visible absorption spectra obtained for a H2PtCl6-loaded microgel at pH 2 at a 1:1 N/Pt molar ratio and for the same sample after NaBH4 reduction. Spectra recorded for the original microgel particles in pure water at pH 2 and 10 are also shown for comparison. The microgel concentration in each case was kept constant at 0.005 wt %. On addition of H2PtCl6, a new absorption peak at 266 nm is observed that is absent in the spectrum of the pure microgel. This feature is characteristic of the electrostatic interactions between the PtCl62- anions and the protonated amine groups of the microgel46 and supports the concept of ionic cross-linking by the metallic anions, as suggested above. In situ reduction using NaBH4 leads to the complete disappearance of the 266 nm peak, which confirms the quantitative formation of Pt nanoparticles that are observed by TEM. The platinum content of the microgel particles was determined by UV-visible spectroscopy. UV-visible spectra of the filtrate solutions obtained after ultrafiltration of the H2PtCl6-loaded microgels were recorded at a 1:1 N/Pt molar ratio. The intensity of the characteristic H2PtCl6 peak at 260 nm (relative to a calibration curve) was used to assess the amount of excess platinic acid present in the supernatant, and hence the metal nanoparticle loading of the microgel particles was calculated by difference. A platinum content of 39 ( 2 wt % was found for the first metalation method, where the H2PtCl6 precursor was added to the hydrophobic latex particles at high pH. For the second method, when H2PtCl6 was added to the hydrophilic swollen microgel particles at low pH, a 35 ( 2 wt % metal loading was calculated. These results confirm that relatively high platinum loadings can be achieved in both cases and suggest that the final loading is, at least to a first approximation, independent of the loading method. These spectroscopic data also agree reasonably well with the loadings estimated by thermogravimetric analysis: a 39 ( 3 wt % Pt content was obtained for microgel particles loaded (46) Bronstein, L. M.; Sidorov, S. N.; Gourkova, A. Y.; Valetsky, P. M.; Hartmann, J.; Breulmann, M.; Co¨lfen, H.; Antonietti M. Inorg. Chim. Acta 1998, 280, 348.

Langmuir, Vol. 23, No. 10, 2007 5767

Figure 8. X-ray diffraction patterns of (a) the pure PDEA latex at pH 7.5 and (b) the Pt nanoparticle-loaded PDEA particles obtained at a 1:1 N/Pt molar ratio.

using the first metalation method,47 which is only 12% lower than the maximum possible loading (51 wt %). X-ray Diffraction (XRD). XRD was used to examine the possible crystallinity of the Pt nanoparticles formed after in situ NaBH4 reduction. Figure 8 shows the X-ray diffraction patterns obtained for the pure latex and the Pt nanoparticle-loaded particles after NaBH4 reduction at a 1:1 N/Pt molar ratio. The broad peak at around 25° is assigned to the polymeric material because it appears in the pure microgel as well. The much sharper peaks observed in the diffractogram of the Pt nanoparticle-loaded microgel at 40, 46, 67.5, 81, 85.5, and 103.5° are characteristic of the crystalline nanodomains of platinum. The fwhm of these peaks can be used to calculate the size of the nanocrystals using the Laue-Scherrer formula; the mean diameter of the Pt nanocrystals was calculated to be around 4.0 nm, which is in good agreement with that estimated above by TEM. It is also noteworthy that these Pt nanoparticles are somewhat larger than those formed within the cores of PDEA-based block copolymer micelles (d < 2 nm) in our previous studies.44 Although the larger size of the microgel-incorporated metal nanoparticles is unfavorable for their efficiency as nanocatalysts, it could be advantageous in applications where catalyst recovery is very important because it could facilitate catalyst separation by centrifugation or filtration.

IV. Conclusions Cross-linked tertiary amine methacrylate-based sterically stabilized latexes prepared by emulsion polymerization have been investigated as pH-sensitive microgel matrices for the in situ formation of nanometer-sized Pt particles. The latex-to-microgel transition in aqueous solution was examined by DLS as a function of solution pH. The latex particles became highly swollen microgels on protonation of the tertiary amine groups below pH 7. The reversible nature of this (de)swelling transition was confirmed, suggesting that the particles retained their colloidal stability during the pH adjustment. The particles were then loaded with H2PtCl6, and Pt nanoparticles were formed within the microgels by in situ reduction using NaBH4. This loading process was followed by DLS, which revealed two effects that strongly influence the swelling behavior of the microgels during metal loading. Addition of platinic acid, H2PtCl6, to the latex particles causes the protonation of the tertiary amine groups, leading to microgel formation. However, the divalent PtCl62- anions act as (47) Allowing for the possible oxidation of platinum upon polymer decomposition, which would result in an oxidized Pt residue after TGA, and given the difference in mass between the platinum and oxygen atoms, a minimum Pt loading of 34 wt % was calculated for this sample.

5768 Langmuir, Vol. 23, No. 10, 2007

an ionic cross-linker for the protonated amine groups, which causes microgel deswelling. The equilibrium between these two opposing effects determines the final size of the microgels after H2PtCl6 complexation. Both the metal salt-loaded microgel and the metal nanoparticle-loaded latex particles possess well-defined equilibrium structure with characteristics that are independent of the synthesis method. UV-visible absorption spectroscopy studies confirmed this polymer-metal salt interaction because an additional peak that is characteristic of these electrostatic interactions was observed. Moreover, this technique also indicated that the in situ reduction of Pt(II) to Pt(0) was essentially complete. Finally, the dimensions of the Pt nanoparticle-loaded microgels were similar to those of the microgel precursor prior to H2PtCl6 complexation, suggesting that even at high metal loadings the Pt nanoparticles have no significant effect on the dimensions of the deswollen latex. X-ray diffraction studies confirmed the formation of Pt nanocrystals of around 4 nm diameter, which is in reasonably good agreement with the Pt diameter estimated by TEM. Two additional important features of in situ metal nanoparticle growth in polymeric templates are the number and (48) (a) Gro¨hn, F.; Kim, G.; Bauer, B. J.; Amis, E. J. Macromolecules 2001, 34, 2179. (b) Gro¨hn, F.; Bauer, B. J.; Amis, E. J. Macromolecules 2001, 34, 6701. (49) (a) Mayer, A. B. R.; Mark, J. E. Colloid Polym. Sci. 1997, 275, 333. (b) Seregina, M. V.; Bronstein, L. M.; Platonova, O. A.; Chernyshov, D. M.; Valetsky, P. M.; Hartmann, J.; Wenz, E.; Antonietti, M. Chem. Mater. 1997, 9, 923. (c) Sulman, E.; Matveeva, V.; Usanov, A.; Kosivtsov, Y.; Demidenko, G.; Bronstein, L.; Chernyshov, D.; Valetsky, P. J. Mol. Catal. A: Chem. 1999, 146, 265.

Palioura et al.

spatial location of Pt nanoparticles. These parameters are currently under investigation using small-angle X-ray scattering (SAXS).28,48 In principle, the Pt nanoparticle-loaded PDEA microgel particles investigated in this study could be used as recoverable catalysts in various organic reactions for the synthesis of highvalue vitamins, cosmetics, and pharmaceuticals. Metal-loaded block copolymer micelles have been previously examined in this context.49 The three advantages that these cross-linked microgel particles offer over block copolymer micelles is their relative ease of synthesis (at 10% solids via aqueous emulsion polymerization), their enhanced colloid stability over a wide pH range, and their use in a wide range of organic solvents. However, one possible disadvantage may be that the Pt nanoparticles are buried within the microgels, which would reduce their accessibility and also retard mass transport via diffusion. The catalytic properties of these metalated microgels are currently under investigation. Acknowledgment. Part of this research was sponsored by NATO’s scientific Affairs Division (Science for Peace Programme), by the Greek General Secretariat of Research and Technology, by the European Union (NMP3-CT-2005-506621), and by the Ministry of Education (Applied Molecular Spectroscopy postgraduate program). S.P.A. is the recipient of a 5-year Royal Society-Wolfson Trust Research Merit Award. LA063359V