Fe Oxide Nanoparticles in Block Copolymer Micelles

Oct 2, 2008 - Electron-transparent specimens for transmission electron microscopy ...... Rugge , A., Ford , W. T., and Tolbert , S. H. Langmuir 2003, ...
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Mixed Co/Fe Oxide Nanoparticles in Block Copolymer Micelles Lyudmila M. Bronstein,*,† Maxim Kostylev,†,¥ Eleonora Shtykova,| Tedi Vlahu,† Xinlei Huang,† Barry D. Stein,‡ Alexei Bykov,⊥ Nicholas B. Remmes,§ David V. Baxter,§ and Dmitri I. Svergun|,∇ Departments of Chemistry, Biology, and Physics, Indiana UniVersity, Bloomington, Indiana 47405, Institute of Crystallography, Russian Academy of Sciences, 59 Leninsky pr., 117333 Moscow, Russia, TVer Technical UniVersity, 22 A. Nikitin St., TVer, 170026, Russia, and EMBL, Hamburg Outstation, Notkestrasse 85, D-22603 Hamburg, Germany ReceiVed July 7, 2008. ReVised Manuscript ReceiVed August 22, 2008 Small iron oxide and Co-doped iron oxide nanoparticles (NPs) were synthesized in a commercial amphiphilic block copolymer, poly(ethylene oxide)-b-poly(methacrylic acid) (PEO68-b-PMAA8), in aqueous solutions. The structure and composition of the micelles containing guest molecules (metal salts) or NPs (metal oxides) were studied using transmission electron microscopy, dynamic light scattering, X-ray photoelectron spectroscopy, and X-ray powder diffraction. The enlarged micelle cores after incorporation of metal salts are believed to be formed by both PMAA blocks containing metal species and penetrating PEO chains. The nanoparticle size distributions in PEO68-b-PMAA8 were determined using small-angle X-ray scattering (SAXS) in bulk. Two independent methods for SAXS data interpretation for comprehensive analysis of volume distributions of metal oxide NPs showed presence of both small particles and larger entities containing metal species which are ascribed to organization of block copolymer micelles in bulk. The magnetometry measurements revealed that the NPs are superparamagnetic and their characteristics depend on the method of the NP synthesis. The important advantage of the PEO68-b-PMAA8 stabilized magnetic nanoparticles described in this paper is their remarkable solubility and stability in water and buffers.

1. Introduction Nanoscale functional polymer colloids have received considerable attention, as they have a potential for drug delivery,1,2 can serve as nanoreactors for formation of nanoparticles with catalytic, magnetic and optical properties,3-11 and as templates for synthesis of organic and inorganic materials.12-14 Polymer colloids such as block copolymer micelles,5,6,15-19 dendrim* To whom correspondence should be addressed. E-mail: lybronst@ indiana.edu. † Department of Chemistry, Indiana University. ‡ Russian Academy of Sciences. § Department of Biology, Indiana University. | Tver Technical University. ⊥ Department of Physics, Indiana University. ∇ EMBL. ¥ Present address: Cornel University, Department of Chemistry and Chemical Biology, Ithaca, NY 14853-1301. (1) Cohen, H.; Levy, R. J.; Gao, J.; Fishbein, I.; Kousaev, V.; Sosnowski, S.; Slomkowski, S.; Golomb, G Gene Ther. 2000, 7, 1896. (2) Pignatello, R.; Bucolo, C.; Puglisi, G. J. Pharmaceut. Sci. 2002, 91, 2636. (3) Spatz, J. P.; Roescher, A.; Mo¨ller, M. AdV. Mater. 1996, 8, 337. (4) Bronstein, L. M.; Linton, C.; Karlinsey, R.; Stein, B.; Svergun, D. I.; Zwanziger, J. W.; Spontak, R. J. Nano Lett. 2002, 2, 873. (5) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. AdV. Mater. 1995, 7, 1000. (6) Moffitt, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 7, 1185. (7) Masuhara,H.;Nakanishi,H.;Sasaki,K.,SingleOrganicNanoparticles;Springer-Verlag: Berlin, Heidelberg, 2003; p 402. (8) Miinea, L. A.; Sessions, L. B.; Ericson, K. D.; Glueck, D. S.; Grubbs, R. B. Polym. Preprints 2003, 44(2), 214. (9) Xu, S.; Zhang, J.; Paquet, C.; Lin, Y.; Kumacheva, E. AdV. Funct. Mater. 2003, 13, 468. (10) Underhill, R. S.; Liu, G. Chem. Mater. 2000, 12, 2082. (11) Zhao, M.; Crooks, R. M. AdV. Mater. 1999, 11, 217. (12) Liu, T.; Burger, C.; Chu, B Prog. Polym. Sci. 2003, 28, 5. (13) Mezzenga, R.; Ruokolainen, J.; Fredrickson, G. H.; E.J.;, K.; Moses, D.; Heeger, A. J.; Ikkala, O. Science 2003, 299(5614), 1872. (14) Fo¨rster, S. Top. Curr. Chem. 2003, 226, 1. (15) Gohy, J.-F.; Willet, N.; Varshney, S.; Zhang, J.-X.; Jerome, R. Angew. Chem., Int. Ed. 2001, 40, 3214. (16) Spatz, J. P.; Moessmer, S.; Moeller, M. Chem.-A Europ. J. 1996, 2, 1552. (17) Liu, S.; Weaver, J. V. M.; Save, M.; Armes, S. P. Langmuir 2002, 18, 8350.

ers11,20,21 or other polymer particles9,10,22,23 can measure 10-90 nm in diameter and can be easily placed in a sensor trap, used as self-assembling building blocks on flat surfaces for colloidal crystal formation24-26 or as catalysts for various organic reactions if they bear catalytic metals. When filled with magnetic materials, polymer colloids display magnetic properties and can serve as elements for magnetic devices,8-10,27-29 as carriers for magnetic drug targeting,30-32 biosensors,33,34 and in vivo imaging.35,36 Amphiphilic block copolymer micelles are often the systems of choice as they allow efficient stabilization of very small particles (18) Moeser, G. D.; Roach, K. A.; Green, W. H.; Laibinis, P. E.; Hatton, T. A Ind. Eng. Chem. Res. 2002, 41, 4739. (19) Uzun, O.; Frankamp, B. L.; Sanyal, A.; Rotello, V. M. Polym. Prepr. 2003, 44(2), 509. (20) Gro¨hn, F.; Bauer, B. J.; Amis, E. J. Macromolecules 2001, 34, 6701. (21) Pellechia, P. J.; Gao, J.; Gu, Y.; Ploehn, H. J.; Murphy, C. J. Inorg. Chem. 2004, 43, 1421. (22) Bronstein, L. M.; Linton, C.; Karlinsey, R.; Stein, B.; Svergun, D. I.; Zwanziger, J. W.; Spontak, R. J. Nano Lett. 2002, 2, 873. (23) Antonietti, M.; Gro¨hn, F.; Hartmann, J.; Bronstein, L. Angew. Chem., Int. Ed. 1997, 36, 2080. (24) Yu, A.; Meiser, F.; Cassagneau, T.; Caruso, F. Nano Lett. 2004, 4, 177. (25) Rugge, A.; Ford, W. T.; Tolbert, S. H. Langmuir 2003, 19, 7852. (26) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (27) Platonova, O. A.; Bronstein, L. M.; Solodovnikov, S. P.; Yanovskaya, I. M.; Obolonkova, E. S.; Valetsky, P. M.; Wenz, E.; Antonietti, M. Colloid Polym. Sci. 1997, 275, 426. (28) Radtchenko, I. L.; Giersig, M.; Sukhorukov, G. B. Langmuir 2002, 18, 8204. (29) Dresco, P. A.; Zaitsev, V. S.; Gambino, R. J.; Chu, B. Langmuir 1999, 15, 1945. (30) Kohler, N.; Sun, C.; Wang, J.; Zhang, M. Langmuir 2005, 21(19), 8858. (31) Sukhorukov, G. B.; Rogach, A. L.; Zebli, B.; Liedl, T.; Skirtach, A. G.; Koehler, K.; Antipov, A. A.; Gaponik, N.; Susha, A. S.; Winterhalter, M.; Parak, W. J. Small 2005, 1(2), 194. (32) Alexiou, C.; Jurgons, R.; Schmid, R. J.; Bergemann, C.; Henke, J.; Erhardt, W.; Huenges, E.; Parak, F. J. Drug Target. 2003, 11(3), 139. (33) Grancharov, S. G.; Zeng, H.; Sun, S.; Wang, S. X.; O’Brien, S.; Murray, C. B.; Kirtley, J. R.; Held, G. A. J. Phys. Chem. B 2005, 109(26), 13030. (34) Chung, S. H.; Hoffmann, A.; Bader, S. D.; Liu, C.; Kay, B.; Makowski, L.; Chen, L. Appl. Phys. Lett. 2004, 85(14), 2971. (35) Romanus, E.; Huckel, M.; Gross, C.; Prass, S.; Weitschies, W.; Brauer, R.; Weber, P. J. Magn. Magn. Mater. 2002, 252(1-3), 387. (36) Flynn, E. R.; Bryant, H. C. Phys. Med. Biol. 2005, 50(6), 1273.

10.1021/la8021276 CCC: $40.75  2008 American Chemical Society Published on Web 10/02/2008

Nanoparticles in Block Copolymer Micelles

Langmuir, Vol. 24, No. 21, 2008 12619

Table 1. Characteristics of Fe- and Co/Fe-Containing PEO68-b-PMAA8 sample notation Fe1 Fe2 CoFe1 CoFe2 CoFe3 CoFe4

metal salt

pH used for metal ion incorporation

oxidation T, °C

(NH4)Fe(SO4)2 · 6H2O (NH4)Fe(SO4)2 · 6H2O CoCl2 · 6H2O (NH4)Fe(SO4)2 · 6H2O CoCl2 · 6H2O (NH4)Fe(SO4)2 · 6H2O CoCl2 · 6H2O (NH4)Fe(SO4)2 · 6H2O CoCl2 · 6H2O (NH4)Fe(SO4)2 · 6H2O

3.3 3.3; 5.0 3.3; 5.0 3.3; 6.0 3.3; 6.0 3.3; 6.0; 7.0

25 50 50 50 80 80

in the micelle core and their solubilization in various media due to the micelle corona. This can be advantageous for catalytic applications and for combination of very small superparamagnetic particles within larger polymer colloids. In our earlier work we explored synthesis of transition metal5,27,37 and metal halide38 nanoparticles in block copolymer micelles both in organic and aqueous media. Magnetic Co nanoparticles of different sizes and shapes were prepared in the polystyrene-b-poly(4-vinyl pyridine) block copolymers in organic media.27 Depending on the reaction conditions, the Co particle properties could be tuned from nonmagnetic to superparamagnetic and to ferromagnetic. Similar approaches were explored by other groups for the synthesis of Co and Co-Co oxide nanoparticles in the polystyrene-b-poly(2-vinyl pyridine) block copolymer micelles.39,40 Another way to incorporate metal precursors into the functional block is to add metal salts in the block copolymer molecular solution followed by microsegregation in the film and formation of the CoFe2O4 nanoparticles in the microsegregated block.41 Recently, core-shell magnetic polymer particles have been used to form water-based magnetic fluids by coating of 7.5 nm magnetite (Fe3O4) with a 9 nm bifunctional polymer layer comprised of an outer hydrophilic poly(ethylene oxide) (PEO) region for colloidal stability and an inner hydrophobic poly(propylene oxide) for absorption of soluble organic compounds from water.42 In general, core-shell inorganic polymer particles are suitable for a number of magnetic applications, but we believe that the so-called “raspberry” morphology5 when many small inorganic particles are formed within larger polymeric colloid can be more appropriate for a subtle control over the structure and properties of superparamagnetic particles. Several examples of raspberry morphology are presented in the literature,28,43-46 but in a number of cases these are submicron and micron particles, which is restricting some important applications. In this paper we report the formation of metal oxide nanoparticles in a commercial amphiphilic block copolymer, poly(ethylene oxide)-b-poly(methacrylic acid) (PEO68-bPMAA8), in aqueous solutions. The structure and composition of the micelles containing guest molecules or particles and their magnetic properties are described in detail using transmission (37) Bronstein, L. M.; Sidorov, S. N.; Valetsky, P. M.; Hartmann, J.; Coelfen, H.; Antonietti, M. Langmuir 1999, 15, 6256. (38) Loginova, T. P.; Kabachii, Y. A.; Sidorov, S. N.; Zhirov, D. N.; Valetsky, P. M.; Ezernitskaya, M. G.; Dybrovina, L. V.; Bragina, T. P.; Lependina, O. L.; Stein, B.; Bronstein, L. M. Chem. Mater. 2004, 16, 2369. (39) Diana, F. S.; Lee, S.-H.; Petroff, P. M.; Kramer, E. J. Nano Lett. 2003, 3(7), 891. (40) Boyen, H.-G.; Kastle, G.; Zurn, K.; Herzog, T.; Weigl, F.; Ziemann, P.; Mayer, O.; Jerome, C.; Moeller, M.; Spatz, J. P.; Garnier, M. G.; Oelhafen, P. AdV. Funct. Mater. 2003, 13(5), 359. (41) Ahmed, S. R.; Kofinas, P. Macromolecules 2002, 35, 3338. (42) Moeser, G. D.; Roach, K. A.; Green, W. H.; Laibinis, P. E.; Hatton, T. A. Ind. Eng. Chem. Res. 2002, 41(19), 4739. (43) Lindlar, B.; Boldt, M.; Eiden-Assmann, S.; Maret, G. AdV. Mater. 2002, 14, 1656. (44) Zhang, J.; Coombs, N.; Kumacheva, E. J. Am. Chem. Soc. 2002, 124, 14512. (45) Pham, H. H.; Kumacheva, E. Macromol. Symp. 2003, 192, 191. (46) Caruso, F.; Susha, A. S.; Giersig, M.; Mohwald, H. AdV. Mater. 1999, 11, 950.

elemental analysis of metal, wt % Fe: 1.12 Fe: 3.65 Fe: 2.10;Co: 0.44 Fe: 3.72; Co: 0.80 Fe: 3.50;Co: 0.76 Fe: 4.2; Co: 1.22

doped oxide composition

surface Fe content by XPS, at %

CoFe5O8.5 CoFe4.9O8.4 CoFe4.8O8.2 CoFe3.6O6.4

0.14 0.19 0.0 0.19 0.0 0.13

electron microscopy (TEM), dynamic light scattering (DLS), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), and magnetometry. As guest particles, we chose iron oxide nanoparticles and Co-doped iron oxide nanoparticles since doping with Co can be used to change magnetic47,48 and even optical49 properties. The investigation of the nanoparticle size distributions in the polymer matrices was carried out by small-angle X-ray scattering (SAXS)50 which was already widely and successfully employed for this purpose as a powerful nondestructive diffraction technique.51-54 In the present paper, two independent methods for SAXS data interpretation are used for comprehensive analysis of volume distributions of metal oxide nanoparticles in PEO68-b-PMAA8 in bulk. The important advantage of the block copolymer stabilized magnetic nanoparticles described in this paper is their remarkable solubility and stability in water and buffers.

2. Experimental Section 2.1. Materials. PEO68-b-PMAA8 (3007EA Goldschmidt) was obtained as a gift and used as received. CoCl2 · 6H2O (Sigma-Aldrich), (NH4)Fe(SO4)2 · 6H2O (Sigma-Aldrich), TBE buffer (1.3 M Tris, 450 mM Boric acid, 25 mM EDTA · Na2 in H2O, Fluka), 0.1 M NaOH and H2O2 (50%, Fisher Chemical) were used as received. Water was purified with a “Barnstead NANOpure water” purification system. 2.2. Syntheses. In a typical experiment of preparation of Co-Fecontaining block copolymer micelles, 0.2 g of PEO68-b-PMAA8 (4.4 × 10-4 mol of PMAA) was added to 60 mL of deionized water at room temperature and stirred for 24 h to form a cloudy solution. The pH of the solution was then raised to 10 using 1.0 M NaOH and stirred where the solution became clear. After 24 h, the pH of the solution was lowered to 3.3 by adding 0.1 M HCl in a dropwise fashion and allowed to stir for 24 h. The solution was purged with argon and a mixture of 0.0174 g of CoCl2 · 6H2O (7.33 × 10-5 mol) and 0.0575 g of (NH4)2Fe(SO4)2 · 6H2O (14.66 × 10-5 mol) was added. This solution was stirred under an argon atmosphere for 24 h after which the solution was observed to have light violet color. The pH of the solution was raised using 0.1 M NaOH to a value specific for each experiment: pH 5.0 for CoFe1; pH 6.0 for CoFe2, CoFe3 and CoFe4 followed (in 24 h) with pH 7.0 for the last sample (see Table 1 for notations). After 24 h stirring, the solution underwent ultrafiltration and then was stirred for another 24 h. After that the (47) Lo, H.-L.; Gung, W. J. Appl. Phys. 1979, 50, 2414. (48) Ichiyanagi, Y.; Yamazaki, J.; Kimishima, Y.; Tachibana, Y. Trans. Mater. Res. Soc. Jpn. 2004, 29(4), 1651. (49) Nair, S. S.; Mathews, M.; Anantharaman, M. R. Chem. Phys. Lett. 2005, 406(4-6), 398. (50) Feigin, L. A.; Svergun, D. I. Structure Analysis by Small-Angle X-ray and Neutron Scattering;Plenum Press: New York, 1987. (51) Svergun, D. I.; Shtykova, E. V.; Kozin, M. B.; Volkov, V. V.; Dembo, A. T.; Shtykova, E. V. J.; Bronstein, L. M.; Platonova, O. A.; Yakunin, A. N.; Valetsky, P. M.; Khokhlov, A. R J. Phys. Chem. B 2000, 104, 5242. (52) Bronstein, L. M.; Linton, C.; Karlinsey, R.; Ashcraft, E.; Stein, B.; Svergun, D. I.; Kozin, M.; Khotina, I. A.; Spontak, R. J.; Werner-Zwanziger, U.; Zwanziger, J. W. Langmuir 2003, 19, 7071. (53) Shtykova, E. V.; Svergun, D. I.; Chernyshov, D. M.; Khotina, I. A.; Valetsky, P. M.; Spontak, R. J.; Bronstein, L. M. J. Phys. Chem. B 2004, 108, 6175. (54) Bronstein, L. M.; Dixit, S.; Tomaszewski, J.; Stein, B.; Svergun, D. I.; Konarev, P. V.; Shtykova, E.; Werner-Zwanziger, U.; Dragnea, B. Chem. Mater. 2006, 18(9), 2418.

12620 Langmuir, Vol. 24, No. 21, 2008 solution was heated in an oil bath to the oxidation temperature (50 °C for CoFe1 and CoFe2 and 80 °C for CoFe3 and CoFe4) and the pH was raised to 10 using 0.1 M NaOH. Then the solution was allowed to stir in the oil bath at the designated temperature for 24 h. The solution was then ultrafiltrated and concentrated. If the solid sample was required, the concentrated solution was placed in a drying oven at 60 °C for 24 h and then in a vacuum oven for 4 h which caused the sample to form solid, dark brown flakes which were then ground into a powder. In the case of the samples containing solely Fe species, 0.08624 g (2.2 × 10-4 mol) of (NH4)2Fe(SO4)2 · 6H2O was added as a single load at pH 3.3 for Fe1 or as two loads for Fe2: one-third of the full salt amount was added at pH 3.3 and then two-thirds were added in 24 h stirring at pH 5.0. Other steps were similar to the above procedure. The oxidation was aided with an addition of 10 drops of 50% H2O2 to the reaction solution at pH 10 and at the designated temperature (25 °C for Fe1 and 50 °C for Fe2) and was allowed stirring at this temperature for 24 h. 2.3. Characterization. Powder diffraction patterns were collected on a Scintag θ-θ powder diffractometer with a Cu KR source (1.54 Å). Electron-transparent specimens for transmission electron microscopy (TEM) were prepared by placing a drop of dilute solution onto a carbon-coated copper grid. Images were acquired at accelerating voltage of 60 kV on a JEOL JEM1010 transmission electron microscope. Micelle sizes were calculated out of 100-120 particles using Scion Image software. Dynamic light scattering (DLS) measurements were carried out in aqueous solutions with the Malvern Instruments Zetasizer Nano-6. DLS experiments were carried out at 90° scattering angle and 25 °C. X-ray photoelectron spectra were obtained using Mg KR (hν ) 1253.6 eV) radiation with a modified ES-2403 Spectrometer (provided by the Institute for Analytic Instrumentation of the Russian Academy of Sciences, St. Petersburg, Russia). The analyzer was operated at a pass electron energy of 100 eV. All data were acquired at an X-ray power of 200 W and an energy step of 0.1 eV. The electron-flood gun accessory was used so that the current of the total emitted electron flux from the flood-gun filament was adjustable from 0 to 100 mA, with the optimum found to be 70 mA at an electron energy of 2 eV. Samples were allowed to outgas for 15-30 min before analysis and were sufficiently stable during examination. Data analysis was performed using a standard RFES-set with Resolver program. The synchrotron radiation X-ray scattering investigations were performed on the X33 camera55 of the European Molecular Biology Laboratory (EMBL) on the storage ring DORIS III of the Deutsches Elektronen Synchrotron (DESY, Hamburg). A MAR Image plate detector was used to record the scattering data in the range of the momentum transfer 0.1 < s < 5.0 nm-1, where s ) 4π sin θ/λ, 2θ is the scattering angle, and λ ) 0.15 is the X-ray wavelength. Three powder samples PEO-b-PMAA-Fe2, PEO-b-PMAA-CoFe2 and PEO-b-PMAA-CoFe4 were measured with exposure time 2 min in a vacuum cuvette to diminish the parasitic scattering. The collected scattering data were primary processed using standard procedures.56 Two independent methods of data interpretation were employed to analyze size distributions in the systems. First, the particle size distributions DV(R) of the metal oxide particles were calculated using an indirect transform program GNOM,57 which was extensively and successfully employed to estimate these structural characteristics in different metal-containing polymer matrices.51-54,58,59 (55) Roessle, M. W.; Klaering, R.; Ristau, U.; Robrahn, B.; Jahn, D.; Gehrmann, T.; Konarev, P.; Round, A.; Fiedler, S.; Hermes, C.; Svergun, D. J. Appl. Crystallogr. 2007, 40, s190. (56) Konarev, P. V.; Volkov, V. V.; Sokolova, A. V.; Koch, M. H. J.; Svergun, D. I. J. Appl. Crystallogr. 2003, 36, 1277. (57) Svergun, D. I. J. Appl. Crystallogr. 1992, 25, 495. (58) Svergun, D. I.; Kozin, M. B.; Konarev, P. V.; Shtykova, E. V.; Volkov, V. V.; Chernyshov, D. M.; Valetsky, P. M.; Bronstein, L. M. Chem. Mater. 2000, 12, 3552. (59) Shtykova, E. V; Shtykova, E. V., Jr.; Volkov, V. V.; Konarev, P. V.; Dembo, A. T.; Makhaeva, E. E.; Ronova, I. A.; Khokhlov, A. R.; Reynaers, H.; Svergun, D. I J. Appl. Crystallogr. 2003, 36, 669.

Bronstein et al. The alternative analysis of the metal oxide cores formed in the amphiphilic block copolymer matrix was performed using the program MIXTURE.56 The program simulates scattering from mixtures containing up to ten different components, i.e. particles with different but simple shapes (spheres, cylinders, etc.), each characterized by its volume fraction, average size and the width of polydispersity distribution, contrast, and, optionally, potential for interparticle interactions. The experimental scattering pattern is fitted by a weighted combination of calculated individual curves from the components and the parameters of the best fit are determined. The repeating distances of the periodical motifs in the crystalline regions dj ) 2π/smax, corresponding to the peak position smax on the scattering patterns, were calculated using program PEAK.56 Magnetometry measurements were obtained using a Quantum Design MPMS-XL magnetometry system. The zero field cooling (ZFC) curve was acquired by cooling the sample from room temperature to 4.5 K in a null field ((0.5 G), applying a 50 G field at 4.5 K, and then measuring the magnetic moment as the temperature was increased. The sample was then cooled from 300 to 4.5 K in the 50 G field and the measurements were repeated for the field cooling (FC) curve. Differences between the ZFC and FC are indicative of hysteretic behavior in the sample and are generally attributed to “blocking” of the magnetic moment in the particles. Below this blocking temperature, the energy barrier associated with the magnetic anisotropy of the particles is substantially greater than the thermal energy and so the sample ceases to behave in a superparamagnetic manner. The blocking temperature for monodisperse, noninteracting particles can be approximated by the formula TB ) KV/25kT where K is the effective anisotropy of the particle, V is the particle volume, and k the Boltzman constant. Conventionally, this temperature is taken to be the position of a peak observed in a zero-field cooled magnetization vs Temperature curve. Larger particles and/or particles with higher effective anisotropy will have higher blocking temperatures. In the case of polydisperse systems, the temperature at which the difference between the ZFC and FC curves becomes negligible represents the blocking temperature of the largest and/or highest anisotropy particles in the sample, and the shape of the ZFC is highly dependent on the distribution of particles. Accordingly, while the ZFCFC curves are not necessarily easy to interpret quantitatively, they do give one significant qualitative insight into the differences in size distribution and anisotropy of the different particles. Above the blocking temperature, the ZFC and FC curves are expected to obey a modified version of the Curie-Weiss law so that χ ) c(1 - (T/Tc)1.5)2/(T - θ) where the factor (1 - (T/Tc)1.5)2 has been added to compensate for the decreasing moment of the superparamagnetic particles as they approach the Curie temperature. In the analysis, the data have been plotted as M-1 vs T, and the data has been fit with the equivalent function in eq 1 where c, θ, and Tc are fit parameters.

M-1 )

T-θ cH[1 - (T⁄Tc)1.5]2

(1)

Notably, there is a transition region between the blocking temperature and the region where Curie-Weiss behavior is observed, and so the minimum temperature for the fit was chosen to be the lowest temperature which gave a reasonable χν2 value (χν2 ≈ 1). The Curie temperature for the superparamagnetic particle is expected to be reduced from its bulk value. The Weiss offset θ can be understood as being due to a molecular field associated with interacting particles. A positive value of θ indicates the molecular field is aiding the applied field and a negative value indicates the molecular field is opposing the applied field. The constant c is the Curie constant and is proportional to the number of particles and the square of the moment per particle. Notably, the constant c should be independent of the effective anisotropy of the particle, as would be expected since the analysis is based on data entirely above the blocking temperature where thermal energies dominate. Finally, the magnetization of each sample was measured as a function of the applied field up to at least (2 T at several different temperature settings between 4.5 and 300 K. For monodisperse particles above the blocking temperature, the magnetization curve

Nanoparticles in Block Copolymer Micelles

Langmuir, Vol. 24, No. 21, 2008 12621

Figure 4. The TEM image of Fe2 after oxidation. Figure 1. Hydrodynamic diameter distributions of PEO-b-PMAA in water at the pH 10 and 3.3.

Figure 2. TEM images of Fe1 containing iron salt (a) and after oxidation of Fe(II) species (b). Inset shows an enlarged micelle.

Figure 3. Schematic representation of the PEO68-b-PMAA8-Fe micelle at low pH.

is expected to be a Langevin curve (M ) M0(coth(µH/kT) - kT/µH) where µ is the moment per particle and M0 is the saturation magnetization. Deviations from the Langevin curve are common and usually attributable to a polydisperse system. A complete model of the magnetization curve is therefore difficult, but even a poor fit of a single Langevin curve can give substantial insight into how the moment per particle compares across samples. All magnetization measurements are presented as emu per gram of magnetic ions (cobalt and iron) to make comparisons across the samples easier.

3. Results and Discussion 3.1. Structure of Metal-Containing PEO68-b-PMAA8 Micelles: DLS, TEM, XRD, and XPS. PEO68-b-PMAA8 contains very short pH block whose amphiphilic properties may be tuned by changing the solution pH. At low pH values this block is

Figure 5. XRD profile of Fe2 after the thermal treatment at 350 °C for 2 h.

hydrophobic. (According to ref 60 pKa for PMAA varies from 5.9 to 6.6 depending on the degree of ionization.) Above pKa the PMAA block becomes hydrophilic. The ability of this block copolymer to adsorb on the growing inorganic particles was used for biomineralization61-63 or silver nanowire formation64 but to the best of our knowledge this block copolymer was not explored as a nanoreactor container for the formation of magnetic nanoparticles. To provide a regular aggregation due to hydrophobic interactions, the pH of the aqueous PEO68-b-PMAA8 solution was first raised to 10 where both blocks are hydrophilic and molecularly soluble and then adjusted to 3.3 where the PMAA block is protonated and rather hydrophobic. The DLS number distributions65 vs hydrodynamic diameters (Dh) of PEO68-b-PMAA8 in water at these two pH values are shown in Figure 1. These data suggest that in both cases block copolymer molecules form individual coils (unimers), so when the pH is dropped to 3.3, no micelles are formed. We believe that absence of micelles is caused by a very short hydrophobic block (containing only eight units) along with a long PEO block providing good water solubility. Because no micelles were formed at low pH and we could not incorporate metal species into the micelle cores,5 the other avenue (60) Poe, G. D.; Jarrett, W. L.; Scales, C. W.; McCormick, C. L. Macromolecules 2004, 37(7), 2603. (61) Marentette, J. M.; Norwig, J.; Stockelmann, E.; Meyer, W. H.; Wegner, G. AdV. Mater. 1997, 9(8), 647. (62) Li, M.; Coelfen, H.; Mann, S. J. Mater. Chem. 2004, 14(4), 2269. (63) Tjandra, W.; Yao, J.; Ravi, P.; Tam, K. C.; Alamsjah, A. Chem. Mater. 2005, 17(19), 4865. (64) Zhang, D.; Qi, L.; Ma, J.; Cheng, H. Chem. Mater. 2001, 13, 2753. (65) Because the DLS intensity distributions are strongly dependent on the presence of large particles (the scattering intensity is proportional to the squared volume, i.e., to R6), we used number distributions of hydrodynamic diameters.

12622 Langmuir, Vol. 24, No. 21, 2008

Bronstein et al.

Figure 6. TEM images of CoFe2 before (a) and after (b) oxidation. Inset shows single micelle (a gray dot) containing metal oxide nanoparticles (darker dots).

was used. As was demonstrated earlier by us66,67 and others,68,69 the interaction of double hydrophilic (fully molecularly soluble) block copolymers with metal ions may induce micellization if one of the blocks is prone to coordinate with metal species. We used this approach to sequester metal ions into PEO68-b-PMAA8 and to form micelles. To incorporate Fe species into the PEO68-b-PMAA8, (NH4)2Fe(SO4)2 · 6H2O was added to the block copolymer solution at pH 3.3 and stirred for 24 h. To prevent a premature oxidation, all procedures were carried out under argon. The choice of the iron salt was determined by the enhanced stability of ammonium iron sulfate toward oxidation. After ultrafiltration allowing removal of unreacted salt, the pH was raised to 10 and H2O2 was added to form iron oxide (normally, Fe2O3).10 Iron content in the block copolymer sample was 1.12 wt.% while complete incorporation of (NH4)2Fe(SO4)2 · 6H2O would result in 5.66 wt % Fe (calculated from the loading). The low degree of iron inclusion can be explained by very low degree of ionization of carboxyl groups at pH 3.3. Figure 2 demonstrates the TEM images of the block copolymer micelles containing (NH4)2Fe(SO4)2 · 6H2O and those after oxidation of Fe(II) ions. The presence of the Fe species provides electron contrast (similar to staining) of the micelles appearing as nearly spherical or slightly oblate dark spots. The mean visible micelle size is 10.2 nm but because the Fe species should be located only in the core, the visible spot should match the micelle core and its vicinity. However, because the fully extended length of the PMAA block is only 1.6 nm (the fully extended length of the block copolymer is 22 nm), the size of the micelle core should not exceed 3.2 nm if it consists of only PMAA blocks. This consideration stimulated the following reasoning. Because the micelle core is formed due to an interaction of Fe(II) ions with the PMAA blocks retaining a globular conformation at the pH 3.3, we believe that the hydrophobic area in the globule is minimized, thus the micelle core may include not only the PMAA blocks but also the PEO chains penetrating the core (especially considering that the hydrophobic block is so short). This suggestion is well supported by formation of hydrogen-bonding interpolymer complexes between PMAA and PEO at low pH,70 increasing their miscibility. Such a scenario may increase the micelle core size as is schematically demonstrated in Figure 3. (66) Bronstein, L. M.; Sidorov, S. N.; Gourkova, A. Y.; Valetsky, P. M.; Hartmann, J.; Breulmann, M.; Colfen, H.; Antonietti, M. Inorg. Chim. Acta 1998, 280, 348. (67) Bronstein, L. M.; Vamvakaki, M.; Kostylev, M.; Katsamanis, V.; Stein, B.; Anastasiadis, S. H. Langmuir 2005, 21(21), 9747. (68) Sanson, N.; Bouyer, F.; Gerardin, C.; In, M. Phys. Chem. Chem. Phys. 2004, 6(7), 1463. (69) Bouyer, F.; Sanson, N.; Destarac, M.; Gerardin, C. New J. Chem. 2006, 30(3), 399. (70) Lowman, A. M.; Cowans, B. A.; Peppas, N. A. J. Polym. Sci. B: Phys. 2000, 38(21), 2823.

Figure 7. TEM image of CoFe4.

Figure 8. Experimental SAXS profiles (circles) and the best fits (solid lines) computed by the program MIXTURE56 to the scattering patterns for Fe2 (1 and 1a), CoFe2 (2 and 2a) and CoFe4 (3 and 3a) systems.

The close look at the Figure 2b (see inset) does not allow one to detect the iron oxide nanoparticles. XRD diffraction pattern of this sample is completely featureless (see Figure S1 in the Supporting Information), demonstrating that the iron oxide nanoparticles are amorphous or too small resulting in extreme line broadening. As was indicated above, the low degree of Fe incorporation is due to low pH. At the same time, the pH increase should be handled with caution because it may lead to iron hydroxide formation or premature oxidation of Fe(II) to Fe(III); both events being undesirable for the Fe2O3 formation.18 To avoid a strong dependence of the micelle size on the pH, when metal salts are incorporated at different pH, one-third of the total amount of the iron salt was added at pH 3.3 to provide cross-linking between the PMAA globules with the Fe(II) cations to create micelle cores. Then two-thirds of the iron salt was added at pH 5 when PMAA chains were more ionized and swollen. In addition, the oxidation was carried out at 50 °C to intensify the growth of iron oxide nanoparticles. These modifications of the synthetic procedure led to an increase of the iron content up to 3.65 wt % and to the change of the micelle morphology. The TEM image of the sample after oxidation (Figure 4) shows both the nearly spherical micelles with a visible diameter of 9.8 nm and rods with a cross-section diameter of 5.8 nm and a length of 15-36

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Langmuir, Vol. 24, No. 21, 2008 12623

Figure 9. Volume distribution functions of the metal oxide nanoparticles in PEO68-b-PMAA8. The numbering of the distributions and colors of the symbols correspond to that in Figure 8. Table 2. Size Characteristics of Metal Oxide Compositions samples 1 2 3

Fe2 CoFe2 CoFe4

Rg′, HM

R, nm

26.73 ( 0.32 19.15 ( 0.05 30.12 ( 0.06

1.0 1.0 1.5

nm. The DLS number distributions (see Figure S2, Supporting Information) show that both Fe1 and Fe2 samples contain micelles (with Dh of about 10-11 nm) and unimers, yet the fraction of unimers is lower in Fe2 probably due to a higher fraction of metal species. The deconvolution of the high resolution XPS Fe 2p spectra (see Figure S3, Supporting Information) demonstrates that in Fe2 only the Fe(III) (Fe 2p3/2 peak at 710.9 eV) species are present revealing complete oxidation of the Fe(II) ions. Again iron oxide nanoparticles cannot be detected from the TEM image and the XRD profile is featureless. We assumed that nanoparticles formed at the ambient temperature are too small and carried out annealing of the Fe2 sample at 350 °C for 2 h. Figure 5 shows the XRD profile of this sample after annealing. A signal at 2θ of 36° can be assigned to maghemite or magnetite crystals but absence of the other peaks reveals a poor degree of ordering. At the same time, peaks at 2θ of 44° and 83° unambiguously demonstrate presence of R-Fe crystals71 (Figure 5). Apparently annealing resulted in a partial iron oxide reduction and formation of R-Fe, an unusual phenomenon for such mild conditions. Normally formation of R-Fe from iron oxides occurs at 900 °C or higher in a reducing atmosphere.72 We think that presence of very small particles facilitates such a transition. Although the above phenomenon is certainly interesting, obviously the annealing does not allow us to crystallize iron oxide nanoparticles within block copolymer micelles. As was reported, doping of iron oxide with Co species improves magnetic properties.47,48 To explore this possibility and to incorporate both Co(II) and Fe(II), a mixture of CoCl2 and (NH4)2Fe(SO4)2 · 6H2O was added in the block copolymer solution at pH 3.3 followed by pH increase to 5 (CoFe1). Oxidation was carried out at pH 10. By loading, the molar ratio of PMAA to (71) Signorini, L.; Pasquini, L.; Savini, L.; Carboni, R.; Boscherini, F.; Bonetti, E.; Giglia, A.; Pedio, M.; Mahne, N.; Nannarone, S. Phys. ReV. B 2003, 68(19), 195423/1. (72) Xue, D. S.; Huang, Y. L.; Ma, Y.; Zhou, P. H.; Niu, Z. P.; Li, F. S.; Job, R.; Fahrner, W. R. J. Mater. Sci. Lett. 2003, 22(24), 1817.

Co was 1 to 6, while the molar ratio of Co to Fe was 1:2. This molar ratio might provide formation of cobalt ferrite (CoFe2O4), however the elemental analysis data presented in Table 1 reveal the composition CoFe5O8.5, demonstrating a low fraction of Co species. (Metal oxide composition is similar in all the Co-doped samples except for CoFe4 which has a slightly higher Co content.) Even if the Co salt is loaded first at pH 3.3, while the Fe salt is added at a higher pH, the composition does not change. We think that in the presence of both salts the interaction of ionized PMAA units with Fe(II) is more favorable than with Co(II). TEM images of CoFe1 (see Figure S4, Supporting Information) micelles are similar to those observed for Fe1. The micelle size is about 10 nm both before and after oxidation, while nanoparticles cannot be detected similar to the pure Fe-containing block copolymers. The DLS hydrodynamic diameter distribution (see Figure S2, Supporting Information) is similar to that of Fe2 with a smaller fraction of unimers. The XPS Fe 2p data for this and other Co-doped iron oxide samples show again the presence of only the Fe(III) species with the Fe 2p3/2 peak position in the range 710.8-711.6 eV. We could not obtain any reliable data on the Co oxidation state due to Auger oxygen peaks, but normally in these conditions Co(II) is not oxidized.73 Because we were unable to detect metal oxide particles from the TEM image, while the XRD profile was featureless, we assumed that NPs are very small. We surmised that the formation of very small particles can be due to low fraction of metal ions in the micelles caused by comparatively low pH. The higher pH should lead to more swollen micelles, facilitating the particle growth. However, the incorporation of Co(II) and Fe(II) ions at pH 3.3 followed by the pH increase to 6 (CoFe2) leads to the results similar to those above: the visible micelle size (Figure 6) is similar to that of CoFe1 both after incorporation of metal ions (9.8 nm) and after oxidation (10.4 nm), although the metal content is higher for CoFe2 than for CoFe1 (Table 1). We believe the constancy of the micelle size is due to first cross-linking of the micelle cores with metal ions while the PMAA block is in a globular form. Although the micelle size does not change, the hydrodynamic diameter distribution (see Figure S2, Supporting Information) of CoFe2 shows no unimers. Another important consequence of the pH increase is formation of larger nanoparticles (of ∼1.5-2.0 nm in diameter, see inset in Figure 6) which can be attributed to a higher fraction of metal ions and to looser micelle cores formed at pH 6. At the same time, XRD diffraction data still do not allow detection of a crystalline structure. To further increase the metal oxide nanoparticle size, we increased the oxidation temperature from 50 (CoFe2) to 80 °C (CoFe3). However, it did not result in the noticeable increase of the particles size (data not shown). We also tested the possibility of further increase of pH for incorporation of metal ions and gradually changed the pH from 3.3. to 6.0 and finally to 7.0. However, if at pH 6 the color of the reaction solution was light yellow (a small fraction of Fe(II) was oxidized to Fe(III)), the pH 7 resulted in a light-brown color indicating the higher fraction of Fe(III) ions, i.e., the premature oxidation. At the same time, such a pH increase resulted in a higher content of both Fe and Co (Table 1) due to higher degree of PMAA ionization. The DLS data (see Figure S2, Supporting Information) show larger and broader distributed micelles (no unimers) with a mean diameter of 20 nm. The larger content of (73) Nunez, N. O.; Tartaj, P.; Morales, M. P.; Bonville, P.; Serna, C. J. Chem. Mater. 2004, 16(16), 3119.

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Figure 10. ZFC and FC curves at 50 Oe applied field for (a) Fe2, (b) CoFe1, (c) CoFe2, (d) CoFe3, and (e) CoFe4. Table 3. Fractional Configuration of Metal Oxide NP Morphologies samples 1

Fe2

2

CoFe2

3

CoFe4

phase

phase content, %

Rin, nm

Rout, nm

solid sphere hollow sphere hollow sphere solid sphere solid sphere hollow sphere hollow sphere solid sphere solid sphere hollow sphere

77 13 10 19 55 17 9 52 40 8

3.2 2.5 2.5 13.5 3.4

1.3-1.4 7.7 20.0 1.2 2.9 8.8 15.0 1.5 3.4 26.7

metal ions and looser micelles at higher pH lead to larger particles (Figure 7) of about 2-3 nm. The XPS quantitative analysis of solid block copolymer samples containing iron oxide or Co-doped iron oxide species (see Table 1 for Fe content) shows that Co cannot be detected (see above), while Fe is detected and its content varies from 0 to 0.19 at %. For CoFe2, the value of 0.19 at % matches 0.80 wt %, i.e., much lower than the Fe content found from the elemental analysis measurements (Table 1). It should be noted that the XPS analysis is the surface method and the average depth of penetration in polymers is about 6-10 nm. Thus, the low Fe content assessed by XPS validates the hypothesis that the metal species are buried in the depth of 6-10 nm or more, which, in a solid state, may account for the approximate size of the PEO corona containing no metal species. Note that the PEO68-b-PMAA8 metal oxide nanoparticles described here have remarkable solubility in water and buffers. All the samples dissolve immediately in water or TBE buffer of different concentrations (20, 40, 60 and 80%) even after 2 years of storage and retain their solution characteristics. 3.2. Nanoparticle and Micelle Characteristics: SAXS. To characterize the NP size more accurately we used SAXS measurements on three solid samples: Fe2, CoFe2, and CoFe4. Figure 8 shows experimental scattering profiles from these

samples. Scattering patterns demonstrate weak maxima, which correspond to the characteristic sizes of about dj ) 43 nm ( 3 nm for the Fe2 and CoFe2 samples, and weakly expressed periodical motifs specific for lamellar organization with dj ) 5.9 nm ( 0.2 nm for CoFe4 system. The particle size distributions computed by GNOM57 from the experimental SAXS data for the metal oxide particles formed in PEO68-b-PMAA8 matrix are presented in Figure 9. Obtained distribution data refer to the metal oxide formation only, since the polymer matrix remains practically invisible for the X-rays due to much lower electron density of the amphiphilic block copolymer versus metal compounds. All DV(R) functions have multimodal character and contain main fraction of small particles in the range 2-3 nm in diameter and detectable amount of larger conglomerates. Radii of gyration Rg and average radii of the small metal oxide populations R calculated from the volume distribution functions DV(R) are presented in Table 2. Among the samples studied CoFe2 shows the smallest Rg, values, while the main fraction of small particles in this specimen has a complicated profile. This fraction obviously can be divided into two subfractions, but the resolution of the method does not allow one to fulfill it. The CoFe4 system contains the largest particles and has wider distribution profile of the main fraction that can be attributed to formation of metal oxide nanoparticles in more swollen micelles (at higher pH) and matches TEM data. For analysis of particle composition allowing one to spot polymeric areas filled with metal or metal oxide nanoparticles ab initio reconstruction of the area shape can be used,74 but in the present work we could not perform it due to obvious polydispersity of the samples. However, we can assume the existence of nanoparticle aggregates (areas filled with NPs) forming hollow bodies in the systems by analogy with our previous work.75 In the cited work, shape simulation of the regions containing Pt nanoparticles showed that spherical micelle cores with the sizes of about 10 nm aggregate forming slightly oblate hollow bodies with outer diameter of about 40 nm. (74) Svergun, D. I Biophys. J. 1999, 76, 2879.

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Langmuir, Vol. 24, No. 21, 2008 12625 Table 4. Magnetic Properties of the NPsa

sample

blocking temp (K)

Curie temp (K)

Fe2 CoFe1 CoFe2 CoFe3 CoFe4