Structured Assemblies of Ferromagnetic Particles through Covalent


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Langmuir 2007, 23, 6879-6882

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Structured Assemblies of Ferromagnetic Particles through Covalent Immobilization on Functionalized Polymer Surfaces Obtained by Surface Segregation Antoine Bousquet,† Emmanuel Ibarboure,† Christine Labrugere,‡ Eric Papon,† and Juan Rodrı´guez-Herna´ndez*,† Laboratoire de Chimie des Polyme` res Organiques (LCPO), UniVersite´ Bordeaux 1, ENSCPB-CNRS, UMR 5629, 16 AVenue Pey Berland, 33607 Pessac-Cedex, France, and Institut de Chimie de la Matie` re Condense´ e de Bordeaux, ICMCB-CNRS, 87 AVenue du Docteur Schweitzer, 33608 Pessac-Cedex, France ReceiVed January 30, 2007. In Final Form: April 24, 2007 We report a strategy to immobilize magnetic particles on polymer surfaces in an organized manner. Surface segregation of binary polymer blends provided surfaces with the desired chemical functions (carboxylic functions). These functional groups were demonstrated to be accessible and were thus able to react with magnetic particles functionalized with amine functions. The presence of a magnetic field during the covalent attachment step in direct surface patterning produced particle chains oriented parallel to the field.

Interest in the phenomenon of spontaneous self-assembly has understandably been growing within the scientific community. Scientists are busily developing their understanding of the process itself and working hard on the design of alternatively organized systems. Recent contributions show, in particular, that controlling such parameters as temperature, pH, and the chemical structure of building blocks can be used to guide self-assembly.1 Some studies use a magnetic field to trigger the formation of nanostructured materials2 composed of either nanocrystals or magnetic particles.3 In the case of particles, the dipole thus generated is used to direct the formation of structured assemblies. The suprastructures include 1D columnar or chain arrays, rings, or even nanowires and 2D patterns.4,5 Magnetic nanoparticles show great potential for applications as medical sensors6,7 and magnetic encoders;8 they can also be used as active supports for biomolecule capture, thereby allowing the extraction6 and purification of biomolecules.9 We describe here the use of an externally applied magnetic field as a means to generate organized hybrid inorganic polymer surfaces after the covalent immobilization of magnetic particles onto a functionalized polymer surface. The latter is covered with carboxylic acid functional groups, as a result of the surface segregation process induced in a blend of polystyrene and an amphiphilic diblock copolymer (PS-b-PAA) additive.10 By taking advantage of their peripheral amine functions, the magnetic † ‡

ENSCPB-CNRS. ICMCB-CNRS.

(1) (a) Bories-Azeau, X.; Armes, S. P.; van den Haak, H. J. W. Macromolecules 2004, 37, 2348. (b) Riess, G. Prog. Polym. Sci. 2003, 28, 1107. (2) Germain, V.; Pileni, M.-P. AdV. Mater. 2005, 17, 1424. (3) Leslie-Pelecky, D. L.; Rieke, R. D. Chem. Mater. 1996, 8, 1770. (4) Dimitrov, A. S.; Takahashi, T.; Furusawa, K.; Nagayama, K. J. Phys. Chem. 1996, 100, 3163. (5) Guo, R.; Arnoux, C.; Palmer, R. E. Langmuir 2001, 17, 7150. (6) Gu, H.; Ho, P.-L.; Tsang, K. W. T.; Wang, L.; Xu, B. J. Am. Chem. Soc. 2003, 125, 15702. (7) Xu, C.; Xu, K.; Zhong, X.; Guo, Z.; Zheng, R.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2004, 126, 3392. (8) Martorana, B.; Carotenuto, G.; Pullini, D.; Zvezdin, K.; Peruta, G. L.; Perlo, P.; Nicolais, L. Sens. Actuators, A 2006, 129, 176. (9) Veyret, R.; Elaissari, A.; Marianneau, P.; Sall, A. A.; Delair, T. Anal. Biochem. 2005, 346, 59. (10) Previous examples of surface segregation can be found in (a) Narrainen, A. P.; Clarke, N.; Eggleston, S. M.; Hutchings, L. R.; Thompson, R. L. Soft Matter 2006, 2, 981. (b) Hester, J. F.; Olugebefola, S. C.; Mayes, A. M. J. Membr. Sci. 2002, 208, 375.

particles were simultaneously aligned at such surfaces and covalently immobilized under a small magnetic load. We demonstrate that the three phenomena described here (i.e., water-induced surface segregation, surface covalent linkage of amine and carboxylic acid functional groups, and magnetic alignment) can be combined and used over a long range to design magnetically induced well-organized hybrid polymer surfaces. The magnetic particles used in this study were composed of a magnetic ferrofluid core, a suspension of γ-Fe2O3 200 nm ferromagnetic particles stabilized by oleic acid in octane, encapsulated in a thin polymer shell. The polymer surface was functionalized with amine functions so that at neutral pH the surface charges were positive, thereby keeping the particles dispersed. The immobilization of such particles on a substrate needs adequate surface chemistry, which often requires a modification of the latter. Until now, this has been generally done by using chemical or physical treatments to modify the elemental composition of nonfunctional polymer surfaces. These methods, however, involve a number of major drawbacks: complex surfaces with a rather undefined chemical composition and in some cases severe mechanical damage. Such problems can be avoided by taking advantage of the spontaneous surface segregation11 occurring in polymer blends when one of the components migrates to the interface. When an amphiphilic diblock copolymer incorporated into a compatible matrix comes into contact with air, the lower-surface-energy hydrophobic block preferentially segregates to the free surface. Water contact, however, guides the surface rearrangement by placing the hydrophilic block at the surface.12 Environmentally driven surface reorganization can thus be used to prepare surfaces enriched with the hydrophilic component as shown in Figure 1i. Films of the blends were prepared by spin-casting THF solutions containing between 10 and 40% of a diblock, polystyrene-b-poly(acrylic acid) (PS36-b-PAA33) in a homopolymer polystyrene (PS700) matrix. The ∼400 nm thick films were placed into contact with water vapor for 4 days at temperatures close to 90 °C. Both XPS and contact angle measurements (11) Muisener, P. A. V. O. R.; Jalbert, C. A.; Yuan, C.; Baetzold, J.; Mason, R.; Wong, D.; Kim, Y. J.; Koberstein, J. T.; Gunesin, B. Macromolecules 2003, 36, 2956. (12) Russell, T. P. Science 2002, 297, 964.

10.1021/la700255e CCC: $37.00 © 2007 American Chemical Society Published on Web 05/16/2007

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Figure 1. (i) Schematic of the surface segregation process of a compatible polymer blend to reveal the PS(black line)-b-PAA(dashed) diblock at the surface. (ii) Contact angle measurements of the surfaces before (left) and after (right) annealing.

Figure 2. XPS spectra of a blend containing 20% diblock PS36PAA37 and 80% PS700 water and air annealed for 4 days at 90 °C. (Inset) O 1s core-level spectra for the blend annealed to water (above) and air (below).

confirmed that this relatively soft treatment could drive the segregation of the poly(acrylic acid) block to the surface. Contact angle measurements for water, depicted in Figure 1ii, show significant changes between nontreated and treated films: the nontreated film has a mostly PS-covered surface, with contact angles of approximately 90°; the treated film, however, contains a PAA-enriched surface increasing the wetting of the interface. Equally, XPS spectra (Figure 2) evidenced a significant increase in oxygen quantity after the water-vapor treatment associated with the migration of the PAA block to the interface. Moreover, the composition-depth profile of the sample exposed to water (Supporting Information) confirmed that the composition of the surface is significantly different from the overall bulk composition.

The large surface excess of the PAA segment, 5.5% gradually reduced down to 1% within the first 10 nm, indicated that the surface segregation phenomenon occurs exclusively near the surface.13 Whereas XPS and contact angle measurements proved the presence of poly(acrylic acid) (PAA) at the surface level, information about both the accessibility and the quantity of acidic functions was obtained by using the methylene blue method.14 This method uses a positively charged dye (i.e., methylene blue (MB)) capable of establishing electrostatic interactions with the PAA, which is a polyanion when in neutral aqueous solution. When the pH is changed to acidic values, the PAA carboxylic acid functional groups become protonated, and MB is thus released. The quantity of MB released at low pH values is therefore proportional to the accessible carboxylic groups at the surface and can be easily evaluated from the UV-vis spectra using the Beer-Lambert law. Figure 3 shows the UV-vis spectra of the MB released from the films containing different amounts of PS-b-PAA diblock copolymer (0-20%). The increase in the maximum absorbance evidenced that the quantity of carboxylic acid functional groups available at the surface increases with the quantity of diblock copolymer used as an additive in the blend. Moreover, this test confirms that such surface carboxylic acid functional groups can complex small cationic molecules and may have the potential to react with other reactants. The covalent immobilization of magnetic particles at the surface was accomplished by reacting the carboxylic acid groups of the PAA block with the amine groups on the particle shell using a water-soluble carbodiimide coupling agent: N-cyclohexyl-N′(2-morpholinoethyl)-carbodiimide methyl-p-toluene-sulfonate (CMTS). The use of water-soluble carbodiimides to bond carboxylic acid groups and amine functions in aqueous media has been previously described.15,16 The films were soaked in a water solution at neutral pH, and the carboxylic acid groups of the PAA block placed at the surface were first activated by reaction with the carbodiimide. Then, the magnetic particles carrying (13) Jalbert, C. J.; Koberstein, J. T.; Balaji, R.; Bhatia, Q.; Salvati, L.; Yilgor, I. Macromolecules 1994, 27, 2409. (14) Chung, A. J.; Rubner, M. F. Langmuir 2002, 18, 1176. (15) Huang, H.; Remsen, E. E.; Wooley, K. L. Chem. Commun. 1998, 1415. (16) Rodriguez-Hernandez, J.; Babin, J.; Zappone, B.; Lecommandoux, S. Biomacromolecules 2005, 6, 2213.

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Figure 3. (i) UV-vis spectra of the released MB for different percentages of diblock within the blend (0 to 20%) and (ii) increase in the absorbance with increasing percentage of diblock in the blend.

amine groups were added and allowed to react overnight and at room temperature. The surfaces were then rinsed with water at two different pH values: first, at pH 7 to remove those particles that could have precipitated during the reaction and second, at pH 2 to 3 to prevent the electrostatic interactions between the negatively charged acidic functions and the positively charged particles. As a consequence, whereas noncovalently linked particles were removed, only those particles that have successfully reacted with the surface remained. A schematic representation of the strategy is depicted in Figure 4i. The magnetic particles immobilized onto the surface both in the presence and in the absence of an external magnetic field were characterized by AFM imaging. In both cases, the covalent attachment of the particles to the polymer surface brought about a single particle layer clearly seen on the polymer film images. This observation showed that by rinsing with water precipitated and/or electrostatically adsorbed particles were successfully removed from the surface. The immobilization of particles in the absence of an applied magnetic field occurs in a random fashion. It has to be noted that, generally, the distribution of particles leaves an empty space that may be related to the electrostatic repulsion between particles of the same charge (i.e., positively charged amine functional groups). According to Schaaf et al., the deposition of particles without any magnetic load is influenced by gravity, thermal motion, and typical colloidal forces such as electrostatic and van der Waals forces.17

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Figure 4. (i) Schematic of the immobilization strategy followed to attach the magnetic particles covalently using a water-soluble carbodiimide as a coupling agent (CMTS). (ii) AFM images (height and phase) of the grafted particles without a magnetic field (5 µm × 5 µm). (iii) AFM images (height and phase) of the grafted particles under a permanent magnetic load (8 mT) using an initial solution containing 0.01% w/w particles (7.5 µm × 7.5 µm). In the presence of a magnetic load, the magnetic dipole of the particles align in the direction of the load and lead to particle chains (1D ordered structure).

The presence of a magnetic field parallel to the polymer surface during the reaction step (carried out under identical conditions) resulted in a modified distribution of particles and moreover creates particle-ordered assemblies over large areas. Under the influence of an external permanent magnet (strength set to 8 mT), the particles align their dipole moments along the direction of the external field. The interparticle magnetic dipole-dipole couplings favor the formation of linear particle chains along the magnetic field flux lines. Even though the images shown in Figure 4iii (height and phase) indicate some residual disorder, they clearly demonstrate the alignment of well-defined 200 nm particles. The immobilization of particles by using water-soluble carbodiimides occurs during the orientation process, generating permanent stabilized structured surfaces. In this letter, we report a strategy to immobilize in an organized manner magnetic particles on polymer surfaces. For that purpose, the surface segregation of binary polymer blends was preferred over currently employed surface treatments to provide surfaces with the desired chemical functions (carboxylic acid functional groups). These functional groups, demonstrated to be accessible by using the MB method, were able to react with amine(17) Ostafin, M.; Zahn, K.; Mann, E. K.; Voegel, J.-C.; Senger, B.; Schaaf, P. Europhys. Lett. 1999, 46, 211.

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functionalized magnetic particles. In the presence of a magnetic field during the covalent attachment step, patterned surfaces with particle chains oriented parallel to the field could be generated. The elaboration of such surfaces can be of interest in numerous applications such as selective cellular adhesion and separation18 in which the placement of recognition sites plays a key role. More generally, surface suprastructured self-assemblies hold great

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promise for the fabrication of magnetic data storage micro- or nanodevices in which the density of recorded information could be significantly increased.19 Supporting Information Available: Experimental details and methods. This material is available free of charge via the Internet at http://pubs.acs.org. LA700255E

(18) Ugelstad, J.; Berge, A.; Ellingsen, T.; Auno, O.; Kilaas, L.; Nilsen, T. N.; Schmid, R. Macromol. Symp. 1988, 17, 117.

(19) Reiss, G.; Hu¨tten, A. Nat. Mater. 2005, 4, 725.