Three-Dimensional Hierarchically Organized Magnetic Nanoparticle

J. Phys. Chem. C 2008, 112, 5397-5404

5397

Three-Dimensional Hierarchically Organized Magnetic Nanoparticle Polymer Composites: Achievement of Monodispersity and Enhanced Tensile Strength Hyon Min Song,† Yong Joo Kim,‡ and Jeong Ho Park*,‡ Center for CMR Materials, Korea Research Institute of Standards and Science, Yuseong P. O. Box 102, Daejeon 305-600, Korea, and DiVision of Applied Chemistry & Biotechnology, Hanbat National UniVersity, San 16-1, Dukmyung-dong, Yuseong, Daejeon 305-719, Korea ReceiVed: October 4, 2007; In Final Form: January 14, 2008

Synthesis of monodisperse polymer-inorganic nanoparticle composites and the study of their mechanical properties are described. Surface-modified MPt (M ) Fe, Ni) nanoparticles undergo hydrosilation reaction with siloxane backbones and make covalent networks by acting as crosslinkers. Monodispersity and the spatial resolution of nanoparticles were identified with atomic force microscopy and transmission electron microscopy. The properties of covalent bonding in the elastomeric network between nanoparticles, allyl-modified dopamine, and siloxane backbones were studied with X-ray photoelectron spectroscopy and IR. The added amount of dopamine-modified FePt nanoparticles affected the crosslinking density in the siloxane backbones. The intensity of Si-H bonding was decreased in IR spectra with increasing density of FePt nanoparticles. Mechanical properties were measured with nanoindentation. FePt nanoparticle polymer composites have more enhanced tensile strength (Er ) 1.56 GPa, H ) 0.126 GPa) than simple elastomers crosslinked with diallyl carbonate (Er ) 0.67 GPa, H ) 0.115 GPa). Further improvement of mechanical strength was achieved with increased amount of dopamine-modified nanoparticles due to the increase in the crosslinking density between siloxane backbones.

Introduction In the recent advances of multifunctional hybrid materials,1 particular attention has been devoted to the nanoparticle polymer composites, which have the combined advantages of organic polymers and inorganic nanoparticles such as flexible properties from polymers and magnetic, optical, and electrical properties from nanoparticles.2,3 When Au and Ag nanoparticles were combined with various polymers with different dielectric functions, surface plasmon resonance effect lead to diverse resonance maximum.4-7 Magnetic nanoparticle polymer composites have been applied in biomedical research applications such as drug delivery8-10 and hyperthermia treatment of cancer cells.11-13 However, aggregation of nanoparticles within the polymer matrix hampered these applications because aggregation or flocculation of nanoparticles affected specific target delivery. Aggregation of magnetic nanoparticles within polymers diminishes the advantage of high surface density of nanoparticles and also limits their applications as magnetic fluids and gels.14 There are several methods to synthesize nanoparticle polymer composites. One of the methods is to introduce nanoparticles into preformed polymer matrix,15,16 while the other is in situ nanoparticle formation within the polymers from metal-ligand monomer precursors.17-22 Polymerization of surface-functionalized nanoparticles,23-25 gas-phase evolution of metal nanoparticles,26 and electrodeposition of preformed nanoparticles into polymers27,28 are also being used to obtain nanoparticle polymer composites. Even though all methods described above are effective for synthesizing nanoparticle polymer composites, the * To whom correspondence should be addressed. Phone: 82-42-8211548. Fax: 82-42-822-1562. E-mail address: [email protected]. † Korea Research Institute of Standards and Science. ‡ Hanbat National University.

polymerization conditions for functionalizing the surfaces of magnetic nanoparticles must be carefully chosen not to form local aggregation of magnetic nanoparticles. Local aggregation is caused from the dipolar magnetic interaction between nanoparticles which is strong enough to disturb self-assembled nanoparticles when the long organic chain is displaced during the polymerization step.29,30 This study describes that well-defined three-dimensional magnetic nanoparticle polymer composites can be constructed using chemically modified dopamine as a strong ligand to the surfaces of superparamagnetic NiPt and FePt nanoparticles. Overall monodispersity was achieved by the covalent bonding between siloxane backbones and the allyl-functionalized dopamines. Dopamine-treated nanoparticles also acted as the crosslinker between the siloxane backbones. It gave rise to the reinforcement of mechanical properties of the siloxane polymers. Compared to Sarkar’s report on the Co and Ni nanoparticle composites synthesized by in situ formation of nanoparticles from metal ion polymer complex,31 this study is focused on the preformed bimetallic magnetic nanoparticles that can be functionalized with organic precursors through hydrosilation reaction in order to give three-dimensional structures. These magnetic nanoparticle polymer composites can be useful in the applications to multifunctional nanomaterials. Experimental General Methods. All chemicals and solvents, unless stated, were purchased commercially and used as obtained without further purification. Air- and water-sensitive reactions employed standard Schlenk techniques under argon atmosphere. N-Allyl3,4-dihydroxyphenylethylamine was prepared using a literature procedure.32 Atomic force microscopy (AFM) images were

10.1021/jp709721g CCC: $40.75 © 2008 American Chemical Society Published on Web 03/19/2008

5398 J. Phys. Chem. C, Vol. 112, No. 14, 2008

Song et al.

Figure 1. AFM (a) topography and (b) phase images of the swollen state of the FePt nanoparticle siloxane polymer composites. Topography and phase images of the deswollen state of FePt (c, d) and NiPt (e, f) nanoparticle polymer composites. TEM images of the as-synthesized FePt nanoparticles (g) and the nanoparticle polymer composites (h). All the scale bars in the AFM images are 200 nm and in the TEM are 50 nm.

SCHEME 1: Surface Modification of MPt (M ) Fe, Ni) Nanoparticles with Allyl-Functionalized Dopamine

Hierarchically Organized Nanoparticle Composites

J. Phys. Chem. C, Vol. 112, No. 14, 2008 5399

Figure 2. Infrared spectroscopy of (a) as-synthesized FePt nanoparticles (black) and dopamine-treated nanoparticles (red). (b) Absorbance spectra of FePt nanoparticle siloxane polymer composites depending on the loading densities of dopamine-treated FePt nanoparticles with 15 mg (red), 25 mg (black), and 40 mg (green) of FePt nanoparticles.

taken in a tapping mode with a silicon nitride tip (Model # OTESPA7 from Veeco) using a Digital Instruments Nanoscope III. To examine the phases of as-synthesized nanoparticles, the nanoparticles were first dispersed in hexane solution, and then the solution was dropped onto mica. Also, topography and surface roughness of siloxane elastomers crosslinked by FePt, or NiPt nanoparticles were measured with AFM. Transmission electron microscopy (TEM) was performed using a JEOL JEM - 1010 electron microscope with a 100-kV accelerating voltage. Synthesis and Characterization of FePt or NiPt Nanoparticles. For the synthesis of nanoparticles, 2.32 mmol (590 mg) of Fe(acac)2 (acac ) acetylacetonate), 2.32 mmol (782 mg) of PtCl4, and 2.90 mmol (750 mg) of 1,2-hexadecanediol were added into the mixture of 20 mL of n-dioctyl ether and 8 g of nonadecane in a 50-mL three-necked round-bottomed flask equipped with a refluxing condenser. The solution was heated for 15 min under nitrogen atmosphere at 100 °C, and then oleic acid (0.5 mL, 1.58 mmol) and oleylamine (0.52 mL, 1.58 mmol) were added into the reaction mixture. The reaction temperature was increased to 300 °C and kept at that temperature for 10 min. Lithium triethylborohydride (2.5 mL, 1.0 M solution in THF) was added dropwise into the mixture. The reaction mixture was further stirred for 30 min. Then, the reaction mixture was cooled down to room temperature with a continuous stirring. The precipitates were collected by centrifugation at 6000 rpm and then washed with hexane. The purification step was repeated

three times to remove the inorganic salts. About 0.185 g of FePt nanoparticles was obtained in 64% yield. The same procedure was applied for the synthesis of NiPt nanoparticles with a starting precursor as Ni(acac)2 (597 mg, 2.32 mmol). The phase purity of the nanoparticles was confirmed using a Rigaku RAD diffractometer (20 kW) utilizing Cu KR radiation (λ ) 1.54056 Å). The lattice parameters were found to be a ) 3.843 Å (FePt) and a ) 3.755 Å (NiPt) with face-centered cubic (fcc, Fm-3m) structures. Surface Modification of Nanoparticles with N-Allyl-3,4dihydroxyphenylethylamine. For the surface modification of nanoparticles, 40 mg of FePt nanoparticles in 10 mL of hexane solution was mixed with 20 mg (0.103 mmol) of N-allyl-3,4dihydroxyphenylethylamine, 5 mL of methanol, and 10 mL of H2O. After the pH value was adjusted to about 4, the mixture was sonicated for 1 h. The mixture was centrifuged to remove the aqueous phase. The obtained pellets were washed with methanol several times. Solvents were removed under vacuum to obtain dopamine-modified FePt nanoparticles. Hydrosilation Reaction for Constructing Nanoparticle Polymer Composites. Dopamine-treated FePt nanoparticles (25 mg), which were already dispersed in the mixtures of hexane (5 mL), polymethylhydrosiloxane (0.08 mL), and dichloro-(1,5cyclooctadiene)platinum (0.2 mg), were placed in a 25-mL oneneck round-bottom flask. Toluene (15 mL) was added to the reaction mixture and then stirred at 80 °C under argon


Song et al.

Figure 3. XPS measurements of the as-synthesized MPt (M ) Fe, Ni) nanoparticles (black), dopamine-treated nanoparticles (red), siloxane elastomers crosslinked by surface-modified MPt nanoparticles (green). (a) C 1s scan, (b) O 1s scan in which cyan and blue lines are deconvolution curves of dopamine-treated FePt nanoparticles. (c) N 1s scan, (d) Pt 4f scan, (e) Fe 2p scan, (f) Si 2p scan.

atmosphere. The gel-like products were observed after about 20 min of stirring. Toluene (20 mL) was added to the gel-like products. To obtain the final nanoparticles composites, the reaction products were filtered and washed with toluene. Fourier Transform-Infrared Spectroscopy (FT-IR) and X-ray Photoelectron Spectroscopy (XPS) Measurements. FTIR spectra of as-synthesized nanoparticles, dopamine-modified nanoparticles, and nanoparticle siloxane polymer composites were taken with a Perkin-Elmer Paragon 1000 FT-IR spectrometer in a transmittance mode. A thin KBr pellet was made with an amount of 10 mg of polymer composites and 70 mg of KBr, which mixture was finely grinded before the thin film formation. All spectra were collected using 512 scans at a resolution of 2 cm-1. Samples for the XPS measurements were perfectly dried under vacuum. Then the samples are used without further treatment for XPS measurements. XPS data were collected with an AXIS Ultra DLD (Kratos Analytical) instrument equipped with a monochromatic Al KR X-ray source (1486.7 eV). All XPS spectra were curve-fitted with the CasaXPS Program (Casa Software Ltd). Nanoindentation Experiment. Nanoindentation of the siloxane elastomers crosslinked by FePt or NiPt nanoparticles was preformed with a Hysitron Triboscope Nanomechanical Test Instrument (Minneapolis, MN). The Berkowitch tip (142.3°) and 90° tips were used to measure the mechanical properties. Loadcontrolled testing mode was performed with the 10 µN/s loading and unloading rates and with the holding time of 5 s. Before the indentation measurement on the nanoparticle polymer composites, the tip was calibrated with the quartz sample using a known reduced elastic modulus (69.6 MPa).32 Unloading curves were used to analyze the reduced elastic modulus and the hardness of the samples.

Results and Discussion FePt and NiPt nanoparticles were synthesized by the usual organometallic precursor method33 with a modification using M(acac)2 (M ) Fe, Ni) and PtCl4 as the starting precursors. The crystal structure of nanoparticles based on the powder X-ray diffraction is the fcc with a cell parameter of a ) 3.843 Å (FePt) and a ) 3.755 Å (NiPt) (Supporting Information). Reaction of diallyl carbonate with dopamine hydrochloride in 1,4-dioxane at room temperature gave the allyl-functionalized dopamines, which were followed by treatment with FePt and NiPt nanoparticles in a weak acidic condition. Normal metal-ligand bonding between a dihydroxy group in dopamine and the metal surface was detected, which was reported in the recent study on the change of metal coordination numbers before and after surface treatment of nanoparticles.34 The resulting dopaminetreated nanoparticles were reacted with polymethylhydrosiloxane in a hydrosilation reaction catalyzed by Pt(1,5-cyclooctadiene)Cl2 at 80 °C (Scheme 1). After unreacted nanoparticles and siloxanes were removed by washing with toluene, the nanoparticle polymer composites were obtained as a swollen state. Topography and phase images obtained by AFM as well as TEM images are shown in Figure 1. The swollen state of FePt nanoparticle polymer composites in a toluene solvent (parts a and b of Figure 1) reveals the surface shapes as was reported in the previous studies on CdSe nanoparticles embedded into PS-b-P2VP (PS, polystyrene; P2VP, poly(2-vinylpyridine)).35 Deswollen states of FePt (parts c and d of Figure 1) and NiPt (parts e and f of Figure 1) nanoparticle polymer composites reveals more clearly the overall morphology of the composites. It is believed that the dopamine-modified nanoparticles are dangling from the siloxane backbones with a certain distance of allyl-dopamine, so that the product is 3-dimensional siloxane



Figure 4. Field- and temperature-dependent magnetization of FePt nanoparticles and FePt-elastomer composites. Field-dependent measurement of (a) FePt nanoparticles and (c) FePt-elastomer composites was performed at 300 K. Temperature-dependent magnetization measurements of (b) FePt nanoparticles and (d) FePt nanoparticle polymer composites were performed with the applied field of 100 Oe. The closed symbol is ZFC, and the open symbol is field-cooled (FC) data.

elastomer networks in which the nanoparticles are acting as the crosslinkers, as is the case in side-chain liquid crystal elastomers.36 The monodispersity of nanoparticles within this polymer nanoparticle composite was identified on the TEM images of the as-synthesized FePt nanoparticles (Figure 1g) and the nanoparticle polymer composites (Figure 1h). This nanoparticle composite is constructed from the forced arrangement of nanoparticles within the siloxane polymers, which is different from the self-assembled arrangement stabilized by the longchain organic surfactants. Dopamine attachment to the surfaces of nanoparticles was confirmed by infrared spectroscopy (IR, Figure 2) and XPS (Figure 3) showing evidence of ligand exchange from oleic acid to allyl-functionalized dopamine.37-40 In Figure 2a, the black line is for the FePt nanoparticles surrounded by oleyl amine/ oleic acid, and the red line is for the dopamine-treated FePt nanoparticles. Amide (1690 cm-1) and allyl (1510 and 1246 cm-1) functionalities of dopamine were introduced with the reduction of long alkyl chain surfactants. The absorbance spectra of FePt nanoparticle siloxane polymer composites (Figure 2b) show the bonding dependence of dopamine-treated FePt nanoparticles on the nanoparticle loading densities. As the density of nanoparticles was increased without changing the amount of the dopamine added, the absorbance intensity of Si-H bond (2190 cm-1) was reduced. It means that more allyl functionalities in dopamine were bonded to the siloxane backbones in a hydrosilation reaction. The peak at 1100 cm-1 is from the

Si-O-Si bonding, and the sharp peak at 1240 cm-1 is from the Si-CH3 bonding. Amide linkage (1690 cm-1) was maintained after the final crosslinking step. XPS measurements of the as-synthesized MPt (M ) Fe, Ni) nanoparticles, dopamine-treated nanoparticles, and siloxane elastomers crosslinked by surface-modified MPt nanoparticles were carried out (Figure 3). A high-resolution C 1s scan (Figure 3a) shows that the long alkyl chain at 283.0 eV and the acid functionality at 288.3 eV of as-synthesized FePt nanoparticles (black) were replaced with allyl-functionalized dopamine (red) with a removal of oleic acid and oleyl amine.41-43 The C peak of the Si-OCH3 bonding in siloxane elastomer crosslinked by this dopamine-treated FePt nanoparticles shows at 287 eV (green). In Figure 3b, the peak at 530.8 eV (black) is due to the oleic acid in as-synthesized FePt nanoparticles. O 1s spectra of the dopamine-treated FePt nanoparticles (red) show a broader peak, which can be deconvoluted into two peaks, one at 529.2 eV of dihydroxy bonding (cyan) and the other at 530.7 eV of the amide linkage (blue). In elastomers crosslinked with FePt nanoparticles (green line), O 1s spectra can be split into 3 peaks, at 530.1 eV (dotted dark blue) from siloxane linkage (Si-O-Si), at 529.5 eV (dotted orange) from dihydroxy bonding, and at 531.3 eV (dotted purple) from oleic acid. On the N 1s scan (Figure 3c), the peak (at 399.0 eV) originated from the oleyl amine in as-synthesized FePt nanoparticles (black) was replaced with the amide bonding of allyl-functionalized dopamine at 397.8 eV in dopamine-treated FePt nano-


Figure 5. (a) Load-displacement curves of simple siloxane elastomer crosslinked by diallyl carbonate (closed symbol) and NiPt nanoparticle siloxane polymer composites (open symbol) with a maximum applied load of 1500 µN. (b) The gradiant indent image was obtained with AFM on the NiPt nanoparticle polymer composite. The scale bar is 2 µm, and the right gradient force bar has a unit of µN.

Song et al. particles (red). It might be interpreted due to the exchange of the oleyl amine surfactant to allyl-functionalized dopamine. In siloxane elastomers crosslinked by dopamine-treated FePt nanoparticles, the N 1s peak (green) moves to 398.1 eV. A Pt 4f scan (Figure 3d) showed that the oxidized states of Pt [at 74.2 eV (4f5/2) and 71.3 eV (4f7/2)] were predominant on the surfaces of as-synthesized FePt nanoparticles (black).44-46 However, in as-synthesized NiPt nanoparticles (red), both the metallic Pt (72.7 and 69.2 eV)) and the oxidized states of Pt (74.2 and 71.3 eV) peaks are observed. The oxidized Pt is thought to be covered on the surfaces of NiPt nanoparticles. In the siloxane elastomers crosslinked by the dopamine-treated FePt nanoparticles (green), two peaks are assigned to the oxidized states of Pt since the bonding energies are similar to the assynthesized nanoparticles appearing at 74.1 and 71.1 eV. From the Fe 2p scan (Figure 3e), two broad peaks centered at 710.2 eV (2p3/2) and 728.2 eV (2p1/2) are observed. The Fe 2p peaks are broad in nature due to the final state effects, and thus chemical shifts are not large enough to distinguish metallic peak from oxidized peaks (+2 and +3) using Al KR as a source if all oxidized states are mixed in a sample. However we can interpret that the two peaks are from the mixture of metallic and oxidized Fe. The peak maximum at 710.2 eV (2p3/2) are close to metallic Fe and the satellite peaks at 718.2 and 732.7 eV due to the Fe3+ oxidation state.47-49 No iron or nickel were detected in the nanoparticle polymer composites due to the high binding energies of Fe, Ni 2p electrons, which are hardly emitted with X-ray when they are underneath the polymers.50 In Figure 3f, the Si 2p peak at 101 eV is due to the oxidized Si in siloxane elastomers crosslinked by dopamine-treated FePt nanoparticles. Magnetic properties of FePt nanoparticles and their polymer composites were studied in order to look at the influences of supporting matrix on the magnetic properties of superparamagnetic particles. Sample for the magnetic measurement was

Figure 6. (a) Nanoindentation curves for siloxane elastomers crosslinked by (black square) diallyl carbonate and (blue circle) dopamine-treated FePt nanoparticles. (b) Reduced elastic modulus (Er) and (c) hardness (H) of two elastomers.



Figure 7. (a) Nanoindentation curves for siloxane elastomers crosslinked by dopamine-treated FePt nanoparticles depending on the densities of nanoparticles. (b) Reduced elastic modulus (Er) and (c) hardness (H) of the elastomers described in (a). The graph shows that the densities of nanoparticles affect the mechanical properties such as more nanoparticles enhance the tensile strength of the elastomers.

prepared with 40 mg of dopamine-modified FePt nanoparticles and 0.08 mL of polymethylhydrosiloxane. The sample was dried in vacuum to remove toluene before the measurement. Similar field-dependent behavior was observed in FePt elastomer composites. They do not reach saturation at the high applied field, which is believed to be due to the lack of interparticle coupling of FePt nanoparticles within the diamagnetic siloxane polymers. On the basis of the field-dependent measurements, polymer composites have more linear dependence of the magnetization on the applied field because there is no strong magnetic order within the polymer composites for the spins in nanoparticles to align in certain anisotropic direction. Temperature-dependent magnetization of FePt nanoparticles also confirms superparamagnetic behavior (Figure 4b). Blocking temperature (TB), over which the net magnetic moment starts to decrease with increasing temperature in zero-field-cooled (ZFC) measurement, was 65 K in FePt nanoparticles (closed symbol, Figure 4b). In the polymer composites crosslinked by FePt nanoparticles, the blocking temperature was lowered to TB of 42 K (Figure 4d). The decrease of blocking temperature is thought to be due to the decrease of dipolar coupling between nanoparticles in the deswollen siloxane polymer medium so that the energy barrier for the nanoparticles to be in the unblocked state is lowered.31,51-53 Mechanical properties of FePt nanoparticle polymer composites were investigated with load-displacement curves in the nanoindentation tests (Figure 5). For comparison, siloxane elastomers crosslinked by diallyl carbonate without using FePt nanoparticles were prepared and tested with the same maximum

load (closed symbol in Figure 5, 1500 µN). Analysis of the unloading curves revealed higher reduced elastic modulus (Er ) 1.5 GPa) and hardness (H ) 0.126 GPa) for FePt nanoparticle composites than simple siloxane elastomers (Er ) 0.67 GPa, H ) 0.115 GPa). These enhanced mechanical properties were also reported with 10 times higher Young’s modulus in other nanoparticle polymer composites such as silica-filled latex films than in the simple latex.54,55 With the other maximum loads, the same trends were observed in FePt nanoparticle polymer composites (blue line, Figure 6a) and elastomers crosslinked by diallyl carbonate (black line, Figure 6a). The average Er value of FePt nanoparticle polymer composites was 1.560 ((0.054) Gpa, and the Er value of elastomers crosslinked by diallyl carbonate was 0.608 ((0.048) GPa (Figure 6b). Hardness measurements also indicated higher tensile strength in FePt nanoparticle polymer composites (0.132 ( 0.003 GPa) than in the elastomers crosslinked by diallyl carbonate (0.109 ( 0.005 GPa) (Figure 6c). Crosslinking density of FePt nanoparticles in the elastomers is also a factor to provide higher mechanical strength to the elastomers (Figure 7). With the fixed amount of siloxane backbones as 80 mg, three different elastomer networks crosslinked with 15, 25, and 40 mg surface-modified FePt nanoparticles were prepared through the hydrosilation reaction. The unloading curves showed clear differences among these three samples. As the loading density of FePt nanoparticles increased, the contact depth of the indentation tip decreased from 1510 nm (at 15 mg FePt) to 1150 nm (at 25 mg FePt) and to 1000 nm (at 40 mg FePt) (Figure 7a). Reduced elastic modulus

5404 J. Phys. Chem. C, Vol. 112, No. 14, 2008 (Er) increased from 1.602 ((0.012) GPa (at 15 mg FePt) to 1.653 ((0.005) GPa (at 25 mg FePt) and to 1.728 ((0.007) GPa (at 40 mg FePt) as the loading density of FePt nanoparticles increased (Figure 7b). Hardness (H) also increased from 0.1357 ((0.003) GPa (at 15 mg FePt) to 0.2294 ((0.012) GPa (at 25 mg FePt) and to 0.3014 ((0.027) GPa (at 40 mg FePt). The increased crosslinking density from the hydrosilation reaction between siloxane backbones and allyl-modified dopamines on the surfaces of nanoparticles leads to more rigid structure of elastomers. Conclusions Elastomeric nanoparticle polymer composites were prepared through a hydrosilation reaction with dopamine-treated FePt and NiPt nanoparticles as crosslinkers. The siloxane networks were constructed with a covalent linkage. The monodispersed arrangement of nanoparticles was observed throughout the networks without local aggregation. Higher Er and H values proved their enforced mechanical properties. The superparamagnetism of the nanoparticles remained after the surface treatment and polymerization reactions in FePt nanoparticle polymer composites. The method described here could also be applied to other nanosized materials such as nanotubes, nanobelts, and nanocubes in order to make monodisperse hybrid composites. Acknowledgment. This work is financially supported by the Ministry of Education and Human Resources Development through the fostering project of the New University of Regional Innovation. Supporting Information Available: X-ray powder diffraction and additional details on AFM and TEM images of nanoparticle polymer composites with height and phase scale bars. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Go´mez-Romero, P.; Sanchez, C. Functional Hybrid Materials; Wiley: New York, 2004. (2) Althues, H.; Henle, J.; Kaskel, S. Chem. Soc. ReV. 2007, 36, 1454. (3) Balazs, A. C.; Emrick, T.; Russell, T. P. Science 2006, 314, 1107. (4) Hao, E.; Lian, T. Chem. Mater. 2000, 12, 3392. (5) Matsui, J.; Akamatsu, K.; Nishiguchi, S.; Miyoshi, D.; Nawafune, H.; Tamaki, K.; Sugimoto, N. Anal. Chem. 2004, 76, 1310. (6) Harada, M.; Einaga, H. Langmuir 2007, 23, 6536. (7) Akamatsu, K.; Tsuboi, N.; Hatakenaka, Y.; Deki, S. J. Phys. Chem. B 2000, 104, 10168. (8) Dobson, J. Drug DeV. Res. 2006, 67, 55. (9) Franc¸ ois, N. J.; Allo, S.; Jacobo, S. E.; Daraio, M. E. J. Appl. Polym. Sci. 2007, 105, 647. (10) Jurgons, R.; Seliger, C.; Hilpert, A.; Trahms, L.; Odenbach, S.; Alexiou, C. J. Phys.: Condens. Matter 2006, 18, S2893. (11) Zhao, D.-L.; Zhang, H.-L.; Zeng, X.-W.; Xia, Q.-S.; Tang, J.-T. Biomed. Mater. 2006, 1, 198. (12) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167. (13) Hergt, R.; Dutz, S.; Mu¨ller, R.; Zeisberger, M. J. Phys.: Condens. Matter 2006, 18, S2919. (14) Grubbs, R. B. J. Polym. Sci. Part A: Polym. Chem. 2005, 43, 4323. (15) Sun, S.; Anders, S.; Hamann, H. F.; Thiele, J.-U.; Baglin, J. E. E.; Thomson, T.; Fullerton, E. E.; Murray, C. B.; Terris, B. D. J. Am. Chem.

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