Magnetic-Field-Induced Formation of Superparamagnetic Microwires

Article pubs.acs.org/JPCC

Magnetic-Field-Induced Formation of Superparamagnetic Microwires in Suspension Maryam Ghazi Zahedi,†,§ Daniela Lorenzo,† Rosaria Brescia,‡ Roberta Ruffilli,‡ Ioannis Liakos,† Teresa Pellegrino,‡ Athanassia Athanassiou,† and Despina Fragouli*,† †

Smart Materials, Nanophysics, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy Nanochemistry, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy § Department of Physics, University of Genova, via Dodecaneso, 33, 16146 Genova, Italy ‡

S Supporting Information *

ABSTRACT: We demonstrate the formation of stable magnetic microwires (MWs) in solution starting from a highly diluted solution of monomer−thermal initiator− superparamagnetic nanoparticles (SMNPs). Under an external magnetic field (MF) the SMNPs get closely packed into wire-like assemblies that become permanently linked due to simultaneous thermal polymerization of the monomer. As the SMNPs assemble in the form of wires under MF, the concentration of the monomer chains adsorbed onto them increases in the near proximity of these assemblies, promoting the polymerization process during heating. This combined process causes the permanent bonding among the SMNPs, forming smooth MWs with metallic character. Detailed microscopic and spectroscopic studies reveal the mechanism of the process and designate the importance of the external MF, the thermal polymerization, and the high dilution factor of the reaction solution for the formation of free-standing uniform wires with controlled size. This method leads to a novel approach to form long magnetic wires with smooth contour and regular shape, which can be used in various fields of applications like in biomedicine, chemistry, fluidics, etc.

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SMNPs is not so efficient owing to flipping magnetic moments caused by thermal fluctuations.9 Furthermore, only a few methods are utilized so far for the formation of stable and dispersed magnetic wires in solutions by MFI assembly.12−15 In fact, 1D wires fabricated by MFI magnetic dipolar coupling of bare individual SMNPs revert gradually after switching off the external MF,13 likely due to the single-domain magnetic features of the constituents with negligible coercivity and remanence.9,13 For this reason various macromolecules or polymeric materials4,9,12 are utilized as permanent coating/ linkers between the NPs during the MFI assembly, resulting in the formation of core−shell hybrid nanocomposites with the presence of the organic part as the main component. Indeed, recently, MFI assembly of solutions of SMNPs functionalized with photoresponsive ligands was utilized for the realization of dynamic superstructures with controllable aspect ratio, using UV light as an external stimulus together with the MF.16 However, such superstructures are not permanent, since as soon as the stimuli are removed, they are destroyed. Other analogous mechanisms are used to obtain permanent connections of 1D structures. Bamboo-like poly(methyl methacrylate) (PMMA)−iron nanowires are formed using MFI assembly during polymerization upon hard X-ray

INTRODUCTION Magnetic nanoparticles (NPs) have drawn significant attention due to their properties, while their incorporation into organic matrices results in the inheritance of their characteristics to the formed hybrid structures. Up to the present, such soft composite materials have been developed and used as magnetic actuators,1 sensors,2 microfluidic mixers,3 etc. Furthermore, the incorporation or the formation of one-dimensional (1D) magnetic nanostructures into polymers has received great attention due to their highly anisotropic properties such as magnetic,1,4 electrical,5 or mechanical.6,7 The directed assembly of magnetic NPs in polymer matrices forming wires is a facile and straightforward technique, and it has been developed through diverse methods, such as magnetic-field-induced (MFI)1,4,6,8 and templating9 assembly processes. The fabrication of metallic wires in the presence of a polymer by MFI assembly leads to the formation of solid films with periodically assembled magnetic wires in the whole volume of the polymer. To induce the assembly, different routes have been exploited including solvent evaporation of a polymer nanocomposite solution4 and radiation-induced1,10 or heat-induced8 polymerization of a mixture of monomer and NPs. Despite the state-of-the-art research in fabrication of wires in polymer matrices, the manufacturing of magnetic wires in solution following the same approach is still a challenge. Unlike ferromagnetic NPs whose self-assembly is dipolar-directed,9,11 in the absence of a magnetic gradient such a process for © XXXX American Chemical Society

Received: August 6, 2014 Revised: November 4, 2014

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synchrotron radiation.17 Moreover, Fe3O4/P(GMA−DVB) peapod-like nanochains have been successfully synthesized by MFI precipitation polymerization using Fe3O4 as building blocks and P(GMA−DVB) as linker.13,18 The above-mentioned studies are mainly focused on the formation of core−shell polymeric nanocomposite structures where the morphology of the wires is neither smooth nor homogeneous or their dimensions are in the nanometer scale. Although this could be well applied in the biological field, the morphology and the dimensions may limit their applicability in mesoscale applications, like in microfluidics and micromixers19−21 or probes for local mechanical properties measurements,22,23 etc. For this reason, other methods like microfabrication,21 templating the core−shell magnetic beads under MF,24 or the coating of nonmagnetic MWs with SMNPs7 have been utilized for the formation of the superparamagnetic MWs following complicated fabrication steps. In the current work, we present a single step fabrication technique for the formation of robust magnetic MWs in solution with smooth contour, using simultaneous heating and applying external MF to a solution containing colloidal cubic SMNPs and methyl methacrylate (MMA) monomer. Heating induces the polymerization of MMA into PMMA polymer that acts as a permanent linker between the SMNPs. In this way, after the removal of the field, MWs with permanently defined shapes are formed. The concentration of both SMNPs and the monomer in the initial solution is the basic factor that controls the formation of separate wires with highly inorganic character. We prove that the formation of MWs is due to the polymer grafting onto SMNPs combined with their MFI alignment process. The proposed method is simple, straightforward, and cost-effective in that excludes the use of complicated procedures previously applied to make robust 1D magnetic structures. Such structures can be used as needle sharp in hyperthermia therapy17 and in various applications such as microfluidic mixers19,20 or micromagnetic stirrers,21 as an alternative to traditional microfabrication techniques to create patterns for separation and detection of biomolecules,17,25,26 colloidal separation,19 etc.

colloidal NPs (0.6 wt % with respect to MMA) was mixed with a defined amount of toluene (10:1 weight ratio of toluene: MMA). An aliquot (250 μL) of the above-mentioned solution was placed in an oven at 80 °C under an external MF of 80 mT for 17 h in order to thermally polymerize the MMA. After the thermal polymerization, the liquid components were removed and the formed wires were washed with chloroform (CHCl3) repeatedly for 3−4 times, using the magnetic separation process. Eventually, the wires were dispersed in chloroform for further investigation. Characterization. Optical microscopy images were acquired by an optical microscope (Nikon eclipse 80i). Scanning electron microscopy (SEM) analyses were carried out in highvacuum conditions with a JEOL JSM-6490LV instrument. Bright-field transmission electron microscopy (BF-TEM), selected area electron diffraction (SAED), energy-filtered TEM (EFTEM), and high-resolution TEM (HRTEM) investigations were performed by a JEOL JEM-2200FS instrument operating at 200 kV, equipped with a CEOS image aberration corrector and an in-column energy filter (Ω type). For EM analyses, 10 μL of the MWs suspension was drop-casted onto polished Si wafers (SEM) and Cu grids covered with a holey C film (TEM). EFTEM mapping was carried out at the carbon K (ΔE = 284 eV, 20 eV slit width) and at the Fe L23 (ΔE = 708 eV, 40 eV slit width) core-loss edges using the three-window method. The statistical analysis of the length of the MWs was done by “ImageJ” image processing software.29 The chemical composition of the MWs was analyzed by Fourier transform infrared spectroscopy (FTIR) using a Bruker Vertex 70 spectrometer. Surface chemistry of both SMNPs and MWs was characterized through X-ray photoelectron spectroscopy (XPS). Briefly, a Specs Lab2 electron spectrometer, equipped with an Mg anode X-ray source (set at 1253.6 eV) and with a Phoibos analyzer Has 3500 (hemispherical energy analyzer), was used. The applied voltage of the Mg Kα X-ray source was 10 kV, and the applied current was 15 mA. The pressure in the analysis chamber was approximately 2 × 10−9 mbar. For the wide scans, the energy pass was set at 90 eV, the energy step at 0.5 eV, and the scan number at 4. For the narrow high-resolution scans the energy pass was set at 60 eV, the energy step at 0.2 eV, and the scan number at 15. All collected spectra were analyzed with Casa XPS software version 2.3.15. The calibration of the binding energy scale was set to correspond to the C 1s peak for the wide scans and to C−C bond for the narrow scans at 285.0 eV as reference, and the binding energy of the O 1s peaks was corrected by the same eV shift as the one used to set the C 1s at 285.0 eV. The Shirley background type was used to determine the O 1s region of the peaks and to create the necessary background where the components were added. Furthermore, the Voigt profile was used to fit the components of the acquired O 1s peaks. To quantify the spectra, a fixed analyzer transmission (FAT) operation mode was used during the XPS experiment. The entrance and exit slit sizes were set to 7 × 20 mm and fully open, respectively. The transmission optimization was set to big X-ray spot size of 5 mm. The transmission correction was applied, by default, through the software, and the escape depth correction was performed by setting its value to −0.7414 in the Intensity Calibration part of this software. The relative sensitivity factors of the detected elements used to calculate the relevant atomic concentrations are Fe 2p = 16; F 1s = 4.26,

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EXPERIMENTAL METHODS Materials. Methyl methacrylate monomer (MMA, purity 99%) and the thermal initiator (TI) 2,2-azobis(isobutyronitrile) were purchased from Sigma-Aldrich. Acetone, 2-propanol, methanol, ethanol, and toluene solvents, iron(III) acetylacetonate (≥99.9% trace metals basis), manganese(II) acetylacetonate, hexadecanetiol (90%), dodecylamine (98%), lauric acid (99%), and benzyl ether (90%) were purchased from SigmaAldrich and used without further purification. Colloidal MnFe2O4 SMNPs with average size of 9 nm were synthesized following a slightly modified27 protocol previously reported;28 2 mmol of Fe(acac)3, 1.25 mmol of Mn(acac)2, 10 mmol of hexadecanediol, 6 mmol of dodecylamine, 6 mmol of dodecanoic acid, and 20 mL of benzyl ether were vigorously mixed in a round-bottom flask, under a nitrogen atmosphere for 60 min at 140 °C, then for 120 min at 210 °C, and finally for 60 min at 300 °C. The product was washed four times with a mixture of acetone, isopropanol, and ethanol and then dissolved in toluene. Wire Formation. The MWs are formed by thermal polymerization of MMA monomer mixed with colloidal SMNPs, TI, in toluene, under an external MF. The initial solution of MMA, TI (0.2 wt % with respect to MMA), and B

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Figure 1. (a) Optical images of free-standing MWs with low (scale bar = 100 μm) and (inset) high magnification (scale bar = 50 μm) together with (b) SEM images with low (scale bar = 5 μm) and (inset) high magnification (scale bar = 1 μm).

Figure 2. (a) BF-TEM image of MWs of SMNPs locked via PMMA partially suspended on a hole in the C support film (scale bar = 200 nm). (b) Elastical EFTEM image (zero-loss) (scale bar = 50 nm) of the portion marked in (a) and (c) corresponding EFTEM map for carbon (scale bar = 50 nm), where the bright areas indicate the presence of carbon. (d) Elastically filtered HRTEM image of another MW region suspended on vacuum (scale bar = 10 nm) and (e) corresponding EFTEM map for carbon (scale bar = 10 nm). (f) BF-TEM image of the edge of a single MW (scale bar = 200 nm) and (g) the corresponding SAED pattern, showing perfect agreement with database powder X-ray diffraction data for MnFe2O4 (JCPDS #74-2403).

A more detailed aspect of the surface morphology is revealed by the TEM study showing that the MWs are composed of SMNPs assembled densely close to each other (Figure 2a,b,d). EFTEM mapping of carbon on peripheral regions of the MWs demonstrates that an extremely low amount of C surrounds individual SMNPs, as clearly seen at the edges of the MWs (Figure 2c). This is also confirmed by the HRTEM images acquired at the borders of the MWs, showing that the organic coating of the edging SMNPs is very thin (Figure 2d,e), even compared to the organic coating of the as prepared colloidal SMNPs (Supporting Information Figure S3). This fact guides us to the conclusion that the process of mixing the colloidal SMNPs with MMA monomer and the subsequent heating process result in the removal of the original capping molecules and their replacement with the much shorter MMA molecules. The magnetic assembly plays an important role for the localized increase of the MMA concentration which further makes possible the polymerization process and the subsequent permanent linking of the SMNPs in the form of MWs. In fact, in the absence of external MF, small beads are formed with an organic coating of similar thickness as the one of the bordered SMNPs of the MWs, much thinner compared to that of the initial capping molecules (Supporting Information Figure S4). Additionally, SAED patterns on portions of individual MWs and individual beads do not show preferential crystalline orientation in either case (i.e., texturing), albeit the dramatically different conditions in which they are formed (compare Figure 2f,g with Figure S4g,h). This is in agreement with the

Mn 2p = 13.6; O 1s = 2.85; C 1s = 1; Cl 2p = 2.36; P 2p = 1.25; Si 2p = 0.865.

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RESULTS AND DISCUSSION The fabrication of magnetic nanocomposites following the thermal polymerization has already been proved to form homogeneous solid films.30 Indeed, in our case, thermal polymerization of MMA-TI-SMNPs solution under an external MF led to the formation of wires in the PMMA solid matrix aligned parallel to the field direction (Supporting Information Figure S1) as also demonstrated in previous studies.8,10 However, if the MMA-TI-SMNPs solution is highly diluted (10:1 weight ratio of solvent: MMA), the obtained system is significantly different, since separate MWs are obtained in solution instead of bulk nanocomposites. As shown in Figure 1, structurally robust MWs with average diameter and length of 0.6 ± 0.3 and 45.0 ± 28.3 μm, respectively, are formed in solution (Supporting Information Figure S2). The SEM study of Figure 1b confirms that the MWs are elongated structures with smooth surface. Neither electrostatic charging nor sample damage was observed for these samples during SEM investigations, hinting at the presence of a low amount of polymer in the MWs. It is noteworthy that without the presence of SMNPs, the initial solution (10:1 weight ratio of toluene:MMA) was not polymerized despite the rest of the parameters were kept precisely the same. C

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Figure 3. FTIR spectra of thermally polymerized PMMA, MWs, MMA−TI−SMNPs mixture (Mix), and SMNPs.

Figure 4. (a, b) Wide scan and (c, d) O 1s regions of XPS spectra of SMNPs and MWs, respectively.

between 1300 and 1900 cm−1 as shown in Figure 3b. In particular, the presence of PMMA polymer in the structure of MWs is confirmed by the presence of the characteristic peak of υ(CO) mode at 1730 cm−1.32 Furthermore, all spectra, except the one of PMMA, present the characteristic IR bands for metal carboxylates in the range of 1300−1700 cm−1.33 The position and separation between the asymmetrical and symmetrical vibration of the ν(COO−) bands can be used to deduce the type of coordination between the carboxylate group and the metallic surface in each case.33 In the case of the mixture, not polymerized sample, there are two absorption

superparamagnetic nature of the SMNPs used, where in the absence of any MF no orientation should be observed. To better understand the mechanism responsible for the formation of the MWs, a detailed chemical analysis of the involved components is conducted. In particular, the FTIR spectra of the formed MWs, SMNPs, thermally polymerized PMMA, and MMA−TI−SMNPs mixture are presented in Figure 3. In all cases, the absorption peaks present between 560 and 580 cm−1 correspond to the Fe−O bond, confirming thus the presence of the metallic NPs.31 The most important differences between the spectra are recorded in the range D

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peaks at 1566 and 1427 cm−1 originating from the symmetrical and asymmetrical stretching of the ν(COO−) bands which demonstrate that the MMA monomer is coordinated with the surface of the metallic SMNPs through carboxylate groups, with the bridging mode.33 The same coordination mode is also observed the case for the MWs, with the two absorption peaks at 1566 and 1415 cm−1. The significant difference in the spectrum of the colloidal SMNPs at the above-mentioned range reveals that the surface interaction of the SMNPs is mainly dictated by the acrylate in the case of the MMA−TI−SMNPs mixture and of the MWs, making MMA or PMMA the principal organic substance present on the surface of the inorganic materials. In addition, the shift of the most predominant band of pure PMMA at 1730 to 1720 cm−1, originating from the υ(CO) mode, in the spectrum of MWs reveals the SMNPs−PMMA chemical linkage through the conjugation of CO with NPs.32 Besides, the vibration of CC double bond at 1643 and 1652 cm−1 ascribed to MMA can be found for MMA−TI−SMNPs mixture and MWs, respectively. However, as it is clearly shown in Figure 3, the ratio of υ(CC) to Fe−O absorption peaks is about 6 times bigger for the MMA−TI−SMNPs mixture in comparison with MWs. This notable decrease in the case of the MWs manifests the reduction of MMA by virtue of its polymerization. The XPS analysis of the fabricated MWs in comparison with the as-prepared colloidal SMNPs is shown in Figure 4. The wide scan spectra of SMNPs (Figure 4a) reveals the presence of Mn, Fe, and O attributed to the SMNPs metallic core, C which together with O is the representative of the organic capping molecules, Si attributed to the incomplete coverage of the silicon wafer substrate by the individual components, and small quantities of other elements attributed to residuals of the colloidal fabrication process. Similar elements are present in the outermost surface (10 nm) of MWs as shown in Figure 4b. In this case the signal from the Si substrate is less, possibly attributed to the higher coverage of the Si substrate by the MWs and to their thickness (ca. 600 nm), which is above the instrumental detection limit (ca. 10 nm). In order to explore the presence of PMMA on MWs, the C/O ratio in both cases is calculated, after normalizing with the signal of silicon oxide due to the substrate (Supporting Information Figure S5). For SMNPs the C/O ratio is 3.27, much different compared to the one of the MWs, which is 2.33. The latter value is very close to the value reported for pure PMMA, which is 2.5.34 This further confirms the presence of PMMA in the structure of MWs, indicating its essential role in keeping SMNPs permanently linked in the form of wires. The O 1s high-resolution spectra are depicted in Figures 4c and 4d for SMNPs and MWs, respectively. The presence of iron oxide is found35 for both cases at approximately 530 eV. A high Si−O−Si peak at approximately 532 eV is detected (Figure 4c) for SMNPs due to the signal from the substrate36 while the CO and C−O−R peaks of SMNPs are due to the oxygen environment of the capping molecules. The spectrum in the case of MWs changes significantly as shown in Figure 4d. In fact, the CO and C−O−R peaks appear in lower binding energies, and this can be attributed to the presence of negatively charged PMMA molecules due to their bonding with SMNPs.37 The absence of Si−O−Si peak for MWs is the result of the deposition of more material (higher surface coverage) with higher thickness, preventing any signal from the substrate.

All the above demonstrates that the formation of MWs in solution occurs due to a synergistic effect between the externally applied MF and the thermal polymerization process of a highly diluted solution. The presence of the solvent causes the reduction of both the rate of polymerization and the average chain length due to the low concentration of monomer molecules in the solution.38,39 Furthermore, some chain transfer to solvent may occur which can reduce the molar mass of the product.40 Therefore, in a highly diluted solution, the polymerization of MMA molecules is inhibited. However, in our case, the presence of SMNPs and the external MF plays a fundamental role in the conversion of this situation. In fact, the efficient interaction between the monomer and the surface of SMNPs in combination with the external MF, which forces them to assemble closely to each other, contributes to the local increase of the monomer concentration and conclusively results in a successful polymerization process exclusively in the close proximity of the aligned SMNPs.

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CONCLUSIONS In the presented study superparamagnetic MWs are formed in a liquid matrix by MFI assembly of SMNPs and simultaneous heat-induced polymerization of MMA around the aligned NPs. The initial MMA−TI−SMNPs toluene solution has a high dilution factor that prevents the formation of solid nanocomposite films during polymerization. The favorable interaction of the SMNPs surface with MMA monomer in combination with the presence of the external MF results in local increase of the MMA−TI concentration in the close proximity of the aligned SMNPs and to the formation of permanently stabilized free-standing MWs in solution. Such a straightforward and simple process gives rise to the formation of smooth MWs of defined shape ready to be used in diverse applications.

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ASSOCIATED CONTENT

* Supporting Information S

Optical microscopy images of magnetic MWs in PMMA film; histograms of size distributions of MWs; HRTEM and zero-loss EFTEM images of as-synthesized SMNPs partially suspended on a hole in the C support film along with EFTEM maps for carbon; BF-TEM images and SAED pattern of the superparamagnetic beads fabricated via heating MMA-TI-SMNPs mixture in toluene at 80 °C for 17 h without MF and EFTEM maps of carbon and Fe; wide scan XPS spectrum of Si wafer and the clarification of the normalization with the signal of silicon oxide. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Tel +3901071781878, Fax +3901071781236 (D.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Mr. Simone Nitti of Istituto Italiano di Tecnologia (IIT) for the synthesis of SMNPs. E

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Based on Superparamagnetic Nanocrystals as Viscosity Sensors in Liquid. J. Appl. Phys. 2011, 110, 064907. (19) Eickenberg, B.; Wittbracht, F.; Stohmann, P.; Schubert, J.-R.; Brill, C.; Weddemann, A.; Hutten, A. Continuous-Flow Particle Guiding Based on Dipolar Coupled Magnetic Superstructures in Rotating Magnetic Fields. Lab Chip 2013, 13, 920−927. (20) Wittbracht, F.; Weddemann, A.; Eickenberg, B.; Zahn, M.; Hutten, A. Enhanced Fluid Mixing and Separation of Magnetic Bead Agglomerates Based on Dipolar Interaction in Rotating Magnetic Fields. Appl. Phys. Lett. 2012, 100, 123507−123507−123504. (21) De Bruyker, D.; Recht, M. I.; Bhagat, A. A. S.; Torres, F. E.; Bell, A. G.; Bruce, R. H. Rapid Mixing of Sub-Microlitre Drops by Magnetic Micro-Stirring. Lab Chip 2011, 11, 3313−3319. (22) Chippada, U.; Yurke, B.; Georges, P. C.; Langrana, N. A. A Nonintrusive Method of Measuring the Local Mechanical Properties of Soft Hydrogels Using Magnetic Microneedles. J. Biomech. Eng. 2008, 131, 021014−021014. (23) Goubault, C.; Jop, P.; Fermigier, M.; Baudry, J.; Bertrand, E.; Bibette, J. Flexible Magnetic Filaments as Micromechanical Sensors. Phys. Rev. Lett. 2003, 91, 260802. (24) Singh, H.; Laibinis, P. E.; Hatton, T. A. Synthesis of Flexible Magnetic Nanowires of Permanently Linked Core−Shell Magnetic Beads Tethered to a Glass Surface Patterned by Microcontact Printing. Nano Lett. 2005, 5, 2149−2154. (25) Rida, A.; Gijs, M. A. M. Dynamics of Magnetically Retained Supraparticle Structures in a Liquid Flow. Appl. Phys. Lett. 2004, 85, 4986−4988. (26) Hayes, M. A.; Polson, N. A.; Garcia, A. A. Active Control of Dynamic Supraparticle Structures in Microchannels. Langmuir 2001, 17, 2866−2871. (27) Fragouli, D.; Bayer, I. S.; Di Corato, R.; Brescia, R.; Bertoni, G.; Innocenti, C.; Gatteschi, D.; Pellegrino, T.; Cingolani, R.; Athanassiou, A. Superparamagnetic Cellulose Fiber Networks via Nanocomposite Functionalization. J. Mater. Chem. 2012, 22, 1662−1666. (28) Zeng, H.; Rice, P. M.; Wang, S. X.; Sun, S. Shape-Controlled Synthesis and Shape-Induced Texture of MnFe2O4 Nanoparticles. J. Am. Chem. Soc. 2004, 126, 11458−11459. (29) Rasband, W. S. ImageJ; U.S. National Institutes of Health: Bethesda, MD, 1997−2014; http://rsb.info.nih.gov/ij/ (accessed August 29, 2013). (30) Li, S.; Qin, J.; Fornara, A.; Toprak, M.; Muhammed, M.; Kim, D. K. Synthesis and Magnetic Properties of Bulk Transparent PMMA/FeOxide Nanocomposites. Nanotechnology 2009, 20, 185607. (31) Ghosh, R.; Pradhan, L.; Devi, Y. P.; Meena, S. S.; Tewari, R.; Kumar, A.; Sharma, S.; Gajbhiye, N. S.; Vatsa, R. K.; Pandey, B. N.; et al. Induction Heating Studies of Fe3O4 Magnetic Nanoparticles Capped with Oleic Acid and Polyethylene Glycol for Hyperthermia. J. Mater. Chem. 2011, 21, 13388−13398. (32) Guo, Z.; Henry, L. L.; Palshin, V.; Podlaha, E. J. Synthesis of Poly(methyl methacrylate) Stabilized Colloidal Zero-Valence Metallic Nanoparticles. J. Mater. Chem. 2006, 16, 1772−1777. (33) Bronstein, L. M.; Huang, X.; Retrum, J.; Schmucker, A.; Pink, M.; Stein, B. D.; Dragnea, B. Influence of Iron Oleate Complex Structure on Iron Oxide Nanoparticle Formation. Chem. Mater. 2007, 19, 3624−3632. (34) Casserly, T. B.; Gleason, K. K. Effect of Substrate Temperature on the Plasma Polymerization of Poly(methyl methacrylate). Chem. Vap. Deposition 2006, 12, 59−66. (35) Wagner, A. J.; Wolfe, G. M.; Fairbrother, D. H. Reactivity of Vapor-Deposited Metal Atoms with Nitrogen-Containing Polymers and Organic Surfaces Studied by In Situ XPS. Appl. Surf. Sci. 2003, 219, 317−328. (36) Simonsen, M.; Sønderby, C.; Li, Z.; Søgaard, E. XPS and FT-IR Investigation of Silicate Polymers. J. Mater. Sci. 2009, 44, 2079−2088. (37) Yilmaz, E.; Sezen, H.; Suzer, S. Probing the Charge Build-Up and Dissipation on Thin PMMA Film Surfaces at the Molecular Level by XPS. Angew. Chem., Int. Ed. 2012, 51, 5488−5492.

REFERENCES

(1) Kim, J.; Chung, S. E.; Choi, S.; Lee, H.; Kim, J.; Kwon, S. Programming Magnetic Anisotropy in Polymeric Microactuators. Nat. Mater. 2011, 10, 747−752. (2) Kaushik, A.; Solanki, P. R.; Ansari, A. A.; Sumana, G.; Ahmad, S.; Malhotra, B. D. Iron Oxide-Chitosan Nanobiocomposite for Urea Sensor. Sens. Actuators, B 2009, 138, 572−580. (3) Li, J.; Zhang, M.; Wang, L.; Li, W.; Sheng, P.; Wen, W. Design and Fabrication of Microfluidic Mixer from Carbonyl Iron-PDMS Composite Membrane. Microfluid. Nanofluid. 2011, 10, 919−925. (4) Fragouli, D.; Buonsanti, R.; Bertoni, G.; Sangregorio, C.; Innocenti, C.; Falqui, A.; Gatteschi, D.; Cozzoli, P. D.; Athanassiou, A.; Cingolani, R. Dynamical Formation of Spatially Localized Arrays of Aligned Nanowires in Plastic Films with Magnetic Anisotropy. ACS Nano 2010, 4, 1873−1878. (5) Fragouli, D.; Das, A.; Innocenti, C.; Guttikonda, Y.; Rahman, S.; Liu, L.; Caramia, V.; Megaridis, C. M.; Athanassiou, A. Polymeric Films with Electric and Magnetic Anisotropy Due to Magnetically Assembled Functional Nanofibers. ACS Appl. Mater. Interfaces 2014, 6, 4535−4541. (6) Robbes, A.-S.; Cousin, F.; Meneau, F.; Dalmas, F.; Boué, F.; Jestin, J. Nanocomposite Materials with Controlled Anisotropic Reinforcement Triggered by Magnetic Self-Assembly. Macromolecules 2011, 44, 8858−8865. (7) Erb, R. M.; Libanori, R.; Rothfuchs, N.; Studart, A. R. Composites Reinforced in Three Dimensions by Using Low Magnetic Fields. Science 2012, 335, 199−204. (8) Lorenzo, D.; Fragouli, D.; Bertoni, G.; Innocenti, C.; Anyfantis, G. C.; Davide Cozzoli, P.; Cingolani, R.; Athanassiou, A. Formation and Magnetic Manipulation of Periodically Aligned Microchains in Thin Plastic Membranes. J. Appl. Phys. 2012, 112, 083927. (9) Yuan, J.; Xu, Y.; Muller, A. H. E. One-Dimensional Magnetic Inorganic-Organic Hybrid Nanomaterials. Chem. Soc. Rev. 2011, 40, 640−655. (10) Fragouli, D.; Torre, B.; Villafiorita-Monteleone, F.; Kostopoulou, A.; Nanni, G.; Falqui, A.; Casu, A.; Lappas, A.; Cingolani, R.; Athanassiou, A. Nanocomposite Pattern-Mediated Magnetic Interactions for Localized Deposition of Nanomaterials. ACS Appl. Mater. Interfaces 2013, 5, 7253−7257. (11) Hill, L. J.; Pyun, J. Colloidal Polymers via Dipolar Assembly of Magnetic Nanoparticle Monomers. ACS Appl. Mater. Interfaces 2014, 6, 6022−6032. (12) Sheparovych, R.; Sahoo, Y.; Motornov, M.; Wang, S.; Luo, H.; Prasad, P. N.; Sokolov, I.; Minko, S. Polyelectrolyte Stabilized Nanowires from Fe3O4 Nanoparticles via Magnetic Field Induced Self-Assembly. Chem. Mater. 2006, 18, 591−593. (13) Ma, M.; Zhang, Q.; Dou, J.; Zhang, H.; Yin, D.; Geng, W.; Zhou, Y. Fabrication of One-Dimensional Fe3O4/P(GMA−DVB) Nanochains by Magnetic-Field-Induced Precipitation Polymerization. J. Colloid Interface Sci. 2012, 374, 339−344. (14) Ho, D.; Peerzade, S. A. M. A.; Becker, T.; Hodgetts, S. I.; Harvey, A. R.; Plant, G. W.; Woodward, R. C.; Luzinov, I.; St. Pierre, T. G.; Iyer, K. S. Magnetic Field Directed Fabrication of Conducting Polymer Nanowires. Chem. Commun. 2013, 49, 7138−7140. (15) Tokarev, A.; Gu, Y.; Zakharchenko, A.; Trotsenko, O.; Luzinov, I.; Kornev, K. G.; Minko, S. Reconfigurable Anisotropic Coatings via Magnetic Field-Directed Assembly and Translocation of Locking Magnetic Chains. Adv. Funct. Mater. 2014, DOI: 10.1002/ adfm.201303358. (16) Das, S.; Ranjan, P.; Maiti, P. S.; Singh, G.; Leitus, G.; Klajn, R. Nanoparticles: Dual-Responsive Nanoparticles and Their SelfAssembly. Adv. Mater. 2013, 25, 492−492. (17) Liou, H.-W.; Lin, H.-M.; Hwu, Y.-K.; Chen, W.-C.; Liou, W.-J.; Lai, L.-C.; Lin, W.-S.; Chiou, W.-A. Synthesis and Characterization of Novel Hybrid Poly(methyl methacrylate)/Iron Nanowires for Potential Hyperthemia Therapy. J. Biomater. Nanobiotechnol. 2010, 1, 50−60. (18) Allione, M.; Torre, B.; Casu, A.; Falqui, A.; Piacenza, P.; Di Corato, R.; Pellegrino, T.; Diaspro, A. Rod-Shaped Nanostructures F

dx.doi.org/10.1021/jp507951f | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

(38) Mathers, R. T.; Meier, M. A. R. Green Polymerization Methods: Renewable Starting Materials, Catalysis and Waste Reduction; WileyVCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2011. (39) Brazel, C. S.; Rosen, S. L. Fundamental Principles of Polymeric Materials; John Wiley & Sons, Inc.: Hoboken, NJ, 2012. (40) Nicholson, J. W. The Chemistry of Polymers, 3rd ed.; The Royal Society of Chemistry: Cambridge, UK, 2006.

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