ARTICLE pubs.acs.org/Langmuir
Free-Standing Poly(L-lactic acid) Nanofilms Loaded with Superparamagnetic Nanoparticles Silvia Taccola,*,†,‡ Andrea Desii,†,‡ Virginia Pensabene,† Toshinori Fujie,†,§,|| Akihiro Saito,§ Shinji Takeoka,§,|| Paolo Dario,†,‡ Arianna Menciassi,†,‡ and Virgilio Mattoli*,† †
Center for MicroBioRobotics@SSSA, Istituto Italiano di Tecnologia, Viale Rinaldo Piaggio 34, 56025 Pontedera (PI), Italy Biorobotics Institute Polo Sant’Anna Valdera, Scuola Superiore Sant’Anna, Viale Rinaldo Piaggio 34, 56025 Pontedera (PI), Italy § Department of Life Science and Medical Bioscience, Graduate School of Advanced Science and Engineering, and European Biomedical Science Institute (EBSI), Organization for European Studies, Waseda University, TWIns, 2-2 Wakamtsu-cho, Shinjuku-ku, 162-8480 Tokyo, Japan )
‡
bS Supporting Information ABSTRACT: Freely suspended nanocomposite thin films based on soft polymers and functional nanostructures have been widely investigated for their potential application as active elements in microdevices. However, most studies are focused on the preparation of nanofilms composed of polyelectrolytes and charged colloidal particles. Here, a new technique for the preparation of poly(L-lactic acid) free-standing nanofilms embeddidng superparamagnetic iron oxide nanoparticles is presented. The fabrication process, based on a spin-coating deposition approach, is described, and the influence of each production parameter on the morphology and magnetic properties of the final structure is investigated. Superparamagnetic free-standing nanofilms were obtained, as evidenced by a magnetization hysteresis measurement performed with a superconducting quantum interference device (SQUID). Nanofilm surface morphology and thickness were evaluated by atomic force microscopy (AFM), and the nanoparticle dispersion inside the composites was investigated by transmission electron microscopy (TEM). These nanofilms, composed of a biodegradable polyester and remotely controllable by external magnetic fields, are promising candidates for many potential applications in the biomedical field.
’ INTRODUCTION In recent years, the development of ultrathin films combining various nanostructures with a polymer matrix has been gaining increasing attention because of their widespread potential applications as nanomechanical sensors, nanoactuators, chemical and biological sensors, electrochemical devices, nanoscale chemical and biological reactors, and drug-delivery systems.15 The most popular method for the fabrication of multilayered nanocomposite thin films is the layer-by-layer (LbL) assembly technique.6 This method is based on the sequential adsorption of oppositely charged organic and inorganic materials (e.g., polyelectrolyte and charged colloidal particles). Freestanding nanocomposite LbL films, realized in conjunction with a sacrificial layer approach, were first reported by Mamedov and Kotov in 2000.7 Since then, this technique has been successfully applied to the encapsulation in polymeric films of various functional nanostructures, such as magnetic nanoparticles, gold nanoparticles, and carbon nanotubes, thus imparting new magnetic, optical, mechanical, or electronic properties to the final structures.79 An interesting review on the topic was authored by Jiang and Tsukruk in 2006.10 The LbL assembly technique presents many advantages, such as simplicity, versatility, and thickness control on the nanoscale. Stable, freely suspended nanofilms have an overall thickness r 2011 American Chemical Society
ranging from 20 to 100 nm depending on the number of polymer bilayers. Usually, to reach these thickness values, a few tenths of bilayers are needed; therefore, this method can be extremely time-consuming because each adsorption step typically requires 1020 min and the total growth of a film can take many hours up to a few days. Different deposition strategies were proposed to reduce the processing time: well-organized multilayer films were obtained by a spin-assisted LbL assembly method (SA-LbL) that combines the conventional LbL assembly with spin coating, reducing the deposition time for each layer. However, this process still requires many steps.11,12 Another major limitation of different LbL assembly strategies is the possibility to use only polyelectrolytes and other charged species in aqueous solutions (e.g., proteins and nanoparticles). Recently, Takeoka and co-workers have proposed a novel, fast, simple fabrication technique suitable for nonelectrolytic polymers based on a combination of the sacrificial layer technique and a single step of spin-assisted deposition.13 In particular, freestanding nanofilms having huge aspect ratio values (up to 106; Received: January 31, 2011 Revised: March 22, 2011 Published: April 01, 2011 5589
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Langmuir tens-of-nanometers-thick polymer membranes having surfaces of few cm2) composed of biodegradable polyesters such us poly(D, L-lactide) (PLA), poly(L-lactic acid) (PLLA), poly(glycolic acid) (PGA), and their copolymers were successfully produced. These materials, which have been widely investigated in traditional clinical applications as degradable sutures, bone screws, pins, and or drug-delivery carriers,1416 could be adapted to many more applications in the biomedical field if presented as nanofilms. Free-standing single-layer PLLA nanofilms were proposed as an innovative alternative to traditional wire for suturing wounds in open and minimally invasive surgery, as nanopatches on gastrointestinal mucosal surfaces, or as flexible cell growth supports.13,17,18 In this framework, the possibility to position and manipulate nanofilms finely within wet or liquid working environments, possibly by using noninvasive external tools, became highly attractive, in particular for biomedical applications. The use of magnetic fields to control microdevices remotely in narrow and delicate districts and apparata of the human body is nowadays a well-accepted approach in surgical and diagnostic procedures, where remote magnetic navigation catheters or magnetic robotic capsules are finely controlled by coupled permanent magnetic fields.19,20 Moreover, innovative methods for targeting nano-objects toward tumor masses with engineered gradient magnetic fields or for positioning magnetic tip guidewires in coronary arteries with permanent magnets are currently studied and tested at the clinical level.21,22 The possibility to integrate magnetic components into PLLA nanofilms represents the first step in the development of magnetic nanosheets with the potential for remotely controlled manipulation by permanent and gradient magnetic fields, thus opening new application scenarios, as already theoretically and experimentally demonstrated.2325 This article reports on the fabrication and characterization of single-layer free-standing PLLA nanofilms embedding superparamagnetic iron oxide nanoparticles (SPIONs) by means of spin-coating deposition. The production of PLLA-SPIONs nanofilms depends on several factors: the stability of the nanoparticles/ polymer dispersion in the selected solvent, the compatibility between the particles and the matrix in the composite, the effect of the spincoating process on the nanocomposite morphology, and so forth. The crucial point in the proposed fabrication method is the identification of the right solvent for PLLA, which could allow a stable colloidal dispersion of SPIONs at the same time. PLLA is soluble in a handful of solvents, most notably chlorinated organic solvents such as dichloromethane (CH2Cl2) and chloroform (CHCl3). The dispersion of SPIONs (coated with different surfactants) in CHCl3 and CH2Cl2 is thus explored in order to select the most stable combination of solvent/nanoparticles. By analyzing both the magnetic properties of the nanoparticles and the final magnetic response of the nanofilms, the saturation magnetization of the different samples and the effective nanoparticle content in the composite are derived, thus providing a quantitative description of the nanofilm behavior in a magnetic field. Furthermore, the effect of each production parameter on the final magnetic nanofilm structure has been accurately investigated.
’ EXPERIMENTAL SECTION Materials. Silicon wafers (400 μm thick, p type, boron doped, Æ100æ, Si-Mat Silicon Materials, Kaufering, Germany), used as substrates for film deposition, were cut (2 cm 2 cm) and treated using an acid washing solution (SPM: 96%:30% H2SO4/H2O2 = 4:1 (v/v)) at 120 C for 10 min and then thoroughly rinsed with deionized (DI) water (18 MΩ cm) in
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order to remove dust and impurities. Poly(vinyl alcohol) (PVA, average Mw = 13 00023 000, 98% hydrolyzed) was purchased from Sigma-Aldrich (St. Louis, MO). Poly(L-lactic acid) (PLLA, Mw = 80 000100 000) was obtained from Polysciences Inc. (Warrington, PA). Superparamagnetic magnetite/maghemite nanoparticles having different types of surface modification and a nominal diameter of 10 nm (fatty acid coated EMG1200, polymer-coated EMG1300, and lipid-coated EMG1400) were purchased from FerroTec Co. (San Jose, CA).
Fabrication of Single-Layer PLLA and PLLA-SPION Nanofilms. Free-standing magnetic PLLA nanofilms and unloaded PLLA nanofilms (used as a control for characterization) were fabricated by a single step of spin-coated assisted deposition using a sacrificial layer approach. For unloaded PLLA nanofilms, the film-deposition process is straightforward: (1) an aqueous solution of PVA (1 wt %) was deposited by spin coating (WS-650 spin processor, Laurell Technologies Corporation, North Wales, PA) on a silicon wafer at 3000 rpm for 20 s, forming the sacrificial layer of water-soluble polymer; (2) the deposition of the single-layer nanofilm was obtained by spinning a solution of PLLA in chloroform (CHCl3) using the same spinning parameters. After each step, the sample was held at 80 C on a hot plate for 1 min to remove the excess solvent. Finally, the polymer-coated wafer was immersed in water: the PVA sacrificial layer was dissolved, thus releasing a freely suspended insoluble PLLA nanofilm. All routines for PLLA nanofilm fabrication were conducted in a clean room (class 10 000) to avoid contamination. The fabrication of magnetic composite nanofilms was performed via the spin-assisted deposition of a stable colloidal solution of SPIONs and PLLA in CHCl3, maintaining the same spinning and drying parameters and using PVA as a water-soluble sacrificial layer. In this study, different PLLA nanofilms were prepared by varying the concentration of the polymer and the number of nanoparticles added to the solution. The samples were referred as PLx-SPy, denoting films prepared using x mg mL1 PLLA (x = 5, 10, 20) and y mg mL1 SPION colloidal solutions (y = 0, 1, 5, 10, 15).
Characterization of Morphological Properties of Nanofilms. For a preliminary investigation of the nanofilm structure, optical images were taken by using a Hirox KH7700 digital microscope (Hirox Co Ltd., Tokyo, Japan) equipped with an MX(G)-10C zoom lens and an OL-140II objective lens, covering a magnification range from 140 to 1400. The nanofilm thickness, topography, and surface roughness were evaluated with a Veeco Innova Scanning Probe Microscope (Veeco Instuments Inc., Santa Barbara, CA) operating in tapping mode using an RTESPA Al-coated silicon probe (Veeco Instruments Inc.) with an elastic modulus of 2080 N m1, a resonance frequency of 235317 kHz, and an average tip radius of 8 nm. All measurements were performed in air, at room temperature, on films collected from the suspended state and dried on a clean silicon wafer. For thickness measurements, SiO2-supported nanofilms were scratched with a needle. The sample was scanned across the edge of the scratch over a 20 μm 20 μm area, recording 128 128 samples. The resulting scan data were elaborated using the Gwyddion SPM analysis tool (http://gwyddion. net). Scan data were leveled with the facet level tool to remove sample tilt, and then the film thickness was evaluated as the difference between the average heights of a region of interest (ROI) selected on the nanofilm surface and the average height of the ROI on the silicon wafer. For roughness measurements, the surface was scanned in tapping mode over 5 μm 5 μm areas, collecting 512 512 samples and recording the topography, phase, and amplitude channels. A watershed algorithm was used to mark SPION grains in the surface scans. A microscopic investigation of a nanoparticle dispersion in the polymeric matrix of nanocomposite nanofilms obtained from the dispersion containing 10 mg mL1 PLLA and different concentrations of SPIONs (1, 5, 10, 15 mg mL1) was performed using transmission electron microscopy (TEM, JM-1011, JEOL, Tokyo, Japan) operating at 5590
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Figure 1. Nanofilm quality assessment. (A) Digital optical microscope image of a PL20-SP5 nanofilm showing the color modulation across the film surface. (B) Released PL20-SP15 nanofilm (2 cm 2 cm) floating over the water surface. 100.0 keV. TEM samples were prepared by collecting the free-standing nanofilms from aqueous media on a 100-mesh copper grid, which had been dried for 12 h in a desiccator prior to the TEM observation. Magnetic Measurements. The magnetic behavior of nanocomposite nanofilms obtained from the dispersion containing 10 mg mL1 PLLA and different concentrations of SPIONs (1, 5, 10, 15 mg mL1) was investigated using superconducting quantum interference measurement device (SQUID) analysis (MPMS-7, Quantum Design, San Diego, CA). The magnetization curves were recorded for dried nanofilms with an average mass of 0.3 mg, corresponding to a stack of 510 nanofilms. Measurements were performed at 37 C under an applied magnetic field ranging from 10 000 to 10 000 Oe.
’ RESULTS AND DISCUSSION Because of the lack of detailed information about the nanoparticle coating, SPION dispersibility in various solvents for PLLA had to be assessed. EMG1200, EMG1300, and EMG1400 nanoparticles were dispersed in CHCl3 and CH2Cl2 with the aid of a vortex mixer. EMG1400 were not dispersible in the chlorinated solvents, whereas EMG1200 was insoluble in CH2Cl2 and poorly dispersible in CHCl3. Colloidal dispersion of EMG1300 in CHCl3 and CH2Cl2 did not show sedimentation after 30 min from the preparation at all investigated concentrations (1, 5, 10, 15 mg mL1). Finally, CHCl3 was selected as the solvent because it is more suitable for the spin coating of polymer solutions thanks to its higher viscosity and boiling point compared to those of CH2Cl2.26 A high concentration of SPIONs was desired to obtain nanofilms with high susceptibility values, compatible with an easier magnetic manipulation. Thus, no separation process, such as centrifugation, filtration, or decantation, was performed on the dispersions. The resulting dispersions were deposited via spin-assisted deposition over a PVA sacrificial layer supported on silicon wafers. The quality of the obtained supported nanofilms was initially checked by digital optical microscopy. No samples showed any holes, cuts, or other discontinuities caused by inefficient solvent evaporation or by a low wettability of the spin-deposited substrate. Nanofilms supported over substrates with an index of refraction different from that of PLLA, such as PVA-coated Si wafers, appeared in bright colors in reflected light optical microscopy under white light illumination (Figure 1A). This effect was due to the interference of the light beam reflected by the film surface with the beam reflected by the underlying substrate, as expected from the thin film with an optical thickness that was less than the wavelength of visible light.27 In nonabsorbing, nonmagnetic nanofilms, the modulation of film color was caused only by the topological changes in the film thickness, resulting in a purely structural color. PLLASPION nanofilms absorbed radiation in the visible range, as
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shown by their characteristic yellow-brown color when suspended (Figure 1B). Nanofilm color in reflected light optical microscopy was not purely structural, so the reflected light spectrum could not be directly correlated to the nanofilm thickness in a quantitative way. Reflected light micrographs of nanofilms revealed a microscopic modulation of the color that was attributed to a periodic variation of the thickness over the whole surface, seemingly along the radial directions perpendicular to the spin axis (Figure 1A). This effect is expected for dilute polymer solutions in highly volatile or “poor” solvents.26 After fabrication and quality control, nanofilms were released from the silicon substrate after immersion in deionized water. In a typical experiment, the PVA sacrificial layer was dissolved and the resulting free-standing nanofilm floated over the water surface because of its hydrophobicity (Figure 1B). PLLA nanofilms were transparent and hardly discernible in water, whereas magnetic composites were colored, with different degrees of intensity depending on the SPION concentration. After the addition of more PVA (0.1 wt %) to the water, acting as a surfactant, it was possible to manipulate the films with metal tweezers or a syringe, injecting and ejecting multiple times without breaking. Even after manipulation, the nanofilms spread in the suspending medium and were completely unfolded. Nanofilms containing SPIONs can be displaced on the water surface by using a permanent magnet. A movie of magnetic nanofilm manipulation is available in the Supporting Information. Once the nanofilms’ macroscopic homogeneity and integrity were assessed, microscopic surface characterization was performed for each fabricated sample to investigate the nanofilms’ surface topology and particle dispersion in the nanocomposite. Flat, homogeneous surfaces were observed by AFM topography in nonmagnetic nanofilms and in nanofilms deposited from lowconcentration SPION dispersions (Figure 2A,B). As the SPION concentration increases, the presence of clusters emerging from the surfaces of the samples was evident (Figure 2CE). The surface morphologies of nanofilms deposited from a dispersion containing the same number of SPIONs and increasing amounts of PLLA have been compared. At a low PLLA concentration, the presence of clusters was more evident (Figure 3). On samples containing well-separated clusters, it was possible to perform cluster size determination using a watershed algorithm. The clusters’ average equivalent disk radius is reported in parentheses in Table 1, confirming the size increase with SPION concentration and the decrease with PLLA concentration. The clustering phenomenon is influenced by several factors. The higher mass fraction of SPIONs in low-concentration PLLA samples, with the consequent reduction in mean particle separation during the evaporative phase of the spinning experiment, could cause particle aggregation by short-range van der Waals dispersive interactions. Instead, the possibility of aggregation caused by magnetic interaction was ruled out: as will be shown by a SQUID analysis, the interparticle magnetic interaction was negligible, as confirmed by the absence of magnetization hysteresis in cluster-containing samples. It has been widely demonstrated that strongly magnetic interacting superparamagnetic particles change their magnetic behavior to a blocked state showing remanence.28 Cluster formation was also influenced by the change in hydrodynamic parameters (viscosity and liquid film thickness) that could influence the aggregation of small particles during the spin coating of a colloidal solution.29 Finally, the presence of polymer can impair the visualization of clusters in thicker nanofilms deposited from polymer-rich solutions. 5591
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Figure 2. AFM surface topography of nanofilms deposited from 5 mg mL1 solutions of PLLA with increasing SPION concentration: (A) PL5-SP0, (B) PL5-SP1, (C) PL5-SP5, (D) PL5-SP10, and (E) PL5-SP15. Clusters can be observed as the SPION concentration increases.
Figure 3. Atomic force microscope scans of nanofilms deposited from 15 mg mL1 solutions of SPIONs with increasing PLLA concentration: (A) PL5SP15, (B) PL10-SP15, and (C) PL20-SP15.
Table 1. Roughness and Clusters Size of PLLA and SPIONPLLA Nanofilms roughness (nm)a SPIONs (mg mL1)
PLLA (5 mg mL1)
PLLA (10 mg mL1)
PLLA (20 mg mL1)
0 1 5 10 15
1.9 2.2 10.8 (88.3)b 35 (179.3)b 27.4 (199.5)b
2.3 3.7 5 17.6 (118)b 31.2 (132.3)b
4.3 4.1 3.1 7.7 7.4
a The roughness was determined from Ra = (1)/(N)Σ|Zi z| from 5 μm 5 μm AFM scans. b Clusters’ average equivalent disk radius (nm).
The effect of particle clustering on surface microroughness, estimated as the average absolute deviation from the mean height value, was investigated. As evidenced in Table 1, in nonmagnetic nanofilms and in nanofilms deposited from low-concentration SPIONs (1 mg mL1), the roughness increased with PLLA concentration. At higher SPION concentration, the presence of clusters inverted this trend. Keeping the PLLA concentration, Ra, constant generally increased with the number of SPIONs in the nanocomposite. In nanofilms containing clusters, the roughness increase was more pronounced.
Generally, the presence of cluster is to be avoided because nanodispersed composites have a higher elastic modulus and tensile strength compared to composites containing micrometric aggregates of nanoparticles.8 However, as already discussed, every fabricated composite nanofilm could be released from the spincoating support and manipulated without breaking. Moreover, preliminary mechanical tests, which will be discussed in other work, showed that the elastic modulus and tensile strength were not influenced negatively by the inclusion of clusters (data not shown). Therefore, as far as the application is concerned (i.e., magnetic manipulation), the presence of clusters inside the nanofilms does not constitute a problem per se. A complete morphological characterization involves the measurement of thickness, which is an important parameter influencing nanofilm adhesion properties.30 As reported in the Experimental Section, a thickness estimation has been performed by AFM measurements (Figure 4A). AFM thickness measurements of unloaded PLLA nanofilms confirmed that a higher concentration and viscosity of the polymeric solution yields thicker nanofilms, as expected from the spin-assisted depositions of polymer solutions. The average thicknesses were measured to be 33 ( 3, 90 ( 6, and 245 ( 9 nm, respectively, for PL5-SP0, PL10-SP0, and PL20-SP0 nanofilms. The incorporation of SPIONs affected the thickness of the nanofilms. As clearly displayed in Figure 4B, for the same 5592
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Figure 4. Thickness of PLLA and SPION-PLLA nanofilms obtained by atomic force microscopy. (A) Example of an AFM scan across a PL10-SP1 nanofilm edge. The film thickness was evaluated as the difference between the average height of an ROI selected on the nanofilm surface (ROI NF) and the average height of the ROI on the silicon wafer (ROI Si). (B) Nanofilm thickness plotted against SPION concentration for different concentrations of PLLA in the deposited solutions: 5 (O), 10 (0), and 20 mg mL1 (Δ). Error bars indicate the standard deviation of nanofilm height in AFM scans (rms roughness).
polymer concentration the average thickness increased with respect to the number of magnetic nanoparticles added to the solution. This effect was partially caused by the polymeric coating of nanoparticles that contributed to increase the viscosity of the dispersion, thus effectively yielding thicker nanofilms. Moreover, the large SPION aggregates emerging from the film surface had a major effect on nanofilm thickness because of their mean vertical dimensions, which are larger than the bare PLLA nanofilm thickness. The proposed single-step deposition method allows the production of nanocomposite thin films with a homogeneous and isotropic structure, whereas LbL assembly leads to anisotropic stratified films in which layers of polyelectrolytes and layers of nanoparticles are organized in a specific predetermined order.7 AFM characterization of PLLA-SPION nanofilms showed that morphology in single-step deposition method can be controlled by tuning the dispersion and deposition parameters (e.g., PLLA and SPION concentrations, spin speed, and time). LbL assembly enables more precise control of the nanofilm morphology: because the thickness of a single layer of polyelectrolyte is typically below 1 nm, the thickness can be controlled on the nanometer scale. Moreover, a different organization of the nanofilm structure can be prepared by changing the number and sequence of the adsorption steps.8 The nanoparticle clustering phenomenon and the increase in surface roughness with respect to the number of nanoparticles embedded in the structure have also been evidenced in nanocomposite thin films deposited by the LbL technique.31 Nanofilms obtained from the spin coating of 10 mg mL1 PLLA solutions with different contents of SPIONs were further investigated by TEM and SQUID. These nanofilms were selected because of their good homogeneity with low cluster content while maintaining a thickness compatible with efficient mucosal tissue adhesion, as reported elsewhere.17 Nanofilms of submicrometric thickness allowed for TEM analysis directly on free-standing films supported over a copper grid, without milling. Bright-field TEM micrographs showed the presence of iron oxide nanoparticles (dark) in the polymeric matrix (bright) as a result of the higher electron absorption of metal oxides compared to that of nonconducting polymers (Figure 5). Samples fabricated from solutions containing 5, 10, and 15 mg mL1 SPIONs incorporated nanoparticles clusters, as evidenced by the presence of large, dark spots in TEM micrographs. Grains observed in AFM micrographs were confirmed to be
Figure 5. Transmission electron microscope micrographs of nanocomposite PLLA nanofilms with increasing SPION concentration: (A) PL10-SP1, (B) PL10-SP5, (C) PL10-SP10, and (D) PL10-SP15.
SPION clusters, ruling out the possibility of a morphological change in the polymeric surface caused by the inclusion of nonclustered nanoparticles. To characterize and quantify the magnetic response of freely suspended nanofilms better, a magnetization hysteresis was evaluated by SQUID. In fact, SPION clustering could in general modify the magnetic behavior of nanoparticles in nanocomposites and colloidal dispersions, depending on the strength of interparticle interactions.28 Hysteresis loops for SPIONs (Figure 6A) and PLLA-SPION nanocomposite films (Figure 6B) showed no remanence or coercivity, thus indicating superparamagnetic behavior, as expected for weakly interacting nanoparticle assemblies. The inclusion of the particles in the polymeric matrix did not significantly alter their magnetic properties, as evidenced in other nanocomposite systems.32 The magnetic behavior of nanocomposites can be described by superparamagnetic modeling, with the saturation magnetization depending only on the nanoparticle number density. The mean magnetic moment (μ) and mass saturation magnetization (MS) of nanoparticles were calculated by fitting SPION magnetization data with the Langevin function, which 5593
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Figure 6. Magnetization hysteresis plots of (A) SPIONs and (B) SPION-loaded nanofilms: PL10-SP1 (O), PL10-SP5 (0), PL10-SP10 (9), and PL10-SP15 (b). Fitting curves are best-fitting Langevin functions.
Table 2. Mass Saturation Magnetization (MS), Mass Magnetic Susceptibility (χ), and Nanoparticle Mass Fraction in the Composites (w) and in the Corresponding Dispersions (wsol) of SPIONs and SPION-PLLA Nanofilms sample
MS (emu g1)
χ (cm3 g1)
w (%)
wsol (%)
EMG1300 PL10-SP15
53.0 48.6
0.101 0.093
100 91.7
60
PL10-SP10
25.2
0.048
47.5
50
PL10-SP5
16.8
0.032
31.5
33
PL10-SP1
6.7
0.012
12.6
9.1
is valid for paramagnetic and superparamagnetic materials μH μH kB T M ¼ MS L ¼ MS coth kB T kB T μH where kB is the Boltzmann constant and T is the experimental temperature (296 K).33 The fit converged properly with μ = 2.33 1016 emu and MS = 53.0 emu g1. The magnetic susceptibility of nanoparticles can be calculated from the first term of the Taylor expansion of the Langevin function: χ¼
M MS μ ¼ ¼ 0:10 cm3 g1 H 3kB T
The saturation magnetization of nanocomposite films (MS nc) was evaluated by fitting the magnetization data of each sample with the Langevin function and fixing the single-particle mean magnetic moment at the previously calculated value of μ = 2.33 1016 emu (Figure 6B). The magnetization evaluation can be used as a measurement of the SPION mass fraction in the nanocomposite because SPIONs are the magnetically active elements in the blend. The mass fraction of nanoparticles in the composites (w) was then calculated as the ratio of the saturation magnetization values: w¼
MSnc MS
The mass magnetic susceptibility of each investigated nanofilm was calculated as the product of SPION susceptibility and SPION mass fraction in the composite (Table 2). As displayed in Table 2, the SPION mass fraction in the composite (w) was similar to the mass fraction in the dispersion (wsol), except for PL10-SP15, which was richer in SPIONs than the corresponding dispersion probably because of the precipitation of larger aggregates during the spin-coating deposition. Magnetic susceptibility is the key factor in the magnetic guidance
of the freely suspended nanofilm in a liquid environment. According to a previously proposed theoretical model for the magnetic manipulation of nanofilms by the displacement of a permanent magnet, it is possible to calculate the maximum drag force and the terminal velocity of a magnetic nanofilm with known susceptibility and geometry.24 In conclusion, the proposed technique allows the fabrication of free-standing, flexible composite nanofilms in a simple, fast single-step process. The inclusion of polymer-coated SPIONs in a PLLA matrix did not alter their magnetic behavior, yielding nanofilms with a high saturation magnetization and magnetic susceptibility. A nanodispersed blend was observed only in films obtained from dispersions containing less than 5 mg mL1 SPIONs, but other magnetic samples showed the inclusion of nanoparticle clusters in the matrix. The presence of clusters, although making composites more magnetically responsive, did not compromise the integrity and manipulability of the free-standing films. In the concentration range between 1 and 10 mg mL1, the nanofilm SPION mass fraction can be regulated with the dispersion concentration. In this way, the susceptibility of the sample can be tuned to the intended application by choosing the nanofilm dimension and SPION weight fraction, controlling only the concentration of polymer and SPIONs in the deposited dispersion. Magnetic nanofilms fabricated from 10 mg mL1 PLLA solutions are particularly suitable for the intended application because of an ideal trade-off among thickness, adhesion properties, structural stability, and magnetic susceptibility. As foreseen in previous papers, magnetic SPIONPLLA nanofilms can be manipulated and precisely positioned within the working environment by using an external magnetic field and could thus provide a novel controllable support in biomedical applications. A further investigation of nanofilm mechanical properties is currently underway in our laboratory and will be reported in future work.
’ ASSOCIATED CONTENT
bS
Supporting Information. A movie showing the manipulation of magnetic nanofilms in water. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected].
’ ACKNOWLEDGMENT We thank Dr. Takeshi Ibe, Mr. Jun Ishiyama, and Prof. Hiroyuki Nishide at the Graduate School of Advanced Science & Engineering, Waseda University, for technical advice and useful discussions concerning the SQUID measurements. This work was supported in part by a JFE (The Japanese Foundation for Research and Promotion of Endoscopy) grant (T.F.) and the Adaptable and Seamless Technology Transfer Program through Target-Driven R&D (A-STEP) from JST, Japan (T.F. and S.T.). ’ REFERENCES (1) Kang, T. J.; Cha, M.; Jang, E. Y.; Shin, J.; Im, H. U.; Kim, Y.; Lee, J.; Kim, Y. H. Adv. Mater. 2008, 20, 3131–3137. (2) Zeng, T.; Claus, R.; Zhang, F.; Du, W.; Cooper, K. L. Smart Mater. Struct. 2001, 10, 780–785. 5594
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dx.doi.org/10.1021/la2004134 |Langmuir 2011, 27, 5589–5595
