Fe3O4 electrospun membranes

Journal of Magnetism and Magnetic Materials 352 (2014) 30–35

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Fabrication and characterization of superparamagnetic poly(vinyl pyrrolidone)/poly(L-lactide)/Fe3O4 electrospun membranes Ioanna Savva a, Demetris Constantinou a, Oana Marinica b, Eugeniu Vasile c, Ladislau Vekas d, Theodora Krasia-Christoforou a,n a

University of Cyprus, Department of Mechanical and Manufacturing Engineering, Nicosia, Cyprus National Center for Engineering of Systems with Complex Fluids, University “Politehnica” Timisoara, Timisoara, Romania c METAV Research & Development, Bucharest, Romania d Center for Fundamental and Advanced Technical Research, Romanian Academy, Timisoara Branch, Timisoara, Romania b

art ic l e i nf o

a b s t r a c t

Article history: Received 11 June 2013 Received in revised form 10 September 2013 Available online 9 October 2013

The fabrication of magnetoactive fibrous nanocomposite membranes based on poly(vinyl pyrrolidone) (PVP), poly(L-lactide) (PLLA) and pre-formed oleic acid coated magnetite nanoparticles (OA Fe3O4) is presented. The aforementioned materials have been prepared by means of the electrospinning technique following a single-step fabrication process. The PVP/PLLA/OA Fe3O4 nanocomposite membranes were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) that provided information on the fiber diameters as well as on the morphological and dimensional characteristics of the OA Fe3O4 nanoparticles embedded within the fibers. The thermal stability of these materials was evaluated by means of thermal gravimetric analysis (TGA) measurements. Finally, vibrational sample magnetometry (VSM) analysis disclosed superparamagnetic behavior at room temperature. The combination of the hydrophilic, biocompatible and photo-crosslinkable PVP with the biodegradable PLLA and the superparamagnetic OA Fe3O4 nanoparticles within these materials allows for the future development of crosslinked fibrous magnetoactive nanocomposites exhibiting high stability in aqueous solutions, with potential use in biomedical and environmental applications. & 2013 Elsevier B.V. All rights reserved.

Keywords: Poly(vinyl pyrrolidone) (PVP) Poly(L-lactide) (PLLA) Magnetoactive polymer-based fibrous nanocomposites Electrospinning Oleic acid-coated magnetite (Fe3O4) nanoparticles Superparamagnetism

1. Introduction During the last years, polymer science has been focusing in the design and development of novel stimuli-responsive polymeric materials, capable of responding to externally applied stimuli such as temperature and pH changes, irradiation, magnetic or electric field [1–5]. Fibrous membranes belonging to the aforementioned class [6], have received considerable attention owing to their potential applicability in many fields including drug delivery and tissue engineering [7–9], optoelectronics [10–12], filtration and bioseparation processes [13,14] and sensing [15,16]. A simple and cost-effective way for fabricating polymeric fibrous materials is by means of the electrospinning process [17–19]. The latter is a versatile fabrication method allowing the production of not only polymeric fibrous materials but also of ceramic and composite (nano)fibrous mats [20,21] in which the fiber diameters range from a few nanometers up to a few micrometers [22]. Consequently, these materials are characterized by high surface-to-volume ratios which in combination with the existing

n

Corresponding author. Tel.: þ 357 228 922 88; fax: þ 357 228 950 81. E-mail address: [email protected] (T. Krasia-Christoforou).

0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.10.005

versatility on materials' design and chemical composition render electrospun polymer-based fibrous membranes ideal for many applications. Magnetoactive polymer-based fibrous membranes are stimuliresponsive composites consisting of magnetic (nano)particles embedded within a polymeric fibrous matrix. Due to the presence of magnetic (nano)particles, the properties of these materials are strongly influenced by the presence of an externally applied magnetic field. In literature examples referring to the fabrication of magnetoactive fibrous materials via electrospinning, magnetite (Fe3O4) is the most commonly used as the magnetoresponsive component [23–30] whereas other inorganic magnetic particles such as FePt [31], CoFe2O4 [32] and Fe [33] particles have been also used. In the present work the fabrication and characterization of membranes consisting of the hydrophilic and biocompatible poly (vinyl pyrrolidone) (PVP) [34], the hydrophobic and biodegradable poly(L-lactide) (PLLA) and pre-formed oleic acid-coated iron oxide nanoparticles (OA Fe3O4) is described for the first time. These fibrous nanocomposites comprised of fibers with mean diameters of
5.5–8.5 μm in which magnetite nanoparticles are uniformly dispersed, demonstrate superparamagnetic behavior at room temperature in the presence of an externally applied magnetic

I. Savva et al. / Journal of Magnetism and Magnetic Materials 352 (2014) 30–35

field owing to the very small diameters of the OA Fe3O4 nanoparticles [35], which are retained during the electrospinning process. The ability of PVP for self-photocrosslinking when exposed under UV irradiation [36,37] combined with the co-existence of both PLLA and superparamagnetic iron oxide nanoparticles (SPIONs), allows for the future development of highly stable in aqueous media crosslinked fibrous magnetoactive nanocomposites, with potential use in the biomedical and environmental fields.

2. Experimental part 2.1. Materials Poly(vinyl pyrrolidone), (PVP, M n ¼1 300 000 g/mol) and poly (L-lactide) (PLLA, M n ¼99 000 g/mol), iron sulfate (II) heptahydrate (97%) and iron chloride (III) tetrahydrate (99%) were purchased from Sigma-Aldrich. Chloroform (CHCl3) and oleic acid (99%) were purchased by Scharlau and Merck respectively. All reagents were used as provided from the manufacturer without further purification. 2.2. Synthesis of oleic acid-coated magnetite nanoparticles (OA Fe3O4) The oleic acid-coated magnetite nanoparticles (OA Fe3O4) were prepared by following an experimental procedure developed by Bica et al. [38–41]. Briefly, magnetite nanoparticles, Fe3O4, obtained by the co-precipitation of Fe2 þ and Fe3 þ ions (salts FeSO4  7H2O; FeCl3  4H2O) in the presence of excess NH4OH, at 80–82 1C in aqueous solution, were subsequently coated by chemisorbed oleic acid monolayer. Repeated flocculation (acetone) – redispersion in light hydrocarbon was used to extract magnetite particles coated with a single surfactant layer and to eliminate the free (non-adsorbed) surfactant. Finally, the dried OA Fe3O4 powder was redispersed in a light hydrocarbon carrier to obtain the purified magnetic nanofluid to be used for the preparation of the magnetoactive membrane. 2.3. Membrane fabrication Membranes comprised of the commercially available homopolymers PVP and PLLA and preformed OA Fe3O4 nanoparticles were fabricated by means of the electrospinning technique. Initially, solutions of PVP (solution concentration: 3, 5, 10 and 15%w/v) and solutions of PVP/PLLA (solution concentration: 10%w/v; weight percentage proportion 70/30 respectively) were prepared in chloroform. The PVP/PLLA solutions were then mixed with different amounts of OA Fe3O4 nanoparticles (5%, 10% and 20% in respect to the total polymer mass) at room temperature. In all cases, the polymer concentration in solution was kept constant (10%w/v) and only the concentration of the OA Fe3O4 varied. The quantities of the reactants used for the preparation of a series of PVP, PVP/PLLA and PVP/PLLA/OA Fe3O4 solutions are summarized in Table 1. The membranes were fabricated by using the electrospinning technique. All experiments were performed at room temperature using an electrospinning equipment comprised of a controlledflow, four-channel volumetric microdialysis pump (KD Scientific, Model: 789252), syringes with specially connected spinneret needle electrodes, a high-voltage power source (10–50 kV) and custom-designed, grounded target collectors, inside an interlocked Faraday enclosure safety cabinet. Systematic parametric studies were carried out by varying the applied voltage, the needle-tocollector distance, the needle diameter and the flow rate so as to

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Table 1 Quantities of the reactants used for the preparation of the PVP, PVP/PLLA and PVP/ PLLA/OA Fe3O4 solutions in chloroform. a/a Sample Code 1 2 3 4 5 7 8 9

PVP.3 PVP.5 PVP.10 PVP.15 PVP/PLLA PVP/PLLA PVP/PLLA PVP/PLLA

Chloroform (mL) PVP (g) PLLA (g) OA Fe3O4 (g)

10 10 10 10 (70/30) 10 (70/30).5 10 (70/30).10 10 (70/30).20 10

0.3 0.5 1.0 1.5 0.7 0.7 0.7 0.7

    0.3 0.3 0.3 0.3

     0.053 0.111 0.250

Entries 1–4: the numbers 3, 5, 10 and 15 denote the polymer solution concentrations (w/v). Entries 5–9: the 70/30 corresponds to the weight percentage proportion of PVP and PLLA respectively. Entries 7–9: the numbers 5, 10 and 20 denote the weight percentages of the OA Fe3O4 nanoparticles in respect to the total polymer mass.

determine the optimum experimental conditions for obtaining fibrous membranes.

2.4. Instrumentation Thermal Gravimetric Analysis (TGA) measurements were performed on a Q500 TA instrument under nitrogen flow at a heating rate of 10 1C/min. Transmission Electron Microscopy (TEM) investigations of the membranes were performed by using a TECNAI F30 G2 S-TWIN microscope operated at 300 kV equipped with energy dispersive X-ray spectrometer (EDX). Samples were placed into a double copper grid (oyster) to be visualized by TEM. The morphological characteristics of the membranes were also determined by Scanning Electron Microscopy (SEM) (Vega TS5136LSTescan) in the absence and presence of OA Fe3O4 nanoparticles. The samples were gold-sputtered (
15 nm) (sputtering system K575X Turbo Sputter Coater – Emitech) prior to SEM inspection. Digimizer Image Analysis Software was used to determine the average diameters of the produced fibers. The magnetic properties of the nanocomposite PVP/PLLA/OA Fe3O4 fibrous membranes were determined at 300 K with a Vibrating Sample Magnetometer (VSM)-Model 880 from ADE technologies USA.

3. Results and discussion 3.1. Membrane fabrication Fibrous membranes comprised of PVP, PVP/PLLA and PVP/PLLA/ OA Fe3O4 were prepared by electrospinning as illustrated in Fig. 1. For determining the optimum electrospinning conditions to obtain fibrous (nanocomposite) membranes, different experimental parameters were systematically varied while maintaining the rest relatively unchanged. Such parameters included the polymer solution concentration, the applied voltage, the delivery rate of the solution, the needle diameter and the needle-tocollector distance. Experimental parametric studies were carried out for the PVP (solution concentration: 3, 5, 10, 15 wt%) and the PVP/PLLA (solution concentration: 10 wt%) systems in the absence and presence of the magnetic nanoparticles. Under specific experimental conditions, fibrous PVP and PVP/PLLA membranes (entries 1–5, Table 2) as well as magnetite-containing membranes with different magnetic loading (entries 7–9, Table 2) were successfully obtained by electrospinning.

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Oleic acid-coated iron oxide nanoparticles (OA.Fe3O4) poly(vinylpyrrolidone), PVP

poly(L-lactic acid), PLLA

Electrospinning/ room temperature

PVP/PLLA Oleic acid-coated Fe3O4 nanoparticles

CHCl3

Fig. 1. Chemical structures and names of the main reagents used for the preparation of PVP, PVP/PLLA and PVP/PLLA/OA Fe3O4 electrospun fibrous membranes and schematic presentation of the methodology followed for the fabrication of PVP/PLLA/OA Fe3O4 nanocomposite membranes by electrospinning.

Table 2 Optimum experimental conditions successfully employed for the fabrication of the PVP and PVP/PLLA fibrous membranes in the absence and presence of OA Fe3O4. a/a

Sample Code

Needle (G)

Needle-toCollector Distance (cm)

Voltage (kV)

Flow rate (mL/h)

1 2 3 4 5 7 8 9

PVP.3 PVP.5 PVP.10 PVP.15 PVP/PLLA PVP/PLLA PVP/PLLA PVP/PLLA

16 20 22 20 20 20 20 20

25 25 30 25 30 25 25 25

15 15 20 20 25 15 10 10

20 15 10 10 10 10 10 10

(70/30) (70/30).5 (70/30).10 (70/30).20

Entries 1–4: the numbers 3, 5, 10 and 15 denote the polymer solution concentrations (w/v). Entries 5–9: the 70/30 corresponds to the weight percentage proportion of PVP and PLLA respectively. Entries 7–9: the numbers 5, 10 and 20 denote the weight percentages of the OA Fe3O4 nanoparticles in respect to the total polymer mass.

3.2. Morphological characterization The morphological characteristics of the membranes were determined by SEM. Initially, parametric studies were carried out for the PVP homopolymer upon systematically increasing the solution concentration from 3 wt% to 15 wt%. According to literature reports [9,17,42] it is expected that at lower polymer solution concentrations, fragmentation of the charged jet occurs leading to the generation of droplets as a result of the applied voltage and the solution's surface tension [43]. As the concentration of the polymer solution increases, a mixture of fibers and beads is obtained, while a further increase causes the morphological transition of the beads from spherical to ellipsoidal and finally to continuous fibers, owing to chain entanglement effects arising in concentrated solutions. Indeed as shown in Fig. 2a, starting from the low concentration solutions (3 and 5 wt%), the co-existence of fibers and beaded structures is observed with the latter being the dominant ones, whereas upon increasing the solution concentration the fibrous morphology dominates. A comparison of the SEM images presented in Fig. 2a corresponding to the 10 and 15 wt% polymer solution concentration respectively, clearly shows that a further increase in the concentration leads to the disappearance of the beaded structures and the generation of continuous fibers with mean diameters of 7.3371.04 μm. The SEM images of the produced PVP/PLLA membranes and of the nanocomposite PVP/PLLA/OA Fe3O4 membranes containing 5, 10 and 20 wt% OA Fe3O4 are also provided in Fig. 2(b) and (c). In all

cases the solution concentration was maintained at 10 wt% since at this concentration continuous fibers were obtained with mean diameters in the range of
5.5 μm–8.5 μm. Upon increasing the magnetic content from 5 to 20 wt%, no significant changes were observed in the morphological characteristics of the fibers in line with previously reported results on electrospun PVP–Fe(0) nanocomposite fibers [33]. TEM was also employed for visualizing the PVP/PLLA/OA Fe3O4 systems. In Fig. 3 transmission electron micrographs of the PVP/ PLLA/OA Fe3O4 nanocomposite membrane containing 20 wt% OA Fe3O4 nanoparticles are depicted. From the TEM bright field images (Fig. 3a–c) it can be clearly seen that the magnetic Fe3O4 nanoparticles embedded within the membranes are spherical with average diameters of approximately 5 nm. By comparing the TEM images and size distribution of the as-prepared OA Fe3O4 nanoparticles provided in a recent publication [35] with the mean diameters of the nanoparticles embedded within the polymer fibers shown in Fig. 3, it can be concluded that no significant agglomeration phenomena occur during the electrospinning process since the dimensions of the embedded nanoparticles are retained within the same range. Moreover, the magnetic nanoparticles are uniformly dispersed in the PVP/PLLA matrix resulting in high homogeneity. The HRTEM image presented in Fig. 3d shows that the nanoparticles are nanocrystals, disclosing the crystalline planes (311) of Fe3O4 with 2.51 Å characteristic interplanar distance. The two EDX spectra presented in Fig. 3 show the presence of Fe, O and C as the major elements in the sample (element Cu comes from the copper grid). The presence of minor elements such as Si and Cl may be attributed to mild sample contamination during the preparation process for TEM investigations. 3.3. Thermal stability The degradation temperature of the PVP/PLLA membranes was determined in the presence and absence of OA Fe3O4 by means of TGA. Fig. 4 provides the TGA thermograms of the pure OA Fe3O4, the pristine PVP/PLLA membranes and the nanocomposite PVP/ PLLA/OA Fe3O4 systems. In the case of the pure OA Fe3O4 the weight loss observed at temperatures below 300 1C, is attributed to the decomposition of the organic stabilizing layer (oleic acid) [44]. As seen in Fig. 4, a two-step decomposition profile is observed in the case of the PVP/PLLA polymer membrane. The first degradation step is observed at around 320 1C, and it is attributed to the decomposition temperature of PLLA in agreement with previously reports [45,46]. PVP degradates at a higher temperature (
400 1C)

I. Savva et al. / Journal of Magnetism and Magnetic Materials 352 (2014) 30–35

3% w/v

5% w/v

5% wt. OA.Fe3O4

10% w/v

10% wt. OA.Fe3O4

33

15% w/v

20% wt. OA.Fe3O4

Fig. 2. SEM images of the pristine PVP (a) and PVP/PLLA (b) polymer membranes as well as of the magnetoactive PVP/PLLA/OA Fe3O4 polymeric membranes (c) in the presence of 5, 10 and 20 wt% OA Fe3O4 nanoparticles.

in line with previously reported findings [47]. The remaining residue observed in the case of the magnetite-containing membrane at higher temperatures (T4400 1C), correspond to the inorganic (Fe3O4) content. 3.4. Magnetic properties Investigation of the magnetic properties of the PVP/PLLA/OA Fe3O4 nanocomposite membranes was carried out by VSM at 300 K. Fig. 5 shows the magnetization versus applied magnetic field strength plots (M¼ f(H)) corresponding to nanocomposite membranes loaded with different amounts of OA Fe3O4. The sigmoidal shape of these plots and the lack of a hysteresis loop demonstrates the superparamagnetic behavior of these materials at ambient temperature. Moreover tunability of the saturation magnetization (Ms) is possible upon vaying the magnetic content within the membranes, retaining at the same time the superparamagnetic

response. Based on literature reports, the saturation magnetization of the bulk (uncoated) Fe3O4 nanoparticles is approximately (
92 emu/g) [48]. The oleic-acid coated magnetite nanoparticles exhibit lower saturation magnetization (
40 emu/g) due to their nanosize dimensions and the presence of the organic, nonmagnetic oleic acid coating. The incorporation of the OA Fe3O4 within the PVP/PLLA fibers results to a further decrease of the Ms owing to the existence of the non-magnetic polymers [48–50].

4. Conclusions Novel magnetoactive PVP/PLLA/OA Fe3O4 fibrous nanocomposite membranes have been prepared for the first time by means of the electrospinning technique. The membranes consisting of microfibers with embedded magnetite nanoparticles uniformly dispersed within the fibers exhibit superparamagnetic behavior at room

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I. Savva et al. / Journal of Magnetism and Magnetic Materials 352 (2014) 30–35

Edx1

Edx2

Fig. 3. TEM bright field (a–c) and HRTEM (d) images and corresponding EDX spectra (Edx1 for ensample and Edx2 acquired with point beam on a nanoparticle from edge of fiber) of the PVP/PLLA/OA Fe3O4 polymeric membranes in the presence of 20 wt% OA Fe3O4 nanoparticles. 100

60

temperature that can be tuned depending on the magnetic content. Future work involves the photo-crosslinking of the aforementioned PVP-containing systems via UV-irradiation for the development of materials exhibiting high stability in aqueous media and the investigation of their applicability in the biomedical and environmental fields.

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Acknowledgments

% mass

80

20

0 100

200

300

400

500

600

Temperature (oC)

Fig. 4. TGA thermograms of OA Fe3O4 nanoparticles, PVP/PLLA and PVP/PLLA/OA Fe3O4 (20 wt% OA Fe3O4).

This work was supported by the University of Cyprus and the Center for Fundamental and Advanced Technical Research from Timisoara of the Romanian Academy. We thank Dr. Athanassios Nicolaides (Department of Chemistry, University of Cyprus) for useful discussions and Ms. Alina Taculescu (Center for Fundamental and Advanced Technical Research from Timisoara of the Romanian Academy) for the synthesis of the OA Fe3O4. References

Magnetization (emu/g)

40

20

0

-20

-40 -1000 -800 -600 -400 -200

0

200

400

600

800 1000

Applied field (kA/m)

Fig. 5. Magnetization curves of the as-prepared OA Fe3O4 nanoparticles and of the PVP/PLLA/OA Fe3O4 nanocomposite membranes containing 5 and 10 wt% OA Fe3O4 measured at 300 K.

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