Design, Optimization Process and Efficient Analysis for Preparation of Copolymer-Coated Superparamagnetic Nanoparticles

INTRODUCTION

Nanomaterials have received much recent attention because they are expected to be used in various applications based on their excellent and unique optical, electrical, magnetic, catalytic, biological, or mechanical properties [1]. Such properties originate from the finely tuned nanoarchitectures and nanostructures of these materials. Application of nanomaterials could help improve the environment and control pollution, which further progresses environmental science and engineering [2].

Polymer nanocomposites are potentially important due to the fact that they offer a number of significant advantages over traditional polymer composites. Magnetic polymer nanocomposites are usually composed of magnetic cores to ensure a strong magnetic response and polymeric shells to provide favorable functional groups and protect the magnetic nanoparticles against environmental degradation effects and isolate these blocks from each other to weaken and/or avoid the magnetic interaction [3]. The use of nanoparticles in polymeric systems has become a subject of interest in engineering applications due to potential dramatic changes in physical properties of composites. These changes in properties come from two aspects of nanoparticles: increased surface area and quantum effects associated with nano-dimensional particle structures [4]. These factors can change or enhance properties such as reactivity, strength, and electromagnetic properties of composites. The presence of nanoparticles also provides improvements in other properties as well, such as erosion resistance, wear resistance, fire resistance, hardness, and environmental resistance.

The application in biomedicine and bioengineering fields is also new progress due to they can be easily collected with the application of a magnetic field; and the coupling of appropriate ligands to such microspheres provides an effective tool to achieve rapid, simple, and specific biological separation [5].

Currently, magnetically controlled drug targeting has become one of the most active areas of cancer research since it offers true possibilities of active drug targeting and selective guidance of anticancer drug molecules to specific cells of the diseased site. The basic challenge in drug delivery is the transfer of drug agents to targeted sites at an appropriate time [6]. Apart from the application in drug controlled release, the magnetic polymer composites have also found wide applications in adsorbents of heavy metal ions and organic pollutants in waste water so that the strong magnetism is in favor of the separation and recycle of adsorbents [7]. Because of comparatively large surface areas, it is likely that nanosized adsorbents with strong affinity can be a useful tool in enhancing the adsorption capacity in drinking water treatment [8]. Applications of nanomaterials in environmental protection have created conditions to improve environment and control pollution, which will bring breakthrough progress to environmental science and engineering.

In this work, poly(maleic anhydride-co-acrylic acid) P(MAH-co-AA) was chosen as a polymer shell onto the magnetite nanoparticle because it offers the possibility to anchor additional functional groups for active site attachment. The morphology, magnetic properties and optimized condition of the magnetic nancomposite were examined with a FTIR, SEM, TEM, XRD and VSM.

Materials and Methods

Materials

All the chemicals were analytical purity and used without further purification. Iron (III) chloride hexahydrate (FeCl₃.6H₂O), iron (II) sulfate heptahydrate (FeSO₄.7H₂O), the ammonium hydroxide (NH₄OH) solution (25 wt%) and oleic acid (OA) were purchased from the Bendosen Laboratory Chemicals,Malaysia. The monomers maleic anhydride (MAH) and acrylic acid (AA) were obtained from Fluka, Sigma-Aldrich, Switzerland. 2,2-azo-bisisobutyronitrile (AIBN),  polyvinylpyrrolidone (PVP), 1,4-dioxane and N,N-methylenebisacrylamide (BIS) were purchased from Sigma-Aldrich, U.S.A. All chemicals are analytical grade without further purification. Deionized water was used throughout the experiments.

Synthesis of Modified Magnetite Nanoparticles

The Fe₃O₄ magnetite nanoparticles (Fe₃O₄-MNPs) were prepared by a co-precipitation method [9]. A bout 0.046 mol of FeCl₃-6H₂O and 0.023 mol of FeSO₄-7H₂O were dissolved into 150 mL deionized water in 250 mL three-necked flask and nitrogen gas was bubbled for 3 min to extrude the air. Then 20 mL ammonium hydroxide was added quickly into the iron solution under vigorous stirring. After 30 min 3 mL oleic acid was added into the mixture to modify the Fe₃O₄ MNPs and the mixture was heated to 80^oC. After 1h, the resulting Fe₃O₄ nanoparticles (black precipitate) were collected from the solution by magnetic separation and washed several times with deionized water and ethanol, then dried under vacuum conditions at 60 ^oC for 12h. The pH range of the synthesis iron oxide NPs should be 8-14 with maintaining molar ratio of Fe³⁺/Fe²⁺ (2:1) under a non-oxidizing condition. It is important to have an oxygen free environment during the synthesis otherwise; magnetite can be further oxidized to ferric hydroxide in the reaction medium. In the absence of any surface modification Fe₃O₄-MNPs tend to aggregate due to strong magnetic dipole-dipole attractions between particles.

Synthesis of Magnetic Fe₃O₄/P(MAH-Co-AA) Core/Shell Nanocomposites

The magnetic Fe₃O₄/P(MAH-co-AA) core/shell nanocomposites were prepared using a dispersion polymerization method. First, in a 150 mL three-neck flask, different concentration of modified Fe₃O₄ nanoparticles were dispersed in 10 mL 1,4-dioxane solution and the solution mixture was ultrasonicated for 20 min, and then mechanically stirred at 45 ^oC under nitrogen. In another flask, 0.4 g MAH, 0.4 mL AA, and 0.15 g PVP as stabilizer were dispersed in 20 mL 1,4-dioxane under ultrasonication for 30 min. Subsequently, the mixture was rapidly added into the previous ferrofluid and the stirring speed was adjusted to 500 rpm. Then 0.02 g AIBN as an initiator and 0.05 g BIS as a cross-linker were added into the reaction mixture.  The resulting medium was transferred into the reactor and sonicated for about 5 min within an ultrasonic water bath for the complete dissolution of AIBN in the polymerization medium.

The polymerization reactor was placed in a water bath and heated to 75^oC for 6 h with a mechanical stirring (500 rpm). The whole process was protected in nitrogen. After the reaction, a brown-colored product was separated from the solution by magnetic separation. The particles were sequentially washed with acetone and deionized water several times and vacuum-dried at 60 ^oC for 24h to obtain the polymer-coated Fe₃O₄ magnetic nanocomposite.

Characterization Methods and Instruments

Fourier transform infrared spectroscopy (FTIR) was used to identify the functional groups present in the synthesized compounds. FTIR spectra were recorded over the range of (400-4000) cm⁻¹ with a Perkin Elmer 1650, FTIR spectrophotometer. The crystal structure of the samples prepared has been confirmed by XRD analysis with a Shimadzu XRD-6000 Lab X wideangle diffractometer. The average particle size and size distribution of the samples were observed by a Hitachi H-7100 transmission electron microscope (TEM) with an acceleration voltage of 200 kV, whereas the particle size distributions were determined using the UTHSCSA Image Tool version 3.00 program. The analysis of the surface morphologies of the samples was performed with a Philips XL-30 environmental scanning electron microscope (SEM). Magnetization measurements were carried out with a Lakeshore (model 7407) vibrating sample magnetometer (VSM) to study magnetic properties of the magnetic nanoparticles under magnetic fields up to 10KG (1T) at room temperature.

Results and Discussion

In this work, AIBN used as the initiator in the dispersion polymerization to synthesize Fe₃O₄/poly(maleic anhydride-co-acrylic acid) magnetic nanocomposite with core/shell structure. The mechanism for polymerization is shown below:

1) Free radical formation from initiator

2) Addition of free radical to monomer

3) Propagation of polymer chain

4) Termination of chain via coupling

The schematic structures for the formation of magnetic Fe₃O₄/P(MAH-co-AA) core/shell nanocomposites is shown in Fig. 1. The first step is the formation of Fe₃O₄ nanoparticles by precipitation and coating with oleic acid as stabilizing agent.  The second step is encapsulation of magnetic nanoparticles into P(MAH-co-AA) by dispersion polymerization  to form  a magnetic core with a polymer shell.

Structural Analysis of Magnetic Fe₃O₄/P(MAH-Co-AA) Core/Shell Nancomposites

Fourier Transform Infrared Spectroscopy

FTIR spectra of the Fe₃O₄ nanoparticles, and magnetic Fe₃O₄/P(MAH-co-AA) core/shell are shown in Fig. 2. For the IR spectrum of Fe₃O₄ nanoparticles (Fig. 2a), the strong IR band of magnetite is Fe-O stretching at 580 cm^-1 and the absorption peak around 3440 cm-1 is attributed to the hydroxyl groups on the surface of Fe₃O₄ or adsorbed water [10-11]. The presence of additional peaks is related to OA-coated magnetite nanoparticles [9]. The characteristic bands of the anhydride cycles are observed at 1860, 1784 cm^-1 due to the C=O bonded with non-conjugated oxygen atoms [12]. The absorption bands of the carboxyl groups of PAA can be noticed at 2960 cm^-1 and 1675 cm^-1 correspond to O-H and C=O stretching vibration respectively in the IR spectrum of the P(MAH-co-AA). In addition, AA and MAH show two peaks at 1169 and 1285 cm^-1 due to the C-O stretching. The results indicate that P(MAH-co-AA) was successfully synthesized. The absorption bands at 580 cm-1 of the Fe–O bond (Fig. 2b), combined with the characteristic peaks at 2960, 1784 and 1675 cm^-1, signify that P(MAH-co-AA) was successfully polymerized and the active groups has been introduced onto the surface of magnetite core. Therefore the magnetite Fe₃O₄ surface was wrapped by the copolymer P(MAH-co-AA) and a novel magnetic Fe₃O₄/P(MAH-co-AA) nanocomposite has been synthesized.

X-ray Diffraction           

Fig. 3 shows the XRD patterns for Fe₃O₄ nanoparticles and P(MAH-co-AA). A series of characteristic peaks for Fe₃O₄ nanoparticles were observed in the XRD pattern at 2θ of 9.6^o, 30.1^o, 35.9^o, 43.8^o, 54.5^o, 57.6^o and 63.6^o. The positions and relative intensities of the reflection peak of Fe₃O₄ nanoparticles agree with the XRD diffraction peaks of standard Fe₃O₄ samples indicating that the black-colored magnetic powders are Fe₃O₄-MNPs [13]. Sharp peaks in Figure 3a suggest that the Fe₃O₄-MNPs have good crystallize structure. It was found that the magnetite crystallites could be well indexed to the inverse cubic spinel structure of Fe₃O₄. The broader peak at 2θ of 20.5o could be assigned to the diffraction scattering of amorphous P(MAH-co-AA) copolymers (Fig. 3b).

The peak intensity of Fe₃O₄/P(MAH-co-AA) nanocomposite is lower than that of pure Fe₃O₄ nanoparticles in order to the Fe₃O₄ nanoparticles are incorporated into polymer  spheres. It is clear from Fig. 4, the peak intensity of polymer increases by decreasing concentration of Fe₃O₄-MNPs and polymer peak is clearer from a-d. It can be explained that polymer thickness increases by decreasing of nanoparticle concentration, therefore peak intensity of polymer better detected. On the other hand reduction peak intensity of polymer is due to polydispertion in high concentration of Fe₃O₄.

The average crystal sizes of the Fe₃O₄ and Fe₃O₄/P(MAH-co-AA) core/shell nanocomposite were calculated from the XRD data using Debye-Scherrer equation, and found to be 9.2 and 15.15 nm, respectively.

Transmission Electron Microscopy

TEM image of the magnetite nanoparticles shows that the sizes of Fe₃O₄ nanoparticles are almost uniform and most of the particles are approximately cubic in shape. As shown in Fig. 5a, the particle size distribution curve of Fe₃O₄ MNPs indicated that mean diameter size was 9.41 nm.

Fig. 5b shows TEM images of polymer coated magnetite nanoparticles. It is clear from the TEM images that Fe₃O₄/P(MAH-co-AA) nanoparticles used in this case have excellent dispersibility and the composite particles are cubic in shape and the magnetic particles are encapsulated in the P(MAH-co-AA) template. No pure polymer particles are observed. It can be seen from Fig. 5a; the pure Fe₃O₄ MNPs were polydisperse and seriously aggregated. After surface polymerization the dispersion of Fe₃O₄ nanoparticles improved greatly, which can be explained by the fact that the interaction among particles might be changed after modification with P(MAH-co-AA), leading to good dispersion. Histogram evaluating size distributions of the polymer coated nanoparticles showed that the average particle size was 17.09 nm, and the results were reasonably consistent with those obtained from XRD. It should be noted that the average size of core/shell nanoparticles was higher than that of naked Fe₃O₄ nanoparticles; therefore it is clear that the average particle size increased after surface modification.

Transmission Electron Microscopy of Nanocomposites at Different Concentration of Fe₃O₄-MNPs

Magnetic polymer nanocomposite with different magnetic content were synthesised by reacting different amounts of Fe₃O₄ nanoparticles. Increasing the concentration of magnetic nanoparticles, results in an increase of polydispersity and the average diameter size from 6.56 to 25.21 nm as shown in Fig. 6. Possibly, the content of the Fe₃O₄-MNPs in the nanocomposites have a considerable effect on the morphology of product due to the existence of static magnetic attraction [14]. Another reason for increasing the magnetic aggregation or magnetic size by increasing magnetite concentration could be that with rising iron oxide concentration agglomeration is favored compared to encapsulation inside the polymer. As result only few large aggregates are faced to many monomer droplets which cannot encapsulate the iron oxide particles. This decreases monodispersity and increases polydispersity.

Scanning Electron Microscopy  

The morphology and structure of the Fe₃O₄ nanoparticles and Fe₃O₄/P(MAH-co-AA) nanocomposite were observed by SEM images as shown in Fig. 7. The results demonstrate that Fe₃O₄ nanoparticles are cubic and highly uniform in size. The SEM micrograph of magnetice Fe₃O₄/P(MAH-co-AA) presented in Fig. 7b shows that the nanocomposite have a porous surface structure. The magnetic nano composites are spherical in shape and have a rough surface due to the pores, which formed during the polymerization process. The porous surface structure is a factor for increasing surface area.

Vibrating Sample Magnetometer

The magnetization curves measured at room temperature for Fe₃O₄ nanoparticles and polymer-coated Fe₃O₄ are compared in Fig. 8. There was no hysteresis in the magnetization for both of samples, suggesting the magnetic particles produced are superparamagnetic and the single-domain magnetite nanoparticles remained in the polymer particles. This can be attributed to the small size of MNPs which were smaller than the superparamagnetic critical size (25 nm). On the other hand when the magnetic component size of the particles is smaller than critical size, the particles will exhibit superparamagnetism [15]. From the magnetization curves, we can also conclude that magnetite nanoparticles were trapped in the polymer matrix without obvious aggregation, which were match with the results of TEM micrographs. The high saturation magnetization of pure Fe₃O₄ indicated the good crystal structure. The saturation magnetization values (M_s) of polymer-coated Fe₃O₄ was smaller than the value for the pure magnetite nanoparticles and decreases from 83.6 to 46.6 emu g^-1, therefore the saturation magnetization reduced after polymer coating onto the surface of Fe₃O₄ core. This was due to the existence of the large amount of diamagnetic of the polymer shells surrounding the magnetite nanoparticles [16]. With such high magnetization and superparamagnetic behaviors, the magnetic polymer particles could be easily and rapidly separated from the solution and easily redispersed by gentle shaking. Therefore these magnetic properties are critical in the applications of the biomedical and bioengineering fields.

Influence of The Fe₃O₄ Content on The Saturated Magnetization

The magnetization curves for magnetic Fe₃O₄/P(MAH-co-AA) nanocomposite with different Fe₃O₄ dosages are shown in Fig. 9. It can be seen from the magnetization curves that the saturation magnetization (Ms) of the MNPs increased from 11.8 to 57.4 emu g^-1, when Fe₃O₄ content increased from 1 to 7 wt% of polymer shell. It indicated that more Fe₃O₄ nanoparticles being trapped in the polymer matrix. These data also indicated that the Fe₃O₄ content in composites systems can be varied. In addition, the increase in Ms can be attributed to the increase in particle size which reduced the surface area. It is clear that the energy of a magnetic particle in an external field is proportional to its size via the number of magnetic molecules in a single magnetic domain. When this energy becomes comparable to the thermal energy, thermal fluctuations will significantly enhance the total magnetic moment at a given field [17]. The saturation magnetization value of 16.3 emu g^-1 was sufficient for magnetic separation with a conventional magnet [18]. Therefore, the magnetization value achieved with the 5 wt% Fe₃O₄ content of the polymer particles was high enough for magnetic separation.

Optimization of Polymer Shell onto the Surface of Magnetic Nanoparticles

The magnetization curves of the polymer-coated Fe₃O₄ nanocomposites at different concentration of polymer recorded with VSM at room temperature are shown in Fig. 10. It can be seen that the saturation magnetization of nanocomposites significantly decreases from 46.2 to 14.2 emu g^-1 with the increase in polymer concentration. Because the weights of nanoparticles used for the measurement of magnetic properties were constant and the decrease of saturation magnetization was due to the increased amount of polymer incorporated in the polymer-coated magnetite, thus the increase in the mass of polymer results in the decrease in the magnetic strength of the nanocomposite.

The reduction of the saturation magnetization can be explained by the presence of polymer on the surface of Fe₃O₄ nanoparticles which may generate a magnetically dead layer so any crystalline disorder within the surface layer cause to a significant decrease in the saturation magnetization of nanoparticles [19]. The saturation magnetization of Fe₃O₄/P(MAH-co-AA) (1:4), Fe₃O₄/P(MAH-co-AA) (1:5) and Fe₃O₄/P(MAH-co-AA) (1:6) were 46.2, 31.6 and 14.2 emu g^-1, respectively.

According to previous part the saturation magnetization value achieved with the Fe₃O₄/P(MAH-co-AA) magnetic nanocomposite below 6 wt% was high enough for magnetic separation. On the other hand, the use of polymer above 5 wt% of Fe₃O₄ duo to decrease in Ms is not sufficient for magnetic separation.

The result demonstrates that the magnetic properties of the magnetic Fe₃O₄/P(MAH-co-AA) nanoparticles are strongly influenced by the polymer shell thickness. In addition, the magnetic properties of the nanocomposite can be easily controlled by varying the starting polymerization during fabrication [20].

Conclusion

 In this study a magnetic nanocomposite has been successfully synthesized by dispertion polymerization method with core/shell structure. The coated OA molecules as surfactant on the nanoparticles surface leads to highly dispersed and high entrapment efficiency of magnetic into a polymer and also smaller particle size. The TEM images reveal that the Fe₃O₄ and Fe₃O₄/P(MAH-co-AA)  core/shell nanoparticles are roughly cubic in shape and uniform in size with a mean diameter of 9.41 nm for Fe₃O₄ nanoparticles and after surface polymerization the mean diameter of nanoparticles (core/shell) increased to 17.09 nm. The X-ray diffraction pattern revealed that the structure and crystallity of Fe₃O₄ core was not changed significantly after coating.

The magnetic behaviors of polymer-coated magnetite nanoparticles exhibit superparamagnetism. All examinations have demonstrated that the Fe₃O₄ nanoparticles coated with surfactants oleic acid act as a core and the polymer layer of P(MAH-co-AA) formed on the Fe₃O₄ surface as a shell.

ACKNOWLEDGEMENTS

The authors are grateful to the Department of Chemistry, Faculty of Sciences for the laboratory facilities.

Conflict of interest

The authors declare that there are no conflicts of interest regarding the publication of this manuscript.

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