Preparation and characterization of polymer coated superparamagnetic magnetite nanoparticle agglomerates

https://doi.org/10.1016/j.jpcs.2009.04.007Get rights and content

Abstract

In order to produce magnetic microparticles (agglomerates), magnetite (Fe3O4) particles were synthesized using coprecipitation of FeSO4·7H2O and FeCl3·6H2O with the presence of poly(methacrylic acid) (PMAA) in aqueous solution.. Transmission electron microscopy (TEM), X-ray diffraction, and vibrating sample magnetometry (VSM) methods were used to characterize the PMAA coated superparamagnetic agglomerates. The influences of various processing parameters such as the process temperature, PMAA content, and the addition of surfactant on the agglomerate size and size distribution of produced magnetic microparticles were investigated. The particle size and size distribution characteristics, (the volume weighted mean size (D[4,3], surface weighted mean size D[3,2], the geometric standard deviation, and span value) of the magnetic agglomerates were determined using the laser diffraction technique. The PMAA coated magnetic agglomerates with surface weighted mean sizes ranging from 1.5 to 3 μm were produced successfully.

Introduction

Both nano- and micron-sized magnetic particles have been used successfully for a wide range of applications including drug delivery, immunoassays, magnetic resonance imaging (MRI) contrast enhancements, hyperthermia, diagnosis, and separation, purification, or detection of proteins, DNA, viruses, cells, or bacteria for their response to magnetic fields [1], [2], [3], [4], [5], [6], [7], [8], [9] Magnetite (Fe3O4) or maghemite (γ-Fe2O3) are usually used as magnetic materials. Magnetic iron oxide particles (Fe3O4 or γ-Fe2O3) suspended in carrier fluids are referred to as magnetic fluids. Recently, a significant progress has been made in the fields of magnetic fluids and magnetic particles. Many of the particles used in the separation technology are superparamagnetic, which can easily be magnetized with an external magnetic field and immediately redispersed once the magnet is removed [10]. Aggregation of magnetic nanoparticles is inevitable because of attractive van der Waals forces between very small particles. Therefore, the surfaces of nanoparticles are coated with various types of stabilizers such as oleic acid, poly(methacrylic acid) (PMAA) or poly(glycerol monoacrylate) in order to obtain stable colloidal suspensions [11], [12], [13], [14], [15], [16]. Micron- or submicron-sized magnetic particles, which are used extensively for many applications, are made of a large number of nanosized maghemite or magnetite primary particles distributed in a polymer matrix. Micron-sized magnetic particles are preferable for some applications such as separation, purification, or detection of proteins, DNA, viruses, cells, or bacteria since the separation of magnetic microparticles from liquid medium can be executed easily and economically compared with that of magnetic nanoparticles [1], [17]. Submicron-sized magnetic polymer microparticles with strong magnetization are selected for applications where negligible sedimentation and a high specific surface area for the immobilization of a large number of biomolecules are required [18]. For these applications, magnetic microparticles should have narrow size distributions and hydrophilic surfaces with proper functional groups and exhibit superparamagnetism and high magnetization [17]. Magnetic microparticles are covered with a polymer coating possessing active groups. It is possible to prepare polymer coated magnetic microparticles appropriate for various types of application by modifying functional groups of the polymer coating [19], [20], [21], [22]. PMAA has lately drawn considerable attention as a coating material because of its capabilities of chelating carboxylic acid (–COOH) groups to retain metal ions and hydrogen bonds leading to a three-dimensional structure [23].

Magnetite (Fe3O4), which is chemically stable, non-toxic, and non-carcinogenic, has a high magnetic saturation value (92 emu/g) compared to maghemite (82 emu/g) [24], [25], [26]. If exposed to oxygen environment magnetite may transform to maghemite. One way to prepare stable magnetite particles is to coat magnetite particles with a polymeric material so that the transformation of magnetite to maghemite (γ-Fe2O3) due to the reaction of the surface Fe(II) cations with adsorbed oxygen can be prevented [27], [28]. Furthermore, the coating of magnetite particles by PMAA provides enhanced stability for the aqueous magnetite suspensions due to combined electrostatic and steric (electrosteric) stabilization. Several methods have been reported for the production of magnetite nanoparticles [29], [30], [31], [32], [33]. The coprecipitation of iron salts in the presence of a base is one of the most used methods for the synthesis of superparamagnetic iron oxide nanoparticles [34], [35], [36]. Various techniques are used for preparing polymer encapsulated iron oxide particles. Some of those techniques are based on, first, precipitating iron oxide particles, and then coating the precipitated particles with polymers. Some researchers adapted in situ synthesis of polymer encapsulated iron oxides, and for this synthesis route the precipitation and coating processes occur simultaneously by precipitating particles in a polymer solution [37]. It is essential to control both the primary particle size and agglomerate (cluster) size of magnetic particles. To obtain superparamagnetic particles, the primary particle size of magnetite should be less than about 12 nm. The agglomerate (cluster) size cannot be too small since very small magnetic particles cannot be captured easily during magnetic separation operations [38]. Large magnetic agglomerates (clusters) with a low specific surface area are not desirable since they do not provide efficient separation. Therefore, it is important to control the agglomerate size of magnetic particles to obtain an optimum product.

In this work, in situ synthesis was used, that is, magnetic (Fe3O4) particles were prepared by the coprecipitation of FeSO4·7H2O and FeCl3·6H2O with the presence of PMAA in aqueous solution. The influence of molar ratio of Fe2+/Fe3+ on the properties of precipitated magnetite was not studied since it was already investigated by various researchers [39], [40]. The influences of various processing parameters such as the process temperature, PMAA content, and the addition of surfactant on the agglomerate size and size distribution of produced magnetic microparticles were investigated by using the laser diffraction technique. Transmission electron microscopy (TEM), vibrating sample magnetometry (VSM), thermal analysis, and X-ray diffraction (XRD) techniques were also used to characterize the PMAA encapsulated magnetic agglomerates.

Section snippets

Materials

PMAA(sodium salt) (30 wt% aqueous solution, MW ca. 9500), Fluka, Cetyltrimethylammonium bromide (CTAB), Aldrich, Igepal®CO-520, Aldrich, were purchased and used without any purification. Other reagents were commercially available analytical grade products.

Preparation of magnetic microparticles

The flow chart for the preparation procedure of the magnetite agglomerates is illustrated in Fig. 1. The procedure is similar to the one reported [37], however, FeSO4 was used instead of FeCl2 and the precipitation reaction was carried out

X-ray diffraction

As shown in Fig. 2, the PMAA encapsulated magnetite microparticles (agglomerates) can be easily captured and separated by a permanent magnet. The XRD pattern of the PMAA encapsulated iron oxide nanoparticles (sample: S3) is illustrated in Fig. 3. All detected XRD peaks can be assigned to the characteristic peaks of Fe3O4. Furthermore, the lattice constant of the sample was calculated as 0.838 nm from the four most intense peaks, indicating that the sample is magnetite. The lattice constant of

Conclusions

PMAA coated magnetite agglomerates consisting of a large number superparamagnetic nanoparticles with a primary particle size of approximately 8 nm were successfully produced by coprecipitating iron salts (FeSO4·7H2O and FeCl3·6H2O) in the presence of poly(methacrylic acid) (PMAA) in aqueous solution. The influences of PMAA content, precipitation temperature, and addition of surfactant on the agglomerate size of PMAA coated magnetite particles were investigated. It has been found that both the

Acknowledgements

The authors would like to thank the Central Laboratory, Middle East Technical University, for zeta potential, particle size, and thermal analysis measurements.

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