Influence of ultrasonically assisted synthesis on particle size of magnetic nanoparticles
Introduction
Magnetite (Fe3O4) and maghemite (γ-Fe2O3) are well-known iron oxide phases among magnetic nanoparticles due to their magnetic properties, chemical stability, and nontoxicity. Magnetic iron oxide nanoparticles have been used successfully for wide range of applications such as targeted drug delivery [1], [2], [3], [4], [5], enzyme and protein separations [6], [7], [8], [9], magnetic resonance imaging (MRI) contrast agent [10], and environmental remediation [11]. When magnetic nanoparticles are exposed to air, they may not be stable, thus it is necessary to coat magnetic nanoparticles with a protective layer such as polymer [12], [13], silica [14], and carbon [11]. Furthermore, magnetic nanoparticles aggregate easily due to attractive van der Waals forces between very small particles. Therefore, the surfaces of nanoparticles are coated with various types of stabilizers such as sodium polyoxyalkylene di-phosphonates, poly(methacrylic acid) or poly(glycerol monoacrylate) in order to obtain stable colloidal suspensions [15], [16], [17], [18], [19]. To obtain monodispersed magnetite particles, sonochemical synthesis was adapted [20]. Wu et al. synthesized Fe3O4 magnetic nanoparticles using ultrasonic chemical co-precipitation technique [21].
The superparamagnetic magnetic particles are preferred for the separation technology. To obtain superparamagnetic particles, the primary particle size of magnetite should be less than about 12 nm. In addition, it is desirable to have magnetic particles with a high surface area for applications such as separation, purification or detection of proteins, cells or bacteria. However, the agglomerate (cluster) size cannot be too small since very small magnetic particles cannot be captured easily during magnetic separation operations [22]. Large magnetic agglomerates with a low specific surface area are not desirable since they do not provide efficient separation. Therefore, it is essential to control both the primary particle size and agglomerate size of magnetic particles to obtain an optimum product. The coating of magnetite particles by polymers provides enhanced stability of magnetite nanoparticles [23], [24], [25], [26]. 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 [27], [28]. 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 the precipitated particles are coated 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 [29], [30].
In this work, magnetite nanoparticles were synthesized via co-precipitation in aqueous solution under ultrasonic agitation and magnetic stirring. Both in situ and ex situ synthesis were adapted for the preparation of magnetite particles. The influence of ultrasonic agitation on the mean size and specific surface area of poly(methacrylic acid) (PMAA) coated magnetic agglomerates during sample preparation was investigated and compared with the use of conventional magnetic stirring. The aim was to decrease the size and increase the specific surface area of PMAA-coated magnetic agglomerates.
Section snippets
Materials
Iron (III) chloride hexahydrate (FeCl3·6H2O, 97%, Aldrich); ethanol (CH3CH2OH, 99.5%, Sigma-Aldrich); ferrous sulfate heptahydrate (FeSO4·7H2O, 99%, analar by British Drug Houses); iron (III) chloride hexahydrate (FeCl3·6H2O), 97%: Aldrich); poly(methacrylic acid) (PMAA) (30 wt% solution in water, Mn=5400, Mw=9500, Aldrich); hydrochloric acid (HCl, 37%, Merck); sodium hydroxide pellets (NaOH, Riedel-de Haën); nitrogen gas (99.998%) were used for the synthesis of iron oxide nanoparticles.
Poly(methacrylic acid)-coated magnetite (Fe3O4) nanoparticles
Preparation of PMAA coated magnetite particles was accomplished either ex situ, that is, the Fe3O4 nanoparticles were prepared first and then subjected to the PMAA solution under agitation or in situ where the iron oxide nanoparticles were synthesized in the presence of PMAA.
In situ preparation of magnetite nanoparticles was done previously in our laboratory via co-precipitation of iron salts under magnetic stirring in the presence of aqueous PMAA solution [31]. The influences of the process
Conclusions
Magnetite nanoparticles were coated with poly(methacrylic acid) (PMAA) by co-precipitation in basic solution under both magnetic stirring and ultrasonic agitation. PMAA-coated magnetite agglomerates with a primary particle size of approximately 10 nm were ex situ synthesized in two steps using both magnetic stirring and ultrasound agitation. There was no influence of the mixing type (magnetic stirring and ultrasonic agitation) on the primary particle size of ex situ synthesized PMAA-coated
Acknowledgments
The authors thank Central Laboratory at Middle East Technical University and Dr. Ibrahim Cam for the particle size and zeta potential measurements.
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