Pharmaceutical Nanotechnology
Preparation and characterization of superparamagnetic iron oxide nanoparticles stabilized by alginate

https://doi.org/10.1016/j.ijpharm.2006.10.006Get rights and content

Abstract

SPION with appropriate surface chemistry have been widely used experimentally for numerous in vivo applications. In this study, SPION stabilized by alginate (SPION-alginate) were prepared by a modified coprecipitation method. The structure, size, morphology, magnetic property and relaxivity of the SPION-alginate were characterized systematically by means of XRD, TEM, ESEM, AFM, DLS, SQUID magnetometer and MRI, respectively, and the interaction between alginate and iron oxide (Fe3O4) was characterized by FT-IR and AFM. The results revealed that typical iron oxide nanoparticles were Fe3O4 with a core diameter of 5–10 nm and SPION-alginate had a hydrodynamic diameter of 193.8–483.2 nm. From the magnetization curve, the Ms of a suspension of SPION-alginate was 40 emu/g, corresponding to 73% of that of solid SPION-alginate. This high Ms may be due to the binding of Fe3O4 nanoparticles to alginate macromolecule strands as visually confirmed by AFM. SPION-alginate of several hundred nanometers was stable in size for 12 months at 4 °C. Moreover, T1 relaxivity and T2 relaxivity of SPION-alginate in saline (1.5 T, 20 °C) were 7.86 ± 0.20 s−1 mM−1 and 281.2 ± 26.4 s−1 mM−1, respectively.

Introduction

Alginate, a natural-occurring polyelectrolytic polysaccharide found in all species of brown algae and some species of bacteria, is a linear polymer composed of α-l-guluronate (G) and β-d-mannuronate (M) units in varying proportions and sequential arrangements, and is biocompatible and biodegradable in tissue (Gombotz and Wee, 1998, Robitaille et al., 1999, Bouhadir et al., 2001). Alginates have also been widely studied for their ability to form gels in the presence of divalent cations (Nesterova et al., 2000, Finotelli et al., 2004, Sreeram et al., 2004). Metal alginate gels are ionotropic in nature, differing from the classical type of gels in which long-chain molecules are held together by simple van der Waals forces, and the macromolecular chains are chelating polyvalent metal ions (Khairou et al., 2002). The design and construction of metal-containing nanoscale arrays is a challenge since such structures are expected to be indispensable in the emerging technologies of this century.

Superparamagnetic iron oxide nanoparticles (SPION) with appropriate surface chemistry have been widely used experimentally applications such as magnetic resonance imaging contrast enhancement (Wang et al., 2001), immunoassay (Medina et al., 2004), hyperthermia (Jordan et al., 2001), magnetic drug delivery (Alexiou et al., 2000), magnetofection (Krotz et al., 2003), cell separation/cell labelling (Olsvik et al., 1994, Bulte et al., 2001), etc. Furthermore, some SPION preparations have already been approved for clinical use, especially for MR imaging, such as Endorem® (diameter 80–150 nm, Advanced Magnetics) and Resovist® (diameter 60 nm, Schering) for liver/spleen imaging (Wang et al., 2001). Recently, functionalized magnetic nanoparticles conjugated with antibodies or receptors including epidermal growth factor receptors (EGFRs) (Suwa et al., 1998), her2/neu (Funovics et al., 2004) and folate (Sonvico et al., 2005) have been widely studied. These applications need special surface coating of the magnetic particles, which has to be not only non-toxic and biocompatible but also allow targetable delivery with particle localization in a specific area. Most work in this field has been done to improve the biocompatibility of the materials, but only a few scientific investigations and developments have been carried out to improve the quality of magnetic particles, their size distribution, their shape and surface so as to characterize them to establish a protocol for their quality control. The nature of surface coatings and their subsequent geometric arrangement on nanoparticles determine not only the overall size of the colloid but also play a significant role in the biokinetics and biodistribution of nanoparticles in the body. Some authors have reviewed the different factors related to the clearance of the iron oxide nanoparticles from blood: particle size, dose, surface charge, coating material, stability in physiological environment, etc. (Gupta and Gupta, 2005). According to the previous studies, the particle size plays a very important effect. The larger the particles, the shorter their plasma half-life. The blood half-life of AMI-25 with a diameter of 80–150 nm is only 6 min and approximately 80% of the injected dose accumulated in the liver and 5–10% in the spleen within minutes of administration. However, the vascular half-life of NC10050 with diameter of 20 nm was up to 3–4 h (Wang et al., 2001). It was reported that the higher the surface charge, the shorter the residence time of SPION in the circulation (Neuberger et al., 2005).

In recent years, several investigations for the preparation of iron oxide nanoparticles with alginate have been developed (Kroll et al., 1996, Llanes et al., 2000, Shen et al., 2003, Nishio et al., 2004). The standard chemical synthesis consists of three steps: (a) gelation of alginate and ferrous ion; (b) in situ precipitation of ferrous hydroxide by the alkaline treatment of alginate; (c) oxidation of ferrous hydroxide with an oxidizing agent such as O2 or H2O2. The methods mentioned above are complex, since it involves multiple recycles to produce iron oxide of about 50% content, needs control of the complicated oxidation reaction, and usually results in a nonmagnetic form of iron oxide. Moreover, the way alginate interacts with iron ions has not yet well been understood. It has been considered that the alginate polymeric backbone is more conformationally restricted and hence it would result in the aggregation of iron (Sreeram et al., 2004). Thus, we attempt to develop a modified two-step coprecipitation method, involving (a) formation of Fe3O4 particles by coprecipitation of ferric and ferrous ion with alkaline solution, and (b) combination of Fe3O4 particles to alginate. Compared with the standard chemical synthesis method, the preparation process is easier and SPION-alginate produced by this method will be of good stability and good magnetism. It was reported that the reason for the stability of dextran-coated iron oxide particles was that a COO terminal of dextran bound to a Fe atom on the core surface (Kawaguchi et al., 2001). Electrostatic repulsion between particles with the same electric charge prevents the aggregation of particles (Ravi Kumar et al., 2004). Meanwhile, alginate has the structure with many carboxyl groups as mentioned above. Hence, we speculated the interaction between COO of alginate and iron ion as well as the electrostatic repulsion may make the SPION-alginate stable.

This study reports our efforts to understand the nature of the iron ion-alginate of SPION-alginate and to develop a SPION-alginate composite, which may be an application in magnetic resonance imaging.

Section snippets

Materials

Three kinds of sodium alginate of pharmaceutical grade, KELTONE® LVCR (with a viscosity average molecular weight of 54,000 and an M/G ratio of 1.5), KELTONE® HVCR (with a viscosity average molecular weight of 160,000 and an M/G ratio of 1.5) and MANUGEL® DMB (with a viscosity average molecular weight of 124,000 and an M/G ratio of 0.6), were kindly donated by ISP Alginate Inc. Ferric chloride hexahydrate, ferrous chloride tetrahydrate and all other chemicals used for this study were of

Content of Fe3O4 in SPION-alginate

The Fe3O4 contents in solid samples of SPION-alginate varied with the type of alginate (Fig. 1a), but rarely changed when the concentration of alginate (LVCR) increased from 1% to 4% (w/w) (Fig. 1b). The content of Fe3O4 in SPION-alginate produced with LVCR alginate is twice as high as that produced with HVCR alginate or DMB alginate. Thus, the low molecular weight of alginate is beneficial for the formation of SPION-alginate with high iron content. The interaction between alginate and iron

Conclusions

SPION-alginate was prepared using a modified two-step coprecipitation method. The results revealed that typical iron oxide nanoparticles were Fe3O4 with a core diameter of 5–10 nm and SPION-alginate had a hydrodynamic diameter of 193.8–483.2 nm. Magnetic measurement showed that Fe3O4 nanoparticles in SPION-alginate were superparamagnetic with an Ms of 40 emu/g, corresponding to 73% of that of solid SPION-alginate (55 emu/g). This high Ms may be due to Fe3O4 nanoparticles binding with alginate

Acknowledgement

The authors are thankful to the National High Technology Research and Development Program of China (863 Program, No. 2003AA326020) for financial support of this study.

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