Preparation and characterization of water-soluble monodisperse magnetic iron oxide nanoparticles via surface double-exchange with DMSA

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Abstract

A simple, but efficient method for preparation of water-soluble iron oxide nanoparticles has been developed. Monodisperse Fe3O4 nanoparticles were synthesized by thermal decomposition of iron-oleate. Surface double-exchange of oleic acid capped monodisperse Fe3O4 nanoparticles with a familiar 2,3-dimercaptosuccinnic acid (HOOC–CH(SH)–CH(SH)–COOH, DMSA) was first performed in chloroform in the presence of triethylamine, and then this process was repeated in ethanol under the same conditions. The resulting Fe3O4 nanoparticles could be transferred into water to form stable magnetic fluid without post-treatment processes such as filtration and re-concentration. TEM images show that water-soluble Fe3O4 nanoparticles remain monodisperse and even form a monolayer of ordered assembly, and the results of TGA, VSM show that Fe3O4 nanoparticles via surface double-exchange possess more DMSA molecules through intermolecular disulfide cross-linking between DMSA, as confirmed by Raman spectra. Zeta potential measurements show that nanoparticles after surface double-exchange are negatively charged in the range of pH = 1–14, and stability assays exhibit their excellent stability in water and other physiological environments.

Introduction

Magnetic nanoparticles have been widely studied because of their many biological applications such as magnetic separation [1], [2], DNA detection [3], magnetic resonance imaging [4], [5], target-drug delivery [6], and magnetic hyperthermia [7], [8]. Among magnetic nanoparticles, iron oxide nanoparticles are of particular interests for applications because of their unique magnetic properties and biocompatibility. For future highly sensitive magnetic nanodevices and biological applications, iron oxide nanoparticles with controlled-shape, -size, and a narrow size distribution are urgent. Very recently, several groups have reported that such high-quality iron oxide nanoparticles could be synthesized by thermal decomposition of different types of iron precursors such as iron acetylacetonate [9], iron pentacarbonyl [10], and iron-oleate [11], [12]. However, the direct products of the above-mentioned approaches are organic-soluble, which to some extent limits their applications in biological fields.

To take advantage of their high-quality properties in biological applications, it is necessary to transfer iron oxide nanoparticles from organic to aqueous solution. Some groups have reported surface modification techniques including surface exchange with cyclodextrin [13], copolypeptides [14], anionic octa(tetramethylammonium)-polyhedral oligomeric silsesquioxane (TMA-POSS) [15], and intercalation of surfactants [16]. Especially, some reports have given prominences to surface exchange with DMSA by which iron oxide nanoparticles modified are fairly stable in water over wide ranges of pH and salt concentrations [4], [5], [17], [18], [19], [20], which makes them preferable in biological applications. From the point of surface chemistry, the success of any surface exchange process relies on a careful balance of the intermolecular forces driving the interaction between the molecules to be exchanged and the outermost layer of the substrate surface. Bruce et al. reported an inexpensive method for the introduction of more amino groups onto the surface of silica-coated iron oxide nanoparticles under various reaction conditions such as solvent systems and temperatures [21]. Here, we developed surface double-exchange of oleic acid capped monodisperse Fe3O4 nanoparticles with DMSA in the presence of triethylamine. The resulting Fe3O4 nanoparticles could be transferred into water to form stable magnetic fluid without post-treatment processes and kept stable for several months, even at high concentration.

Section snippets

Materials

1-octadecene was purchased from Alfa Aesar. 4-Morpholineethanesulfonic acid (MES) was purchased from Pierce and RPMI-1640 was purchased from Gibco. Note that RPMI-1640 used in our experiments contains 10% (v/v) fetal calf serum. The other chemicals were analytical reagents and purchased from Shanghai Chemical Reagent Corporation, China. All chemicals were used as received. Distilled water was used for all the experiments.

Synthesis of monodisperse Fe3O4 nanoparticles

Known method was followed to synthesize monodisperse Fe3O4 nanoparticles

TEM

Monodisperse Fe3O4 nanoparticles (MNP-1) were synthesized by thermal decomposition of iron-oleate in the presence of oleic acid. TEM image (Fig. 1a) shows that MNP-1 is monodisperse with about 10 nm average diameter and well-dispersed in chloroform. According to the ED pattern (Fig. 1b), the d-spacing can be calculated in the following equation:Lλ=dRwhere L is the distance between the test sample and the film (L = 137 cm), λ the wavelength of electron beam (λ = 0.0251 A°) and R is the radius of the

Conclusions

Oleic acid capped monodisperse Fe3O4 nanoparticles have been successfully transferred into aqueous solution via surface double-exchange with DMSA in the presence of triethylamine. The characterization results show that surface double-exchange is an efficient method that can introduce more DMSA molecules onto the surface of nanoparticles through intermolecular disulfide cross-linking between DMSA. By this means, we obtain stable nanoparticles with negative charges in the range of pH = 1–14.

Acknowledgments

This work has been carried out under financial support of the National Natural Science Foundation of China (Nos. 60571031, 60501009 and 90406023) and National Basic Research Program of China (Nos. 2006CB933200 and 2006CB705600). The author would like to thank Mr. A.Q. Xu, from the Analysis and Testing Centre of Southeast University for technique assistances.

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