Oleic acid coating on the monodisperse magnetite nanoparticles
Introduction
Magnetic nanoparticles have been of great interests because of their extensive applications in high-density data storage, biochemistry, hyperthermia, in vivo drug delivery, MR contrast reagent [1], [2], [3], [4], [5], [6], [7]. To apply magnetic nanoparticles in various potential fields, it is very important to control the size and shape, and to keep the thermal and chemical stability by surface modification [8]. This modification generally will play a key role on the properties and applications of the magnetic nanoparticles in bio-solutions or tissue environments [9]. The magnetic structure of the surface layer usually is greatly different from that in the body of nanoparticle, and the magnetic interactions in the surface layer could have a notable effect on the magnetic properties of nanoparticles [10], [11]. Understanding the interaction between the surfactant and the nanoparticle is critical and essential to synthesis and application of nanoparticles.
Oleic acid (OA) is a commonly used surfactant to stabilize the magnetic nanoparticles synthesized by the traditional coprecipitation method, and some studies [12], [13] have proved that the strong chemical bond formed between the carboxylic acid and the amorphous iron and amorphous iron oxide nanoparticles. However, it is hardly to know the interaction between the single nanoparticle and surfactant from the “compositive” results given by these kinds of size and shape widely dispersed nanoparticles systems. For the nanoparticles with different sizes, the surface effects are significantly various due to the difference of volume fraction of surface atoms within the whole particle. Excitingly, the chemical routes of synthesis monodisperse magnetic nanoparticles by thermal decomposition method have obtained outstanding results [14], [15], [16], [17]. These monodisperse nanoparticles coated with OA may provide a factual system to get the exact information of the interaction and adsorption model at the interface. In the present work, monodisperse Fe3O4 nanoparticles with diameter of 7 and 19 nm were synthesized by the seed-mediated high temperature thermal decomposition of iron(III) acetylacetonate (Fe(acac)3) precursor method. The chemical structure of the surfactant adsorbed on the magnetite nanoparticles has been identified, and the model of OA molecules adsorbed on the nanoparticles surface was discussed.
Section snippets
Experimental
Magnetite nanoparticles were prepared according to the Sun's method [16]. Such nanoparticles then serve as seeds to grow larger nanoparticles in the seed-mediated growth process. It is worth to note that the OA as the surfactant added in the reaction mixture before the Fe3O4 nuclei produced, but not as usual way that the surfactant was modified after the Fe3O4 synthesised [18], [19], [20].
In a typical synthesis, Fe(acac)3 (2 mmol), 1,2-hexadecanediol (10 mmol), benzyl ether (20 ml), oleic acid (6
Results and discussion
XRD patterns in Fig. 1 reveal the nanocrystal nature of the two samples. The position and relative intensity of all peaks match well with standard Fe3O4 powder diffraction data, indicating that each sample is Fe3O4 crystal.
To understand the adsorption mechanism of the OA on the surface of Fe3O4 nanoparticles, Fourier transform infrared measurements were carried out on the pure oleic acid and the composite Fe3O4 nanoparticles coated with OA. Fig. 2 shows the typical FTIR spectrum of the pure
Conclusions
Monodisperse magnetite nanoparticles coated with OA provided a factual system to get the exact information about the interaction and adsorption model at interface. Study shows the adsorption of OA molecules on the nanoparticles were by chemisorption in all cases, and the OA molecular coated on the particles surface with a single layer structure. Furthermore, the two distinct of surfactant desorbed on the particle surface implied that there were two kinds of different binding energies between
Acknowledgments
The authors are grateful to the 863 Hi-Tech Research and Development Program (2002AA302210) and Shanghai Nano Program (0249 nm071) for financial support.
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