Investigation of formation of silica-coated magnetite nanoparticles via sol–gel approach

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Abstract

A systematic investigation of the formation of silica-coated magnetite particles via sol–gel approach was conducted using transmission electron microscope (TEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). The results show that reaction parameters including the type of alcohol, the volume ratio of alcohol to water, the amount of catalyst and the amount of precursor play important roles on the formation of silica-coated magnetite particles, and the obtained silica-coated magnetite nanoparticles possess superparamagnetic property.

Introduction

During the last decade, owing to the emergence of a new generation of high technology materials, the number of research groups involved in nanomaterials has increased exponentially [1]. Nanomaterials are implicated in several domains including electronics, sensors and biotechnology, etc. This is, to some extent, due to their novel properties of nanomaterials, which differ from both the isolated atoms and the bulk phase. Among various nanostructured materials, magnetic nanoparticles such as magnetite, maghemite, nickel and cobalt, due to their remarkable phenomena such as supeparamagnetism, high field irreversibility and high saturation field, have attracted growing interest in magnetic nanostructure materials for their unique properties [2] and potential applications in various fields, especially in biomedicine and bioengineering such as magnetically assisted drug delivery [3], cell isolation [4], MRI contrast agents [5], immunoassay [6] and biomacromolecule purification [7]. However, most of these applications require magnetic nanoparticles to be embedded in a nonmagnetic matrix to avoid aggregation and sedimentation of magnetic nanoparticles as well as to endow them with particular surface properties for specific application.

Coating magnetic nanoparticles with silica is becoming a promising and important approach in the development of magnetic nanoparticles for both fundamental study and technology application. First, silica formed on the surface of magnetic nanoparticles could screen the magnetic dipolar attraction between magnetic nanoparticles, which favors the dispersion of magnetic nanoparticles in liquid media and protects them from leaching in an acidic environment. Second, due to the existence of abundant silanol groups on the silica layer, silica-coated magnetic nanoparticles could be easily activated to provide the surface of silica-coated magnetic nanoparticles with various functional groups. Finally, the most important is that the silica layer provides a chemically inert surface for magnetic nanoparticles in biological systems. In general, there are four major methods for preparation of silica-coated magnetic nanoparticles. The first method, known as sol–gel process, relies on the use of the of silicon alkoxides as the source of silica matrix. In this method, silica phase are formed on colloidal magnetic nanoparticles in a basic alcohol/water mixture [8]. The second approach is based on either in situ formation of magnetic nanoparticles inside the pores of pre-synthesized silica using metal compounds (i.e. salts, complex or alkoxides) as the source of magnetic phase under certain conditions [9]. The third method is called aerosol pyrolysis, in which silica-coated magnetic nanoparticles were prepared by aerosol pyrolysis of a precursor mixture composed of silicon alkoxides and metal compound in a flame environment [10]. More recently, the fourth method, i.e. W/O microemulsion method, has been proposed for the preparation of silica-coated magnetic nanoparticles [11]. In this method, non-ionic surfactants were used to formation of inverse microemulsion for preparation or suspension of magnetic nanoparticles, and silica were formed around the magnetic nanoparticles by hydrolysis and condensation of tetraethyl orthosilicate (TEOS). Among these methods, sol–gel process has been adopted more widely for preparation of silica-coated magnetic nanoparticles for its advantages compared with the other methods, i.e. relatively mild reaction condition, low cost and surfactant-free. Ohmor and Matijevic [12] prepared spindle-type hematite particles with length of about 400 nm, and coated them directly with silica by through sol–gel process. Using surfactant stabilized magnetite nanoparticles as seeds, Philipse et al. [13] and Lu et al. [14] respectively prepared silica-coated magnetite particles through sol–gel process. Up to now, although there are many reports on preparation of magnetite-silica composite particles, most of the reported composite particles showed ill-defined structure and morphology as a result of ignoring pretreatment of magnetite before coating with silica [15], [16], [17], [18], [19], and few work has been directed to systemic investigation of the formation of well-defined silica-coated magnetite nanoparticles in sol–gel approach. In this paper, with an aim to give a direction for preparation of uniform silica-coated magnetite particles with core-shell structure, we study systematically the formation of silica-coated magnetite nanoparticles by utilization of transmission electron microscope (TEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM).

Section snippets

Materials

FeCl3·6H2O and FeCl2·4H2O were purchased from Fluka. Sodium hydroxide (NaOH) (Shanghai FeiDa Trade Company), trisodium citrate, tetraethyl orthosilicate (TEOS), methanol, ethanol, iso-propanol, propanol and ammonia aqueous (25 wt.%) (Shanghai Chem Reagent Co.) were all used as supplied. In this study, only distilled water was used.

Colloidal iron oxide nanoparticle synthesis

Magnetic nanoparticles were prepared using the method already described [20], based on the chemical coprecipitation of Fe2+ and Fe3+ by adding a concentrated solution

Synthesis of magnetite nanoparticles

Fig. 1 is TEM image and electron diffraction pattern of the synthesized magnetite nanoparticles, which shows that most of the particles are quasi-spherical with an average diameter of 15 nm. Fig. 2 shows the XRD pattern of magnetic nanoparticles. The experimental d spacing obtained from the XRD pattern and ASTM data are listed in Table 1. From Table 1, the experimental data was found similar to the ASTM data cards of the Fe3O4. Based on the d spacing and the preparation method, it can be

Conclusion

A systematic study of the formation of silica-coated magnetite particles via sol–gel approach was made. The results show that the reaction parameters including the type of alcohol, the volume ratio of alcohol/water, the amount of ammonia aqueous and TEOS on the formation of silica-coated magnetite nanoparticles. With the increase of polarity of alcohol, the shape and morphology of silica-coated magnetite particles become more irregular except that poor coating efficiency when methanol was used.

Acknowledgement

We are grateful to the support of NSFC (Nos. 50173005, 50343019) and STCSM (03JC14012).

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