Synthesis of high magnetization hydrophilic magnetite (Fe₃O₄) nanoparticles in single reaction—Surfactantless polyol process

Abstract

High magnetization hydrophilic magnetite nanoparticles have been synthesized in two different batches with mean particle sizes of 32.3 and 9.2 nm by inexpensive and surfactant-free facile one-pot modified polyol method. In the synthesis, polyethylene glycol was used as a solvent media and it has been found to play a key role to act as a reducing agent as well as a stabilizer simultaneously. It was shown that the size of the nanoparticles can be effectively controlled by modifying the reaction parameters such as reaction temperature, time and polyol/metal precursor ratio. X-ray diffraction and energy dispersive spectroscopy studies confirm the formation of a pure magnetite phase without the presence of any other phases. Transmission electron microscopy and the Fourier transform infrared spectroscopy results reveal that the particle size and surface adsorption properties are very much dependent on reaction parameters. The magnetic properties of the samples measured by physical property measurement system have shown that the as-synthesized magnetite nanoparticles possess a high magnetization of 85.87 emu/g at 300 K and 91.7 emu/g at 5 K with negligible coercivities. The structural and magnetic characterizations of these polyol coated, hydrophilic, monodisperse, superparamagnetic nanoparticles clearly indicate that they are suitable for biomedical applications.

Introduction

Magnetite (Fe₃O₄) nanoparticles with excellent structural and magnetic properties are considered to be highly promising magnetic materials for biomedical applications due to their non-toxicity and high chemical stability [1]. It means, magnetic nanoparticles to be used in bioapplication systems should possess some of the physical features like small size, uniform size distribution, superparamagnetic nature with high magnetic moment besides the solubility in water so as to make them suitable for applications such as targeted drug delivery, hyperthermia and magnetic resonance imaging enhancement [2]. In addition, magnetic nanoparticles are also used as magnetic labels for applications such as cell separation, manipulation and biomolecule detection in which the particle sizes can have the flexibility to be slightly larger to accommodate the presence of multiple ligands on the particle surface and to achieve multivalent interactions. In this latter case, the magnetic labels often come as nanoparticle encapsulated biocompatible polymer beads with the properties of large magnetization and good chemical stability [3]. These labels/beads, as a result of the coupling of the many adjacent encapsulated magnetic spins, tend to exhibit good magnetic performance so as to generate sufficient levels of stray fields for efficient detection on successful bio-interaction. Nevertheless, since the applicability of the magnetic nanoparticles in the biosystems demand high saturation magnetization and biocompatibility as basic requirements, the development of magnetite nanoparticles, which are known for their high biocompatibility and high magnetization, has therefore become an obvious choice for intense investigations.

Recently, with the advent of several wet chemical methods for the synthesis of nanoparticles, many groups have worked to synthesize magnetite nanoparticles using different methods such as solvothermal [4], sol–gel [5], coprecipitation [6], thermal decomposition [7], citrate gel [8], etc. Though all these methods provide great benefits in the control of size, component and dispersion, they however suffer from certain disadvantages too such as consumption of large amounts of surfactant, long synthesis times, etc. Moreover, some of these methods may even require deoxygenated protection and process in organic phase to yield hydrophobic nanoparticles which further need additional modifications to be hydrophilic for use in bio-applications. Besides, toxic gases like carbon monoxide might be emitting in the case of thermal decomposition method. However, the polyol method which involves reduction of metal salts with a diol, typically ethylene glycol, diethylene glycol, or a mixture of both is believed to be one of the most appropriate methods for synthesis of hydrophilic nanoparticles as it amply provides the possibility to control the experimental conditions kinetically and to scale up easily [9]. Also, by using the polyol process we can dispense the use of surfactant, because the polyethylene glycol (PEG) plays a triple role as high-boiling solvent, reducing agent, and stabilizer to efficiently control the particle growth and prevent inter-particle aggregation due to steric interactions, in addition to its hydrophilic properties [10]. Thus, there have been some efforts to this effect by some of the research groups and reported the results on the synthesis of magnetite nanoparticles using different kinds of polyol such as ethylene glycol, diethylene glycol, tri-ethylene glycol, tetra-ethylene glycol and propylene glycol [11], [12], [13], [14]. In these works, apart from using different kinds of polyol, deoxygenated protection of the reaction and in some cases additives and long time reactions were also used to obtain monodisperse, water soluble magnetite nanoparticles with saturation magnetizations in the range from 45 to 77 emu/g.

Herein, we report a one-pot facile polyol method for the preparation of high magnetization hydrophilic Fe₃O₄ nanoparticles without using any surfactant and deoxygenated conditions, while realizing the possibility for controlling the size of the nanoparticles by varying some of the reaction parameters. The structure, composition, size, surface coating, thermal and magnetic properties of the synthesized nanoparticles were examined in detail and discussed the obtained results.