Polymer-assisted size control of water-dispersible iron oxide nanoparticles in range between 15 and 100 nm

https://doi.org/10.1016/j.colsurfa.2014.10.001Get rights and content

Highlights

  • Coated Fe3O4 nanoparticles were synthesized by the precipitation oxidation method.

  • Starch can be employed as an effective control agent to tune the nanoparticle size.

  • Starch coated nanoparticles are water dispersible and forms a ferrofluid.

  • Nature of the employed polymer defines the efficiency of the size-control process.

Abstract

Starch-coated Fe3O4 nanoparticles were synthesized by the precipitation–oxidation of ferrous hydroxide method. Starch was employed as a kinetic control agent, and the effect of the polymer on both size and aggregation of the Fe3O4 nanoparticles was studied. The size of the as-prepared magnetite nanoparticles was tuned from 15 to 100 nm by changing the time of addition of a starch solution on the reaction system. Also, the starch-coating over Fe3O4 nanoparticles assures good water-dispersibility, stability, and possible biocompatibility. Transmission and scanning electron microscopies (TEM, SEM), X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and magnetic measurements were used to characterize the prepared samples.

Kinetic control assays were also done with polyethylene glycol and polyvinyl alcohol in order to study the influence of the polymer nature in the size and aggregation process of the Fe3O4 nanoparticles. For this work, the effect is more pronounced for voluminous polymers, with large electrosteric hindrance produced by increased polar groups per monomer, like starch.

Introduction

Magnetic iron oxide nanoparticles (IONPs) systems (mainly magnetite, Fe3O4, and maghemite, γ-Fe2O3) have been synthetized and studied for several years, but recently they have gained special consideration and attraction due to their technological applications associated to their nanosize-related magnetic properties [1]. These technological applications include recording media devices, chemical sensors [2], magnetic ferrofluids employed in seals, dampers or loudspeakers, as well as many biomedical uses such as biomolecule separation, drug targeting, magnetic hyperthermia, and contrast agents for nuclear magnetic resonance images [3], [4].

Novel and diverse routes to synthesize IONPs have been reported in the last years, like thermal decomposition of organo-metallic precursors [5], polyol process [6], [7] or reverse micelles [8]. However, the classical procedures in aqueous media remain as the most popular methods because of their operational and economic viability. Co-precipitation of Fe(II)/Fe(III) ions is by far the most employed method to synthesize Fe3O4 nanoparticles [1], [9]. Precipitation–oxidation of ferrous hydroxide, first reported by Sugimoto and Majitevic in 1980 [10], is also a widely applied aqueous-based synthesis route [11], [12], [13], [14], [15]. The differences between the iron sources in both procedures trigger distinct mechanisms for the Fe3O4 nanoparticles formation, yielding products of different sizes. Typically, co-precipitation produces nanoparticles between 5 nm and 15 nm [9], [16] while precipitation–oxidation can generate products as large as 1000 nm [11]. For the latter method, the manipulation of the synthesis conditions, like the ratio between the concentrations of the oxidant and the iron precursor, defines the nanoparticle size [11], [14], [15]. Despite this widespread use and the long-range of diameters achievable during the synthesis, both methods do not provide good control over crystallinity and size distribution of the nanoparticles in comparison with the most recently reported methods. In this sense, the search of alternatives and modifications that can improve these classical aqueous-based procedures has become a challenge of particular interest, especially after the technological applications revealed in the last years.

In order to develop a successful synthetic procedure, two main goalshave to be achieved: first, the size and shape distribution must be as predictable and narrow as possible, and second, aggregation of the nanoparticles must be avoided. The incorporation of stabilizing agents during nanoparticles formation can help to accomplish both objectives. Diverse materials have been studied as stabilizing agents for IONPs. Taking advantage from its various carboxylic groups, citric acid has been employed in both co-precipitation [17] and oxidation precipitation [18] methods, with good results. Employing the latter synthetic method, Jing et al. [18] reported that citric acid not only avoids nanoparticle aggregation, but different concentrations of this compound can also control and define the nanoparticle size. Core–shell structures with gold or SiO2, and polymers like polyethylene glycol, polyethylene imine or chitosan have also been tested in order to procure useful products for technological applications [1].

In this sense, natural starch appears as a very promising material for the controlled synthesis of IONPs. The size and presence of multiple functional groups may provide the adequate interaction strength with IONPs and at the same time prevent further changes of the nanoparticle, exerting an electrosteric function as protection agent [19]. Beside, starch results an attractive material due to its commercial availability at a low cost, and for being environmentally friendly [20], [21].

At the moment, starch-coated IONPs obtained by the co-precipitation method have been developed for As(V) removal in environmental remediation [22], [23], for controlled release of cisplatin [24] or as magnetic resonance contrast agents in biomedicine [25].

In this study, we investigate the influence of starch in the synthetic process of Fe3O4 nanoparticles by the oxidation–precipitation of ferrous hydroxide method. The addition of starch as a kinetically control agent can affect both the size and size distribution of the Fe3O4 nanoparticles, and the aggregation state of the obtained products. Also, we compared the performance of other two polymers as control agents in order to relate the chemical properties of the stabilization agent with the final products of the Fe3O4 nanoparticles synthesis.

Section snippets

Iron oxide nanoparticles (IONPs) syntheses

All the reagents used in this work were analytical grade without further purification. Iron (II) sulphate heptahydrate (FeSO4·7H2O, ≥99%, Sigma–Aldrich), sodium nitrate (NaNO3, ≥99%, Anedra), sodium hydroxide (NaOH, 99%, Anedra), soluble starch (reagent grade, Mallinckrodt), polyethylene glycol 6000 (PEG, 99%, Anedra), polyvinyl alcohol (PVA, fully hydrolyzed, Sigma–Aldrich). Deionized water was used in all experiments.

IONPs were prepared by the precipitation–oxidation of ferrous hydroxide

Starch-coated IONPs

As an alternative to the Fe(II)/Fe(III) co-precipitation method, the synthesis of magnetite nanoparticles by ferrous hydroxide precipitation–oxidation have some features that make it more suitable to accomplish certain objectives. For instance, particle sizes in the range between 12 and 100 nm can be easily controlled simply by the addition of starch as a capping agent during the first steps of the reaction. In the well-documented SX sample synthesis procedure (without stabilization agents) [10]

Conclusions

A chemical control process mediated by a polymer was proposed to define the size and aggregation of Fe3O4 nanoparticles synthetized by the precipitation–oxidation of ferrous hydroxide method. The addition of starch at different times after the beginning of the synthesis can stop the nanoparticle growth, and yield starch-coated Fe3O4 nanoparticles of different size. Also, starch hydrophilic-coating assures good water-dispersion stability and possible biocompatibility.

The kinetic control assays

Acknowledgments

We acknowledge the financial support from National Research Council (CONICET) and National Agency for the Promotion of Science and Technology (ANPCyT) from Argentina, and the support from PEDECIBA (PNUD/URU/97016), CSIC and ANII from Uruguay. Laboratorio de Biomateriales belongs to the Centro Interdisciplinario en Nanotecnología, Química, y Física de Materiales, Espacios Interdisciplinarios, UdelaR. We would like also to thank D. Muraca (LMBT, Campinas, Brazil) & M. M. Gómez-Hermida (UNAL,

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