Structural and magnetic properties of nanoscale iron oxide particles synthesized in the presence of dextran or polyvinyl alcohol

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Abstract

Magnetite/maghemite nanoparticles were synthesized in the presence of either dextran or polyvinyl alcohol, yielding cluster- and necklace-like aggregates, respectively. Magnetization, Mössbauer spectroscopy, and microscopy measurements indicate that the arrangement of the particles within the aggregates affects the magnetic properties of the particles resulting in smaller particles in the clusters having higher superparamagnetic blocking temperatures than larger particles in the necklaces.

Introduction

The recent development of biocompatible, functionalized ferrofluids and ferromagnetic particles has led to a range of new biomedical and diagnostic applications e.g. Refs. [1], [2], [3], [4]. New studies utilizing these particles and ferrofluids in unique ways are continually appearing in the scientific literature and there are now many companies, which produce these products both for research and clinical applications.

In virtually all cases, the magnetic properties of these particles play an important role in the effectiveness of the application and affect the behavior of the particles and ferrofluids in applied fields. However, very few studies have examined the magnetic properties of these particles — particularly, those produced by chemical synthesis techniques. As part of a study to produce gel-based ferrofluid standards for MRI analysis, we have synthesized nanoscale magnetite (Fe3O4)/maghemite (γ-Fe2O3) particles in the presence of organic polymers. The resulting organic–inorganic composites form stable aqueous suspensions. Here we present the method of preparation and the detailed characterization of the structural and magnetic properties of these nanoscale magnetic particles. We show that the arrangement of the particles within clusters plays a major role in determining the observed magnetic properties.

Section snippets

Synthesis

Magnetic nanoparticles were prepared in the presence of one of two types of polymers. The polymers used were dextran (40 kDa) and polyvinyl alcohol (PVA) (30–70 kDa) both from Sigma Chemical Company. Aqueous suspensions of magnetic particles were prepared under identical reaction conditions by coprecipitation of Fe(III) and Fe(II) in the presence of NH4OH and polymer. The following method of preparation is similar to those described in Refs. [5], [6].

A solution of a mixture of Fe(III) and Fe(II)

Results

Fig. 1 shows TEM images of the Fe-dextran and Fe-PVA preparations. Electron dense particles are clearly seen as dark regions in the micrograph. The polymer components of the materials have too low an electron density to contribute to contrast in the images. The images show evidence for well-formed crystal faces on some of the particles. The particles in the Fe-dextran preparation appear to form cluster-like aggregates whereas the particles in the Fe-PVA preparation tended to form necklace-like

Discussion

The TEM images strongly suggest that the use of a polymer in the material synthesis limits particle size as both the Fe-dextran and Fe-PVA preparations generate particles that are significantly smaller than the control preparation in which no polymer is present. The type of polymer also appears to affect particle size as the Fe-dextran preparation consists of smaller particles than the Fe-PVA preparation. A similar effect of polymer type on resulting particle size has been seen previously by

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