Application of factor analysis to XPS valence band of superparamagnetic iron oxide nanoparticles

doi:10.1016/j.apsusc.2011.07.123

Applied Surface Science

Volume 257, Issue 24, 1 October 2011, Pages 10863-10868

https://doi.org/10.1016/j.apsusc.2011.07.123 Get rights and content

Abstract

X-Ray photoelectron spectra of nano-sized superparamagnetic iron oxide nanoparticles were examined with the aim to discriminate the different degree of iron oxidation. Careful analysis of the valence band regions reveals the presence of both Fe₃O₄ and Fe₂O₃. The application of factor analysis enabled us to extract the relative molar concentrations of these oxides in the nanoparticles. This is of particular interest in improving the magnetic properties of iron oxide nanoparticles whose superparamagnetic character can be optimized to obtain better contrast in images from nuclear magnetic resonance. As a result, the factor analysis allows tuning the nanoparticle synthesis conditions in order to obtain the optimal magnetic properties for imaging. Results obtained by the XPS valence band analysis were compared to the transmission electron microscopy, X-ray diffraction and Raman measurements.

Graphical abstract

. Superparamagnetic iron oxide nanoparticles were characterized by Raman and X-ray photoelectron spectroscopy to quantify the amount of maghemite and magnetite in the nanoparticles.

Highlights

► Magnetic iron oxide nanoparticles. ► Spectroscopic analysis of the nanoparticles. ► Presence of magnetite e maghemite phases. ► Quantification of the phases by analysis of the XPS valence band.

Introduction

Superparamagnetic iron oxide nanoparticles (SPIO) have a wide range of applications, including high-density magnetic storage, catalytic and separation processes [1], magnetic resonance imaging (MRI) [2], in vivo imaging of tumour progression [3], [4] and drug delivery [5].

Iron oxide nanoparticles have been clinically used as imaging agents for MRI. Recently a number of research groups have investigated them as drug carriers while retaining their imaging functions [6], [7]. Fe₃O₄ nanoparticles (NP) exhibiting superparamagnetism and high saturation magnetization values [8] are used as contrast agents to enhance relaxation differences between healthy and pathological tissues. In addition, a unique advantage of iron oxide nanoparticle is that they can be delivered in a targeted manner to a desired region by applying an external magnetic field. For these reasons, a certain number of research groups are trying to use FeO-NPs as carriers for drug delivery [9], [10]. Recently this is gaining a high relevance for the application in medicine and in particular for the cure of tumour affections.

SPIO are normally composed by iron oxide, in particular magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃). These two oxides show similar magnetic properties, but maghemite has a lower saturation magnetization [11]. On the other hand, magnetite can be also oxidized to hematite (α-Fe₂O₃) by molecular oxygen and water, which lead it to the maximum oxidation state [12]. This causes the formation of magnetic-inactive Fe₂O₃ with a decrease of the system efficiency. Besides magnetization also nanometric size is required and various methods have been developed to synthesize Fe₃O₄ with dimensions in this range [13]. Unfortunately the magnetic properties of magnetite NPs highly depend upon the synthesis procedure [14], [15]. In fact, iron possesses different oxidation states giving origin to different forms of iron oxides. In magnetite iron ions are arranged in an inverse spinel structure with cubic O²⁻ ions in octahedral crystalline structure. Hematite is formed by Fe₂O₃ in a structure where Fe³⁺ ions are coordinated by hexagonal O²⁻ ions. Maghemite can be considered as an oxidized form of magnetite where the previous forms coexists in a rather complex crystalline arrangement. In nature the more diffuse form of iron oxide are magnetite and hematite. The synthesis of iron oxide NPs leads to aggregates formed by different stoichiometries between iron and oxygen composed prevalently by a mixture of maghemite and magnetite. Different crystalline structures directly influence the physical properties of the iron oxide. Hematite is an antiferromagnetic while magnetite is ferromagnetic. The first is an intrinsic semiconductor [16] while the second at room temperature has a high conductivity because of a rapid electron hopping between octahedral Fe²⁺ and Fe³⁺ ions [17]. This is why an accurate characterization of the chemical components of Fe based nanoparticles is important to determine the NP physical properties.

In this paper we present an accurate data manipulation of the XPS valence band region based on the Factor Analysis. This last was applied to the XPS valence band (VB) regions of SPIO nanoparticles. This statistical approach allowed us to calculate the relative molar concentrations of both magnetite and maghemite in the iron oxide nanoparticles.

Section snippets

Materials and methods

SPIO nanoparticles were prepared by thermodecomposition of iron acetylacetonate in the presence of oleic acid and oleylamine and the final products were soluble in toluene and kept under argon after purification [18]. The mean dimension of these particles as revealed by TEM is around 10 nm. Samples were stored in toluene solution, deposited on the substrate under nitrogen atmosphere and then analyzed directly to avoid contaminations. Two SPIO nanoparticles samples were analyzed: freshly prepared

Result and discussion

In Fig. 1A a transmission electron microscopy image of the SPIO NPs is reported. It appears evident that the nanoparticles present primary a truncated octahedral shape. The size distributions show in Fig. 1B reveal a mean value of around 10 nm.

The X-ray diffraction pattern of the SPIO nanoparticles showed in Fig. 2 shows weak diffraction peaks assigned to the reflection planes of the octahedral crystalline structure phase. A possible explanation of the so low diffraction signal is the formation

Conclusions

The XPS valence band analysis is a powerful tool for the characterization of the iron oxide species. Using the FA applied to the XPS valence band region an estimation of the amount of maghemite and magnetite present in SPIO nanoparticles is possible. This is of fundamental importance for the control of the nanoparticles quality in order to maximize the magnetization properties for SPIO nanoparticles for in vivo imaging. Raman and XPS characterizations put in evidence the presence of maghemite

Acknowledgement

This research was performed in the framework of the Nanosmart-PAT research project.

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Application of factor analysis to XPS valence band of superparamagnetic iron oxide nanoparticles

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Materials and methods

Result and discussion

Conclusions

Acknowledgement

Acad. Radiol.

Adv. Drug. Deliv. Rev.

J. Magn. Mater.

J. Magn. Magn. Mater.

Surf. Sci.

J. Am. Chem. Soc.

Bioconjug. Chem.

Nat. Biotechnol.

Nat. Biotechnol.

Angew. Chem. Int. Ed.

Small

Mol. Pharm.

Magnetic Materials

Ann. Biomed. Eng.