Magnetic, optical and relaxometric properties of organically coated gold–magnetite (Au–Fe3O4) hybrid nanoparticles for potential use in biomedical applications

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Abstract

We present the magnetic, optical and relaxometric properties of multifunctional Au–Fe3O4 hybrid nanoparticles (HNPs), as possible novel contrast agents (CAs) for magnetic resonance imaging (MRI). The HNPs have been synthesized by wet chemical methods in heterodimer and core–shell geometries and capped with oleylamine. Structural characterization of the samples have been made by X-ray diffraction and transmission electron microscopy, while magnetic properties have been investigated by means of Superconducting Quantum Interference Device-SQUID magnetometry experiments. As required for MRI applications using negative CAs, the samples resulted superparamagnetic at room temperature and well above their blocking temperatures. Optical properties have been investigated by analyzing the optical absorbtion spectra collected in UV–visible region. Relaxometric measurements have been performed on organic suspensions of HNPs and Nuclear Magnetic Resonance (NMR) dispersion curves have been obtained by measuring the longitudinal 1/T1 and transverse 1/T2 relaxation rates of solvent protons in the range 10 kHz/300 MHz at room temperature. NMR relaxivities r1 and r2 have been compared with ENDOREM®, one of the commercial superparamagnetic iron oxide based MRI contrast agents. MRI contrast enhancement efficiencies have been investigated also by examining T2-weighted MR images of suspensions. The experimental results suggest that the nanoparticles' suspensions are good candidates as negative CAs.

Highlights

► Au–Fe3O4 superparamagnetic Hybrid NanoPrticles (HNPs) enhance contrast in MRI. ► HNPs are expected to have optical activities through observed SPR phenomena. ► HNPs have relatively high magnetic anisotropy originating from Au/Fe3O4 interface. ► Magnetic dipolar interactions have been observed between particles in powders.

Introduction

Recently, multifunctional hybrid magnetic nanoparticles (HNP) attracted much attention, since they offer unique advantages with respect to mono-functional NPs. For instance the HNPs can be constituted of a magnetic part like magnetite (Fe3O4) or maghemite (γ-Fe2O3), whose magnetic properties are well known and a metal component like Au which is non-magnetic but shows surface plasmon phenomena and optical activity. As a whole the HNPs have both magnetic and optical properties, and thus behave as multifunctional materials [1]. This multifunctionality is at the origin of possible simultaneous use of HNPs in different biomedical applications, such as biosensing, optical imaging, MRI, magnetic fluid hyperthermia (MFH) and others [2], [3]. Moreover, from a fundamental point of view the HNPs offer the opportunity to investigate the interactions between the magnetic and plasmonic excitations [4].

Magnetic Resonance Imaging (MRI) is one of the best non-invasive techniques for the diagnosis of diseases, for example in the study of tumors where their extension can be distinguished from healthy tissues by local contrast difference in images. The image contrast in MRI depends mainly on i) proton density, ii) spin–lattice (T1) and spin–spin (T2) nuclear relaxation times, and iii) diffusion coefficients, differently weighted along different parts of the body. The so-called contrast agents (CAs), which are paramagnetic or superparamagnetic nanoparticle suspensions, are used in order to improve the image contrast of the interested region of the body allowing better definition and location of inflammation, cerebral damages, tumors, etc. [5], [6]. The paramagnetic CAs are known as T1-relaxing or positive contrast agents because the images are characterized by brilliant spots in the regions where they are delivered. The superparamagnetic CAs are known as T2-relaxing or negative contrast agents since they decrease the MRI signal intensity in the tissues by shortening T2 times of nearby protons resulting in darker points in the image. Such positively or negatively enhanced contrast allows often a better diagnosis of the investigated pathology.

In this work, we report on the characterization of potential contrast agents based on Fe3O4 NPs and Au–Fe3O4 HNPs, with particular focus on HNP samples. First, the synthesis of samples is briefly explained and the morphology of the synthesized samples is described, such characteristics determining the optical and magnetic properties of HNPs. The optical properties of HNPs are presented through measurements of optical absorption spectroscopy. Magnetic characterization data are also reported and the role of the magnetic anisotropy on the physical properties of samples is discussed. Finally, concerning the MRI contrast enhancement abilities of HNP suspensions, NMR dispersion (NMRD) profiles, obtained by relaxometric studies, are presented comparatively with a commercial contrast agent.

Section snippets

Synthesis and structural characterization

We investigated three samples, one constituted by a magnetite Fe3O4 core alone and two Au–Fe3O4 hybrid HNPs with dimer-like and core–shell topologies, respectively (see Fig. 1). In these two latter geometries the magnetic and optical properties can be controlled by changing microscopic parameters such as size of magnetic core and particle coating. All particles are capped with oleic acid and oleylamine, two organic molecules included as a surfactant during the synthesis process, which inhibit

Conclusions

In this paper we investigated Au–Fe3O4 HNPs synthesized in dimer and core–shell geometry, together with spherical Fe3O4 nanoparticles. It was shown that Au–Fe3O4 HNPs have optical activities through Au and can be used as MRI contrast agents, meaning that they can be classified as multifunctional materials. All the samples presented superparamagnetic behavior at room temperature, with ZFCFC thermal irreversibility, blocking temperature and closed hysteresis loop. Further investigation of the

Acknowledgments

This work was partly supported with the funds of Foundation Cariplo Project 2010–0612 and also supported by Italian MIUR through FIRB RBPR05JH2P “Rete ItalNanoNet”. M. Mariani is thanked for help in the experimental measurements and A. Capozzi for helping in fitting the ac data. We also thank to Serhat Döker at Department of Chemistry in Hacettepe University for AAS measurements on suspensions.

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