iron-oxide nanoparticles as excellent MRI contrast enhancement agents

Journal of Magnetism and Magnetic Materials 331 (2013) 17–20

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Core/shell structured iron/iron-oxide nanoparticles as excellent MRI contrast enhancement agents Hafsa Khurshid a,n, Costas G. Hadjipanayis b, Hongwei Chen c, Wanfeng Li a, Hui Mao c, Revaz Machaidze b, Vasilis Tzitzios d, George C. Hadjipanayis a a

Department of Physics and Astronomy, University of Delaware, 217 sharp lab, Newark, DE 19716, United States Department of Neurological Surgery, Emory University School of Medicine Atlanta, GA 30322, United States c Department of Radiology, Emory University School of Medicine Atlanta, GA 30322, United States d Institute of Materials Science, ‘‘Demokritos’’ 15310 Athens, Greece b

a r t i c l e i n f o

abstract

Article history: Received 3 May 2012 Received in revised form 25 September 2012 Available online 16 November 2012

We report the use of metallic iron-based nanoparticles for magnetic resonance imaging (MRI) applications. Core/shell structured iron-based nanoparticles prepared by thermally decomposing organo-metallic compounds of iron at high temperature in the presence of hydrophobic surfactants were coated and stabilized in the aqueous solvent using the newly developed polysiloxane PEO–b– PgMPS (poly(ethylene oxide)–block–poly (g methacryloxypropyl trimethyl oxysilane)) diblock copolymers. Particles are well suspended in water and retain their core–shell morphology after coating with the copolymer. In comparison to the conventionally used iron-oxide nanoparticles, core/shell structured iron/iron-oxide nanoparticles offer a much stronger T2 shortening effect than that of iron-oxide with the same core size due to their better magnetic properties. Published by Elsevier B.V.

Keywords: Core–shell nanoparticles Iron nanoparticles Magnetic resonance imaging

1. Introduction Magnetic nanoparticles have emerged as a potential multifunctional clinical tool that can provide cancer cell detection when used as MRI contrast agents as well as therapy by targeted cancer cell delivery of agents (antibodies, drugs, and small molecule inhibitors) or local hyperthermia generation by heating the nanoparticles using an alternating magnetic field [1–3]. Recently, chemically synthesized magnetic nanoparticles have been found to be highly suitable for these applications because of their monodispersity and unique magnetic and microstructural properties which are suited for biomedical applications. Ironoxide nanoparticles (IONPs) are particularly attractive for in vivo MRI applications mainly because of their large saturation magnetization, which is higher than that of the Gd-chelates that are used routinely in diagnostic imaging. In principle, higher magnetization may increase the MRI contrast with the applied magnetic field due to the stronger interferences with the relaxation times of water that is the source in MRI signal. Additionally, for applications that require particle manipulation by an external magnetic field at a distance, it is essential that the particle saturation magnetization be as high as possible. Furthermore, using nanoparticles with larger magnetization it is possible to administer

n

Corresponding author. Tel.: þ1 3028313515. E-mail address: [email protected] (H. Khurshid).

0304-8853/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jmmm.2012.10.049

hyperthermia treatments at a safe biological frequency range (f o1.2 MHz) because of their higher ferromagnetic resonance (FMR) frequency [4,5]. In principle, nanoparticles composed of elemental iron may provide greater magnetization than IONPs. Although nanoparticles made from elemental Fe may be highly reactive in an aqueous environment, issues of biocompatibility, solubility, and stability can be addressed by coating the nanoparticles with a biocompatible polymer. In this work, we report the use of core/shell structured iron/iron-oxide nanoparticles (iron in the core and iron-oxide in the shell) for these applications. Our results demonstrate that core/shell structured iron/ iron-oxide nanoparticles offer a much stronger T2 shortening effects than that of iron-oxide nanoparticles at a comparable core size, suggesting that these particles have the potential to be more powerful contrast-enhancing media than currently used iron oxides.

2. Experimental details Core/shell structured iron/iron-oxide nanoparticles were obtained by a process reported elsewhere [6]. Briefly, iron pentacarbonyl was decomposed at high temperature in an air free environment in 1-octadecene, oleylamine (OAm) and oleic acid (OA), under vigorous stirring. The schematic of reaction route is shown in Fig. 1. This synthesis technique produced core/shell structured

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Fig. 1. Schematic of the reaction route followed for the nanoparticle synthesis (left) and ligand exchange using diblock copolymer to achieve water dispersibility (right).

iron/iron-oxide nanoparticles. The dark nanoparticle solution was precipitated by the addition of absolute ethanol and separated by a strong laboratory magnet and dispersed in hexane. Particle size can be controlled by varying the different reaction parameters; details have been published elsewhere [6]. Being capped with OA and OAm, these particles exhibited organophilic behavior. During ligand exchange, particles were precipitated and dried and then dispersed in tetrahydofuran (THF). A newly developed biocompatible poly(ethylene oxide)– block–poly (g methacryloxypropyl trimethyl oxysilane) (PEO–b– PgMPS) diblock copolymer [8] was used to achieve water dispersibilty in these particles via ligand exchange. This polysiloxane containing diblock copolymer that is comprised of both a hydrophobic segment with ‘‘surface anchoring moiety’’ (silane group) and a hydrophilic segment with PEO was obtained by the reversible addition fragmentation chain transfer (RAFT) polymerization using the PEO macromolecular chain transfer agent as reported by Chen et al. [8]. It has been used to transform and solubilize the hydrophobic iron-oxide or pure iron nanoparticles into an aqueous medium. Fig. 1 shows a schematic illustration of ligand exchange. Fe/gFe2O3 nanoparticles in THF were mixed with polymer solution. Afterwards, the mixture was dropped into water with magnetic stirring. To remove the extra polymer, the particle suspension was washed by letting it flow through two poles of a strong electromagnet. The free polymer passed through the two poles easily while polymer coated particles were collected from the poles afterwards. All MRI experiments were performed on a 1.5 T MR scanner. The magnetic nanoparticle solution samples with different concentrations were placed in the iso-center of the magnet for MRI scans. To measure the transverse relaxation time T2 of each sample, a multi-echo spin echo (SE) sequence was used with a repetition time (TR) of 2000 ms and 20 s echo delay time (TE) starting at 10 ms with increments of 10 ms. The value of the T2 relaxation time was calculated from the measured average signal intensity values (I) at different TE values using the equation: I ¼ K 2 expTE=T 2 and non-linear exponential curve fitting where K2 ¼ r(H)f(v) (r(H)¼spin (proton) density, f(v) is the signal arising from fluid flow).

3. Results and discussions Particle’s size and microstructure was investigates by TEM imaging and selected area diffraction pattern. Conventional bright field TEM studies (Fig. 2(A)) revealed a very clear inner contrast variation, suggesting the existence of a core/shell type morphology in these particles. Such a structure arises as a result of surface oxidation of the initially formed metallic iron nanoparticles in air or during the washing process [10]. The absence of such contrast in smaller particles can be explained by the complete oxidation of those particles. The typical shell thickness is observed to be 2–3 nm which justifies the complete oxidation of the small nanoparticles. The high resolution TEM image (Fig. 2(C)) shows

the randomly oriented grains of iron-oxide in shell and single crystal iron core. A selected area diffraction pattern from Fig. 2(A) is shown in Fig. 2(B) and is indexed to the mixture of metallic iron and to that of the ferrimagnetic iron-oxide; magnetite and or maghmite (Fe3O4, gFe2O3 respectively). Because of their very similar microstructure, it is not possible to distinguish between magnetite and maghmite based on their diffraction analysis. However, Mossbauer analysis indicated that during high temperature synthesis and with a particles size less than 5 nm, the predominant phase of iron-oxide is gFe2O3 [15]. Field and temperature dependent measurements of magnetization were performed using a vibrating sample magnetometer (VSM) and a superconducting quantum interference device (SQUID) magnetometer on samples deposited on a Si substrate. The saturation magnetization (Ms) is determined by extrapolating the magnetization curve to infinite fields assuming M  1/H2. It has been reported before in our previous work about the core/ shell structured Fe/gFe2O3 that the room temperature saturation magnetization increases with average particle size and a saturation magnetization as high as 150 emu/g can be obtained with an average particle size of 18 nm [6]. However, for highly magnetic nanoparticles magneto-static attractive forces overcome steric and electrostatic repulsive forces and cause particles to agglomerate in solution [11]. This challenge can be answered by synthesizing nanoparticles below a superparamagnetic size limit. In this study, the superparamagnetic nanoparticles used for MRI studies have a bimodal size distribution of 8.971.2 and 270.5 nm and room temperature saturation magnetization of 42 emu/g. The value of the particle specific magnetization (equal to the saturation magnetization Ms in emu/cc times the density) as a mass-weighted average of the relative contributions [16] is given by Ms,p mp ¼ M s,c mc þ M s,Ox mOx

ð1Þ

Here mp is the total mass of the particle, mc is the mass of the Fe core, and ms is the mass of the oxide shell; Ms,p, MsFe, and Ms,Ox are the specific saturation magnetizations of the particles, bulk Fe, and bulk iron-oxide. Assuming the bulk values of saturation magnetization (217 emu/g for MsFe, 78 emu/g for Ms,Ox) and density (7.86 g/cm3 for rc and 5.23 g/cm3 for rOx), the calculated saturation magnetization (Ms) of oleic coated particles with 5 nm Fe core and 2 nm oxide shell is 98 emu/g [14]. The lower value of magnetization in our particles can be explained by the nonuniformity of the iron core and small oxide particles present in the sample. Further, magnetization decreases due to the added amount of organics at the particle surface that contribute to its total mass [6,12]. In the case of small particles, the higher surface to volume ratio results in a much higher contribution from oxide layer and a greater amount of surfactant at surface when compared with the larger particles. However, the value of saturation magnetization observed experimentally in these particles (42 emu/g for Fe/gFe2O3 nanoparticles, size  9 nm) is much smaller than the 78 emu/g bulk saturation magnetization for gFe2O3. Berkowitz et al. [12] and Luborsky [13] have reported the existence of nonmagnetic surface layers (magnetically dead

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Fig. 2. TEM bright field image of particles coated with OA and OAm showing a core/shell morphology in these particles (A) along with corresponding selected area diffraction indicating crystalline core and shell (B) and core/shell morphology at high resolution (C).

Fig. 3. TEM morphology of particles coated with OA/OAm (A) and surface modified with diblock copolymer (B).

Fig. 4. Magnetization versus temperature at an applied field of 100 Oe, indicating, a superparamagnetic response in these particles (left) with the inset showing room temperature hysteresis loop and A comparison of T2-weighted MRI contrasts in Fe/gFe2O3 and IONPs using a clinical 1.5 T MRI scanner (right). (A) Fe/gFe2O3 nanoparticles with different Fe concentrations lead to signal drops in T2-weighted MRI and (B) Fe/gFe2O3 nanoparticles exhibits higher r2 (i.e., 1/T2) relaxivity when compared to ironoxide at the same core size.

layers) around Fe particles dispersed in mercury and around acicular g-Fe2O3 particles due to spin canting in the outer shell when particle size is smaller than 10 nm. Fig. 4 shows the magnetization dependence on field at room temperature. The coercivity, which is negligible at room temperature and increases to 1200 Oe at 10 K, indicates superparamagnetic behavior in these particles. The temperature dependence of magnetization in the zero field cooled (ZFC) and field cooled (FC) curves at a 100 Oe

applied field indicates a blocking temperature of 100 K. It is believed that the small kink in the ZFC curve corresponds to the blocking temperature of iron-oxide nanocrystals in the shell. The magnetocrystalline energy is not large enough for the spins of the particles to overcome thermal energy and hence the sample shows paramagnetic behavior at room temperature. The calculated blocking temperature for 5 nm (core size) iron particles using a bulk value of anisotropy (4.8  105 erg/cc) is 10 K. However, the observed

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blocking temperature of 100 K would correspond to an anisotropy value of 5.9  106 erg/cc. It has been reported in the past that core/shell (iron/iron-oxide) nanoparticles exhibit a much higher magnetocrystalline anisotropy than those of their bulk values because of surface effects [7,13,14]. Particles were well suspended in water after coating with PEO–b–PgMPS diblock copolymer, and remained stable and highly dispersed in water for more than two months. Transmission electron microscopy (Fig. 3) reveals the particles retain their core–shell morphology after copolymer coating. Selected area electron diffraction (SAED) confirms electron diffraction rings which correspond to the metallic Fe core and Fe-oxide shell. To assess the relaxation properties and MRI contrast-enhancing effect of these nanoparticles, phantoms containing Fe/gFe2O3 nanoparticles and IONPs of various sizes were made as described in the experimental section. The transverse relaxivity rate (r2 ¼1/ T2) was calculated from the measurement of T2 relaxation time of nanoparticles at different concentrations at a magnetic field strength of 1.5 T on a clinical MRI scanner. A set of T2-weighted spin–echo images showed the strong T2 contrast from Fe/gFe2O3 nanoparticles and IONPs (darkening effect) in contrast to the H2O phantom (Figure 5A, upper panel) with TR of 2000 ms and 20 TEs starting at 10 ms with increments of 10 ms. It can be seen very clearly that in comparison to conventionally used IONPs, Fe/gFe2O3 nanoparticles offers almost four times stronger T2 shortening effect than that of IONPs of the same core size. Such a stronger T2 shortening effect, typically from stronger magnetic susceptibility, leads to spin dephasing and substantial MRI signal drop, which generated a ‘‘darkening’’ contrast, as seen in T2-weighted MRI images (Fig. 4). The transverse relaxivity of Fe/gFe2O3 nanoparticles with a 5 nm core size almost matches that of IONPs with 20 nm core size which is in the range of Feridex, an iron-oxide based contrast agent previously used in clinic for liver imaging [9]. This observation suggests the potential use of smaller iron nanoparticles to achieve the similar T2weighted MRI contrast-enhancing effect of larger IONPs. The advantage of using smaller particles may be important for in vivo delivery of nanoparticles as smaller particles may penetrate tissue and navigate through delivery barriers easier than larger magnetic nanoparticles.

4. Conclusion In summary, highly crystalline core/shell structured iron/ironoxide nanoparticles have been synthesized by thermally

decomposing organometalic compounds of iron at high temperature in the presence of hydrophobic surfactants. Water dispersibilty was achieved using PEO–b–PgMPS diblock copolymers via ligand exchange. Particles remain well suspended in water and retain their core/shell morphology after coating with diblock copolymer. A relaxometery measurement of the transverse relaxation time T2 of the Fe/gFe2O3 nanoparticles solution at 1.5 T confirms that the Fe/gFe2O3 nanoparticles are an excellent T2 contrast agent for MRI. In comparison to the conventionally used IOPNs, Fe/gFe2O3 nanoparticles offer a much stronger T2 shortening effect than that of IONPs with the same core size due to their more favorable magnetic properties.

Acknowledgments This work was supported by NSF DMR-0302544

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