Local Magnetic and Electronic Structure of the Surface Region of Postsynthesis Oxidized Iron Oxide Nanoparticles for Magnetic Resonance Imaging

Abstract

Iron oxide nanoparticles (FeO x -NP) are applied in medicine as contrast agents in magnetic resonance imaging (MRI) where they reduce the spin−spin relaxation time (T 2 -time) of absorbing tissue. Hence, control of their magnetic properties is essential for these applications. Magnetic properties strongly depend on the particle size and shape as well as the surface functionalization of the iron oxide nanoparticles. Especially, structural and magnetic disorder in the region close to the surface (1−2 nm) lead usually to a reduced magnetization compared to the corresponding bulk material. Therefore, X-ray magnetic circular dichroism (XMCD) in the total electron yield (TEY) mode is used to investigate local magnetic and electronic properties of the surface region of monodisperse, spherical FeO x -NPs (Fe 3 O 4 /γ-Fe 2 O 3 ) before and after the postsynthetic treatment in oxygen-rich environment. Charge transfer multiplet calculations of the XMCD spectra are performed to analyze the contributions of Fe 2+ and Fe 3+ at different lattice sites, i.e., either in octahedral or tetrahedral environment. The analysis of the XMCD data reveals that both, the magnetization of the nanoparticle surface region as well as their maghemite to magnetite ratio, are strongly increased after tempering in an oxidative environment, which likely causes rearrangement of their crystalline order. The magnitude and the kinetics of these variables depend strongly on the particle size. In addition, after thermal annealing a reduced spin canting is extrapolated from the lower magnetic coercivity, which confirms that a structural rearrangement takes place.

Oxidation of magnetite nanoparticles: impact on surface and crystal properties

“…The reduced saturation magnetisation compared to bulk magnetite can be explained by a disordered structure near the surface leading to canted spins in the surface layer. 20,[60][61][62][63] Our mild oxidation experiment reveals no changes in saturation magnetisation as well as in the shape of the magnetisation curve in the first two hours (Fig. 2).…”

“…The negative coercivity as well as negative remanence magnetisation can be explained by site-specific surface anisotropy, noncollinear spin structure or antiferromagnetic coupling of particle core and shell. 20,49,61,64,65 The saturation magnetisation of nanoparticles oxidised in nitric acid decreases much faster (Fig. 3).…”

“…13 Additionally, great separation capabilities and affinities to biomolecules are requested for bioseparation engineering. 14,15 However, even biomedical in vivo applications such as hypothermia, 16 drug delivery 17 and as contrast agents for magnetic resonance imaging 5,[18][19][20] are possible for MNP as they have been approved by the US Food and Drug Administration (FDA). 21 On the other hand, for catalytic applications, such as Fenton chemistry, a high reactivity of MNP is desired.…”

The UE46 PGM-1 beamline at BESSY II

“…• Charge order of high-Tc Superconductors (Blanco-Canosa et al, 2013;Comin et al, 2014;da Silva Neto et al, 2014;Fink et al, 2009;Ghiringhelli et al, 2012) • Coupling of electronic / lattice degrees of freedom in multiferroic materials (Glavic et al, 2013;Partzsch et al, 2011;Schierle et al, 2010;Schmitz-Antoniak et al, 2013;Skaugen et al, 2015) • Microcrystals of novel materials (Leininger et al, 2011;Matsuda et al, 2015) • Interfacial electronic properties in heterostructures (Frano et al, 2013;Wadati et al, 2009) • Element-speci c magnetic hysteresis loops (Radu et al, 2012) • Single molecular magnets (Bernien et al, 2015;Hermanns et al, 2013) • Electronic depth pro les • Electronic ground states and phase transitions in correlated materials (Schmitz et al, 2014;Strigari et al, 2013;Willers et al, 2012Willers et al, , 2011 • Magnetic clusters in carbon nanotubes (Shiozawa et al, 2015) • Nanoparticles for medical applications (Graf et al, 2015) • Magnetic semiconductors (Khalid et al, 2014) …”

An X-ray magnetic circular dichroism (XMCD) study of Fe ordering in a synthetic MgAl2O4-Fe3O4(spinel-magnetite) solid-solution series: Implications for magnetic properties and cation site ordering

“…The main peak for the most magnetite-rich sample (SP14) is very similar to that for end-member natural magnetite ( Figure 2 in Pearce et al, 2006) in that only a weak shoulder is present on the low energy side of the main peak; this feature is considered to be typical of endmember magnetite which lacks any significant oxidation towards a phase containing the maghemite molecule (Zhu et al, 2015; see also Fig. 3 in Graf et al, 2015). With increasing Mg-content the XAS spectra show that the feature on the low energy side of the main L 3 peak becomes progressively better resolved and more intense; note that the spectrum for the Mg-rich sample SP5 is similar to that for a natural magnesian spinel (Mgt 0.20 , BM1983,595) (Cressey et al,1993;Henderson et al, 1996).…”