The ε-AlxFe2-xO3 nanomagnets as MRI contrast agents: Factors influencing transverse relaxivity

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Vol. 589; p. 124423
• Main Authors: Kubíčková, Lenka, Kaman, Ondřej, Veverka, Pavel, Herynek, Vít, Brázda, Petr, Vosmanská, Magda, Kmječ, Tomáš, Dvořák, Petr, Kubániová, Denisa, Kohout, Jaroslav
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20-02-2020
• Subjects: Core-shell nanoparticles; Epsilon polymorph iron oxide; Magnetic nanoparticles; Magnetic resonance imaging; Mössbauer spectroscopy; Relaxometry; Core-shell nanoparticles; Magnetic resonance imaging; Epsilon polymorph iron oxide; Relaxometry; Magnetic nanoparticles; Mössbauer spectroscopy
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/j.colsurfa.2020.124423

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•Nanoparticles of ε-Al0.23Fe1.77O3 are in blocked state with coercivity ∼1.5 T at RT.•Transverse relaxivity r2 is strongly affected by surface modification of particles.•Its temperature dependence is due to combination of MAR and SDR mechanisms.•Relaxivity is also influenced by magnetization history of nanoparticle suspensions.•Silica-coated particles at 0.35 mM(Fe) exert no effect on rat stem-cell viability.
Most magnetic materials studied as negative contrast agents for magnetic resonance imaging (MRI) enter the superparamagnetic state with decreasing size of their particles, which is distinguished by thermal fluctuations of particle magnetic moment. In the present study, a novel type of contrast agents based on non-superparamagnetic nanoparticles (nanomagnets) of aluminum-doped epsilon polymorph of Fe2O3 is suggested and used as a model system to elucidate the role of magnetic blocking and other factors in the transverse relaxation of water. Specifically, the dependence of the transverse relaxivity r2 on the magnetic field (0.47–11.75 T), temperature (278−348 K), magnetization, surface modification (silica or citrate) and thickness of silica coating (6−21 nm) is analyzed for the ε-AlxFe2-xO3 nanoparticles with x = 0.23 and the median size of 21 nm. These nanoparticles evinced higher magnetization than the undoped material and were in the blocked state in the temperature range under study. Further, irreversible magnetization processes were revealed by SQUID magnetometry in aqueous suspension of coated clusters of magnetic crystallites that resulted in the field dependence of r2. The temperature dependence of r2 was interpreted by the combination of two different regimes – motional averaging and static dephasing regimes. Slow diffusion of water molecules inside the silica shells was suggested to explain only moderate decrease of r2 with increasing the coating thickness. Moreover, the performance of the contrast agent was demonstrated not only in ultra-high-field MRI at 11.75 T but also by imaging in vivo on a mouse model at 1 T. Finally, preliminary evaluation of cytotoxicity on rat mesenchymal stem cells did not reveal any significant effects. Due to their low toxicity and high relaxivity, ε-AlxFe2-xO3 nanoparticles present a promising starting material for further biomedical applications involving MRI, multimodal contrast agents and theranostic carriers.