Speaker
Description
Magnetic nanoparticles of magnetite and its substituted variants, including functional derivatives, show promising capabilities in emerging diagnostic imaging methods and novel therapeutic interventions. In addition to the rapid development of synthesis and functionalization methods, a comprehensive understanding of their fundamental physical characteristics and the complex link among composition, microstructure, magnetic properties, and relaxation characteristics is pivotal for the rational design of well-defined magnetic nanoparticle systems. The magnetic behaviour of nanoparticles deviates from bulk materials not only due to finite-size and surface effects but also by the occurrence of metastable states, such as non-equilibrium cation distribution. This study aims to explore the impact of cation distribution on the magnetic properties of Zn$_x$Fe$_{3-x}$O$_4$ nanoparticles, paving the way for their potential applications in theranostics.
The nanoparticles with varying Zn concentrations ($x = 0$, $0.05$, $0.33$, and $0.36$ according to X-ray fluorescence) were prepared using controlled two-step thermal decomposition at relatively low temperature. Their morphology and size distribution were analysed via transmission electron microscopy, revealing log-normal distributions with mean diameters ranging from $5.1$ nm to $14.4$ nm. X-ray diffraction confirmed the spinel structure for all samples. EXAFS was employed to analyse the local environment of Zn and Fe atoms. Local probe methods, solid-state NMR spectroscopy and Mössbauer spectroscopy, along with DC magnetic measurements, provided insights into the distribution of Zn and Fe cations over the tetrahedral (A) and octahedral [B] sites that significantly influences the predominant A-B magnetic interactions, causing changes in the magnetic structure.
High specific surface area of nanoparticles allows quick oxidation during synthesis, yielding mixed maghemite/magnetite composition. The surfactants used during synthesis stabilized the magnetite nanoparticles for prolonged periods, as evidenced by differences in MS spectra between as-prepared and purified samples. Even smallest concentration of Zn substitution was found to suppress the Verwey transition, a well-known effect in magnetite characterized by a dramatic change in structural, electronic, and magnetic properties around $120$ K. The presence of Zn atoms in the structure led to an overall increase of saturated magnetization and transversal relaxivity, with highest observed value slightly higher than those previously reported for other contrast agents with iron oxide cores.
Acknowledgements
The work is supported by Operational Programme Johannes Amos Comenius (FerrMion, CZ.02.01.01/00/22_008/0004591).
