Effect of synthetic conditions on the structure and magnetic properties of iron oxide nanoparticles in diethylene glycol medium

Ultra-small iron oxide nanoparticles (USNPs) have attracted particular attention in the past 15 years as perspective contrast agents for MRI. Unfortunately, the synthesis of such small nanoparticles with high contrast properties and water dispersibility is still challenging. This paper presents a study on the influence of synthetic conditions on the structure and the properties of hydrophilic iron oxide nanoparticles obtained by a simple single-step thermal decomposition in diethylene glycol at 230–235 °C. The samples were studied using X-ray diffraction, Mössbauer and infra-red spectroscopy, transmission electron microscopy, vibrating sample magnetometry, MRI, and dynamic light scattering. All the obtained samples are of spinel structure (Fd-3 m), specific to both magnetite and maghemite. With an increase in the synthesis time from 1 to 8 h, the crystallite size of the series with C(Fe(acac)₃) = 30 mM changed from 1.8 ± 0.2 to 4.7 ± 0.5 nm, the average size according to TEM changed from 3.3 ± 0.8 to 3.8 ± 0.4 nm, and the saturation magnetization from 13.9 ± 0.3 to 83.3 ± 1.7 A•m²/kg, which is close to the values of bulk iron oxide. The same tendency was revealed with the increase in the concentration of C(Fe(acac)₃) from 30 to 120 mM for 1 h of synthesis. An increase in the synthesis time for 60- and 120-mM solutions did not significantly change the crystallite size and the magnetic properties. It was shown that the samples obtained using this approach have unexpectedly high values of r₂-relaxivity, up to 235 mM⁻¹•s⁻¹, which the highest published for USNPs. The studied method of water-soluble USNPs is promising for use in creating T₂-contrast agents for MRI.