Ultrasound-assisted co-precipitation synthesis of GdFeO₃ nanoparticles: structure, magnetic and MRI contrast...

Superparamagnetic nanocrystals of gadolinium orthoferrite (GdFeO₃) with close to isometric morphology were successfully synthesized by heat treatment of gadolinium and iron(III) hydroxides obtained by direct co-precipitation with and without ultrasonic irradiation. The obtained samples were investigated by PXRD, low-temperature nitrogen adsorption-desorption isotherm measurements, HRTEM and VSM. It was established that ultrasonication during co-precipitation led to a decrease in the average size of GdFeO₃ crystallites obtained after heat treatment by approximately 19%, an increase in their BET specific surface area by more than two times, a decrease in the degree of their aggregation by about five times and an improvement in their magnetic properties due to the increase in phase homogeneity. The colloidal solutions of the GdFeO₃ nanoparticles synthesized using ultrasound were investigated by ¹H NMR to measure the T₁ and T₂ relaxation times of water protons at 0.47 T. The resulting relaxivities r₁ and r₂ were approximately recalculated at 1.5, 3 and 4.7 T on the basis of a semi-statistical ad hoc method by analyzing the literature data for a number of structurally similar compounds with reported relaxivity values at different NMR frequencies. According to the experimental and predicted values of the r₂/r₁ ratio, the investigated GdFeO₃ sample may be classified as a T₁-contrast agent for MRI at 0.47 and 1.5 T, as a T₁-T₂ dual-modal contrast agent at 3 T and as a T₂-contrast agent at 4.7 T.