Esterification synthesis of iron oxide nanoparticle tracers for magnetic particle imaging (MPI)

The potential for producing iron oxide magnetic particle imaging (MPI) tracers using an alternative synthesis method based on esterification of iron oleate with oleyl alcohol was evaluated. The defined reaction mechanism allows monitoring of reaction progress with Fourier Transform Infrared (FTIR) spectroscopy. The influence of reaction temperature and precursor flow rate on esterification reactions of Fe(II) oleate for tuning iron oxide nanoparticle size and dispersity was studied, identifying conditions for producing larger nanoparticles suitable as MPI tracers. Increasing temperature and decreasing flow rates were found to increase the resulting nanoparticle size and reduce dispersity. Furthermore, the effect of the iron source used to prepare the iron oleate precursor was evaluated by characterization of nanoparticle magnetic properties, composition, and MPI performance. Although the nature of the precursor did not appear to affect nanoparticle morphology or growth, it influenced magnetic properties and MPI performance. Saturation magnetization was close to the bulk value of magnetite and the discrepancy between physical and magnetic diameters was lowest for nanoparticles synthesized with oleates prepared using Fe(II) or Fe(III), as opposed to nanoparticles synthesized using an oleate prepared with a 1 : 2 molar mixture of Fe(II) and Fe(III). X-ray diffraction characterized that nanoparticles synthesized using the Fe(III) oleate are the most crystalline, followed by Fe(II) and the 1 : 2 Mix, respectively. Mössbauer spectroscopy was used to verify iron oxide phases, suggesting nanoparticles synthesized using the Fe(III) oleate consist of a mixture of γ-Fe₂O₃ and Fe₃O₄, in contrast to those obtained from the Fe(II) and 1 : 2 Mix oleates, which consisted of a mixture of wüstite, γ-Fe₂O₃ and Fe₃O₄. Characterization of MPI performance using a MOMENTUM™ scanner demonstrated the capability of the esterification reaction to yield high-quality monodisperse MPI tracers.