Size- and shape-controlled iron oxide nanocrystals as MPI tracers based on a heating up synthesis technique

Abstract

A challenging aspect of the success of MPI is not only the development of sophisticated imaging scanners but also the synthesis of advanced tracers. Iron oxide nanocrystals (NCs) are   promising candidates due to fascinating magnetic properties, including blocking temperature, coercivity, saturation magnetization, magnetic domain size, and exchange coupling effects. To achieve higher tracer sensitivity leading to better temporal and spatial resolution in a MPI scanner, NCs should have a steeper slope in the linear part of the magnetization curve and an approximate step function. The magnetic properties of magnetic NCs strongly depend on size, shape, and crystal structure. Therefore, we developed a sophisticated synthesis technique based on the thermal decomposition of iron oleate in order to control these parameters. The oleate was synthesized by use of a precursor system - Fe(II)CO₃ and Fe₂(III)(CO₃)₃. The method allows the synthesis of highly monodisperse NCs in various well-defined sizes and morphologies on a gram scale.

For a strong signal generation in MPI, a special oxidation treatment after synthesis was performed to maximize the magnetite phase's content and increase the saturation magnetization. And moreover the uniformity and monodispersity of the sample make a strong signal generation in MPS. Based on the seeded emulsion polymerization, a phase transfer of the hydrophobic NCs into water, followed by subsequent encapsulation with a polystyrene shell, provides water solubility and biocompatibility. Thereby realizing the option for surface functionalization with e.g., antibodies or even the formation of defined clusters.