(174o) Evolution of Magnetic Properties, Heating Rate, and Mpi Performance of Iron Oxide Nanoparticles during Post-Synthesis Oxidation

Iron oxide nanoparticles are attractive in a wide range of applications in both biological and physical sciences since they exhibit spontaneous magnetization that can be controlled by an applied magnetic field. Motivated by their distinct properties, tremendous progress has been made on the synthesis of iron oxide nanoparticles focusing on physical size and shape control. However, not as much effort has been devoted to control and improve their magnetic properties. In cases where magnetic properties are characterized, often only their saturation magnetization and the presence of superparamagnetism are reported. To date, the assumption has been that properties such as heating rate scale directly with physical diameter. However, emerging understanding suggests that this assumption is incorrect, particularly for large particles.

A few groups have recognized this problem and have developed methods to obtain monodisperse magnetite nanoparticles with improved magnetic properties using post-synthesis oxidation. Recently, Kemp et al.¹ demonstrated an effective way to obtain particles with equal magnetic and physical diameter ranging from 15 nm to 30 nm using post-synthesis oxidation. However, only the magnetic diameter evolution and saturation magnetization were analyzed. Performance of iron oxide nanoparticles in many applications, such as nanoscale thermal cancer therapy, is expected to scale with the particle’s response to an applied time-varying magnetic field, which in turn depends on magnetic properties such as the magnetocrystalline anisotropy energy. Here we evaluate how magnetic properties of iron oxide nanoparticles evolve during post-synthesis thermal oxidative treatment. Aliquots taken at points along the post-synthesis thermal oxidative treatment were analyzed for physical size and morphology using TEM and magnetic properties using SQUID magnetometry. For particles that are 19 nm in physical diameter with starting magnetic diameter of 10 nm, the magnetic diameter increased to 19 nm after 3 hours of post-synthesis oxidation. Magnetic characterization of the final aliquot indicated a small interaction temperature (2.8 K), a magnetic anisotropic constant (11 KJ/m³) close to that of bulk magnetite (13.5 KJ/m³), and no exchange bias. However, the saturation magnetization was 32 Am²/kg, which is less than that of bulk magnetite (86 Am²/kg). Results will be presented for particles with physical diameters ranging from 20 to 30 nm, exploring the effect of physical diameter on evolution of magnetic properties after post-synthesis oxidation.

1 Kemp, S. J., Ferguson, R. M., Khandhar, A. P. & Krishnan, K. M. Monodisperse magnetite nanoparticles with nearly ideal saturation magnetization. Rsc Advances 6, 77452-77464, doi:10.1039/c6ra12072e (2016).