The Role of Diffusion-Controlled Growth in the Formation of Uniform Iron Oxide Nanoparticles with a Link to Magnetic Hyperthermia

Uniform superparamagnetic iron oxide nanoparticles were obtained by coprecipitation under synthesis conditions that guarantee diffusion-controlled growth. Study of nanoparticle crystal structure formation by HRTEM showed that at the earlier stage of the reaction some nanoparticles consist of crystalline core and amorphous surface layer, whereas resulting particles display a high degree of crystalline order. This result suggests that nanoparticles are formed from fusion of noncrystalline primary particles of iron (hydr)oxide. Slow addition of iron salts to excess ammonia restricts the amount of primary particles; as a result, their diffusion is the limiting step of the reaction, which provides the formation of uniform nanoparticles. Importantly, 5 min reaction product shows the same polydispersity and heating efficiency as the final product. Thus, monodispersity determines the particle properties and facilitates the control of heat generation for a given amplitude and frequency of AMF. Magnetic dipole interactions between single nanoparticles lead to the formation of dense aggregates (multicore particles) at the beginning of the reaction. The dispersions of separated multicore particles with hydrodynamic size of about 85 nm shows higher heating efficiency than dispersion of as-prepared nanoparticles. The increase of aggregate size leads to a decrease of heating efficiency to the value of as-prepared nanoparticles due to a demagnetizing effect.