Magnetic nanoparticles based on iron oxides: synthesis, structural architectures and functional applications

The synthesis of iron oxide nanoparticles (IONPs) significantly influences their characteristics, which are sensitive to factors such as pH, atmospheric conditions, temperature, and surface coatings. Therefore, it is essential to understand how different synthesis methods affect their structure, composition, and magnetic behavior. This review examines some of the main synthesis methods used to prepare IONPs and discusses several parameters that influence their size, morphology, crystallinity, phase composition, and magnetic response. The role of heat treatment in modulating oxidation states and phase transformations among Fe3O4, γ-Fe2O3, and α-Fe2O3, is also analyzed, considering the effects of temperature, treatment time and processing atmosphere. The impact of functionalization or surface coatings on colloidal stability, biocompatibility, and structural or morphological integrity is also considered. Core–shell architectures and their role in tailoring magnetic and interfacial properties are also briefly reviewed. Additionally, key structural, morphological, magnetic, and surface characterization techniques are described to establish clear relationships between structure and properties. Finally, an overview of current and emerging applications is provided, along with a discussion of challenges regarding synthesis reproducibility, long-term stability, toxicity assessment, and technology transfer, highlighting future directions for the rational design of magnetic nanoparticles intended for emerging applications.