SPION clusters with porous silica shell: Synthesis, core-shell structure, magnetic properties, biocompatibility and MRI application

Abstract

This study focuses on the synthesis of superparamagnetic iron oxide nanoparticle clusters (SPIONc) coated with porous silica (silica@SPIONc) and on examining the effects of thermal annealing on their structural, magnetic and MRI relaxometric properties. The objective is to investigate how thermal treatments of these partly isolated SPION clusters with a deficit of oxygen, protected by a porous silica shell, influence their magnetic behavior. The goal is to assess their suitability for biomedical applications, particularly as MRI contrast agents and multifunctional theranostic materials. A novel aspect of this study is the discovery of unexpected thermal effects on the properties of the samples, caused by the specific structural conditions of the iron oxide nanoparticle clusters. The synthesis process, combining a wet colloidal chemical method with sol-gel silica deposition, was optimized to create uniform core-shell nanoclusters. XRD and SAED analyses confirm the presence of spinel-structured maghemite/magnetite (γ-Fe2O3/Fe3O4) nanocrystals within the SPIONc. Based on the results observed in the FTIR spectra, we conclude that the samples are likely in the γ-Fe2O3 (maghemite) phase. Magnetization studies at 300 K demonstrated superparamagnetic behavior, characterized by zero coercivity and magnetic remanence, consistent with the Langevin model. The compactly arranged superparamagnetic nanocrystals (≈10 nm diameter) within the silica@SPIONc cores exhibited a high magnetic moment (mnc ≈ 1.3 ∗ 106 μB), facilitating rapid alignment under external magnetic fields. Annealing at 300 °C enhances inter-particle interactions and lowers the blocking temperature, whereas annealing at 600 °C raises the blocking temperature and reduces magnetic coupling due to surfactant decomposition. This approach demonstrates a novel strategy for tuning the magnetic properties of SPION-based materials through controlled thermal treatments, broadening their functional adaptability. Superparamagnetic iron oxide nanoparticle cluster coated with porous silica exhibits high transverse relaxivity (r2 ≈ 345 mM−1s−1) and low cytotoxicity, establishing them as promising candidates for advanced MRI contrast agents. © 2025 Elsevier Ltd and Techna Group S.r.l.