Structure-property coupling in cracked iron oxide nanoparticles: Synthesis conditions, magnetic properties, MRI relaxivity and biocompatibility

Abstract

This study reports a simple and environmentally friendly synthesis of hematite nanoparticles with tunable magnetic properties, including quasi-superparamagnetic behavior and a suppressed Morin transition. The hematite nanoparticles were synthesized via high-temperature hydrolysis of aqueous FeCl3, with synthesis durations of 3 h (sample S1) and 6 h (sample S2). XRD analyses confirmed the formation of phase-pure hematite, while TEM imaging revealed a distinctive cracked-spherical morphology with an average diameter of ≈ 35 nm and surface cracks 1–2 nm wide. The critical size for superparamagnetism and the suppression of the Morin transition below 150 K in hematite (α-Fe2O3) remain insufficiently studied. Magnetic properties revealed a size- and surface-dependent magnetic behavior. Sample S1 exhibited quasi-superparamagnetic behavior, characterized by a high blocking temperature (TB ≈ 275 K) and an irreversibility temperature exceeding room temperature (Tirr > 300 K). In contrast, sample S2 displayed a strongly suppressed Morin transition at TM ≈ 130 K. These results show that hematite nanoparticles ≈ 35 nm in size approach the superparamagnetic threshold. We attribute the modified magnetic response to enhanced surface disorder and strain associated with the cracked morphology, supported by XRD strain analysis (Williamson–Hall), HRTEM evidence of structural disorder and comparative discussion with non-cracked hematite. In addition to their tunable magnetic properties, the hematite nanoparticles demonstrated a promising potential for biomedical applications, exhibiting transverse and longitudinal MRI relaxivities (r2 ≈ 4.58 mM−1s−1 and r1 ≈ 0.075 mM−1s−1, respectively) and low cytotoxicity. This work highlights the importance of surface morphology and particle size in controlling the magnetic behavior of hematite nanostructures and their potential use as MRI contrast agents.