Ultra-low gadolinium doping in multi-core iron oxide enables efficient dual-mode MRI and magnetic hyperthermia: a structure–function study

Abstract

We present a novel strategy for engineering multifunctional nanoplatforms for cancer theranostics by employing ultra-low gadolinium (Gd3+) doping to optimize the performance of maghemite (γ-Fe2O3) “nanoflowers” for both magnetic resonance imaging (MRI) and magnetic hyperthermia treatment (MHT). Controlled Gd3+ doping up to 1.7 mol% was sufficient to significantly alter the material properties while preserving the γ-Fe2O3 phase and hierarchical multi-core architecture. X-ray photoelectron spectroscopy (XPS) revealed that doping induces critical surface defects, specifically a gradual increase in surface Fe2+ species and non-lattice oxygen with increasing Gd3+ content, indicating redox imbalance and the formation of oxygen vacancies. Electron paramagnetic resonance (EPR) measurements confirmed that these defects enhance magnetic anisotropy and spin disorder, while SQUID magnetometry showed that all samples retained superparamagnetic behavior despite a non-monotonic decrease in saturation magnetization. Under external alternating magnetic fields (AMF), the Gd0.011Fe1.989O3 sample exhibited the highest MHT performance, with Intrinsic Loss Power (ILP) values reaching up to 2.73 nH m2 kg−1. Simultaneously, MRI relaxometry at 7 T demonstrated that low-level Gd3+ doping markedly improved both longitudinal (r1) and transverse (r2) relaxivities. The Gd0.022Fe1.978O3 sample achieved an exceptional r2 value of 253.3 mM−1 s−1, with an r2/r1 ratio exceeding 220, making it a powerful T2-weighted MRI contrast agent. Importantly, the Gd0.011Fe1.989O3 sample showed a tunable balance, with a favorable r2/r1 ratio suitable for dual-mode T1/T2 MRI imaging and MHT. These findings underline the novelty of operating in an ultra-low Gd regime, where defect engineering and tailored multi-core architecture synergistically optimize the structure–property–function relationship, paving the way for safer and more effective theranostic nanoplatforms.