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 Gd_0.011Fe_1.989O₃ 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 Gd_0.022Fe_1.978O₃ 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 Gd_0.011Fe_1.989O₃ 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.