Multifunctional lanthanide-based iron oxide luminescent-magnetic nanoparticles: synthesis, properties and biomedical...

Multifunctional nanocomposites that integrate lanthanide photophysics with iron oxide magnetism represent a rapidly expanding class of materials for precision bioimaging and theranostics. Lanthanide-based iron oxide nanoparticles (Ln-IONPs) uniquely combine the narrow f-f emissions, long-lived excited states, and UC/DC luminescence of lanthanides with the superparamagnetism, high relaxivity, and magnetic field responsiveness of Fe₃O₄/γ-Fe₂O₃. However, their rational design remains challenging due to interfacial energy transfer, concentration-dependent cross-relaxation, and fluorescence quenching induced by iron oxide. This review provides the first systematic overview of Ln-IONPs, covering their fundamental luminescent and magnetic mechanisms, ligand and host engineering, and factors influencing their optical and magnetic performance, such as particle size, dopant distribution, surface chemistry, pH, temperature, and solvent effects. Synthetic strategies, including coprecipitation, thermal decomposition, hydro(solvo)thermal growth, sol-gel assembly, microwave-mediated crystallization, and polymer-integrated architectures, are critically evaluated with respect to phase control, heterointerface engineering, and quenching suppression via inert buffer layers. We further highlight recent advances in multimodal bio-applications, including high-resolution UC imaging, magnetically guided drug delivery, photothermal/photodynamic therapy, and integrated opto-magnetothermal theranostics. Persistent challenges, such as optimizing core-shell energy barriers, balancing luminescence efficiency with magnetic moment retention, and improving in vivo stability and clearance, are discussed alongside future opportunities for designing next-generation Ln-IONPs with enhanced optical-magnetic coupling and clinical translational potential.