Opto-Magnetic Nanomaterials—From Synthesis Design to Biomedical Applications

Sodium metal fluorides (NaMF_x, M = metal ion) have received considerable attention because of their unique magnetic, optical, ferroelectric, and electrochemical properties. For instance, upconverting NaLnF₄ (Ln = lanthanides) materials have been suggested for a wide range of applications including the fields of biomedicine, optoelectronics, and solar energy conversion. NaMnF₃ has been studied for its potential application as magnetic resonance bioimaging contrast agents, ferromagnets, and active material for batteries. The growing attention toward such materials has prompted the development of novel synthesis methods for a more reliable and efficient access to these systems. In this regard, microwave-assisted approaches provide unique advantages over traditional synthesis methods (solvothermal, co-precipitation and thermal decomposition) reliant on convectional heating: namely, significantly shorter reaction durations, more rigid reaction conditions, and thus a higher degree of reproducibility.<br/>We here report the rapid and straightforward microwave-assisted synthesis of NaGdF₄ nanoparticles and NaMnF₃ sub-micron particles as candidates for magnetic resonance imaging (MRI). Herein, T₁ contrast is generated thanks to the paramagnetic properties of the Gd³⁺ and Mn²⁺ ions, respectively. Tuning of the metal-ion-ratio (using metal trifluoroacetates as precursors) was found key when seeking control over morphology and crystalline phase of the resulting products. For instance, NaMnF₃ particles were obtained with tunable morphologies, yielding rods and ribbons in addition to the commonly seen plate-like structures by varying the Na⁺-to-Mn²⁺ ratio. The selective synthesis of NaGdF₄ in either of its crystalline phases (cubic α or hexagonal β) by adjustment of the Na⁺-to-Gd³⁺ ratio allowed the influence of host crystallinity on the T₁ contrast behaviour to be investigated, unveiling superior performance of cubic-phase NaGdF₄ nanoparticles. The developed microwave-assisted approach further allows for doping of these nanoparticles with luminescent lanthanide ions (e.g., upconverting dopant pair Yb³⁺/Er³⁺) to endow the probes with additional photoluminescence capabilities. As such, selective doping and host material choice in core/multi-shell architectures has the potential to leverage the optical properties of Yb³⁺/Er³⁺ in conjunction with the magnetic properties of ions such as Gd³⁺ and Dy³⁺, ultimately broadening the application potential of these materials to multifunctional optical/MRI/CT imaging probes.