The remarkable optomagnetic properties of the rare-earth elements (RE) make RE-based materials ideal for biomedical applications, including diagnostic (for instance, imaging or thermal sensing) and therapeutic (for instance, drug delivery and photodynamic therapy) approaches. This is due the unique electronic properties of the f-elements allowing for upconversion and near-infrared emission under near-infrared excitation as well as high magnetic moments.
Yet, challenges remain, including low photoluminescence efficiency of small nanoparticles (NPs) and the need for more reliable and fast synthesis routes. As material chemists, we tackle these challenges with new designs of RE-NPs by chemically controlled synthesis, application-oriented surface chemistry, and understanding of structure-property-relationships. Sodium rare-earth fluorides (NaREF4) are our favorite materials, and we developed a fast and reliable microwave-assisted synthesis approach allowing crystalline phase and size control in the sub 15nm realm. Such control is crucial for the understanding of fundamental structure-property relationships and to optimize their optical and magnetic properties, when aiming for the design of next-generation optical probes or contrast agents for magnetic resonance imaging. For instance, NaGdF4 NPs are gaining interest as alternative MRI contrast agent, while co-doping with RE3+ ions endows the NPs with luminescent properties for applications as optical probes. The hexagonal crystalline phase of NaGdF4 is known as the more efficient host material for upconversion emission (that is the emission of one higher energy photon following excitation with two or more lower energy photons). In contrast, we observed that the cubic counterpart of NaGdF4 shows superior performance as MRI contrast agent.
Having a fast and reliable synthesis route towards NaREF4 NPs on hand, we now explore various nanoparticle architectures and compositions.