Challenges in Controlled Doping of NaMnF ₃ :Yb ^{3 <b>+</b>} , Er ^{3 <b>+</b>} Nanoparticles

NaMnF₃:Yb³⁺, Er³⁺ upconversion nanoparticles (UCNPs) are highly attractive for multimodal biomedical imaging because they combine a single red emission band with magnetic properties, allowing their use as <i>T</i> ₁ contrast agents in magnetic resonance imaging. However, compared to other magnetic or red-emitting UCNPs, their synthesis remains challenging, so they are rarely used despite their great potential. Syntheses based on hydrothermal approaches usually yield rather large or aggregated and often polydisperse and polymorphic particles. Reported thermal decomposition methods, in principle more suited to yield highly crystalline, small, and monodisperse UCNPs, are questionable because of the unclear phase and doping state of the NaMnF₃ particles despite their strong red emission. This is due to the parallel formation of red luminescent NaYbF₄ particles, which has not been sufficiently investigated. In this work, different thermal decomposition routes are studied, and the particles are analyzed to optimize them for high dopant integration and strong red luminescence at 655 nm. An optimized synthesis route for well-defined phase-pure particles in a size range from 20 to 25 nm was developed. This approach is based on separately preparing Mn²⁺, Er³⁺, and Yb³⁺ oleates and then introducing them into the reaction, followed by a purification step. This leads to phase-pure UCNPs and substantial increases in dopant concentrations as well as luminescence compared to reported synthesis routes. After their hydrophilic functionalization with polyacrylic acid, the ability of the particles to serve as magnetic resonance imaging (MRI) contrast agents with a high Mn²⁺ content was demonstrated by NMR relaxometry.