# Challenges in Controlled Doping of NaMnF3:Yb3+ , Er3+ Nanoparticles

## TL;DR

Researchers developed a better method to synthesize NaMnF3:Yb3+, Er3+ nanoparticles for biomedical imaging by improving their size control and luminescence.

## Contribution

A new thermal decomposition method was developed to produce phase-pure, monodisperse UCNPs with enhanced red luminescence and high Mn2+ content.

## Key findings

- An optimized synthesis route produced 20-25 nm phase-pure UCNPs with strong red emission at 655 nm.
- The method increased dopant concentrations and luminescence compared to previous approaches.
- Hydrophilic functionalization enabled the particles to act as effective MRI contrast agents.

## Abstract

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 T
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.