Improving dispersibility of lanthanide-doped upconversion nanoparticles for luminescence applications in medium polarity solvents

The dispersibility of lanthanide-doped upconversion nanoparticles (UCNPs) is related to the polarity of solvents. Typically, the as-synthesized UCNPs are poorly dispersed in medium and high polarity solvents. Here, we investigated the dispersibility of oleate-capped UCNPs in different solvents and improved the dispersibility of the nanoparticles in medium polarity solvents by a ligand exchange process. 2-naphthoxyacetic acid and alpha-hydroxyhippuric acid were selected as ligands to improve the dispersion of UCNPs in long-chain alcohol and ester solvents, respectively. The improvement of dispersibility was confirmed by dynamic light scattering measurements and transmission electron microscope images. In parallel, the luminescence features of the UCNPs dispersed in different solvents were measured to understand the influence of solvents on the luminescence lifetime, red-green emission ratio and power dependent properties of UCNPs. Additionally, the ligand exchange rendered an improved dispersibility of UCNPs in medium polarity media, which prevented aggregation of UCNPs in polydimethylsiloxane (PDMS) matrix improving transmittance of UCNPs-polydimethylsiloxane (PDMS). This study provides an effective method to improve the dispersion of UCNPs in organic polymer media expanding the applicability of nanoparticles in displays and anticounterfeiting.