Lanthanide-doped NaYF4 UCNPs and Na4Sn2S6 ligands were synthesized according to established protocols (see Methods and Supporting Information for details). We initially attempted ligand exchange by directly adding the Sn2S6 4-ligand to a toluene dispersion of UCNPs. However, after stirring for several hours, no nanoparticle aggregation was observed, indicating that this method was ineffective in facilitating ligand exchange.