Oleic Acid Binding and Exchange Dynamics on NaYF4 Nanocrystals

Lanthanide-doped NaYF4 nanocrystals (NCs) are widely used for their exceptional photoluminescence properties. Their performance and processability are critically dependent on surface-bound ligands such as oleic acid. While these ligands stabilize NCs during synthesis and in nonpolar solvents, applications often require post-synthetic ligand exchange. Here, we investigate the protonation state, surface ion binding, and exchange dynamics of oleic acid on NaYF4 NCs using a variety of analysis techniques, including infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR), and thermogravimetric analysis (TGA). Our methods distinguish bound and free ligand populations, quantify surface coverage, and detail how common purification steps alter ligand density. These findings offer key insights into NC surface chemistry and provide a foundation for rational NC purification and ligand-exchange strategies, with implications for optimization of colloidal stability and functionalization.