Effect of Surface Modification on the Luminescence of Individual Upconversion Nanoparticles

Abstract

AbstractLanthanide‐doped upconversion nanoparticles (UCNPs) hold promise for single‐molecule imaging owing to their excellent photostability and minimal autofluorescence. However, their limited water dispersibility, often from the hydrophobic oleic acid ligand during synthesis, is a challenge. To address this, various surface modification strategies' impact on single‐particle upconversion luminescence are studied. UCNPs are made hydrophilic through methods like ligand exchange with dye IR806, HCl or NOBF4 treatment, silica coating (SiO2 or mesoporous mSiO2), and self‐assembly with polymer of DSPE‐PEG or F127. The studies revealed that UCNPs modified with NOBF4 and DSPE‐PEG exhibited notably higher single‐particle brightness with minimal quenching (3% and 8%, respectively), followed by SiO2, F127, IR806, mSiO2, and HCl (84% quenching). HCl disrupted UCNPs's crystal lattice, weakening luminescence, while mSiO2 absorbed solvent molecules, causing luminescence quenching. Energy transfer to IR806 also reduced the brightness. Additionally, a prevalence of upconversion red emission over green is observed, with the red‐to‐green ratio increasing with irradiance. UCNPs coated with DSPE‐PEG exhibited the brightest single‐particle luminescence in water, retaining 48% of its original emission due to a lower critical micelle concentration and superior water protection. In summary, the investigation provides valuable insights into the role of surface chemistry on UCNPs at the single‐particle level.