Hydrothermal Synthesis of NaYF4:Yb,Er Upconversion Nanoparticles and Modification of Their Surfaces for Biosensing

NaYF4:Yb,Er upconversion nanoparticles are obtained via hydrothermal synthesis. To give nanoparticles hydrophilic properties without major changes in their photophysical characteristics, their surfaces have been modified by replacing the oleate shell with L-cysteine molecules. No fading of the luminescence of the modified upconversion nanoparticles is observed for months.