NIR-II Upconversion Photoluminescence of Er³⁺ Doped LiYF₄ and NaY(Gd)F₄ Core-Shell Nanoparticles

The availability of colloidal nano-materials with high efficiency, stability, and non-toxicity in the near infrared-II range is beneficial for biological diagnosis and therapy. Rare earth doped nanoparticles are ideal luminescent agents for bio-applications in the near infrared-II range due to the abundant energy level distribution. Among them, both excitation and emission range of Er³⁺ ions can be tuned into second biological window range. Herein, we report the synthesis of ∼15 nm LiYF₄, NaYF₄, and NaGdF₄ nanoparticles doped with Er³⁺ ions and their core-shell structures. The luminescent properties are compared, showing that Er³⁺ ions with single-doped LiYF₄ and NaYF₄ nanoparticles generate stronger luminescence than Er³⁺ ions with doped NaGdF₄, despite the difference in relative intensity at different regions. By epitaxial growth an inert homogeneous protective layer, the surface luminescence of the core-shell structure is further enhanced by about 5.1 times, 6.5 times, and 167.7 times for LiYF₄, NaYF₄, and NaGdF₄, respectively. The excellent luminescence in both visible and NIR range of these core-shell nanoparticles makes them potential candidate for bio-applications.