Unexpected luminescence enhancement of upconverting nanocrystals by cation exchange with well retained small particle size

Highly luminescent upconversion nanoparticles (UCNPs) with small sizes are highly desirable for bioapplications. A facile in situ cation exchange strategy has been developed to greatly enhance the UC luminescence of hexagonal phase NaYF₄ NPs while maintaining their small particle size and shape. Via a cation exchange treatment by hot-injecting Gd³⁺ precursors into the as-prepared NPs solution without pre-separation, the naked-eye visible UC emission of the NPs was enhanced about 29 times under 980 nm near infrared (NIR) excitation with unchanged particle size. The cation exchange process was further demonstrated for the case of NaYF₄ nanorods (NRs). After the cation exchange, the nanorod was broken into two NPs with stronger emission. The cation exchanged hydrophobic UCNPs were further encapsulated with poly(amino acid) and successfully applied for targeted cancer cell UC luminescence imaging.