Dopant-Controlled Synthesis of Water-Soluble Hexagonal NaYF₄ Nanorods with Efficient Upconversion Fluorescence for Multicolor Bioimaging

A novel strategy is proposed to directly synthesize water-soluble hexagonal NaYF₄ nanorods by doping rare-earth ions with large ionic radius (such as La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, and Gd³⁺), and the dopant-controlled growth mechanism is studied. Based on the doping effect, we fabricated water-soluble hexagonal NaYF₄: (Yb, Er)/La and NaYF₄: (Yb, Er)/Ce nanorods, which exhibited much brighter upconversion fluorescence than the corresponding cubic forms. The sizes of the nanorods can be adjusted over a broad range by changing the dopant concentration and reaction time. Furthermore, we successfully demonstrated a novel depth-sensitive multicolor bioimaging for in vivo use by employing the as-synthesized NaYF₄: (Yb, Er)/La nanorods as probes.