Water-Soluble NaYF4:Yb,Er(Tm)/NaYF4/Polymer Core/Shell/Shell Nanoparticles with Significant Enhancement of Upconversion Fluorescence

Hexagonal phase nanoparticles of NaYF4:Yb,Er and NaYF4:Yb,Tm (∼8.5 ± 0.8 nm) were synthesized. Significant enhancements of upconversion fluorescence were observed after coating with an undoped NaYF4 shell (core/shell ∼ 11.1 ± 1.5 nm). The hydrophobic core/shell nanoparticles were rendered hydrophilic by a subsequent amphiphilic coating of PAA. These nanostructures have great potentials as bio-probes.