Water-soluble NaYF4:Yb,Er(Tm)/NaYF4/polymer core/shell/shell nanoparticles with significant enhancement of upconversion fluorescence

Abstract

Hexagonal bulk nanoparticles of NaYF4:Yb,Er(Tm) were prepared by solid-state or hydrothermal reaction. These hexagonal phase NaYF 4:Yb,Er(Tm) nanoparticles of small size and narrow size distribution were hydrophobic with lower fluorescence compared to the bulk counterparts. These hydrophobic C/S nanoparticles rendered hydrophobic by a layer of amphiphilic polymer. The polymer coated C/S nanoparticles synthesized contain an amphiphilic layer, 25% octylamine, and 40% isopropylamine modified poly(acrylic acid) (PAA). The organic surfactant with high vibrational energy on the nanoparticle surface quench fluorescence. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of NaYF4:Yb,Er(Tm) confirm single crystallinity. The results show major upconversion fluorescence enhancements of 7.4 times from NaYF4:Yb, Er C/S and 29.6 times from NaYF4:Yb, Tm C/S.