Synthesis of Stable Block-Copolymer-Protected NaYF4:Yb3+, Er3+ Up-Converting Phosphor Nanoparticles

Abstract

Near-infrared-to-visible up-conversion of light by rare earth ion-doped nanophosphors (NaYF4:Yb3+,Er3+) opens new possibilities for improved biolabeling. A major obstacle to applications of up-converting nano phosphors (UCNPs) has been obtaining samples stable in serum media for biological applications. Previous researchers have made UCNPs stable in DI water, but not serum media. In this study, hexagonal phase nanophosphors were prepared using one-step cothermolysis utilizing oleic acid (OA) and trioctyl phosphine (TOP) ligands. Two routes to polymer surface modification of the UCNPs were studied: direct ligand exchange using poly(acrylic acid) (PAA) and amphiphilc copolymer encapsulation via flash nano-precipitation (FNP). FNP-coated UCNPs were produced using three block-copolymers: poly(ethylene glycol)-block-poly(caprolactone) (PEG-b-PCL), poly(ethylene glycol)-block-poly(lactic-coglycolic acid) (PEG-b-PLGA), and poly((ethylene glycol)-block-lactic acid) (PEG-b-PLA). Both surface modification routes produced colloidally stable UCNP dispersions in DI water. However, for the first time, we report the successful preparation of colloidal UCNPs stable in buffers and serum media (Leibovitz L-15 media with added fetal bovine serum) using FNP and PEG surface coatings. The stabilizing block-copolymer layer added ca. 15 nm to the diameter of the phosphor crystals. UCNPs assembly of amorphous PLA or PLGA is strikingly different than for crystallizable PCL. These polymer-modified UCNPs provide promising new materials for applications in bioimaging and photodynamic therapy.