Optimisation of Amphiphilic-Polymer Coatings for Improved Chemical Stability of NaYF4-based Upconverting Nanoparticles

NaYF4 nanoparticles codoped with Yb3+ and Tm3+ exhibit upconversion fluorescence in near-infrared and visible spectral range. Consequently, such upconverting nanoparticles (UCNPs) can be used as contrast agents in medical diagnostics and bioassays. However, they are not chemically stable in aqueous dispersions, especially in phosphate solutions. Protective amphiphilic-polymer coatings based on poly(maleic anhydride-alt-octadec-1-ene) (PMAO) and bis(hexamethylene)triamine (BHMT) were optimised to improve the chemical stability of UCNPs under simulated physiological conditions. Morphologies of the bare and coated UCNPs was inspected with transmission electron microscopy. All samples showed intense UC fluorescence at ~800 nm, typical for Tm3+. The colloidal stability of aqueous dispersions of bare and coated UCNPs was assessed by dynamic light scattering and measurements of zeta potential. The dissolution of UCNP in phosphate-buffered saline at 37 °C, was assessed potentiometrically by measuring the concentration of the dissolved fluoride. Protection against the dissolution of UCNPs was achieved by PMAO and PMAO crosslinked with BHMT.