Chemical and colloidal stability in complex aqueous media are among the main challenges preventing nanoparticles from successfully entering into the biomedical field. Small core-shell upconversion nanoparticles (UCNPs) NaYF4:Yb,Er@NaYF4 of 12 nm in diameter with a high surface-to-volume ratio are utilized to demonstrate that self-assembling phospholipid bilayers (PLMs) have several benefits compared to common ligand-exchange and ligand-addition particle coatings such as poly(acrylic acid) and amphiphilic polymers. An efficient hydrophobic barrier against water quenching and toward particle disintegration is formed by PLM. Particles with this functionalization have a higher upconversion luminescence in aqueous media in contrast to common surface ligands. They attract with better colloidal stability in phosphate buffer, in a wide pH range, in high ionic solutions, and in complex cell media, as is required for biological applications. Moreover, kidney cells (NRK) are not affected by these stable PLM-coated UCNPs as first cell viability tests reveal.
Keywords: Luminescence; nanoparticle; phospholipids; surface chemistry; upconversion.