Decoration of up-converting NaYF4:Yb,Er(Tm) nanoparticles with surfactant bilayer. A versatile strategy to perform oil-to-water phase transfer and subsequently surface silication

Up-conversion rare-earth nanoparticles (UCNPs) are potential alternatives to down-conversion fluorescent quantum dots for biolabeling, and the prerequisite for biological applications is the enhancement of surface hydrophilicity and physiological stability of UCNPs. In this work, we demonstrated a facile method to modulate the surface functionality of original hydrophobic UCNPs by forming a surfactant bilayer, which made UCNPs dispersible in water. After oil-to-water phase transfer, more than 30% up-conversion luminescence was preserved. It was found that the key to avoiding the aggregation of bilayer-modified UCNPs was the concentration of surfactant. High concentration of surfactant led to bilayer-modified individual UCNPs, whereas insufficient surfactant generated the water-dispersible aggregates of UCNPs, namely UCNP superstructures. Various commercially available surfactants, such as cationic cetyltrimethylammonium bromide, anionic sodium dodecyl sulfate, and nonionic polyethylene glycol tert-octylphenyl ether, were practical, making the current method versatile for anchoring functional groups on UCNPs. Moreover, bilayer-modified UCNPs could act as a platform to perform surface silication, thus further improving the biocompatibility and stability of UCNPs.