A general synthesis method for small-size and water-soluble NaYF4:Yb, Ln upconversion nanoparticles at high temperature†

In this work, we developed a general synthesis method for the water-soluble lanthanide ion-doped NaYF₄ upconversion (UC) nanoparticles (NPs) using triethylene glycol (TEG) as a high temperature solvent and diacid as a surfactant. Since the boiling point of the TEG is as high as 289.4 °C, the synthesis temperature can be correspondingly increased to a higher temperature that is a bit lower than this one. Therefore, water-soluble UCNPs with a small size (<50 nm) can be easily prepared. The nanocrystal growth temperature could be elevated from 180 °C to 285 °C due to the superiority of the high-temperature polar solvent environment introduced in this work. The temperature-dependent nanocrystal growth mechanism and luminescent properties of UCNPs are deeply explored. Different chain length diacids, e.g., three-carbon chain length propanedioic acid (PDA), six-carbon chain length hexanedioic acid (HDA) and nine-carbon chain length azelaic acid (AA), were used in this work to prepare the water-soluble UCNPs in polar solution, and we finally found that the fluorescent intensity and water-stability are inversely proportional to the carbon chain length of the ligand. PDA was proved to be an optimum surfactant to prepare the most stable water-soluble UCNPs. As a result, the water-soluble UCNPs we prepared can also be successfully applied in the upconversion luminescent cell imaging. This study opens up new avenues for the synthesis of water-soluble UCNPs with small sizes and provides more opportunities for their applications in fields such as biological imaging and biological detection.