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Upconverting materials, which can be efficiently excited by near infrared light and emit strong visible light through a process termed'upconversion fluorescence', have shown great potential for use in biological labelling and imaging. Some upconverting nanoparticles such as NaYF 4 doped with lanthanide ions have been synthesized; however, these nanoparticles are not soluble in water, not biocompatible and do not have functional chemical groups for conjugation of biomolecules, and as a result their bioapplications are very limited unless some surface modifications are performed. Here we report a method for one-pot synthesis of polyethylenimine/NaYF 4 nanoparticles doped with lanthanide ions, which are water soluble and biocompatible. The amino groups of polyethylenimine existing on the nanoparticles can be used for attachment of biomolecules. The nanoparticles showed a spherical shape with an …