The luminescent rare-earth nanomaterials have great potential in the biological and biomedical applications due to their unique optical characteristics. An infrared-to-visible up-conversion NaYF4: Yb3+, Er3+ nanoparticle was synthesized by hydrothermal method. More intense green up-conversion luminescence was obtained due to using citrate as a chelator. Different sizes of up-conversion particles (UCNPs) in the range of 40 to 1000 nm were available by adjusting the molar ratio of fluoride to Ln3+. The UCNPs were coated by silica and functionalized with amino groups for a conjugation with biomolecules. The UCNPs prepared might be promising for bioassay, bioimaging and diagnostics, especially in vivo environment and living cells.