There is a great technological interest in synthesis of lanthanide doped upconverting nanoparticles (UCNPs) with controlled crystal phase, morphology and intense luminescence properties suitable for biomedical use. A conventional approach for synthesis of such particles comprises decomposition of organometallic compounds in an oxygen-free environment, followed either with a ligand exchange, or biocompatible layer coating. Biocompatible NaY0.65Gd0.15F4:Yb,Er nanoparticles used in this study were synthesized through chitosan assisted one-pot hydrothermal synthesis and were characterized by X-ray powder diffraction (XRPD), Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS) and photoluminescence measurement (PL). Due to the presence of the amino groups at their surface, excellent biocompatibility and notably low cytotoxicity against MRC-5 cells (line of normal human fibroblasts) and A549 cells (human lung cancer cells) were detected using MTT assay. Furthermore, upon 980 nm laser irradiation, particles were successfully used in vitro for labeling of both, MRC-5 and A549 cells
