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There is a great interest for the synthesis of rare earth (RE) doped up-converting nanoparticles (UCNPs) which morphological and luminescence properties are well suited for application in optoelectronics, forensics, security and biomedicine. The synthesis of such particles usually comprises decomposition of organometallic compounds in an oxygen-free environment followed with coating of biocompatible layer or ligands exchange. In this work hydroxyl-carboxyl (HO-C) type of chelators (citric acid and sodium citrate) are used for the stabilization of NaYF4:Gd,Yb,Er UCNPs during solvothermal treatment of rare earth nitrate salts with different fluoride sources (NaF, NH4F and NH4HF2). The x-ray powder diffraction (XRPD) showed that all powders contain the mixture of cubic and hexagonal NaYF4:Gd,Yb,Er phase in nano and micro-sized particles respectively. However, the content of later one prevails in samples obtain when Nacitrate is used as chelator, regardless of which fluoride source is used for precipitation. Additionally, variation of the particles size and shape is detected with a variation of fluoride type. All particles have hydrophilic surface due to retention of citrate ligands and emit intense green light emission centered at 519 and 539 nm (2H11/2,4S3/2→4I15/2) when excited with near infrared light.

NaYF4 doped with Rare Earth elements such as Yb and Er is well known as a compound with the remarkable up-conversion photoluminesce and wide application. In this work, the influence of additional co-doping of NaYF4:Yb,Er with gadolinium (0, 15 and 30 mol%) was explored. The syntheses were performed solvothermally using the mixture of water/ethanol as medium and polyvinylpyrrolidone (PVP) as capping ligand. The obtained powders were subjected to X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy and photoluminesce (PL) analyses. Structural refinement shown that NaYF4:Yb,Er particles generally crystallized in a cubic form (Fm-3m), while the additional presence of Gd3+ in the crystal lattice lead to the pure hexagonal phase crystallization (P63/m). Morphological analyses revealed that all powders are composed from nanodimensional particles with the size in the range of 50-70 nm, while FTIR spectra confirmed the presence of PVP functional groups on the particles surfaces. The strongest effect of Gd3+ doping was notices in the PL spectra, where the up-conversion response increases with the mol% of this dopant.