Enhanced upconversion luminescence in NaGdF4:Yb,Er nanocrystals by Fe3+ doping and their application in bioimaging

The visible green and red upconversion emissions in Er3+/Yb3+ doped beta-NaGdF4 nanoparticles were enhanced by tridoping with Fe3+ ions (0-40 mol%). XRD, XPS, ICP-AES and EDS data demonstrated successful incorporation of Fe3+ ions in NaGdF4:Yb3+/Er3+ nanoparticles. The effect of Fe3+ tridoping on the upconversion luminescence in NaGdF4:Yb3+/Er3+ NPs was investigated in detail. The green and red emission intensities were enhanced by 34 and 30 times, respectively. The maximum emission was observed in a sample containing 30 mol% Fe3+ ions. A possible mechanism for the enhanced upconversion emission is proposed. In addition, a layer of silica was coated onto the surface of UCNPs to improve the biocompatibility. Folic acid was covalently linked to the silica coated UCNPs to form UCNP@SiO2-FA nanoprobes, which have been successfully applied to the fluorescent imaging HeLa cells.