Preparation of bi-functional NaGdF4-based upconversion nanocrystals and fine-tuning of emission colors of the nanocrystals by doping with Mn2+

Synthesis of fluorescent and magnetic bi-functional NaGdF4-based upconversion nanocrystals and a study of the effect of doping Mn2+ on the fluorescent and magnetic properties of the nanocrystals have been carried out. It is found that the desired bi-functional nanocrystals can be synthesized successfully by a simple thermal decomposition method and the doping by Mn2+ has a large effect on the properties of the nanocrystals. The doping by Mn2+ can fine-tune the emission of the nanocrystals. When Mn2+ is doped in the nanocrystal NaGdF4:Yb, Er, the red upconversion emission of the nanocrystals increases and green upconversion emission decreases or disappears. This makes the nanocrystal good for bioimaging since pure red upconversion emission can penetrate tissue deeply with high signal/noise ratio. Moreover, the doping with Mn2+ can enable the nanocrystals to exhibit superparamagnetic properties. The HeLa cell imaging experiments reveal that the nanocrystals used as luminescent probes to label the cancer cells show strong upconversion luminescence. These results suggest that the developed nanocrystals, which have strong pure red upconversion emission and superparamagnetic properties, could serve as an upconversion luminescence and magnetic resonance (MR) probe for dual-modality bioimaging.