Synthesis of biocompatible uniform NaYF4:Yb3+,Er3+ nanocrystals and their characteristic photoluminescence

For biological application, lanthanide ion doped upconverting nanocrystals should be modified to be biocompatible. Here, we show a viable and efficient procedure for producing biocompatible NaYF₄:Yb³⁺,Er³⁺ upconverting nanocrystals. The uniform NaYF₄:Yb,Er upconverting nanocrystals were firstly synthesized by a mild chemical procedure, which were subsequently coated with a layer of polyethylene-glycol (PEG) to be biocompatible. The photoluminescent intensity of the PEG coated NaYF₄:Yb,Er nanocrystals varies nonlinearly with increasing the thickness of the PEG coating. In particular, it was noted that the Intensity Ratio of Red to Green Emission (IRRGE) of PEG coated NaYF₄:Yb,Er was highly depended on the excitation power density: IRRGE keeping almost constant with increasing the excitation power density below 826 W/cm², but remarkably increasing when increasing the excitation power density above 826 W/cm². For this unique phenomenon, the excitation and emission mechanisms related to PEG coating were discussed.