Synthesis of Yb/Er co-doped MnF2 nanocrystals with bright red up-converted fluorescence

MnF2 nanocrystals co-doped with Yb3+/Er3+ ions were synthesized via a polyethyleneimine (PEI)-based method. These nanocrystals exhibited unique red up-converted fluorescence at 660 nm under excitation at 970 nm, because Mn2+ ions in the host efficiently induced an energy transfer (4S3/2- > 4F9/2) of Er3+. Considering that the light of wavelength 600-1100 nm has a high penetration depth in tissues, such PEI-coated up-converted nanocrystals have the potential to be good candidates for deep tissue imaging.