Liang Zhao, Artem Kutikov, Jie Shen, Chunying Duan, Jie Song and Gang Han . Stem Cell Labeling using Polyethylenimine Conjugated (a-NaYbF4:Tm3+)/CaF2 Upconversion Nanoparticles. Theranostics (featured on cover). 2013; 4(3):249-257.