Bioimaging probes, which can emit near-infrared (NIR) light and are excitable with NIR light, are promising for deep tissue imaging in vivo. Lanthanide-doped phosphors, such as Y2O3:Tm,Yb; Y2O3:Ho,Yb; and Y2O3:Er,Yb, are candidates for the probes. We enhanced the NIR emission of the three kinds of lanthanide-doped phosphor by using Li-ion doping. The probes were applied to cellular imaging.