Optimal concentration and incubation time of silica nanoparticles for labeling of MSCs. UCBMSCs were incubated for more than 4 h with 200 or 400 μg/ml silica nanoparticles, which yielded visible fluorescent signals. However, no effective fluorescence occurred at 100 μg/ml. The majority of cells showed cytoplasmic fluorescence after 4 h of incubation. Magnification, ×400.