..and as the first paper says, ZnO nanoparticles are NOT toxic if silica coated, so devil is in the details here. It should also be noted that no in vivo toxicity has been demonstrated from non-coated nanoparticles, although there's a theoretical potential for it.

In the second study ZnO nanoparticles were injected DIRECTLY INTO THE BLOOD of rats with no control for non-nano particles. Crazy they even got it published.