Polyethylenimine (PEI)-DNA nanoparticles are used to invoke specific therapeutic response in dysfunctional cells. The efficacy of PEI gene delivery is partly attributed its buffering capacity, which enables endosomal/lysosomal escape based on the hypothesized “proton sponge effect”. However, hypothesis highly debated, and mechanistic picture endosomal remains unclear. In our previous work, prep...