It is well known that ROS are generated on the surface of nano-materials, which further induces the cells to generate more ROS . ROS generation is a cumulative process, whereby only reaching a certain value of damage can cause significant permanent intracellular oxidative damage. We further investigated the influence of MSNs on intracellular ROS after exposing E. coli to different doses of MSNs. As shown in Fig 4B, when the concentrations of MSNs were in the range of 1–100 μg/mL, the levels of ROS in E. coli increased only slightly. On the other hand, MSN concentrations in the range of 100–1000 μg/mL caused the generation of significant amounts of ROS. These results indicated that MSNs, just like other nanomaterials such as fullerene, graphene and its derivatives, could cause an increase of intracellular ROS, which might finally cause damage to the cells [42–44]. Nel et al. did a considerable amount of work on the toxicity of nanomaterials and found that the generation of ROS might be associated with surface defects, electron—hole pair generation, chemical dissolution and release of toxic metal ions from nanomaterials . Judging by the SEM images (Fig 1D), we also found that the MSNs possessed surface defects, and we thus speculated that MSNs could catalyze ROS formation due to their discrete crystal planes and surface defects, which can result in the production of active electrons on their surface. The next reaction step might be that the excited electrons turned oxygen molecules into superoxide anions (O2.-), which eventually generated further ROS via disproportionation .