2,7-Dichlorodihydrofluorescein diacetate (DCFH-DA) (Sigma-Aldrich, USA) was added to 50 mL LB media as a 10mM stock in DMSO, and incubated at 37°C under constant orbital shaking at 220 rpm for 20 min, after which the cells were washed twice with PBS and dispersed in PBS solutions containing 0, 1, 10, 100, or 1000 μg/mL of MSNs. Subsequently, the PBS solutions containing the cells and the MSNs were incubated at 37°C under shaking at 220 rpm for another 4 h, after which the PBS supernatants were transferred into the wells of a 96-well plate and evaluated on a fluorescent spectrophotometer (SpectraMax M3, Molecular Devices, USA) using an excitation wavelength of 488 nm and an emission wavelength of 525 nm. The results are expressed as the means ± SD of six parallel measurements.