Synergistic Oxidative Stress of Surface Silanol and Hydroxyl Radical of Crystal and Amorphous Silica in A549 Cells

The surface silanol groups and the induced hydroxyl radical (·OH) by two silica samples (Quartz and Nano-SiO₂) were studied. The Fourier transform infrared spectroscopy (FTIR) and potentiometric titration were adopted to qualitatively and quantitatively determine the surface silanol groups and the surface site density of Quartz and Nano-SiO₂. Electron paramagnetic resonance (ESR) spectroscopy and fluorescence spectra were used to measure the ·OH generated by the silica particles. The confocal fluorescence microscopy and flow cytometry were adopted to evaluate the intracellular reactive oxygen species (ROS). Additionally, superoxide dismutase (SOD), glutathione (GSH), and cell viability were tested for the assessment of oxidative stress in A549 cells induced by the surface silanol and ·OH of silica particles. The result showed that only the amorphous Nano-SiO₂ exhibited the bending of Si–OH at 953 cm⁻¹ in the FTIR spectra. The surface Si–OH site density of Quartz and Nano-SiO² were 0.31 and 1.14 sites/nm₂. The ·OH could form from the surface-catalyzed homolytic cleavage of H₂O₂ by silanol and the Si–O· dangling bonds, of which was remarkable for the crystal quartz particle. The Quartz with more ·OH production was less toxic to the A549 cells, while the dramatic increase of the intracellular ROS, depletion of GSH, and high cell mortality were observed in the A549 cells after the exposure to Nano-SiO₂. The high concentration of surface silanol contributions to the toxic effects in A549 cells more than the ·OH for Nano-SiO₂. Nonspecific for silica particles, both the surface silanol and ·OH formed on the surface effects the viability of A549 cells. Moreover, the surface silanol and the generated ·OH of silica particles exerted synergistic effect on the oxidative stress in A549 cells.