Size-dependent hydroxyl radicals generation induced by SiO2 ultra-fine particles: The role of surface iron

Oxidative stress and hydroxyl radicals (center dot OH) play important roles in adverse health effects caused by inhalable ambient air particles (PM10). The ultra-fine fraction of PM10 has been hypothesized as one of the critical contributors to center dot OH generation. Both in vivo and in vitro studies have shown that ultra-fine particles (UFPs) or nano-particles generate more center dot OH than larger particles with identical mass and composition. Both the surface area and surface-adsorbed redox-active metals have been suggested as factors to determine the oxidative capacity of UFPs. In this study, the center dot OH-generating capability of different sizes of SiO2 particles was investigated in order to determine which factor influences particle-induced center dot OH generation. The amount of center dot OH generated in both acellular and cellular systems was quantified using a capillary electrophoresis method following exposure to SiO2 particles with diameters of 14, 100, and 500 nm. The amount of center dot OH was strongly dependent on particle size, and a significant enhancement was observed only with 14 nm particles. Further studies indicated a close association between center dot OH and iron ion concentration (R (2) = 0.812, p < 0.01). Washed particles, with their surface iron being removed, did not generate center dot OH. The iron-containing leachate from these washings was able to enhance center dot OH production as untreated particles did. Therefore, the presence of adsorbed iron on the surface of the SiO2 particles is presented as a possible mechanism of UPFs-induced center dot OH generation. The SiO2 acted as an inert substrate, and the surface of ultra-fine SiO2 particles acted as a carrier for iron.