Impact of particle size of zinc oxide nanoparticles on its bioaccumulation and oxidative stress responses

Abstract

<p indent=0mm>The extensive production and use of zinc oxide nanoparticles (ZnO NPs) could inevitably increase their release into aquatic environment, presenting potential hazards to aquatic organisms. It is necessary to understand the bioaccumulation and aquatic toxicity of ZnO NPs for assessing their ecological risks. One major toxicity mechanism for ZnO NPs is oxidative stress. Previous studies have demonstrated that particle size is one of the most important factors determining aquatic toxicity of ZnO NPs, but the effects of particle size of ZnO NPs on their bioaccumulation and induction of oxidative stress responses are still unclear. The aim of this study was to investigate the impact of different particle sizes of ZnO NPs on their bioaccumulation and induction of oxidative stress biomarkers changes in <italic>Daphnia magna</italic>. Three types of ZnO NPs, having different primary particle sizes (30, 50 and <sc>90 nm),</sc> were exposed to <italic>Daphnia magna</italic> for <sc>24 h,</sc> afterward Zn accumulation and four oxidative stress biomarkers in <italic>Daphnia magna</italic> including total superoxide dismutase (T-SOD), catalase (CAT), glutathione (GSH) and malondialdehyde (MDA) were measured. Meanwhile, the differences of oxidative damage in <italic>Daphnia magna </italic>induced by 30, 50 and <sc>90 nm</sc> ZnO NPs at unit internal exposure dose (per unit Zn accumulation) were compared. MDA is a good general indicator of lipid peroxidation and its accumulation can reflect the degree of oxidative damage in <italic>Daphnia magna</italic>. The results showed that the percentage of immobilisation of <italic>Daphnia magna </italic>exposed to <sc>90 nm</sc> ZnO was higher than those exposed to 30 and <sc>50 nm</sc> ZnO after <sc>24 h</sc> of exposure. Compared to <sc>30 nm</sc> ZnO treated groups, the accumulated Zn in <italic>Daphnia magna </italic>exposed to 50 and <sc>90 nm</sc> ZnO immediately increased in the first exposure period (within the first <sc>6 h).</sc> After <sc>6 h</sc> of exposure, the Zn accumulation in <italic>Daphnia magna</italic> of <sc>90 nm</sc> ZnO was significantly higher than that of <sc>50 nm</sc> ZnO. The results indicated that <sc>90 nm</sc> ZnO could enter and accumulate in <italic>Daphnia magna</italic> more easily than 30 and <sc>50 nm</sc> ZnO. The oxidative stress biomarkers measurements demonstrated that the MDA concentrations per unit Zn accumulation in <italic>Daphnia magna</italic> for <sc>90 nm</sc> ZnO were significantly higher than those for 30 and <sc>50 nm</sc> ZnO. The results indicated that the degree of oxidative damage in <italic>Daphnia magna </italic>induced by <sc>90 nm</sc> ZnO was more serious than 30 and <sc>50 nm</sc> ZnO treated groups, which may be related to the fact that the hydrodynamic diameters of <sc>90 nm</sc> ZnO were significantly smaller than those of 30 and <sc>50 nm</sc> ZnO. In addition, the level of T-SOD activity, CAT and GSH in <italic>Daphnia magna</italic> induced by <sc>90 nm</sc> ZnO changed significantly at most time points. In summary, our findings showed that <sc>90 nm</sc> ZnO with smaller hydrodynamic diameter was more likely to enter and accumulate in <italic>Daphnia magna</italic>, and resulted in more serious oxidative stress in <italic>Daphnia magna</italic>. These results suggested that we should not only consider primary particle sizes of nanoparticles, but also pay more attention to their hydrodynamic diameters in aquatic environmental systems for assessing the environmental risks of nanoparticles.