In-situ and ex-situ EPS-corona formation on ZnO QDs mitigates their environmental toxicity in the freshwater microalgae Chlorella sp

The current work seeks to understand how the interactions between ZnO QDs and extracellular polymeric substances (EPS) may vary based on the types of EPS (loosely and tightly bound) and modes of eco-corona formation (In-situ or ex-situ). In-situ eco-corona refers to formation of an EPS layer on the QDs during the interactions with the algae whereas the ex-situ condition refers to forming the layer before the interactions. ZnO QDs were added at 0.25, 0.5, and 1.0 mg/L concentrations for pristine, in-situ, and ex-situ corona treatments with the cells. Pristine ZnO QDs induced significant oxidative stress in algal cells, as evident from increased levels of reactive oxygen species (ROS), malondialdehyde (MDA), superoxide dismutase (SOD), and catalase activity. This decreased the photosynthetic efficiency and caused significant growth inhibition in algae. In contrast, both the in-situ and ex-situ corona treatments with loosely bound and tightly bound EPS reduced the oxidative stress, improved the photosynthetic efficiency, and diminished growth inhibition effects. This study asserts the importance of EPS in reducing the toxicity of ZnO QDs, while maintaining the fluorescence activity. This ensures the sustainable usage of the ZnO QDs without any harm to aquatic ecosystems.