Extracellular polymeric substances altered the physicochemical properties of molybdenum disulfide nanomaterials to mitigate its toxicity to Chlorella vulgaris

Although the toxic effects of two-dimensional nanomaterials (2D-NMs) have been widely reported, the influence of extracellular polymeric substances (EPS) on the environmental fate and risk of 2D-NMs in aquatic environments is largely unknown, and the processes and mechanisms involved remain to be revealed. Herein, we investigated the impact of EPS secreted by microalgae (Chlorella vulgaris (C. vulgaris)) on the environmental transformation and risk of molybdenum disulfide (MoS₂). We found that the attachment of EPS increased the thickness of MoS₂ (from 2 nm to 5 nm), changed it from a monolayer sheet to a fuzzy multilayer structure, and promoted the formation of defects on MoS₂. The blue-shift of the peak associated with the plasmon resonances in the 1 T phase and the generation of electron-hole pairs suggested that EPS altered the surface electronic structure of MoS₂. EPS interacted mainly with the S atoms on the 1 T phase, and the attachment of EPS promoted the oxidation of MoS₂. The reduction in hydrodynamic diameter (D_h) and the decrease in zeta potential indicated that EPS inhibited the agglomeration behavior of MoS₂ and enhanced its dispersion and stability in aqueous media. Notably, EPS reduced the generation of free radicals (superoxide anion (•O₂⁻), singlet oxygen (¹O₂), and hydroxyl radicals (•OH⁻)). Furthermore, EPS mitigated the toxicity of MoS₂ to C. vulgaris, such as attenuated reduction in biomass and chlorophyll content. Compared to pristine MoS₂, MoS₂ + BG11 + EPS exhibited weaker oxidative stress, membrane damage and lipid peroxidation. The adsorption of EPS on MoS₂ surface reduced the attachment sites of MoS₂, making MoS₂ less likely to be enriched on the cell surface. The findings have significant contribution for understanding the interactions between EPS and MoS₂ in aquatic ecosystems, providing scientific guidance for risk assessment of 2D-NMs.