Ecotoxicity and environmental safety assessment of two-dimensional niobium carbides (MXenes)

Two-dimensional (2D) MXenes have gained great interest in water treatment, biomedical, and environmental applications. The antimicrobial activity and cell toxicity of several MXenes including Nb₄C₃T_x and Nb₂CT_x have already been explored. However, potential side effects related to Nb-MXene toxicity, especially on aquatic pneuma, have rarely been studied. Using zebrafish embryos, we investigated and compared the potential acute toxicity between two forms of Nb-MXene: the multilayer (ML-Nb₄C₃T_x, ML-Nb₂CT_x) and the delaminated (DL-Nb₂CT_x, and DL-Nb₄C₃Tx) Nb-MXene. The LC₅₀ of ML-Nb₄C₃T_x, ML-Nb₂CT_x, DL-Nb₂CT_x, and DL-Nb₄C₃T_x were estimated to be 220, 215, 225, and 128 mg/L, respectively. Although DL-Nb₂CT_x, and DL-Nb₄C₃Tx derivatives have similar sizes, DL-Nb₄C₃T_x not only shows the higher mortality (LC₅₀ = 128 mg/L Vs 225 mg/L), but also the highest teratogenic effect (NOEC = 100 mg/L Vs 200 mg/L). LDH release assay suggested more cell membrane damage and a higher superoxide anion production in DL-Nb₄C₃T_x than DL-Nb₂CT_x,. Interestingly, both DL-Nb-MXene nanosheets showed insignificant cardiac, hepatic, or behavioral toxic effects compared to the negative control. Embryos treated with the NOEC of DL-Nb₂CT_x presented hyperlocomotion, while embryos treated with the NOEC of DL-Nb₄C₃T_x presented hyperlocomotion, suggesting developmental neurotoxic effect and muscle impairment induced by both DL-Nb-MXene. According to the Fish and Wildlife Service (FSW) Acute Toxicity Rating Scale, all tested Nb-MXene nanosheets were classified as "Practically not toxic". However, DL-Nb₄C₃Tx should be treated with caution as it might cause a neurotoxic effect on fauna when it ends up in wastewater in high concentrations.