Increasing the carbon chain length of imidazolium ionic liquids impacts their toxicity on daphnids

Ionic liquids emerged as promising environmentally friendly alternatives to volatile organic compounds offering reduced volatility and enhanced stability. However, their unavoidable introduction into natural environments led to ecological harm particularly to aquatic species. To address this effect-based methods are crucial for the early detection of environmental pollutants and mechanistic understanding of their actions. In this study, three methylimidazolium ionic liquids with varying carbon chain length (1-ethyl-3, 1‑butyl‑3 and 1-hexyl-3) were assessed on their impact on daphnids as a key model organism in ecotoxicology. Combining methods such activities of key enzymes and phenotypic endpoints such as feeding and mortality, revealed notable changes highlighting the sensitivity of these organisms to ionic liquids. The longer chain length resulted in higher mortality; however, this was not reflected in ingestion rates in neonates which was decreased by 1-ethyl-3-methylimidazolium. In addition, activity of peptidase was decreased across all ionic liquids and acid phosphatase was increased only in 1-ethyl-3-methylimidazolium and 1-hexyl-3-methylimidazolium. Finally, glutathione-S-transferase was significantly increased in 1-hexyl-3-methylimidazolium. This study demonstrates that increasing the carbon chain length of the IL results in the most significant changes in enzyme activity. Overall, the integration of daphnid-based assays provides valuable insights into the toxicological effects and environmental risks associated with emerging pollutants such as ionic liquids. This approach underscores the importance of employing advanced methodologies for effective environmental monitoring and protection of aquatic ecosystems.