Thymol-Loaded Biogenic Silica Nanoparticles in an Aquatic Environment : The Impact of Particle Aggregation on Ecotoxicity

Thymol, a monoterpene phenol, is used as a natural biocide. To circumvent its chemical instability, we propose use of thymol-loaded biogenic silica nanoparticles (BSiO₂#THY NPs); however, the toxicity of this system for aquatic organisms is unknown. Thus, the present study aimed to evaluate the toxicogenetic effects induced by thymol, BSiO₂ NP, and BSiO₂#THY on Artemia salina and zebrafish (Danio rerio) early life stages. We also investigated the impact of BSiO₂ aggregation in different exposure media (saline and freshwater). Based on the median lethal concentration at 48 h (LC50_48h), BSiO ₂#THY (LC50_48h = 1.06 mg/L) presented similar toxic potential as thymol (LC50_48h = 1.03 mg/L) for A. salina, showing that BSiO₂ had no influence on BSiO₂#THY toxicity. Because BSiO₂ aggregated and sedimented faster in A. salina aqueous medium than in the other medium, this NP had lower interaction with this microcrustacean. Thus, BSiO₂#THY toxicity for A. salina is probably due to the intrinsic toxicity of thymol. For zebrafish early life stages, BSiO₂#THY (LC50_96h = 13.13 mg/L) was more toxic than free thymol (LC50_96h = 25.60 mg/L); however, BSiO₂ NP has no toxicity for zebrafish early life stages. The lower aggregation of BSiO₂ in the freshwater medium compared to the saline medium may have enhanced thymol's availability for this aquatic organism. Also, BSiO₂#THY significantly induced sublethal effects as thymol, and both were genotoxic for zebrafish. In conclusion, although BSiO₂#THY still needs improvements to ensure its safety for freshwater ecosystems, BSiO₂ NP seems to be a safe nanocarrier for agriculture. Environ Toxicol Chem 2021;40:333–341.