Methylcellulose coating markedly reduces tissue bioaccumulation of Fe3O4 nanoparticles in the green toad (Bufotes sitibundus).

The increasing environmental release of iron oxide (Fe3O4) nanoparticles has raised concerns regarding their bioaccumulation and potential ecological impacts in aquatic and semi-aquatic ecosystems. In this study, we investigated whether surface modification with methylcellulose (MC) can reduce the tissue bioaccumulation of Fe3O4 nanoparticles in the green toad (Bufotes sitibundus). Uncoated Fe3O4 nanoparticles were synthesized via a co-precipitation method, and MC-coated Fe3O4 nanocomposite was prepared. Physicochemical characterization using dynamic light scattering and atomic force microscopy showed an increase in hydrodynamic diameter from approximately 80.6 nm for uncoated Fe3O4 to 158 nm for MC/Fe3O4, indicating improved colloidal stability following surface modification. Adult B. sitibundus specimens (n = 135) were exposed for 28 days under waterborne conditions to uncoated or MC-coated Fe3O4 nanoparticles at nominal concentrations of 400, 600, 800, and 1000 mg L-1. Iron concentrations in liver and muscle tissues were quantified using ICP-OES. Exposure to uncoated Fe3O4 resulted in significant, concentration-dependent increases in tissue iron levels compared to controls, with the highest accumulation observed in the liver (11,541 ± 910 mg kg-1 dry weight at 1000 mg L-1). In contrast, MC coating markedly reduced tissue bioaccumulation, with liver iron concentrations decreasing to 2819 ± 450 mg·kg-1 at the same exposure level, corresponding to an approximate 75% reduction. Regression analyses revealed strong dose-body burden relationships (R² = 0.92 for liver and R²= 0.85 for muscle), and accumulation was consistently higher in liver than in muscle. Although the nanoparticle concentrations used in this study exceed those typically reported in current environmental settings, our findings demonstrate that methylcellulose coating substantially lowers the bioavailability and tissue accumulation of Fe3O4 nanoparticles in an amphibian model under controlled exposure conditions. These results highlight the importance of surface modification as a safer-by-design strategy for reducing the hazard potential of engineered nanomaterials. From a methodological perspective, this study contributes to improved hazard characterization frameworks for nanoparticle risk assessment.