Functional Modification of Ferroferric Oxide Nanoparticle Regulates the Uptake, Oxidative Stress, Tissue Damage, and Metabolic Profiles in <i>Eisenia fetida</i>

Ferroferric oxide nanoparticles (Fe₃O₄ NPs) are widely utilized as nanoenabled agrochemicals and soil remediation agents, with functional modification significantly enhancing their stability and biocompatibility. However, excessive use of Fe₃O₄ NPs may pose unassessed ecological risks in soils, particularly concerning the regulatory role of two most common surface modifiers as polyvinylpyrrolidone (PVP) and citric acid (CA) which influence the interactions of NPs with soil organisms and potential toxicity. This study evaluated the nanotoxic effects of bare Fe₃O₄ NPs (B-Fe₃O₄ NPs), CA-Fe₃O₄ NPs, and PVP-Fe₃O₄ NPs on <i>Eisenia fetida</i> in soil ecosystems. After 7 days of exposure, the B-, CA- and PVP-Fe₃O₄ NPs decreased the weight of the earthworms, caused oxidative stress and tissue damage. Functional Fe₃O₄ NPs showed increased accumulation in earthworms while alleviating oxidative stress and homeostatic imbalance by accelerating the activation of related enzymes. Moreover, hyperspectral and pathological observations indicated that CA and PVP modifications effectively alleviated tissue damage caused by Fe₃O₄ NPs via an improvement in NP biocompatibility, dispersion and stability evidenced by the levels of inositol metabolites, which has been upregulated more significantly by B-Fe₃O₄ NPs. Significant metabolic disturbances were observed, indicating that functional modifications forced earthworms to adjust amino acid metabolism and consume more energy to detoxify and repair damage. This work supplements the toxic assessment of Fe₃O₄ NPs and provides crucial insights for optimizing the safety of NPs through functionalization.