Fullerenes, particularly C60, have attracted significant attention due to their unique physicochemical properties, including high surface area, electron affinity, and reactivity. However, their increasing use in industrial and consumer applications raises concerns about their potential toxicity in biological and environmental systems. This review examines the toxicity of fullerenes, focusing on aggregation behavior, oxidative stress mechanisms, and ecotoxicological effects.

Aggregation behavior plays a critical role in determining the environmental fate and biological interactions of fullerenes. In aqueous environments, pristine C60 tends to form stable colloidal aggregates known as nC60, with sizes ranging from 50 to 500 nm. The formation of these aggregates depends on factors such as ionic strength, pH, and the presence of natural organic matter. Studies have shown that higher ionic strength promotes aggregation, while organic coatings can stabilize fullerene suspensions. In biological systems, aggregation influences cellular uptake and distribution. Larger aggregates may be less readily internalized by cells compared to smaller clusters or individual molecules, altering their bioavailability and potential toxicity.

Oxidative stress is a primary mechanism underlying fullerene toxicity. The carbon cage structure of fullerenes can generate reactive oxygen species (ROS) through electron transfer reactions, particularly under photoexcitation. In vitro studies using mammalian cells have demonstrated that exposure to C60 leads to increased ROS production, lipid peroxidation, and depletion of antioxidant defenses such as glutathione. The degree of oxidative damage depends on surface functionalization. For instance, hydroxylated fullerenes (fullerenols) exhibit lower oxidative potential compared to pristine C60 due to their increased solubility and reduced electron-accepting capacity. However, even modified fullerenes can induce oxidative stress at higher concentrations or prolonged exposure times.

Ecotoxicological studies reveal varied effects of fullerenes across different organisms. In aquatic systems, nC60 has been shown to affect microbial communities, with exposure concentrations as low as 0.1 mg/L inhibiting bacterial growth. Algae exhibit sensitivity to fullerenes, with reduced photosynthetic efficiency observed at concentrations above 1 mg/L. Invertebrates such as Daphnia magna display acute toxicity, with 48-hour LC50 values reported between 0.8 and 7.5 mg/L depending on the aggregation state and water chemistry. Fish studies indicate sublethal effects, including gill damage and altered swimming behavior, at environmentally relevant concentrations.

Soil organisms are also susceptible to fullerene exposure. Earthworms exhibit reduced growth and reproduction when exposed to C60-contaminated soils at concentrations exceeding 10 mg/kg. Plant studies show mixed results, with some species experiencing inhibited root growth at high fullerene concentrations, while others demonstrate no adverse effects. The variability in responses highlights the importance of soil composition and organic matter content in modulating fullerene bioavailability.

Environmental persistence is another concern. Fullerenes are resistant to biodegradation, leading to potential long-term accumulation in ecosystems. Sedimentation of aggregated nC60 can result in localized high concentrations in benthic environments, posing risks to sediment-dwelling organisms. The interaction of fullerenes with other pollutants may also enhance toxicity, as they can act as carriers for hydrophobic contaminants, increasing their bioavailability.

Regulatory gaps exist regarding the environmental risk assessment of fullerenes. Current data on chronic exposure and multi-generational effects remain limited, complicating the establishment of safe exposure thresholds. Standardized testing protocols are needed to account for the influence of environmental variables on fullerene behavior and toxicity.

Mitigation strategies for fullerene pollution include the development of biodegradable analogs and improved wastewater treatment methods. Coagulation-flocculation processes have shown efficacy in removing fullerene aggregates from water, while advanced oxidation techniques can degrade functionalized derivatives. However, the effectiveness of these methods varies with fullerene type and water chemistry.

In summary, fullerenes exhibit complex toxicity profiles influenced by their aggregation state, surface chemistry, and environmental conditions. Oxidative stress is a consistent mechanism of harm across biological systems, while ecotoxicological effects vary widely among species. Addressing knowledge gaps in long-term environmental impacts and improving risk assessment frameworks will be essential for the sustainable use of these nanomaterials. Future research should prioritize realistic exposure scenarios and interdisciplinary approaches to better predict and mitigate fullerene toxicity in natural ecosystems.