The contributions of metal impurities and tube structure to the toxicity of carbon nanotube materials

Due to the existence of considerable quantities of metallic and carbonaceous impurities, the key factor and mechanism for the reported toxicity of carbon nanotubes (CNTs) are unclear. Here, we first quantify the contribution of metal residues and fiber structure to the toxicity of CNTs. Significant quantities of metal particles could be mobilized from CNTs into surrounding fluids, depending on the properties and constituents of the biological microenvironment, as well as the properties of metal particles. Furthermore, electron spin resonance measurements confirm that hydroxyl radicals can be generated by both CNTs containing metal impurities and acid-leachable metals from CNTs. Several biomolecules facilitate the generation of free radicals, which might be due to the participation of these biomolecules in redox cycling influenced by pH. Among several major metal residues, Fe has a critical role in generating hydroxyl radicals, reducing cell viability and promoting intracellular reactive oxidative species. Cell viability is highly dependent on the amount of metal residues and iron in particular, but not tube structure, while the negative effect of CNTs themselves on cell viability is very limited in a certain concentration range below 80 μg ml−1. It is crucial to systematically understand how these exogenous and endogenous factors influence the toxicity of CNTs to avoid their undesirable toxicity. Carbon nanotubes (CNTs) are a class of materials that have stimulated a great deal of interest among researchers due to their unique chemical and electronic properties. However, the apparent toxicity of CNTs has raised concerns about their use in research and applications. Chunying Chen and Yuliang Zhao at the Chinese Academy of Sciences in Beijing and their co-workers have re-examined the safety profile of CNTs, highlighting the role that metallic impurities left over from nanotube synthesis play in their toxicity. The researchers found that these metallic particles readily leach out of the nanotubes, including into simulated biological fluids mimicking those found in the lung, stomach and intestines. Iron particles leached into acidic fluids caused particular safety concerns—electron-spin resonance analysis showed that leached iron particles were generating cytotoxic free radical species at levels much higher than the nanotubes themselves. Cell viability studies demonstrated a clear negative correlation between the amount of metal residue in the nanotube sample and the health of the cultured cells. The key factor and mechanism for reported toxicity of CNTs are unclear. Here we firstly quantify the contribution of metal residues and fiber structure to the toxicity of CNTs. Significant quantities of metal can be mobilized from CNTs into surrounding fluids, depending on the properties and constituent of biological microenvironment and metal particles. Hydroxyl radicals were generated by CNT containing metal impurities and leachable metal, while several inherent biomolecules facilitate the generation of free radical. Cell viability is highly dependent on the amount of metal residues and iron in particular but not tube structure, while the negative effect of CNT was limited in a certain concentration range below 80 μg ml−1.