Chemical Valence of Metal Impurity in Single-Wall Carbon Nanotubes Dominate Their Cytotoxicity

Metal impurities are believed to play a role in carbon nanotube (CNT) toxicity, however, redox properties of metal impurities can vary remarkably. It thus becomes necessary to investigate whether altered chemical valence of metal impurities in CNTs elicits diverse cytotoxicity effects. Studies on this issue are scarce partly due to the technical challenges of characterizing the chemical valence of metal impurities. Herein, synchrotron radiation-based technology was applied to qualify and quantify the chemical species and valence of metal impurities in two single wall CNTs and to in situ map intracellular metal distribution after CNT exposure. Surprisingly, the CNT containing a much lower amount of metal displayed significantly higher cytotoxicity compared with the higher metal one. Further analysis indicated that the CNT with lower metal content possessed higher amounts of zero-valent iron (Fe⁰) and cobalt (Co⁰). Results from electron spin resonance spectroscopy further confirmed that CNT with lower metal content generated more hydroxyl radicals, which was attributed to the high amounts of Fe⁰ and Co⁰. Taken together, our findings with state-of-the-art synchrotron radiation techniques demonstrated that chemical valence rather than the quantity of metal impurities in CNTs determine their cytotoxicity, thereby revealing a novel aspect of the cytotoxicity mechanism of CNTs.