What amount of metallic impurities in carbon nanotubes is small enough not to dominate their redox properties?

As carbon nanotubes become a part of everyday life, the paramount questions about their toxicity persist. While it has been proven that a significant part of the toxicity of carbon nanotubes is due to the redox activity of the residual metallic impurities present within them, there is still no response to the fundamental question of where the borderline is that would render the impurities 'redox invisible'. Herein we investigate the electrochemical response of carbon nanotubes containing different amounts of impurities towards the reduction of an important biomarker, hydrogen peroxide, and the oxidation of an important impurity marker, hydrazine. We found that the borderline between being redox active/inactive for iron-based impurities lies in the middle-ppm range.