The role of surface functionalization in the solar light-induced production of reactive oxygen species by single-walled carbon nanotubes in water Original Research Article

Risk assessment of engineered nanomaterials necessitates the need for information on reactivity and persistence of these materials in the environment. To this end, we report on reactivity of single-walled carbon nanotubes (SWCNTs) that: (i) have been functionalized by acid oxidation to contain carboxylic acid groups, (ii) have been further functionalized by esterification with polyethylene oxide groups, or (iii) were unmodified (i.e., unfunctionalized), with all materials used as received from the manufacturer. Aqueous colloidal dispersions of both types of functionalized SWCNTs generated reactive oxygen species (ROS) including singlet oxygen (1O2), superoxide anion image, and hydroxyl radicals (radical dotOH) in light within the solar spectrum (λ = 300–410 nm) under oxic conditions. Under similar experimental conditions (i.e., several days under light near solar intensity) but with a surfactant added as a dispersing agent, aqueous dispersions of unfunctionalized SWCNTs exhibited no measurable ROS production. Defects in the surface caused by functionalization, as well as differences in amorphous carbon and metal impurity content within different SWCNT preparations, may facilitate some ROS production. Adding β-nicotinamide adenine dinucleotide to dispersions of carboxylated SWCNTs resulted in image generation in the absence of light, suggesting dark reaction electron transfer as a potential mechanism of toxicity.