Phytotoxicity of carbon nanotubes and nanodiamond in long-term assays with Cactaceae plant seedlings

We present the results of long-term phytotoxicity assays of single-walled and multi-walled carbon nanotubes (SWNTs and MWNTs, respectively) and nanodiamond (ND) employing cactus species Ferocactus latispinus, Melocactus matanzanus and Parodia ayopayana. They are small and slow growing plants, which is convenient for long-term assays using the substrate volumes as small as a few milliliters, contrary to the commonly reported experiments with crop species. The soil-based experiments described allow for a better match of the real environmental situation. After a few months (up to 40 weeks) observations of seedling growth, we concluded that pristine arc-discharge SWNTs exhibit the strongest phytotoxic effect as compared to pristine MWNTs, purified SWNTs and ND, which can be attributed to a high amount of impurities (including remainders of the catalyst used for their synthesis) in as-prepared SWNTs. We found that after first three months of exposure, nanomaterials might produce seemingly favorable effect on seedling growth, which can invert after a more prolonged exposure. Therefore, careful evaluation of the possible phytotoxic effects must include observations for several months, whereas shorter assays might produce misleading results.