Redox-Active Nickel in Carbon Nanotubes and Its Direct Determination

The presence of residual metal-catalyst impurities in carbon nanotubes is responsible for their toxicity. It is important to differentiate between the total amount of impurities and the redox-active (bioavailable) amount of such impurities because only the bioavailable impurities exhibit toxic effects. Herein, we report a simple and specific method for quantifying the amount of redox-active Ni present in various commercial samples of CNTs. It is based on the electrochemical oxidation of Ni(OH)2 that is formed in alkaline solutions when Ni impurities are opened to the surrounding environment. Metallic Ni impurities play an extremely active role in toxicological assays as well as in undesired catalytic processes, and thus a method to rapidly quantify the amount of redox-active Ni is of great importance.