Oxidation debris in microwave functionalized carbon nanotubes: Chemical and biological effects

Dilute base-washing has been shown to remove oxidation debris by converting the acidic groups to their dissolvable conjugate salts. In this paper we present for the first time the effect of base-washing of microwave induced carboxylated carbon nanotubes (CNTs). Three multiwalled carbon nanotubes (MWCNTs) with different dimensions were studied, and the biological/toxicological properties were evaluated with human THP-1 cells. The carbon to oxygen ratio, the BET surface area, the ratio of D–G bands based on Raman measurements as well as the solubility changed with base wash. Most dramatic was in increase in carboxylic groups on the surface and a corresponding decrease in phenolic and lactonic groups. However, zeta potential, particle size and colloidal stability at different pH showed no significant alterations with the wash. The analysis of the wash showed the presence of graphitic carbon but the absence of small CNT fragments. Toxicity measurements and IL-1β release from the THP-1 cells in the presence of MWCNTs also showed no alteration with base wash. These results demonstrated that the oxidation under microwave conditions generated relatively less debris and fragmentation, and this is a good approach to acid functionalization of CNTs.