Affinity Capillary Electrophoresis Method to Assess Carboxylation of Multi-walled Carbon Nanotubes

• Published in: Analytica chimica acta Vol. 1027; pp. 149 - 157
• Main Authors: Davis, Tyler A, Patberg, Shannon M., Lowry, David T., Sargent, Linda, Stefaniak, Aleksandr B., Holland, Lisa A.
• Format: Journal Article
• Language: English
• Published: 29.03.2018
• ISSN: 0003-2670; 1873-4324
• DOI: 10.1016/j.aca.2018.03.034

Carboxylation of multi-walled carbon nanotubes is hypothesized to reduce the toxicity of these nanomaterials; however, literature reports are conflicting and the degree of carboxylic acid functionalization in different studies is not well quantified. The extent of carboxylation of multi-walled carbon nanotubes after acid treatment is quantified using affinity capillary electrophoresis. A polytryptophan peptide that contains a single arginine residue (WRWWWW) serves as a ligand in affinity capillary electrophoresis to assess the degree of carboxylation. The formation of peptide-nanotube ligand-receptor complex allows for the detection of the complex with a common UV absorbance detector based on light scatter rather than molecular absorbance. Dissociation constants (K D ) are obtained by observing the migration shift of the WRWWWW peptide in background electrolyte at increasing concentrations of multi-walled carbon nanotubes. The K D values obtained from triplicate analyses of a single preparation of commercially available carboxylated multi-walled carbon nanotubes had a precision of 20% RSD (1.2 ± 0.2 mg/L). Preparations of multi-walled carbon nanotube remade from powdered samples ( n = 3) generated K D values with 20% RSD (1.1 ± 0.2 mg/L) as well. When applied to commercially available multi-walled carbon nanotubes with different degrees of carboxylation, the capillary electrophoresis method yielded K D values that reflected higher levels of carboxylation. Zeta potential measurements of these preparations were not significantly different. The utility of the capillary electrophoresis method for evaluating acid treatment protocols was demonstrated by comparing K D values obtained for multi-walled carbon nanotubes subject to six different acidification times. While K D values were significantly different for acidification times ranging from 15 minutes to 3 hours, none of the zeta potential measurements of these samples were significantly different. This work is significant to research involving carbon nanotube toxicity because it provides a new metric to rapidly characterize carbon nanotubes obtained from different vendors, synthesized in laboratories using different procedures, or subject to different acidification protocols. Affinity capillary electrophoresis is a cost-effective, rapid and simple alternative to current technologies used to distinguish the degree of carbon nanotube carboxylation.