pH-responsive reversible dispersion of biocompatible SWNT/graphene–amphiphile hybrids

Abstract

The present work reports synthesis of cholesterol based peptide carboxylates as efficient dispersing agents for single walled carbon nanotubes (SWNTs) as well as graphene in water. Variation of the amino acids within the peptide moiety exhibited interesting changes in their SWNTs dispersion efficacy. The dipeptide carboxylate comprising of two alanine residues showed 80% SWNTs dispersion which is ∼2 fold higher than that obtained by using the common surfactant, SDBS. The dipeptide amphiphiles also efficiently dispersed the 2D-allotrope of carbon, graphene, in water. As to our objective, the terminal carboxylate moiety in these cholesterol based carboxylates exhibited pH-sensitivity towards the reversible solubilization and precipitation of the nanohybrids. Acidification of the nanohybrids with HCl converted the carboxylates to the water insoluble carboxylic acids leading to the precipitation of carbon nanomaterials. Most importantly, addition of an equivalent amount of NaOH resulted in the restoration of stable aqueous dispersion of SWNT/graphene. This reversible precipitation–dispersion cycle was performed time and again. The conversion of the carboxylate salt to the corresponding acid and vice versa is the main reason for such reversible switching between precipitation and dissolution under acidic and basic pH. Indeed the presence of the amino acid/peptide moiety in the structure of cholesterol carboxylates was found to be indispensable for efficient dispersion of carbon nanomaterials. Significant stability of these SWNT dispersions was observed in the presence of high salt and protein concentration. Moreover, the nanohybrids were highly biocompatible with mammalian cells, which increases their future prospects in biomedicine.