Cyclodextrins as Surfactants for Solubilization and Purification of Carbon Nanohorn Aggregates

Nano‐ to micrometer‐scale surface roughness contributes to the hydrophobicity of materials as discussed often in terms of superhydrophobicity. We report here that as little as 1 wt% of α , β , and γ ‐cyclodextrins (CDs) can disperse carbon nanohorn aggregates (CNHa) as surfactants in water by binding on their pointed tips. The binding mode was visually demonstrated using single‐molecule atomic resolution real‐time electron microscopy (SMART‐EM). The SMART‐EM data provided an estimation of the equilibrium constant of the CD binding to be close to that of adamantane ammonium chloride to β ‐CD. A qualitative study on the rate of decomplexation of α , β , and γ ‐CDs from CNHa suggests a substantial mechanistic difference in their binding mode to the CNH tips. A sequence of α ‐CD complexation and decomplexation allows us to purify CNHa by selectively removing graphitic ball‐shaped impurity (GB). Upon treatment with NaNH 2 , the purified CNHa produce GB‐free amino‐CNHa where the tips are functionalized with NH 2 groups.