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 NaNH₂ , the purified CNHa produce GB-free amino-CNHa where the tips are functionalized with NH₂ groups.