Nanodiamonds in sugar rings: an experimental and theoretical investigation of cyclodextrin-nanodiamond inclusion complexes

We report on the noncovalent interactions of nanodiamond carboxylic acids derived from adamantane, diamantane, and triamantane with beta- and gamma-cyclodextrins. The water solubility of the nanodiamonds was increased by attaching an aromatic dicarboxylic acid via peptide coupling. Isothermal titration calorimetry experiments were performed to determine the thermodynamic parameters (K-a, Delta H, Delta G and Delta S) for the host-guest inclusion. The stoichiometry of the complexes is invariably 1 : 1. It was found that K-a, Delta G and Delta H of inclusion increase for larger nanodiamonds. Delta S is generally positive, in particular for the largest nanodiamonds. beta-Cyclodextrin binds all nanodiamonds, gamma-cyclodextrin clearly prefers the most bulky nanodiamonds. The interaction of 9-triamantane carboxylic acid shows one of the strongest complexation constants towards gamma-cyclodextrin ever reported, K-a = 5.0 x 10(5) M-1. In order to gain some insight into the possible structural basis of these inclusion complexes we performed density functional calculations at the B97-D3/def2-TZVPP level of theory.