Adsorption behavior of β-cyclodextrin onto gold nanoparticles.

Native and modified cyclodextrins (CDs) are routinely used as a capping agent upon the preparation of gold nanoparticles (AuNP); however, their role in the synthesis remains unresolved. Here, the adsorption behavior of β-cyclodextrin (β-CD) onto AuNP was studied by all-atom molecular dynamics (MD) simulations. We found that β-CD binding onto the AuNP surface occurs through multiple non-covalent interactions, among which non-covalent binding of the aliphatic carbon moieties and the hydroxyl oxygen atoms plays a vital role. Surprisingly, the MD analysis revealed that the most preferred binding mode of the β-CD molecules onto the gold surface corresponds to the toroid side geometry. Only minor populations of β-CD molecules were observed, in which the adsorbate molecules are bound onto AuNP by either its primary or secondary rims, respectively. The adsorbed β-CDs formed the multiple-layer coating on AuNP, composed up to three adsorbate layers. We found that for AuNP of the size of 2.9 nm, the maximum water-protecting efficiency of the adsorbate coating occurs upon reaching a value of 40 β-CD molecules, the further increase in the adsorbate content resulted in some self-aggregation of β-CD molecules onto the gold surface. Our findings provide the molecular-level understanding of the adsorption behavior of β-CD onto gold, which is relevant for various applications of CD-capped gold nanostructures.