Zinc-Coordinated Hierarchical Organization of Ligand-Stabilized Gold Nanoclusters for Chiral Recognition and Separation.

Three-dimensional organization of d- or l-tryptophan and mercaptopropionic acid-stabilized gold nanoclusters has been achieved by complexation of ligands using zinc ions. Powder X-ray diffraction and transmission electron microscopy analyses substantiated the crystalline nature of the assembly of atomic nanoclusters. The hierarchical arrangement of the nanoclusters exhibited superior optical properties (namely, enhanced photoluminescence and excited-state lifetime) as compared to the non-assembled nanoclusters. Furthermore, photoluminescence of the crystalline assembly of nanoclusters served as a visual marker for chiral recognition of d and l enantiomers of tryptophan, with subsequent separation of the corresponding enantiomer. A theoretical structure based on various experimental observations has also been proposed herein. The mechanistic aspect of the chiral separation is proposed to have occurred through attachment of d or l-tryptophan to the coordinatively unsaturated zinc ions, thus forming super complexes. The degree of stabilization of the super complexes is dictated by a "three-point versus two-point" interaction between the enantiomers and the chiral selector.