Enhanced Enantioselectivity in the Synthesis of Mercury Sulfide Nanoparticles through Ostwald Ripening
Jumpei Kuno, Kazuhiro Miyake, Shohei Katao, Tsuyoshi Kawai, Takuya Nakashima
Abstract
Chirality in nanoparticles (NPs) has recently emerged as an active area of research owing to its wide range of potential applications. NPs with intrinsic chirality in their inorganic core are of particular interest owing to their giant optical activity. Mercury sulfide (HgS) NPs with intrinsic chirality in their core crystalline structure were prepared in the presence of a chiral thiol, penicillamine (Pen), as a capping ligand, leading to the preferential formation of NPs with an enantiomeric core. The synthesis of chiral HgS NPs proceeded via the transient formation of NPs with an achiral metacinnabar phase, which was transformed into a chiral cinnabar phase along with the growth of NPs of size over 5.5 nm. The reaction temperature had an effect on the optical activity of HgS NPs, which was enhanced by 75% compared to that obtained in the first report, giving the highest optical activity for the semiconductor NPs reported so far. This enhancement was discussed in conjunction with the evolution of NPs possessing a combination of crystal handedness and surface ligand coordination pattern. The Pen ligand was considered to afford two chiral bidentate coordination modes with opposite handedness using a pair of thiol and either carboxylate or amine. These two coordination modes selectively stabilize the HgS crystal core with opposite handedness to each other to give NPs with different thermodynamic stability. The less stable NPs disappeared in accord with the growth of more stable NPs with opposite handedness through the Ostwald ripening process, resulting in the enhancement of NPs’ enantiopurity.