Chiral Symmetry Breaking in Magnetoelectrochemical Etching with Chloride Additives

Theory of Chiral Electrodeposition by Chiral Micro-Nano-Vortices under a Vertical Magnetic Field -1: 2D Nucleation by Micro-Vortices

Remarkable chiral activity is donated to a copper deposit surface by magneto-electrodeposition, whose exact mechanism has been clarified by the three-generation model. In copper deposition under a vertical magnetic field, a macroscopic tornado-like rotation called the vertical magnetohydrodynamic (MHD) flow (VMHDF) emerges on a disk electrode, inducing the precessional motions of various chiral microscopic MHD vortices: First, chiral two-dimensional (2D) nuclei develop on an electrode by micro-MHD vortices. Then, chiral three-dimensional (3D) nuclei grow on a chiral 2D nucleus by chiral nano-MHD vortices. Finally, chiral screw dislocations are created on a chiral 3D nucleus by chiral ultra-micro MHD vortices. These three processes constitute nesting boxes, leading to a limiting enantiomeric excess (ee) ratio of 0.125. This means that almost all chiral activity of copper electrodes made by this method cannot exceed 0.125. It also became obvious that chirality inversion by chloride additive arises from the change from unstable to stable nucleation by the specific adsorption of it.

Breaking of Odd Chirality in Magnetoelectrodeposition

Electrodeposition under magnetic fields (magnetoelectrodeposition; MED) can induce surface chirality on copper films. The chiral signs of MED films should depend on the magnetic field polarity; namely, the reversal of the magnetic field causes the opposite chiral sign. This represents odd chirality for the magnetic field polarity. However, odd chirality was broken in several MED conditions. This paper makes a survey of breaking of odd chirality in the MED conditions such as low magnetic fields, specific adsorption of chloride ions, micro-electrode, and cell rotation. These results indicate that the ordered fluctuation of magnetohydrodynamic micro-vortices induces the breaking of odd chirality and that the random fluctuation results in the disappearance of surface chirality.

Fluctuation Effects of Magnetohydrodynamic Micro-Vortices on Odd Chirality in Magnetoelectrolysis

The magnetic field dependence of chiral surface formation was investigated in magnetoelectrodeposition (MED) and magnetoelectrochemical etching (MEE) of copper films. The MED and MEE was conducted in magnetic fields of up to 5 T, which were parallel or antiparallel to the ionic currents. The MED films prepared in high magnetic fields of 5 and 3 T exhibited odd chirality for magnetic field polarity, as expected on the basis of the magnetohydrodynamic (MHD) vortex model. However, the films prepared in the lower fields of 2.5 and 2 T exhibited breaking of odd chirality. Similar magnetic field dependence was observed in the surface chirality of MEE films. These results imply that the fluctuation in the self-organized state of micro-MHD vortices is responsible for the breaking of odd chirality.

Chiroptical study of the bimetal-cysteine hybrid composite: interaction between cysteine and Au/Ag alloyed nanotubes

The coupling between noble metal nanostructures and chiral molecules gives rise to strong chiroptical responses in the range from the ultraviolet (UV) to visible spectrum. In this work, cysteine-modified Au/Ag alloyed nanotubes (ANTs) have been prepared by coupling cysteine with Au/Ag ANTs. The chiroptical responses strongly depend on the chirality of cysteine and show clear mirrored behaviours. In contrast to Ag- or Au-cysteine chiral hybrid nanorods, the cysteine-modified Au/Ag ANTs exhibit higher chiroptical responses due to a stronger local electromagnetic field. The induced CD signals emerge in the interband absorption region of Au/Ag ANTs rather than in the local surface plasmon bands, which can be attributed to both the extended helical network conformation on the surface of Au/Ag ANTs and the near-field enhancement effect of plasmonic nanotubes. This confirms that Coulomb interaction induces coupling between cysteine and Au/Ag ANTs, which allows cysteine molecules to form an extended helical network on the surface of Au/Ag ANTs. Furthermore, the cysteine-modified Au/Ag ANTs also show excellent chiral recognition for amino acids in catalytic electrochemical reactions due to the presence of chiral active sites and the steric effect of large groups.

Surface Chirality in Rotational Magnetoelectrodeposition of Copper Films

Chiral surface formation was investigated in rotational magnetoelectrodeposition (RMED) of copper films, where an electrochemical cell was rotated in magnetic fields. The RMED was conducted with clockwise or anticlockwise rotation in the magnetic fields parallel or antiparallel to the ionic currents. The rotational frequencies were 0.5–6 Hz, and the magnetic fields were 2–5 T. The chiral behaviors are divided into four types: type I has chirality depending on the magnetic field polarity, type II has chirality depending on the rotational direction, and type III has chirality depending on both directions. Type IV represents chiral symmetry breaking, where the RMED films exhibit only L activity in any magnetic field polarity and rotational direction.