Chiral Nanoparticle‐Induced Enantioselective Amplification of Molecular Optical Activity

Enantiodifferentiation is of fundamental importance in chiral chemistry and substantially requires high optical activity (OA) of enantiomers; but the enantiomeric OA is typically weak due to subwavelength molecular dimension, leading to a lack of sensitive enantiodifferentiation. A new approach is devised to evidently amplify the enantiomeric OA by anchoring axially chiral molecules containing the binaphthyl chromophore on silver chiral nanoparticles (AgCNPs) with a sub‐5 nm helical pitch (P). Compared to the OA of dissolved enantiomers, that of (R)‐ and (S)‐ enantiomers can be enantioselectively amplified on right‐handed and left‐handed AgCNPs, respectively, in as high as one order of magnitude. The enantioselective amplification is probably ascribed to the enantiospecific adsorption‐induced change in the dihedral angle of the binaphthyl chromophore, resulting from chirality transfer from chiral footprints of the AgCNPs to the binaphthyl chromophore through the Ag–S bicontacts. The enantioselective amplification tends not to occur as long as P > 5 nm or on achiral Ag nanoparticles, due to a lack of the chirality transfer. This work imposes the significant application of enantiodifferentiation, which is on practical demand for producing single‐enantiomer pharmaceutics and pesticides with no fatal adverse effect, on the emerging chiral metamaterials composed of metallic CNPs.     

Ultrabroadband Optical Superchirality in a 3D Stacked‐Patch Plasmonic Metamaterial Designed by Two‐Step Glancing Angle Deposition

Low‐cost and large‐scale fabrication of 3D chiral metamaterials is highly desired for potential applications such as nanophotonics devices and chiral biosensors. One of the promising fabrication methods is to use glancing angle deposition (GLAD) of metal on self‐assembled dielectric microsphere array. However, structural handedness varies locally due to long‐range disorder of the array and therefore large‐scale realization of the same handedness is impossible. Here, using symmetry considerations a two‐step GLAD process is proposed to eliminate this longstanding problem. In the proposed scheme, the unavoidable long‐range disorder gives rise to microscale domains of the same handedness but of slightly different structural geometries and ultimately contributes to a broad‐band response. Experimentally, a record‐breaking superchiral response of circular dichroism signal of ≈11° is demonstrated and an average polarization rotation angle of 27° in the visible region on ≈1 cm² sample is observed. Computer‐aided geometric reconstruction with experimental parameters unambiguously reveal the presence of strong structural anisotropy and chirality in the prepared stacked‐patch plasmonic chiral metamaterial; microscopic spectral analyses combined with full‐wave electromagnetic simulations coherently provide deeper insights into the measured circular dichroism and optical activity. The observed chiroptical response can also be flexibly controlled by adjusting the deposition parameters for various potential applications.