Enantioselective Surface-Enhanced Raman Scattering by Chiral Au Nanocrystals with Finely Modulated Chiral Fields and Internal Standards.

The chiral discrimination of enantiomers is crucial for drug screening and agricultural production. Surface-enhanced Raman scattering (SERS) is proposed for discriminating enantiomers benefiting from chiral plasmonic materials. However, the mechanism of enantioselective SERS is unclear, and fluctuating SERS intensities may result in errors. Herein, this work demonstrates a reliable SERS substrate using chiral Au nanocrystals with finely modulated chiral fields and internal standards. Chiral electromagnetic fields are enhanced after modulation, which is conducive to increasing the difference in the enantiomeric SERS intensity, as evidenced by the experimental and simulation results. Furthermore, the SERS stability is improved by the corrective effect of the internal standards, and the relative standard deviation is significantly reduced. Using finely modulated chiral fields and internal standards, L- and D-phenylalanine exhibit a stable six times difference in SERS ratio. Theoretical simulations reveal that linearly polarized light can also excite the chiral fields of chiral Au nanocrystals, indicating non-chiral far-field light is converted into chiral near-field sources by chiral Au nanocrystals. Thus, the mechanism of enantioselective SERS can be elucidated by the scattering difference of chiral molecules in chiral near fields. This study will pave the way for the development of enantioselective SERS and related chiroptical technologies.

Engineering the Intrinsic Chirality of Plasmonic Au@Pd Metamaterials for Highly Sensitive Chiroplasmonic Hydrogen Sensing.

Metal helicoid nanoparticles with intrinsic 3D chiral structures have emerged as a new class of plasmonic metamaterials with outstanding chiroplasmonic properties. Despite the considerable potential of metal helicoid nanoparticles in chiroplasmonic sensing, their sensing capabilities remain elusive, stressing the need for the rational chirality engineering of helicoid nanoparticles. In this report, Au@Pd helicoid nanoparticles with engineered chiroplasmonic properties and integrated hydrogen sensing capabilities are rationally synthesized. As chiroplasmonic metamaterials, the Au@Pd helicoid nanoparticles exhibit unprecedented sensitivity for hydrogen chiroplasmonic sensing in the visible range. A significant circular dichroism red-shift as large as 206.1 nm can be achieved when they are exposed to hydrogen. Such a high sensitivity outperforms all the plasmonic hydrogen sensors in the visible range. Besides sensitivity, the chiroplasmonic sensing platform shows a good linear range of 1.5-6.0% hydrogen concentration with higher figure of merit, excellent selectivity, and good reusability. To further demonstrate its applicability, this chiroplasmonic hydrogen sensing platform is utilized to investigate hydrogen absorption and desorption kinetics on Pd. This study heralds a new paradigm for plasmonic hydrogen sensing and highlights the tremendous potential of utilizing helicoid nanoparticles as chiroplasmonic sensing metamaterials by chirality engineering.

Surface Topographical Engineering of Chiral Au Nanocrystals with Chiral Hot Spots for Plasmon-Enhanced Chiral Discrimination.

Surface roughness in chiral plasmonic nanostructures generates asymmetrical localized electromagnetic fields, which hold great promise for applications in chiral recognition, chiroptical spectroscopic sensing, and enantioselective photocatalysis. In this study, we develop a surface topographical engineering approach to precisely manipulate the surface structures of chiral Au nanocrystals. Through carefully controlling the amounts of l- or d-cystine (Cys) and the seed solution in the growth process, we successfully synthesize chiral Au nanocrystals with highly disordered, ordered, and less ordered wrinkled surfaces. An underlying principle governing the relationship between surface roughness, orderliness, and chiroptical response is also proposed. More importantly, the chiral ordered wrinkles on the surfaces of the nanocrystals generate asymmetrical localized electronic fields with enhanced intensity, which achieve excellent plasmon-enhanced chiral discrimination ability for penicillamine (Pen) enantiomers. This work offers exciting prospects for manipulating the surface structures of chiral nanocrystals and designing highly sensitive plasmon-enhanced enantioselective sensors with chiral hot spots.

Synthesis of Chiral Au Nanocrystals with Precise Homochiral Facets for Enantioselective Surface Chemistry.

Metal surfaces with intrinsic chirality play an irreplaceable role in many significant enantioselective chemical processes such as enantioselective catalysis, sensing, and separation. Nonetheless, current methods for the precise preparation of such chiral surfaces suffer with issues of unscalable production and low surface areas. Herein, we report the synthesis of chiral Au nanoparticles with precisely determined homochiral facets. Though a scalable wet chemical method, {125̅8}R and {85̅12}S high-Miller-index facets are obtained with the l- and d-chiral Au nanocrystals, respectively. The growth process of these homochiral facets is investigated, and a new nanocrystal growth pathway is revealed. More importantly, the remarkable enantioselective recognition properties of these homochiral surfaces are demonstrated and enable an efficient electrochemical method for chiral discrimination of l-/d-tryptophan. These results provide a foundation of fundamental studies of heterogeneous enantioselective processes and may pave way for the development of nanocatalysts for enantioselective chemistry.