Superchiral fields generated by nanostructures and their applications for chiral sensing

Chirality is ubiquitous in natural world.Although with similar physical and chemical properties,chiral enantiomers could play different roles in biochemical processes.Discrimination of chiral enantiomers is extremely important in biochemical,analytical chemistry,and pharmaceutical industries.Conventional chiroptical spectroscopic methods are disadvantageous at a limited detection sensitivity because of the weak signals of natural chiral molecules.Recently,superchiral fields were proposed to effectively enhance the interaction between light and molecules,allowing for ultrasensitive chiral detection.Intensive theoretical and experimental works have been devoted to generation of superchiral fields based on artificial nanostructures and their application in ultrasensitive chiral sensing.In this review,we present a survey on these works.We begin with the introduction of chiral properties of electromagnetic fields.Then,the optical chirality enhancement and ultrasensitive chiral detection based on chiral and achiral nanostructures are discussed respectively.Finally,we give a short summary and a perspective for the future ultrasensitive chiral sensing.

Enhanced Circular Dichroism of Z-Shaped Nanorod in Rectangular Nanohole Arrays

Chiral nanostructures have attracted much attention due to the valuable applications in biochemistry, medicine industries, and photonic devices. In this study, we propose an ease-of-fabrication planar nanostructure that consists of rectangular nanohole arrays in which the Z-shaped nanorod is arranged. Theoretically, such chiral nanostructure supports significant absorption circular dichroism (CD) compared with the Z-shaped nanorod because charge distributions are tuned after the introduction of the rectangular frame. Meanwhile, the Z-shaped nanorod directs the flow of current on the rectangular frame, thereby generating the effective quadruple electron oscillation in the Z-shaped nanorod. A novel mode also emerges when an identical Z-shaped nanorod is added into the rectangular hole. The studies will provide a novel approach to enhance the CD effect of planar structures. .

Chiral metal nanostructures:synthesis,properties and applications

Chirality,the property that an object cannot coincide with its mirror image arising from lack of mirror symmetry,is ubiquitous in nature at various length scales.The physical and chemical properties are strongly related to the nature of chiral complexes,playing a significant role in various fields such as photonics,biochemistry,medicine and catalysis.In particular,the recent flexible design of chiral metal nanostructures offers one platform for deeply understanding the origin of chirality and one roadmap for the precise construction of chiral nanomaterials directed by the applications.Herein,we summarize the different geometries and classical synthetic approaches to chiral noble metal nanomaterials.Moreover,chiroptical properties and potential applications of chiral metal nanostructures are discussed as well.Finally,the opportunities and challenges toward the synthesis and application of chiral metal nanostructures are proposed.

Synergistic regulation of intermolecular interactions to control chiral structures for chiral recognition

Understanding the regulatory mechanism of self-assembly processes is a necessity to modulate nanostructures and their properties. Herein, we have studied the mechanism of self-assembly in the C3 symmetric 1,3,5-benzentricarboxylic amino acid methyl ester enantiomers(TPE) in a mixed solvent system consisting of methanol and water. The resultant chiral structure was used for chiral recognition. The formation of chiral structures from the synergistic effect of multiple noncovalent interaction forces was confirmed by various techniques. Molecular dynamics simulations were used to characterize the time evolution of TPE structure and properties in solution. The theoretical results were consistent with the experimental results. Furthermore, the chiral structure assembled by the building blocks of TPE molecules was highly stereoselective for diamine compounds.

On-Surface Stereochemical Characterization of a Highly Curved Chiral Nanographene by Noncontact Atomic Force Microscopy and Scanning Tunneling Microscopy

A highly distorted chiral nanographene structure composed of triple corannulene-fused[5]helicenes is prepared with the help of the Heck reaction and oxidative photocyclization with an overall isolated yield of 28%.The complex three-dimensional(3D)structure of the bowl-helix hybrid nanostructure is studied by a combination of noncontact atomic force microscopy(AFM)and scanning tunneling microscopy(STM)on the Cu(111)surface,density functional theory calculations,AFM/STM simulations,and high-performance liquid chromatography-electronic circular dichroism analysis.This examination reveals a molecular structure in which the three bowl-shaped corannulene bladesd position themselves in a C3-symmetric fashion around a highly twisted triphenylene core.The molecule appears to be shaped like a propeller in which the concave side of the bowls face away from the connected[5]helicene motif.The chirality of the nanostructure is confirmed by the direct visualization of both MMM and PPP enantiomers at the single-molecule level by scanning probe microscopies.These results underline that submolecular resolution imaging by AFM/STM is a powerful real-space tool for the stereochemical characterization of 3D curved chiral nanographene structures.

Ultra-stable silica-coated chiral Au-nanorod assemblies： Core-shell nanostructures with enhanced chiroptical properties

Chiral nano-assemblies with amplified optical activity have attracted particular interest for their potential application in photonics, sensing and catalysis. Yet it still remains a great challenge to realize their real applications because of the instability of these assembled nanostructures. Herein, we demonstrate a facile and efficient method to fabricate ultra-stable chiral nanostructures with strong chiroptical properties. In these novel chiral nanostructures, side-by-side assembly of chiral cysteine-modified gold nanorods serves as the core while mesoporous silica acts as the shell. The chiral core-shell nanostructures exhibit an evident plasmonic circular dichroism （CD） response originating from the chiral core. Impressively, such plasmonic CD signals can be easily manipulated by changing the number as well as the aspect ratio of Au nanorods in the assemblies located at the core. In addition, because of the stabilization effect of silica shells, the chiroptical performance of these core-shell nanostructures is significantly improved in different chemical environments.

Fabrication of chiral plasmonic oligomers using cysteine-modified gold nanorods as monomers

Generation of circular dichroism （CD） beyond the UV region is of great interest in developing chiral sensors and chiroptical devices. Herein, we demonstrate a simple and versatile method for fabrication of plasmonic oligomers with strong CD response in the visible and near IR spectral range. The oligomers were fabricated by triggering the side-by-side assembly of cysteine-modified gold nanorods. The modified nanorods themselves did not exhibit obvious plasmonic CD signals; however, the oligomers show strong CD bands around the plasmon resonance wavelength. The sign of the CD band was dictated by the chirality of the absorbed cysteine molecules. By adjusting the size of the oligomers, the concentration of chiral molecules, and/or the aspect ratio of the nanorods, the CD intensity and spectral range were readily tunable. Theoretical calculations suggested that CD of the oligomers originated from a slight twist of adjacent nanorods within the oligomer. Therefore, we propose that the adsorbed chiral molecules are able to manipulate the twist angles between the nanorods and thus modulate the CD response of the oligomers.

Logic-Gated Plasmonic Nanodevices Based on DNA-Templated Assembly

Life has evolved numerous elegant molecular machines that recognize biological signals and affect mechanical changes precisely to achieve specific and versatile biofunctions.Inspired by nature,synthetic molecular machines could be designed rationally to realize nanomechanical operations and autonomous computing.We constructed logic-gated plasmonic nanodevices through coassembly of two gold nanorods(AuNRs)and computing elements on a tweezer-shaped DNA origami template.After recognition of various molecular inputs,such as DNA strands,glutathione,or adenosine,the geometry and plasmonic circular dichroism(CD)signals of the AuNR–origami nanodevices produced corresponding changes.Then we designed and characterized a set of modular Boolean logic-gated nanodevices(YES,NOT,AND,OR)and proceeded to construct a complicated 3-input circuit capable of performing Boolean OR-NOT-AND operations.Our plasmonic logic devices transduced external inputs into conformational changes and near-infrared(NIR)chiral outputs.This DNA-based self-assembly strategy holds great potential for applications in programmable optical modulators,molecular information processing,and bioanalysis.