Chiral Surface and Geometry of Metal Nanocrystals

Chirality is a basic property of nature and has great importance in photonics, biochemistry, medicine, and catalysis. This importance has led to the emergence of the chiral inorganic nanostructure field in the last two decades, providing opportunities to control the chirality of light and biochemical reactions. While the facile production of 3D nanostructures has remained a major challenge, recent advances in nanocrystal synthesis have provided a new pathway for efficient control of chirality at the nanoscale by transferring molecular chirality to the geometry of nanocrystals. Interestingly, this discovery stems from a purely crystallographic outcome: chirality can be generated on high-Miller-index surfaces, even for highly symmetric metal crystals. This is the starting point herein, with an overview of the scientific history and a summary of the crystallographic definition. With the advance of nanomaterial synthesis technology, high-Miller-index planes can be selectively exposed on metallic nanoparticles. The enantioselective interaction of chiral molecules and high-Miller-index facets can break the mirror symmetry of the metal nanocrystals. Herein, the fundamental principle of chirality evolution is emphasized and it is shown how chiral surfaces can be directly correlated with chiral morphologies, thus serving as a guide for researchers in chiral catalysts, chiral plasmonics, chiral metamaterials, and photonic devices.     

Chiral Ligand‐Free,Optically Active Nanoparticles Inherently Composed of Chiral Lattices at the Atomic Scale

Bulk metals lack chirality. Recently, metals have been sculptured with metastable chirality varying from the micro‐ to nano‐scale. The manipulation of molecular chirality could be novelly performed using metals composed of chiral lattices at atomic scales (i.e., chiral nanoparticles or CNPs) if one could fundamentally understand the interactions between molecules and the chiral metal lattices. The incorporation of chiral ligands has been generally adapted to form metal CNPs. However, post‐fabrication removal of chiral ligands usually causes relaxation of the metastable chiral lattices to thermodynamically stable achiral structures, and thus the coexisting chiral ligands will unavoidably disturb or screen the interactions of interest. Herein, a concept of metal CNPs that are free of chiral ligands and consist of atomically chiral lattices is introduced. Without chiral ligands, shear forces applied by substrate rotation along with the translation of incident atoms lead to imposing the metastable chiral lattices onto metals. Metal CNPs show not only the chiroptical effect but the enantiospecific interactions of chiral lattices and molecules. These two unique chiral effects have resulted in the applications of enantiodifferentiation and asymmetric synthesis. Prospectively, the extension in composition space and constituent engineering will apply alloy CNPs to enantiodiscrimination, enantioseperation, bio‐imaging, bio‐sensing, and asymmetric catalysis.     

Using optical vortex to control the chirality of twisted metal nanostructures

We discovered for the first time that light can twist metal to control the chirality of metal nanostructures (hereafter, chiral metal nanoneedles). The helicity of optical vortices is transferred to the constituent elements of the irradiated material (mostly melted material), resulting in the formation of chiral metal nanoneedles. The chirality of these nanoneedles could be controlled by just changing the sign of the helicity of the optical vortex. The tip curvature of these chiral nanoneedles was measured to be <40 nm, which is less than 1/25th of the laser wavelength (1064 nm). Such chiral metal nanoneedles will enable us to selectively distinguish the chirality and optical activity of molecules and chemical composites on a nanoscale and they will provide chiral selectivity for nanoscale imaging systems (e.g., atomic force microscopes), chemical reactions on plasmonic nanostructures, and planar metamaterials.     

Artificial Chiral Probes and Bioapplications

The development of artificial chiral architectures, especially chiral inorganic nanostructures, has greatly promoted research into chirality in nanoscience. The nanoscale chirality of artificial chiral nanostructures offers many new application opportunities, including chiral catalysis, asymmetric synthesis, chiral biosensing, and others that may not be allowed by natural chiral molecules. Herein, the progress achieved during the past decade in chirality-associated biological applications (biosensing, biolabeling, and bioimaging) combined with individual chiral nanostructures (such as chiral semiconductor nanoparticles and chiral metal nanoparticles) or chiral assemblies is discussed.     

Formation of chiral surface with enantiomeric tartaric acid on gemini-structured self-assembled monolayers

Chiral surfaces were prepared by L, D, and Meso-tartaric acids (TAs) adsorbed on gemini-structured self-assembled monolayers (SAMs) composed of ethylenebis [(12-mercaptododecyl) dimethyl ammonium bromide] (HS-gQA-SH). The formation and structure of the chiral surfaces were characterized by surface plasmon resonance spectroscopy (SPR) and Fourier transform infrared-reflection adsorption spectroscopy (FTIR-RAS). The thickness of enantiomeric TA layers on the HS-gQA-SH SAM was estimated to be c.a. 5-6 angstroms regardless of their chirality, in good agreement with the height of TA molecules anchoring on the surface with two COOH groups. All the TAs on the HS-gQA-SH SAM exhibit the same ionization state independent of their chirality in their vibration bands of carboxylic groups. We attempted a second-layer adsorption of the enantiomeric TAs on L-TA monomolecular layer (L-TA SAM) precomposed on the HS-gQA-SH. A strong affinity between first and second TA layers resulted in the film growth when their chirality is identical (i.e., L-TA on L-TA SAM). We found the structure of second L-TA layer was completely different from that of the first layer, where a crystalline-like L-TA phase was found as a predominant component. Our results imply a preferential crystalline growth of chiral molecules on the same chiral surface, which may lead to a work for optical resolution into two enantiomers at a solid-liquid interface.     

Chiral pinwheel clusters lacking local point chirality

The supramolecular pinwheel cluster is a unique chiral structure with evident handedness. Previous studies reveal that the chiral pinwheels are composed of chiral or achiral molecules with polar groups, which result in strong intermolecular interactions such as hydrogen-bonding or dipole interactions. Herein, it is shown that the simple linear aromatic molecule, pentacene, can be self-assembled into large chiral pinwheel clusters on the semimetal Bi(111) surface, due to enhanced intermolecular interactions. The pentacene pinwheels reveal two levels of organizational chirality: the chiral hexamers resulting from asymmetric shifting along the long molecular axis, and chiral arrangement of six hexamers with a rotor motif. Furthermore, a new relation between the local point chirality and organizational chirality is identified from the pinwheels: the former is not essential for the latter in 2D pinwheel clusters of the pentacene molecule.     

Kinetic Controlled Chirality Transfer and Induction in 2D Hydrogen-Bonding Assemblies of Glycylglycine on Au(111)

Chirality transfer is of vital importance that dominates the structure and functionality of biological systems and living matters. External physical stimulations, e.g. polarized light and mechanical forces, can trigger the chirality symmetry breaking, leading to the appearance of the enantiomeric entities created from a chiral self-assembly of achiral molecule. Here, several 2D assemblies with different chirality, synthesized on Au(111) surface by using achiral building blocks – glycylglycine (digly), the simplest polypeptide are reported. By delicately tuning the kinetic factors, i.e., one-step slow/rapid deposition, or stepwise slow deposition with mild annealing, achiral square hydrogen-bond organic frameworks (HOF), homochiral rhombic HOF and racemic rectangular assembly are achieved, respectively. Chirality induction and related symmetry broken in assemblies are introduced by the handedness (H-bond configurations in principle) of the assembled motifs and then amplified to the entire assemblies via the interaction between motifs. The results show that the chirality transfer and induction of biological assemblies can be tuned by altering the kinetic factors instead of applying external forces, which may offer an in-depth understanding and practical approach to peptide chiral assembly on the surfaces and can further facilitate the design of desired complex biomolecular superstructures.     

Enantiomeric interactions between nucleic acid bases and amino acids on solid surfaces

Molecular interaction between nucleic acid bases and amino acids is a fundamental process in biology. The adsorption of the molecules on surfaces provides the opportunity to study such interactions in great detail by exploiting the high-resolution imaging capabilities of scanning tunnelling microscopy (STM). The chemisorption of prochiral molecules, such as adenine, on a metal surface causes the adsorbed species to become chiral. Subsequent interactions with inherently chiral molecules may then lead to the formation of diastereoisomers, if the enantiomeric interaction process is sufficiently strong. In the case of adenine adsorption on Cu[110], the chiral adsorbates form homochiral chains. Here, we show that the adenine chain direction is fully correlated with the chirality, and that the alpha-amino acid, phenylglycine, shows a strong chiral preference in its interaction with these chains. STM images clearly demonstrate that S-phenylglycine (R-phenylglycine) binds only to chains rotated 19.5 degrees (anti-) clockwise from the [001] direction. Closer examination reveals that the enantiomeric interaction involves double rows of phenylglycine molecules and the adenine chains. This is the first observation at the molecular level of diastereoisomeric interaction, and demonstrates that STM is a powerful method for studying the details of these interactions.     

Formation of chiral branched nanowires by the Eshelby Twist

Manipulating the morphology of inorganic nanostructures, such as their chirality and branching structure, has been actively pursued as a means of controlling their electrical, optical and mechanical properties. Notable examples of chiral inorganic nanostructures include carbon nanotubes, gold multishell nanowires, mesoporous nanowires and helical nanowires. Branched nanostructures have also been studied and been shown to have interesting properties for energy harvesting and nanoelectronics. Combining both chiral and branching motifs into nanostructures might provide new materials properties. Here we show a chiral branched PbSe nanowire structure, which is formed by a vapour-liquid-solid branching from a central nanowire with an axial screw dislocation. The chirality is caused by the elastic strain of the axial screw dislocation, which produces a corresponding Eshelby Twist in the nanowires. In addition to opening up new opportunities for tailoring the properties of nanomaterials, these chiral branched nanowires also provide a direct visualization of the Eshelby Twist.     

Chiral Materials: Progress,Applications, and Prospects

Chirality is a universal phenomenon in molecular and biological systems, denoting an asymmetric configurational property where an object cannot be superimposed onto its mirror image by any kind of translation or rotation, which is ubiquitous on the scale from neutrinos to spiral galaxies. Chirality plays a very important role in the life system. Many biological molecules in the life body show chirality, such as the “codebook” of the earth's biological diversity-DNA, nucleic acid, etc. Intriguingly, living organisms hierarchically consist of homochiral building blocks, for example, l -amino acids and d -sugars with unknown reason. When molecules with chirality interact with these chiral factors, only one conformation favors the positive development of life, that is, the chiral host environment can only selectively interact with chiral molecules of one of the conformations. The differences in chiral interactions are often manifested by chiral recognition, mutual matching, and interactions with chiral molecules, which means that the stereoselectivity of chiral molecules can produce changes in pharmacodynamics and pathology. Here, the latest investigations are summarized including the construction and applications of chiral materials based on natural small molecules as chiral source, natural biomacromolecules as chiral sources, and the material synthesized by design as a chiral source.     

Single-molecule chiral recognition on a surface by chiral molecular tips

Chiral surfaces attract increasing interest due to their vital role in a variety of scientific fields, such as chiral separation and heterogeneous enantioselective catalysis. The most urgent issue in research on such two-dimensional chirality is a lack of methodologies that recognize molecular chirality on a surface. Here we show that the chiral molecular tips enable for the first time discrimination of enantiomers on a single-molecule basis. The chiral selectivity is attributed to favorable chemical interactions that the molecular tips form with only one of two enantiomers. The stereoselective observation reveals spatial distribution of the enantiomers on a surface at the molecular level. The chiral molecular tips open a way for control of organization of enantiomers toward the advanced functionality of these chiral surfaces through knowledge on pivotal roles of chirality on molecular assemblies as shown here.     

From individual to collective chirality in metal nanoparticles

Recent reports have illustrated the promising potential of chiral metal nanostructures, which exploit the characteristic localized surface plasmon resonance of metal colloids, to produce intense optical activity. In this article we review the concepts, synthetic methods, and theoretical predictions underlying the chirality of metal colloids with a particular emphasis on the size range of 10–100 nanometers. The formation of individual colloidal nanoparticles with a chiral morphology and a plasmonic response remains elusive; however, collective chirality and the associated optical activity in nanoparticle assemblies is a promising alternative that has seen a few recent experimental demonstrations. We conclude with a perspective on chiral nanostructures built up from achiral anisotropic metal particles.     

Supramolecular Helices: Chirality Transfer from Conjugated Molecules to Structures

Different scales of chirality endow a material with many excellent properties and potential applications. In this review, using π‐conjugated molecules as functional building blocks, recent progress on supramolecular helices inspired by biological helicity is summarized. First, induced chirality on conjugated polymers and small molecules is introduced. Molecular chirality can be amplified to nanostructures, superstructures, and even macroscopic structures by a self‐assembly process. Then, the principles for tuning the helicity of supramolecular chirality, as well as formation of helical heterojunctions, are summarized. Finally, the potential applications of chiral structures in chiral sensing and organic electronic devices are critically reviewed. Due to recent progress in chiral structures, an interdisciplinary area called “chiral electronics” is expected to gain wide popularity in the near future.     

Chirality and Chiroptical Effects in Plasmonic Nanostructures: Fundamentals,Recent Progress,and Outlook

Strong chiroptical effects recently reported result from the interaction of light with chiral plasmonic nanostructures. Such nanostructures can be used to enhance the chiroptical response of chiral molecules and could also significantly increase the enantiomeric excess of direct asymmetric synthesis and catalysis. Moreover, in optical metamaterials, chirality leads to negative refractive index and all the promising applications thereof. In this Progress Report, we highlight four different strategies which have been used to achieve giant chiroptical effects in chiral nanostructures. These strategies consecutively highlight the importance of chirality in the nanostructures (for linear and nonlinear chiroptical effects), in the experimental setup and in the light itself. Because, in the future, manipulating chirality will play an important role, we present two examples of chiral switches. Whereas in the first one, switching the chirality of incoming light causes a reversal of the handedness in the nanostructures, in the second one, switching the handedness of the nanostructures causes a reversal in the chirality of outgoing light.     

Site-Selective Chiral Growth of Anisotropic Au Triangular Nanoplates for Tuning the Optical Chirality

Site-selective chiral growth of anisotropic nanoparticles is of great importance to realize the plasmonic nanostructures with delicate geometry and desired optical chirality; however, it remains largely unexplored. This work demonstrates a controlled site-selective chiral growth system based on the seed-mediated growth of anisotropic Au triangular nanoplates. The site-selective chiral growth involves two distinct underlying pathways, faceted growth and island growth, which are interswitchable upon maneuvering the interplay of chiral molecules, surfactants, and reducing agents. The pathway switch governs the geometric and chirality evolution of Au triangular nanoplates, giving rise to tailorable circular dichroism spectra. The ability to tune the optical chirality in a controlled manner by manipulating the site-selective chiral growth pathway opens up a promising strategy for exploiting chiral metamaterials with increasing architectural complexity in chiroptical applications.     

Plasmonic circular dichroism of Peptide-functionalized gold nanoparticles

Nature is remarkable at tailoring the chirality of different biomolecules to suit specific functions. Chiral molecules can impart optical activity to achiral materials in the form of the particle's electronic transition frequency. Herein, we used peptides of differing secondary structures (random coil and α-helix) to artificially create optically active chiral gold nanoparticles through peptide-nanoparticle interactions as observed by circular dichroism (CD) spectroscopy. This interaction produces a CD signal at the plasmon resonance frequency (～520 nm) of the chiral peptide-nanoparticle complex. Aggregation of the peptide-coated nanoparticles using metal ions results in a red-shifted plasmonic CD response. Our results suggest that chiroptical properties of nanomaterials can be engineered using peptides.     

Absolute configuration of monodentate phosphine ligand enantiomers on Cu(111)

Scanning tunneling microscopy (STM) has been employed to investigate the chirality of monophosphine compounds that are highly efficient chiral ligands in transition-metal-catalyzed organic transformations. The absolute configuration of 1-(2-diphenyphosphino-1-naphthyl)isoquinoline enantiomers with axial chirality was discriminated directly by the "marker" group, PPh(2) substitutes. Although the two enantiomer molecules adsorb on a Cu(111) surface and form well-defined (4 x 4) structures, the positions of PPh(2) substitutes in the molecular adlayers are different. The mirror symmetry between two adlayers is demonstrated. On the basis of STM results, structural models are proposed to interpret the chiral adsorption. The results presented here supply a straightforward method for axial chirality analysis in adsorbed adlayers by STM.     

Controlling Chirality across Length Scales using DNA

Nano‐objects with chiral properties attract growing interest due to their relevance for a wide variety of technological applications. For example, chiral nano‐objects may be used in characterization platforms that involve chiral molecular recognition of proteins or in the fabrication of nanomechanical devices such as screw‐gears or nanoswimmers. Spatial ordering of emitters of circularly polarized light might greatly benefit from the utilization of chiral shapes. Tools developed in DNA nanotechnology now allow precise tailoring of the chiral properties of molecules and materials at various length scales. Among others, they have already been applied to control the handedness of helical shapes (configurational chirality) or the chiral positioning of different‐sized nanoparticles at the vertices of tetrahedra (compositional chirality). This work covers some of the key advances and recent developments in the field of chiral DNA nanoarchitectures and discusses their future perspectives and potential applications.     

Surface-enhanced Raman optical activity on adenine in silver colloidal solution

We report the collection of Raman optical activity (ROA) spectra of adenine in silver colloidal solution, that is, surface-enhanced Raman optical activity (SEROA) using considerably shorter data acquisition times, reduced excitation power, and lower concentration, as compared to classical ROA measurements on molecules of biological interest so far reported in the literature. These improvements in experimental parameters for ROA measurements can be explained by enhanced Raman signals in the local optical fields of the silver nanoparticles and by at least 1 order of magnitude higher values for circular intensity differences (CIDs), as compared to classical ROA that has been suggested before and theoretically discussed in terms of large field gradients near a metal surface. The measured ROA effect for adenine can be understood in terms of adsorption-induced chirality in the prochiral molecules on the silver nanoparticles. Surface-enhanced Raman optical activity offers potential capabilities for sensitive, rapid, stereochemical characterization of basic building blocks of biopolymers, such as amino acids and nucleosides, as well as biologically active molecules, in particular, also for probing organization and self-assembling of such molecules on metal surfaces.     

Supramolecular Chiral 2D Materials and Emerging Functions

Chiral materials are widely applied in various fields such as enantiomeric separation, asymmetric catalysis, and chiroptical effects, providing stereospecific conditions and environments. Supramolecular concepts to create the chiral materials can provide an insight for emerging chiro-optical properties due to their well-defined scaffolds and the precise functionalization of the surfaces or skeletons. Among the various supramolecular chiral structures, 2D chiral sheet structures are particularly interesting materials because of their extremely high surface area coupled with many unique chemical and physical properties, thereby offering potential for the next generation of functional materials for optically active systems and optoelectronic devices. Nevertheless, relatively limited examples for 2D chiral materials exhibiting specific functionality have been reported because incorporation of molecular chirality into 2D architectures is difficult at the present stage. Here, a brief overview of the recent advances is provided on the construction of chiral supramolecular 2D materials and their functions. The design principles toward 2D chirality and their potential applications are also discussed.