Plasmonic Chirality and Circular Dichroism in Bioassembled and Nonbiological Systems: Theoretical Background and Recent Progress

Nature is chiral, thus chirality is a key concept required to understand a multitude of systems in physics, chemistry, and biology. The field of optics offers valuable tools to probe the chirality of nanosystems, including the measurement of circular dichroism, the differential interaction strength between matter and circularly polarized light with opposite helicity. Simultaneously, the use of plasmonic systems with giant light-interaction cross-sections opens new paths to investigate and manipulate systems on the nanoscale. Consequently, the interest in chiral plasmonic and hybrid systems has continually grown in recent years, due to their potential applications in biosensing, polarization-encoded optical communication, polarization-selective chemical reactions, and materials with polarization-dependent light–matter interaction. Experimentally, chiral properties of nanostructures can be either created artificially using modern fabrication techniques involving inorganic materials, or borrowed from nature using bioassembly or biomolecular templating. Herein, the recent progress in the field of plasmonic chirality is summarized, with a focus on both the theoretical background and the experimental advances in the study of chirality in various systems, including molecular-plasmonic assemblies, chiral plasmonic nanostructures, chiral assemblies of interacting plasmonic nanoparticles, and chiral metal metasurfaces and metamaterials. The growth prospects of this field are also discussed.     

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.     

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.     

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.     

Chiromagnetic Plasmonic Nanoassemblies with Magnetic Field Modulated Chiral Activity

Chiral plasmonic nanoassemblies, which exhibit outstanding chiroptical activity in the visible or near‐infrared region, are popular candidates in molecular sensing, polarized nanophotonics, and biomedical applications. Their optical chirality can be modulated by manipulating chemical molecule stimuli or replacing the building blocks. However, instead of irreversible chemical or material changes, real‐time control of optical activity is desired for reversible and noninvasive physical regulating methods, which is a challenging research field. Here, the directionally and reversibly switching optical chirality of magneto‐plasmonic nanoassemblies is demonstrated by the application of an external magnetic field. The gold‐magnetic nanoparticles core–satellite (Au@Fe₃O₄) nanostructures exhibit chiral activity in the UV–visible range, and the circular dichroism signal is 12 times greater under the magnetic field. Significantly, the chiral signal can be reversed by regulating the direction of the applied magnetic field. The attained magnetic field‐regulated chirality is attributed to the large contributions of the magnetic dipole moments to polarization rotation. This magnetic field‐modulated optical activity may be pivotal for photonic devices, information communication, as well as chiral metamaterials.     

Chiral Nanoceramics

The study of different chiral inorganic nanomaterials has been experiencing rapid growth during the past decade, with its primary focus on metals and semiconductors. Ceramic materials can substantially expand the range of mechanical, optical, chemical, electrical, magnetic, and biological properties of chiral nanostructures, further stimulating theoretical, synthetic, and applied research in this area. An ever-expanding toolbox of nanoscale engineering and self-organization provides a chirality-based methodology for engineering of hierarchically organized ceramic materials. However, fundamental discoveries and technological translations of chiral nanoceramics have received substantially smaller attention than counterparts from metals and semiconductors. Findings in this research area are scattered over a variety of sources and subfields. Here, the diversity of chemistries, geometries, and properties found in chiral ceramic nanostructures are summarized. They represent a compelling materials platform for realization of chirality transfer through multiple scales that can result in new forms of ceramic materials. Multiscale chiral geometries and the structural versatility of nanoceramics are complemented by their high chiroptical activity, enantioselectivity, catalytic activity, and biocompatibility. Future development in this field is likely to encompass chiral synthesis, biomedical applications, and optical/electronic devices. The implementation of computationally designed chiral nanoceramics for biomimetic catalysts and quantum information devices may also be expected.     

Reconfigurable Plasmonic Chirality: Fundamentals and Applications

Molecular chirality is a geometric property that is of great importance in chemistry, biology, and medicine. Recently, plasmonic nanostructures that exhibit distinct chiroptical responses have attracted tremendous interest, given their ability to emulate the properties of chiral molecules with tailored and pronounced optical characteristics. However, the optical chirality of such human-made structures is in general static and cannot be manipulated postfabrication. Herein, different concepts to reconfigure the chiroptical responses of plasmonic nano- and micro-objects are outlined. Depending on the utilized strategies and stimuli, the chiroptical signature, the 3D structural conformation, or both can be reconfigured. Optical devices based on plasmonic nanostructures with reconfigurable chirality possess great potential in practical applications, ranging from polarization conversion elements to enantioselective analysis, chiral sensing, and catalysis.     

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.     

Solvent Mediated Nanoscale Quasi-Periodic Chirality Reversal in Self-Assembled Molecular Networks Featuring Mirror Twin Boundaries

Grain boundaries in polycrystals have a prominent impact on the properties of a material, therefore stimulating the research on grain boundary engineering. Structure determination of grain boundaries of molecule-based polycrystals with submolecular resolution remains elusive. Reducing the complexity to monolayers has the potential to simplify grain boundary engineering and may offer real-space imaging with submolecular resolution using scanning tunneling microscopy (STM). Herein, the authors report the observation of quasi-periodic nanoscale chirality switching in self-assembled molecular networks, in combination with twinning, as revealed by STM at the liquid/solid interface. The width of the chiral domain structure peaks at 12–19 nm. Adjacent domains having opposite chirality are connected continuously through interdigitated alkoxy chains forming a 1D defect-free domain border, reflecting a mirror twin boundary. Solvent co-adsorption and the inherent conformational adaptability of the alkoxy chains turn out to be crucial factors in shaping grain boundaries. Moreover, the epitaxial interaction with the substrate plays a role in the nanoscale chirality reversal as well.     

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.     

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.     

Synthesis of nanostructured SiC using the pulsed laser deposition technique

We report the new results on the direct synthesis of nanostructured silicon carbide (SiC) materials using the pulsed laser deposition technique. Scanning electron microscopy images revealed that SiC nanoholes, nanosprouts, nanowires, and nanoneedles were obtained. The crystallographic structure, chemical composition, and bond structure of the nanoscale SiC materials were investigated using X-ray diffraction, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman scattering spectroscopy. The transverse optical mode and longitudinal optical mode in Raman spectra were found to become sharper as the substrate temperature was increased, while the material structure evolved from amorphous to crystalline.     

Low-dimensional, hinged bar-code metal oxide layers and free-standing, ordered organic nanostructures from turbostratic vanadium oxide

Both low-dimensional bar-coded metal oxide layers, which exhibit molecular hinging, and free-standing organic nanostructures can be obtained from unique nanofibers of vanadium oxide (VO(x)). The nanofibers are successfully synthesized by a simple chemical route using an ethanolic solution of vanadium pentoxide xerogel and dodecanethiol resulting in a double bilayered laminar turbostratic structure. The formation of vanadium oxide nanofibers is observed after hydrothermal treatment of the thiol-intercalated xerogel, resulting in typical lengths in the range 2-6 microm and widths of about 50-500 nm. We observe concomitant hinging of the flexible nanofiber lamina at periodic hinge points in the final product on both the nanoscale and molecular level. Bar-coded nanofibers comprise alternating segments of organic-inorganic (thiols-VO(x)) material and are amenable to segmented, localized metal nanoparticle docking. Under certain conditions free-standing bilayered organic nanostructures are realized.     

Near-field microscopy: throwing light on the nanoworld

Optical microscopy with nanoscale resolution, beyond that which is possible with conventional diffraction-limited microscopy, may be achieved by scanning a nanoantenna in close proximity to a sample surface. This review will first aim to provide an overview of the basic principles of this technique of scanning near-field optical microscopy (SNOM), before moving on to consider the most widely implemented form of this microscopy, in which the sample is illuminated through a small aperture held less than 10 nm from the sample surface for optical imaging with a resolution of ca. 50 nm. As an example of the application of this microscopy, the results of SNOM measurements of light-emitting polymer nanostructures are presented. In particular, SNOM enables the unambiguous identification of the different phases present in the nanostructures, through the local analysis of the fluorescence from the polymers. The exciting new possibilities for high-resolution optical microscopy and spectroscopy promised by apertureless SNOM techniques are also considered. Apertureless SNOM may involve local scattering of light from a sample surface by a tip, local enhancement of an optical signal by a metal tip, or the use of a fluorescent molecule or nanoparticle attached to a tip as a local optical probe of a surface. These new optical nanoprobes offer the promise of optical microscopy with true nanometre spatial resolution.     

Fabrication and morphological control of ion-induced zinc nanostructures

An efficient method for the fabrication of zinc (Zn) nanostructures (nanoneedles and nanofibers) of controllable density and morphology without any catalyst, hazardous chemicals or external heat supply has been investigated. By varying the ion irradiation time and the ion current density, morphological control and the density of Zn nanostructures were successfully achieved using a fast and viable ion irradiation technique. Scanning (SEM) and transmission electron microscopy (TEM) results revealed that the sputtered surface was almost entirely covered with densely distributed conical and needle-like protrusions with linear shaped (sometimes curved) nanostructures (such as nanoneedles and nanofibers) with diameters and lengths of about 20-50 nm and several hundred nanometers, respectively. Detailed analysis of selected area electron diffraction (SAED) patterns with TEM analysis indicates that the Zn nanofibers were polycrystalline in nature. A possible mechanism of the formation of Zn nanostructures is briefly discussed. These aligned arrays of Zn nanoneedles/nanofibers could be a promising material for the fabrication of zinc oxide nanostructures by subsequent oxidation of Zn nanostructures and their future application in nanodevices. Thus, it is believed that this ion irradiation technique could open up a new approach for the fabrication of many kinds of nanomaterials of controllable density.     

Supramolecular Nanotube Reactors for Production of Imine Polymers with Controlled Conformation,Size, and Chirality

A series of supramolecular nanotubes with inner diameters of 1, 4, 9, 12, 16, and 29 nm are prepared from amino acid lipids. The hydrophobic channels of the nanotubes act as reactors for the formation of imine polymers by not only effectively encapsulating the benzaldehyde and diacetyleneamine precursors of the imine monomers but also markedly accelerating imine formation. The nanotube inner diameter determines whether the imine monomers self‐assemble into nanoparticles, nanotapes, nanocoils, or twisted nanofibers in the channels. UV‐induced polymerization of the diacetylene units in the imine nanostructures followed by decomposition of the nanotubes into molecular dispersions of the constituent amino acid lipids results in expulsion of the polymerized imine nanostructures with retained conformation. The isolated nanocoils and twisted nanofibers retain the helicity and circular dichroism induced by the nanotubes, which exhibits supramolecular chirality, even though the components of the imine monomers are achiral. These supramolecular nanotubes with tunable diameters and functionalizable surfaces can be expected to be useful for the production of polymers with controlled conformation, size, and chirality without the need for rational design or chemical modification of the monomers or optimization of the polymerization conditions.     

Structural and Optical Properties and Emerging Applications of Metal Nanomaterials

Nanomaterials possess intriguing optical properties that depend sensitively on size, shape, and material content of the structures. Controlling such structural characteristics of the nanostructures allows the tailoring of their physical and chemical properties, e.g. optical, electronic, and catalytic, to achieve what is desired lot specific applications of interest. This review will cover the development of various shapes for silver and gold nanomaterials with emphasis on their relation to optical properties. Examples of various modern synthetic methods and characterization techniques are highlighted. The influence of the metal nanomaterial＇s shape and optical absorption on surface enhanced Raman scattering （SERS） and a final note on new emerging applications of metal nanostructures are also discussed.     

Chirality and Surface Bonding Correlation in Atomically Precise Metal Nanoclusters

Chirality is ubiquitous in nature and occurs at all length scales. The development of applications for chiral nanostructures is rising rapidly. With the recent achievements of atomically precise nanochemistry, total structures of ligand-protected Au and other metal nanoclusters (NCs) are successfully obtained, and the origins of chirality are discovered to be associated with different parts of the cluster, including the surface ligands (e.g., swirl patterns), the organic–inorganic interface (e.g., helical stripes), and the kernel. Herein, a unified picture of metal–ligand surface bonding-induced chirality for the nanoclusters is proposed. The different bonding modes of M–X (where M = metal and X = the binding atom of ligand) lead to different surface structures on nanoclusters, which in turn give rise to various characteristic features of chirality. A comparison of Au–thiolate NCs with Au–phosphine ones further reveals the important roles of surface bonding. Compared to the Au–thiolate NCs, the Ag/Cu/Cd–thiolate systems exhibit different coordination modes between the metal and the thiolate. Other than thiolate and phosphine ligands, alkynyls are also briefly discussed. Several methods of obtaining chiroptically active nanoclusters are introduced, such as enantioseparation by high-performance liquid chromatography and enantioselective synthesis. Future perspectives on chiral NCs are also proposed.     

Nanostructured biointerfaces

Colloidal lithography is used to create nanostructured interfaces suitable for studying and interacting with cellular biosystems. Large areas of patterned surface can be produced. We investigate the use of plasma etching for transfer of the pattern of individual colloidal particles into the substrates to create short-range ordered arrays of topographic and/or chemical nanostructures. Colloidal masks perform differently to traditional photoresist masks and local redeposition of material around the particle has significant impact on the resultant structures. The colloidal particles can be reshaped to allow the fabrication of flat-topped structures. Topographic and chemical nanostructures can be used to template the self assembly of macromolecules. The phase separation of thin films of PS-PMMA symmetric block copolymers above nanostructure sites is aligned at the surface nanostructure by topographic features. PLL-PEG assembly at alkanethiol-modified nanoscale chemical patterns of gold/silicon allows the production of nanoscale protein patterns. Patterns of ferritin 100 nm in diameter are demonstrated.