Chirality Transfer from Sub-Nanometer Biochemical Molecules to Sub-Micrometer Plasmonic Metastructures: Physiochemical Mechanisms,Biosensing, and Bioimaging Opportunities

Determining the structural chirality of biomolecules is of vital importance in bioscience and biomedicine. Conventional methods for characterizing molecular chirality, e.g., circular dichroism (CD) spectroscopy, require high-concentration specimens due to the weak electronic CD signals of biomolecules such as amino acids. Artificially designed chiral plasmonic metastructures exhibit strong intrinsic chirality. However, the significant size mismatch between metastructures and biomolecules makes the former unsuitable for chirality-recognition-based molecular discrimination. Fortunately, constructing metallic architectures through molecular self-assembly allows chirality transfer from sub-nanometer biomolecules to sub-micrometer, intrinsically achiral plasmonic metastructures by means of either near-field interaction or chirality inheritance, resulting in hybrid systems with CD signals orders of magnitude larger than that of pristine biomolecules. This exotic property provides a new means to determine molecular chirality at extremely low concentrations (ideally at the single-molecule level). Herein, three strategies of chirality transfer from sub-nanometer biomolecules to sub-micrometer metallic metastructures are analyzed. The physiochemical mechanisms responsible for chirality transfer are elaborated and new fascinating opportunities for employing plasmonic metastructures in chirality-based biosensing and bioimaging are outlined.     

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.     

Circular Dichroism Studies on Plasmonic Nanostructures

In recent years, optical chirality of plasmonic nanostructures has aroused great interest because of innovative fundamental understanding as well as promising potential applications in optics, catalysis and sensing. Herein, state‐of‐the‐art studies on circular dichroism (CD) characteristics of plasmonic nanostructures are summarized. The hybrid of achiral plasmonic nanoparticles (NPs) and chiral molecules is explored to generate a new CD response at the plasmon resonance as well as the enhanced CD intensity of chiral molecules in the UV region, owing to the Coulomb static and dynamic dipole interactions between plasmonic NPs and chiral molecules. As for chiral assembly of plasmonic NPs, plasmon–plasmon interactions between the building blocks are found to induce generation of intense CD response at the plasmon resonance. Three‐dimensional periodical arrangement of plasmonic NPs into macroscale chiral metamaterials is further introduced from the perspective of negative refraction and photonic bandgap. A strong CD signal is also discerned in achiral planar plasmonic nanostructures under illumination of circular polarized plane wave at oblique incidence or input vortex beam at normal incidence. Finally perspectives, especially on future investigation of time‐resolved CD responses, are presented.     

Induced chirality through electromagnetic coupling between chiral molecular layers and plasmonic nanostructures

We report a new approach for creating chiral plasmonic nanomaterials. A previously unconsidered, far-field mechanism is utilized which enables chirality to be conveyed from a surrounding chiral molecular material to a plasmonic resonance of an achiral metallic nanostructure. Our observations break a currently held preconception that optical properties of plasmonic particles can most effectively be manipulated by molecular materials through near-field effects. We show that far-field electromagnetic coupling between a localized plasmon of a nonchiral nanostructure and a surrounding chiral molecular layer can induce plasmonic chirality much more effectively (by a factor of 10(3)) than previously reported near-field phenomena. We gain insight into the mechanism by comparing our experimental results to a simple electromagnetic model which incorporates a plasmonic object coupled with a chiral molecular medium. Our work offers a new direction for the creation of hybrid molecular plasmonic nanomaterials that display significant chiroptical properties in the visible spectral region.     

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.     

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.     

Assembled Plasmonic Asymmetric Heterodimers with Tailorable Chiroptical Response

Directed nanocrystal (NC) heteroassemblies could potentially achieve tailorable multiplex circular dichroism (CD) bands. Here, for the first time, we developed assembly of nanoparticle (NP)‐nanorod (NR) chiral heterodimers with chiral molecules to explore their chiroptical activities. The experimental results revealed that plasmonic CD responses were in the region from 520 to 750 nm, which was in agreement with the theoretical simulation. Importantly, the CD band could be regulated by controlling the gaps between adjacent NCs and altering the building blocks of the assemblies. These results show that the plasmonic chiroptical response of NP‐NR heterodimers could come from the finger‐crossed chiral construction of adjacent NC in the heterodimers and the formation of plasmonic hot‐spots in the assemblies could further enhance the plasmonic CD. This work provides a new opportunity to create heterogeneous nanoscale plasmonic objects with tailorable chiroptical response for application in biosensors, in vivo chiral medical carriers and negative refractive index materials.     

手性聚苯胺的制备及其电磁学性能研究

采用二次掺杂法制备了具有手征特性的聚苯胺,利用傅立叶红外光谱(FT-IR)、X射线衍射仪(XRD)、圆二色谱仪(CD)和紫外一可见光分光光度计(UV-vis)等分析手段对聚苯胺的结构、性能和手征特性进行了表征.结果表明,通过过硫酸铵作引发剂、盐酸掺杂获得了导电的盐酸掺杂态聚苯胺(PANI-HCI);通过氨水脱掺杂后得到本征态聚苯胺(EB),EB通过手性樟脑磺酸(CSA)诱导,形成了手征性螺旋构型聚苯胺.电磁性能测试表明,与非手性聚苯胺相比,手性聚苯胺具有较优越的吸波性能.     

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.     

Ligand-Assisted Self-Assembly of 3D Perovskite Nanocrystals into Chiral Inorganic Quasi-2D Perovskites (n = 3) with Ligand-Ratio-Dependent Chirality Inversion

Chiral inorganic quasi-2D perovskites are prepared by self-assembling 3D perovskites in solution for the first time. The quasi-2D perovskite synthesized is a pure-phase perovskite with <n> = 3 and is periodically arranged, which is a big breakthrough in quasi-2D inorganic perovskites.  With the individual chiral CsPbBr₃ nanocrystals (NCs) assemble into quasi-2D perovskite, the g-factor significantly improved (≈5 × 10⁻³). In addition, the chiroptical activity of quasi-2D perovskites is explored to be improved with the lateral size increasing. In the first stage of assembly, chiral optical activity is increased due to the lateral size-dependent optical activity, while the changes in the later stages are attributable to the chiral morphology. Interestingly, chirality inversion is found to be correlated to the number of ligands. It is believed that different conformers of chiral ligands caused by steric hindrance of the original ligand oleylamine result in opposite circular dichroism (CD) polarities. The chirality inversion phenomenon is universal, regardless of the choice of ligands. This work opens up a new path for the synthesis of quasi-2D perovskites and provides more opportunities for the modulation of chiral optical activity.     

Chiral Plasmonic Hydrogen Sensors

In this article, a chiral plasmonic hydrogen‐sensing platform using palladium‐based nanohelices is demonstrated. Such 3D chiral nanostructures fabricated by nanoglancing angle deposition exhibit strong circular dichroism both experimentally and theoretically. The chiroptical properties of the palladium nanohelices are altered upon hydrogen uptake and sensitively depend on the hydrogen concentration. Such properties are well suited for remote and spark‐free hydrogen sensing in the flammable range. Hysteresis is reduced, when an increasing amount of gold is utilized in the palladium‐gold hybrid helices. As a result, the linearity of the circular dichroism in response to hydrogen is significantly improved. The chiral plasmonic sensor scheme is of potential interest for hydrogen‐sensing applications, where good linearity and high sensitivity are required.     

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.

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Complementary Y-shaped chiral metamaterial with giant optical activity and circular dichroism simultaneously for terahertz waves

Abstract

A complementary Y-shaped chiral metamaterial (CYCMM) is proposed for the realization of giant optical activity and circular polarization with strong circular dichroism (CD) simultaneously for terahertz (THz) waves. It is demonstrated that the proposed CYCMM can achieve 90° linear polarization rotation around 2.13 THz and a giant CD effect at 2.38 THz through full-wave simulations. The mechanism of the giant CD effect and optical activity is illustrated by simulated surface current distribution. Due to strong optical activity and the CD effect, the proposed CYCMM is useful for the development of integrated terahertz spectroscopic and imaging devices.     

Chiroptical effects in planar achiral plasmonic oriented nanohole arrays

Chiroptical effects are routinely observed in three dimensional objects lacking mirror symmetry or quasi-two-dimensional thin films lacking in-plane mirror symmetry. Here we show that symmetric plasmonic planar arrays of circular nanoholes produced strong chiroptical responses at visible wavelengths on tilting them with respect to the incident light beam due to the collective asymmetric nature of their surface plasmon excitations. This extrinsic chiroptical effect can be stronger than the local chiroptical response in arrays of intrinsically chiral nanoholes and may be useful for chiral sensing and negative refraction.     

Chiroptically Active Plasmonic Nanoparticles Having Hidden Helicity and Reversible Aqueous Solvent Effect on Chiroptical Activity

The geometrical prerequisite for forming a helix is P (helical pitch) > d (wire diameter). Limited by the current development of nanofabrication techniques, it is difficult to minimize d and consequently P to the sub‐10 nm molecule‐comparable scale, preventing the study of chiral plasmonics at dimensions approaching the physical limit. Herein, glancing angle deposition is operated at substrate temperature of 0 °C and high speed of substrate rotation to generate silver nanoparticles (AgNPs) with nominal P < d. The AgNPs have intrinsic chiroptical activity characterized by circular dichroism (CD), originating from the hidden helicity. With increasing P from 3 to 66 nm, the plasmonic mode barely shifts but shows a logarithmic increase in CD amplitude. Immersing AgNPs in water causes the plasmonic mode to redshift and rise in CD amplitude, i.e., a water effect on chiroptical activity. Hydrophilic AgNP arrays with low array porosity show a reversible water effect, but hydrophobic Ag nanospiral arrays with P > d and high array porosity have an irreversible water effect. This work introduces a cost‐effective, facile approach to minimize P to sub‐10 nm at a regular substrate temperature, paving the way to study chiral plasmonics approaching the physical limit and exploit chirality‐related bioapplications typically operated in aqueous solutions to tackle significant health and environmental problems.     

Ultraviolet–Visible Chiroptical Activity of Aluminum Nanostructures

Ultraviolet (UV)‐resonant metals (e.g., aluminum) typically have low melting point to cause a fabrication difficulty in helical sculpture to generate plasmons with chiroptical activity in the UV region. In this work, using glancing angle deposition (GLAD), two new methods are devised to generate crystalline chiral Al nanostructures that have stable chiroptical response in the UV–visible region originating from intrinsic helical structures. One approach involves fast substrate rotation during GLAD to fabricate Al nanoparticles (AlNPs) with hidden helicity; another is to deposit an achiral Al thin film on a host of plasmonic chiral NPs, such that the helical structures are duplicated from the chiral host to the achiral guest of Al nanocappings. The host@guest helicity duplication is a new GLAD methodology to generate chiroptically active plasmons, which can be generally adapted to diverse plasmonic metals for tailoring plasmonic chiroptical activity flexibly in the UV–visible region. More importantly, this work offers those two new methods to generate UV‐active plasmonic chiral substrates, which can markedly enhance chiroptical activity of biomolecules. It would open a door to develop surface‐enhanced chiroptical spectroscopies for sensitively monitoring stereobiochemical information, which is of prominent interest in understanding a wide range of homochirality‐determined biological phenomena.     

Preparation of chiral poly(diacetylene) films from 10,12-pentacosadiynoic acid using circularly polarized light

By irradiating evaporated 10,12-pentacosadiynoic acid (p-DA) monomer film with circularly polarized light (CPL), we prepared chiral poly(diacetylene) [PDA] film. The circular dichroism (CD) was obtained reproducibly, depending on the rotational direction of the CPL. The induced chirality showed the dependence on the substrate temperature used for the preparation of evaporated p-DA monomer films, and it was stable after the transition to red-phase by annealing. Results suggest that side chain of polymer made a significant contribution to the formation of red-phase chiral PDA.     

Chiral guanosine 5′-monophosphate-capped gold nanoflowers: Controllable synthesis, characterization, surface-enhanced Raman scattering activity, cellular imaging and photothermal therapy

Plasmonics and chirality in metal nanomaterials are intriguing and inspiring phenomena. Nanoscale chirality of metal nanomaterials has emerged as a hot topic in the past several years. Generally, most plasmon-induced circular dichroism (CD) responses of nanomaterials (> 10 nm) have been artificially created by modifying pre-made achiral nanomaterials with chiral agents, because the in situ generation of plasmon-induced CD responses of nanomaterials with larger size (> 10 nm) is not easy. Herein, we report a simple one-pot green synthesis of chiral gold nanoflowers (GNFs) with abundant petal-shaped tips in the chiral reduction environment arising from the presence of chiral guanosine 5??-monophosphate (5??-GMP) and the chiral reducing agent L-ascorbic acid (L-AA). Different reducing agents can impact the shape and chirality of the products. In addition, the size and chirality of the GNFs can be controlled by adjusting the reaction time. The as-synthesized GNFs have good biocompatibility and can be used for surface-enhanced Raman scattering (SERS) enhancement, cellular dark-field imaging and photothermal therapy.      

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.     

Flexible Chirality in Self‐Assembled N‐Annulated Perylenedicarboxamides

N‐annulated perylenedicarboxamides 1–3 form supramolecular polymers with a strong tendency to aggregate. The bundles of fibers formed generate a spontaneous anisotropy that conditions the chiroptical features of the described molecules in solution; a strong linear dichroism effect accompanies the circular dichroism (CD) outcome. There is no influence of the point chirality existing at the side chains of 1 and 2 , and these molecules present the same chiroptical features as achiral 3 . Mechanical rotary stirring increases the CD response and the sign of the dichroic signal changes with the stirring direction. Theoretical calculations indicate that the self‐assembly of 1–3 in helical columnar stacks generates atropisomers by the restricted rotation of the H‐bonded benzamide units. Molecular mechanics/molecular dynamics calculations predict a feasible intrastack stereomutation of the helical aggregates due to the rapid rupture/formation of the amide H‐bonds. This oscillating helicity, together with the fact that right‐ and left‐handed helices are predicted to be mostly isoenergetic, justifies the negligible contribution of the molecular chirality embedded in the paraffinic side chains of 1 and 2 . The reported CD behavior contributes to shed light on the physical processes promoting flexible macroscopic chirality that, in turn, can be utilized for the spectroscopic visualization of torsional flows generated in a vortex.