A Chirality-Based Quantum Leap

There is increasing interest in the study of chiral degrees of freedom occurring in matter and in electromagnetic fields. Opportunities in quantum sciences will likely exploit two main areas that are the focus of this Review: (1) recent observations of the chiral-induced spin selectivity (CISS) effect in chiral molecules and engineered nanomaterials and (2) rapidly evolving nanophotonic strategies designed to amplify chiral light-matter interactions. On the one hand, the CISS effect underpins the observation that charge transport through nanoscopic chiral structures favors a particular electronic spin orientation, resulting in large room-temperature spin polarizations. Observations of the CISS effect suggest opportunities for spin control and for the design and fabrication of room-temperature quantum devices from the bottom up, with atomic-scale precision and molecular modularity. On the other hand, chiral-optical effects that depend on both spin- and orbital-angular momentum of photons could offer key advantages in all-optical and quantum information technologies. In particular, amplification of these chiral light-matter interactions using rationally designed plasmonic and dielectric nanomaterials provide approaches to manipulate light intensity, polarization, and phase in confined nanoscale geometries. Any technology that relies on optimal charge transport, or optical control and readout, including quantum devices for logic, sensing, and storage, may benefit from chiral quantum properties. These properties can be theoretically and experimentally investigated from a quantum information perspective, which has not yet been fully developed. There are uncharted implications for the quantum sciences once chiral couplings can be engineered to control the storage, transduction, and manipulation of quantum information. This forward-looking Review provides a survey of the experimental and theoretical fundamentals of chiral-influenced quantum effects and presents a vision for their possible future roles in enabling room-temperature quantum technologies.

Triskelion Structured Colloidal Quantum Dots

We show, using density functional theory and ab initio molecular dynamics, that certain small colloidal quantum dots with a mixed nanocrystal core capped with achiral surface ligands spontaneously form a triskelion (from the Greek, three-legged) structure with (approximate) C₃ symmetry that can be dynamically stable at room temperature when additionally capped with small amine ligands. Furthermore, the nanocrystal core also forms a triskelion structure. The focus of our study is a colloidal quantum dot with a Cd₁₆Se₇Te₃ core (and a charge of +12) capped with negatively charged surface ligands to achieve charge neutrality-in the simplest instance, 12 Cl^--to form the colloidal quantum dot Cd₁₆Se₇Te₃Cl₁₂. The small size of the core (for which almost all atoms are surface atoms), the high positive charge that destabilizes the core, the mixed (Cd/Te) composition that creates mechanical strain in the core, and the inclusion of precisely three Te atoms in the predominantly Se core all play critical roles in the spontaneous formation of the triskelion structure.

Self-Assembly of a Chiral Cubic Three-Connected Net from the High Symmetry Molecules C60 and SnI4

The design of new chiral materials usually requires stereoselective organic synthesis to create molecules with chiral centers. Less commonly, achiral molecules can self-assemble into chiral materials, despite the absence of intrinsic molecular chirality. Here, we demonstrate the assembly of high-symmetry molecules into a chiral van der Waals structure by synthesizing crystals of C₆₀(SnI₄)₂ from icosahedral buckminsterfullerene (C₆₀) and tetrahedral SnI₄ molecules through spontaneous self-assembly. The SnI₄ tetrahedra template the Sn atoms into a chiral cubic three-connected net of the SrSi₂ type. Our results represent the remarkable emergence of a self-assembled chiral material from two of the most highly symmetric molecules, demonstrating that almost any molecular, nanocrystalline, or engineered precursor can be considered when designing chiral assemblies.

Induction of Chirality in Two-Dimensional Nanomaterials: Chiral 2D MoS2 Nanostructures

Two-dimensional (2D) nanomaterials have been intensively investigated due to their interesting properties and range of potential applications. Although most research has focused on graphene, atomic layered transition metal dichalcogenides (TMDs) and particularly MoS₂ have gathered much deserved attention recently. Here, we report the induction of chirality into 2D chiral nanomaterials by carrying out liquid exfoliation of MoS₂ in the presence of chiral ligands (cysteine and penicillamine) in water. This processing resulted in exfoliated chiral 2D MoS₂ nanosheets showing strong circular dichroism signals, which were far past the onset of the original chiral ligand signals. Using theoretical modeling, we demonstrated that the chiral nature of MoS₂ nanosheets is related to the presence of chiral ligands causing preferential folding of the MoS₂ sheets. There was an excellent match between the theoretically calculated and experimental spectra. We believe that, due to their high aspect ratio planar morphology, chiral 2D nanomaterials could offer great opportunities for the development of chiroptical sensors, materials, and devices for valleytronics and other potential applications. In addition, chirality plays a key role in many chemical and biological systems, with chiral molecules and materials critical for the further development of biopharmaceuticals and fine chemicals, and this research therefore should have a strong impact on relevant areas of science and technology such as nanobiotechnology, nanomedicine, and nanotoxicology.

A series of intrinsically chiral gold nanocage structures

We present a series of intrinsically chiral gold nanocage structures, Au_9n+6, which are stable for n ≥ 2. These structures consist of an Au_9n tube which is capped with Au₃ units at each end. Removing the Au₃ caps, we obtain a series of intrinsically chiral gold nanotube structures, Au_9n, which are stable for n ≥ 4. The intrinsic chirality of these structures results from the helicity of the gold strands which form the tube and not because an individual Au atom is a chiral center. The symmetry of these structures is C₃ and substructures of gold hexagons with a gold atom in the middle are particularly prominent. We focus on the properties of Au₄₂ (C₃) and Au₁₀₅ (C₃) which are the two smallest gold nanocage structures to be completely tiled by these Au₇ "golden-eye" substructures. Our main focus is on Au₄₂ (C₃) since gold clusters in the 40-50 atom regime are currently being investigated in gas phase experiments. We show that the intrinsically chiral Au₄₂ cage structure is energetically comparable with previously reported achiral cage and compact Au₄₂ structures. Cage structures are of particular interest because species can be encapsulated (and stabilized) inside the cage and we provide strong evidence that Au₆@Au₄₂ (C₃) is the global minimum Au₄₈ structure. The intrinsically chiral gold nanocage structures, which exhibit a range of size-related properties, have potential applications in chiral catalysis and as components in nanostructured devices.

Chiral Optical Properties of Tapered Semiconductor Nanoscrolls

Large surface-to-volume ratio, one-dimensional quantum confinement, and strong optical activity make chiral nanoscrolls ideal for the detection and sensing of small chiral molecules. Here, we present a simple physical model of chiroptical phenomena in multilayered tapered semiconductor nanoscrolls. Our model is based on a linear transformation of coordinates, which converts nanoscrolls into flat but topologically distorted nanoplatelets whose optical properties can then be treated analytically. As an illustrative application example, we analyze absorption and circular dichroism spectra of CdSe nanoscrolls using an eight-band model of CdSe. We show that the optical activity of the nanoscrolls originates from the chiral distortion of their crystal lattice and determine selection rules for the optically active interband transitions. The results of our study may prove useful for the modeling and design of semiconductor nanoscrolls and nanoscroll-based materials.

Single-Handed Helical Carbonaceous Nanotubes: Preparation, Optical Activity, and Applications

Carbon-based nanomaterials have been widely studied in the past decade. Three approaches have been developed for the preparation of single-handed helical carbonaceous nanotubes. The first approach uses the carbonization of organopolymeric nanotubes, where the organic polymers are polypyrrole, 3-aminophenol-formaldehyde resin, and m-diaminobenzene-formaldehyde resin. The second approach uses the carbonization of aromatic ring-bridged polybissilsesquioxane followed by the removal of silica. Micropores exist within the walls of the carbonaceous nanotubes. The third approach uses the carbonization of organic compounds within silica nanotubes. This hard-templating approach drives the formation of helical carbonaceous nanotubes containing twisted carbonaceous nanoribbons. All of these helical carbonaceous nanotubes exhibit optical activity, which is believed to originate from the chiral π-π stacking of aromatic rings. They can be used as chirality inducers, and for lithium-ion storage.

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.

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.

Amplification of chiroptical activity of chiral biomolecules by surface plasmons

Chiral molecules are shown to induce circular dichroism (CD) at surface plasmon resonances of gold nanostructures when in proximity to the metal surface without direct bonding to the metal. By changing the molecule-Au separation, we were able to learn about the mechanism of plasmonic CD induction for such nanostructures. It was found that even two monolayers of chiral molecules can induce observable plasmonic CD, while without the presence of the plasmonic nanostructures their own CD signal is unmeasurable. Hence, plasmonic arrays could offer a route to enhanced sensitivity for chirality detection.

Chirality of the 1,4-phenylene-silica nanoribbons at the nano and angstrom levels

We reported the preparation of chiral 1,4-phenylene-silicas, using a sol-gel transcription approach, by self-assembly using low-molecular-weight gelators as templates. The silicas exhibited chirality at both the nano and angstrom levels. However, the relation between the chirality at the nano level and that at the angstrom levels has not been well studied. In this study, chiral 1,4-phenylene-silica nanoribbons were prepared by the self-assemblies of three chiral cationic gelators derived from amino acids as templates. These samples were characterized using field-emission scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and circular dichroism. The results indicated that the handedness of the nanoribbons and the stacking of the aromatic rings were controllable. Although the nanoribbons exhibited left-handedness at the nano level, the stacking of the aromatic rings could exhibit left- or right-handedness. The handedness of the nanoribbons at the nano level was controlled by the organic self-assembly of the gelator. However, the stacking of the aromatic rings seemed to be controlled by the gelator itself.

Multifaceted prismatic silver nanoparticles: synthesis by chloride-directed selective growth from thiolate-protected clusters and SERS properties

We describe the synthetic preparation of well-defined symmetric multifaceted prismatic silver nanoparticles with chemically controlled faceting advantageous for strong and tunable surface-enhanced Raman scattering, SERS. These silver nanoparticles, that have been termed nanoflowers, AgNFls for their characteristic morphologies, have been prepared by a one-pot aqueous reaction under ambient conditions. AgNFl faceting is synthetically controlled by selective nanoparticle growth driven by chloride ions. Selective chloride binding to the surface of growing AgNFls results in nanoparticle enlargement predominantly at the points of their highest energy. These growth points are located at the tips of prismatic polygons in precursor prismatic morphologies that have been produced from thiolate-protected silver clusters whose coalescence is triggered with a strong base. For the practical aspects of AgNFl synthesis, concentrations of thiol and a strong base were found to be the key variables reliably controlling the extent of AgNFl faceting, as well as the kinetics of AgNFl formation and their stability. The selective growth of AgNFls progresses slower compared to that of non-faceted prisms: fewer nuclei can form leading to larger AgNFls with the diameter ranging from 130 to 2250 nm and asperity sizes on the order of 20 to 100 nm. Self-assembly of AgNFls yields columnar stacking. AgNFls were demonstrated to function as a promising substrate for surface-enhanced Raman scattering. SERS measurements were performed for a series of AgNFls with variable faceting, where the enhancement factors of 4.6 × 10(8) and 425 have been achieved for dry solid films and aqueous dispersions of non-aggregated AgNFls with single-particle enhancement, respectively. These SERS results are promising, especially in combination with that AgNFl nanoscale asperities can be conveniently tailored synthetically. Overall, AgNFls offer valuable opportunities for a system with synthetically variable nanoscale asperities.

Chiral electromagnetic fields generated by arrays of nanoslits

Using a modal matching theory, we demonstrate the generation of short-range, chiral electromagnetic fields via the excitation of arrays of staggered nanoslits that are chiral in two dimensions. The electromagnetic near fields, which exhibit a chiral density greater than that of circularly polarized light, can enhance the chiroptical interactions in the vicinity of the nanoslits. We discuss the features of nanostructure symmetry required to obtain the chiral fields and explicitly show how these structures can give rise to detection and characterization of materials with chiral symmetry.

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.

On the nature and importance of the transition between molecules and nanocrystals: towards a chemistry of "nanoscale perfection"

This paper discusses the importance of the transition between molecular compounds and nanocrystals. The boundary between molecular and nanocrystals/nanoclusters can be defined by the emergence of the bulk phase; atoms in the core of the nanoclusters that are not bound to ligands. This transition in dimensions and structural organization is important because it overlaps with the boundary between atomically defined moieties (molecules can be isolated with increasing purity) and mixtures (nanocrystals have a distribution of sizes, shapes, and defects; they cannot be easily separated into batches of structurally identical species). Passing through this boundary, as the size of a structure increases beyond a few nanometres, the information about the position of each atom gradually disappears. This loss of structural information about a chemical structure fundamentally compromises our ability to use it as a part of a complex chemical system. If we are to engineer complex functions encoded in a chemical language, we will need pure batches of atomically defined (truly monodisperse) nanoscale compounds, and we will need to understand how to make them and preserve them over a broad range of length scales, compositions, and timeframes. In this review we survey most classes of monodisperse nanomaterials (mostly nanoclusters) and highlight the recent breakthroughs in this area which might be spearheading the development of a chemistry of "nanoscale perfection".

Circular dichroism and UV-Vis absorption spectroscopic monitoring of production of chiral silver nanoparticles templated by guanosine 5'-monophosphate

Herein we report the chemical reduction of silver ions incorporated into chiral supramolecular nanostructures by NaBH(4) in buffered (basic) and unbuffered conditions. In situ self-assembly of guanosine 5'-monophosphate (5'-GMP) templated by Ag(I) and generation of silver nanoparticles (NPs) were continuously monitored by CD and UV-Vis spectroscopy measurements. 5'-GMP has been identified as an efficient chiral organic ligand to complex silver ions into a hierarchical helical nanostructure and is a useful capping agent for stabilizing silver NPs with a size diameter lower than 20 nm. The observation of opposite signed bands in the CD spectra of Ag(I)/5'-GMP complexes at different pH has suggested the existence of opposite-handed supramolecular helical structures depending on pH. Both helical supramolecular structures induce chirality in the silver NPs during their growth of the same handedness as shown by the CD signals in the plasmon resonance band.

Plasmon-induced CD response of oligonucleotide-conjugated metal nanoparticles

Non-chiral metal nanoparticles conjugated with chiral oligonucleotide molecules demonstrated a circular dichroism (CD) at the plasmonic wavelengths due to aggregation effects.

Chiral inorganic nanoparticles: origin, optical properties and bioapplications

Chirality of inorganic nanoparticles (NPs) is an emerging and hot topic in nanoresearch in the past several years. Many novel and interesting properties of chiral NPs have been explored and studied, which highlight their importance in both fundamental research and potential applications. This review summarizes recent progress in the study of origins, optical properties and bioapplications of chiral NPs, and future developments in this research area are also discussed.

Chiral CdTe Quantum Dots

Quantum dots (QDs) are fluorescent semiconductor (e.g. II-VI) nanocrystals, which have a strong characteristic spectral emission. This emission is tunable to a desired energy by selecting variable particle size, size distribution and composition of the nanocrystals. QDs have recently attracted enormous interest due to their unique photophysical properties and range of potential applications in photonics and biochemistry.The main aim of our work is develop new materials based chiral quantum dots (QDs) and establish fundamental principles influencing the structure and properties of chiral QDs. Here we report the quantum efficiency control in cysteine capped CdTe quantum dots (QDs) by varying ratios of enantiomeric stabilizers. We also demonstrate that the circular dichroism (CD) of CdTe QDs can be introduced by utilizing the mixture of penicilamine and cysteine stabilizers of the same chirality. This approach results in QDs with the enhanced CD activity, but causes a decrease in the quantum yield and widening of the emission due to the presence of chiral defects at the nanoparticle surface. We believe that these new QDs could find important applications as fluorescent assays and sensors (or probes) in asymmetric synthesis, catalysis, enantioseparation, biochemical analysis and medical diagnostics.

Metal-metal-interaction-facilitated coordination polymer as a sensing ensemble: a case study for cysteine sensing

A detailed investigation of the absorption and CD signals of Ag(I)-cysteine (Cys) aqueous solutions at buffered or varying pH has allowed us to suggest that coordination polymers are formed upon mixing Ag(I) and Cys bearing a Ag(I)-Cys repeat unit. The formation of the coordination polymers are shown to be facilitated by both the Ag(I)···Ag(I) interaction and the interaction between the side chains in the polymeric backbone. The former allows for an immediate spectral sensing of Cys with enantiomeric discrimination capacity with both high sensitivity and selectivity, and the contribution of the side-chain/side-chain interaction serves to guide extended sensing applications by means of modulating this interaction. With our preliminary data on the corresponding Cu(I)-Cys and Au(I)-Cys systems that exhibited similar spectral signals, we conclude that the M(I)-SR coordination polymers (M = Cu, Ag, or Au) could in general function as spectral sensing ensembles for extended applications. This sensing ensemble involves the formation of coordination polymers with practically no spectral background, thus affording high sensing sensitivity and selectivity.

Nanoparticles in liquid crystals and liquid crystalline nanoparticles

Combinations of liquid crystals and materials with unique features as well as properties at the nanoscale are reviewed. Particular attention is paid to recent developments, i.e., since 2007, in areas ranging from liquid crystal-nanoparticle dispersions to nanomaterials forming liquid crystalline phases after surface modification with mesogenic or promesogenic moieties. Experimental and synthetic approaches are summarized, design strategies compared, and potential as well as existing applications discussed. Finally, a critical outlook into the future of this fascinating field of liquid crystal research is provided.

Size-dependent transition to high-symmetry chiral structures in AgCu, AgCo, AgNi, and AuNi nanoalloys

A class of nanomaterials possessing the highest degree of chiral symmetry, the chiral icosahedral symmetry, is found by a combination of global optimization searches and first-principle calculations. These nanomaterials are core-shell nanoalloys with a Cu, Ni, or Co core and a chiral Ag or Au shell of monatomic thickness. The chiral shell is obtained by a transformation of an anti-Mackay icosahedral shell by a concerted rotation of triangular atomic islands which breaks all mirror symmetries. This transformation becomes energetically favorable as the cluster size increases. Other chiral nanoalloys, belonging to a different structural family of C(5) group symmetry, are found in the size range between 100 and 200 atoms. High-symmetry chiral nanoalloys associate strong energetic stability with potential for applications in optics, catalysis, and magnetism.

High yield synthesis of pure alkanethiolate-capped silver nanoparticles

One-phase, one-pot synthesis of Ag(0) nanoparticles capped with alkanethiolate molecules has been optimized to easily achieve a pure product in quantitative yield. We report the synthesis of dodecanethiolate-capped silver particles and the chemophysical, structural, and morphologic characterization performed by way of UV-vis, (1)H NMR, and X-ray photoelectron (XPS) spectroscopies, X-ray powder diffraction (XRD) and X-ray absorption fine structure analysis (XFAS), electron diffraction and high-resolution transmission electron microscopy (HR-TEM), and scanning and transmission electron microscopy (SEM and TEM). Depending on the molar ratio of the reagents (dodecylthiosulphate/Ag(+)), the mean Ag(0) particle size D(XRD) is tuned from 4 to 3 nm with a narrow size distribution. The particles are highly soluble, very stable in organic solvents (hexane, toluene, dichloromethane, etc.), and resistant to oxidation; the hexane solution after one year at room temperature does not show any precipitation or formation of oxidation byproducts.

Chiral luminescent CdS nano-tetrapods

The utilisation of chiral penicillamine stabilisers allowed the preparation of new water soluble white emitting CdS nano-tetrapods, which demonstrated circular dichroism in the band-edge region of the spectrum.

Plasmonic circular dichroism of chiral metal nanoparticle assemblies

We describe from the theoretical point of view a plasmonic mechanism of optical activity in chiral complexes composed of metal nanoparticles (NPs). In our model, the circular dichroism (CD) signal comes from the Coulomb interaction between NPs. We show that the CD spectrum is very sensitive to the geometry and composition of a chiral complex and also has typically both positive and negative bands. In our calculations, the strongest CD signals were found for the helix geometry resembling helical structures of many biomolecules. For chiral tetramers and pyramids, the symmetry of a frame of a complex is very important for the formation of a strong CD response. Chiral natural molecules (peptides, DNA, etc.) often have strong CD signals in the UV range and typically show weak CD responses in the visible range of photon energies. In contrast to the natural molecules, the described mechanism of plasmonic CD is able to create strong CD signals in the visible wavelength range. This plasmonic mechanism offers a unique possibility to design colloidal and other nanostructures with strong optical chirality.

Synthesis of glycal-based chiral benzimidazoles by VO(acac)2-CeCl3 combo catalyst and their self-aggregated nanostructured materials

VO(acac)(2)-CeCl(3) combo catalyst has been developed for chemoselective cyclocondensation cum oxidation under mild reaction conditions toward synthesis of a new class of optically pure compounds, 2-(2'-C-3',4',6'-tri-O-benzyl/methyl-glycal)-1H-benzimidazoles. It involves an operationally simple synthetic protocol efficient for the syntheses of a wide range of chiral benzimidazoles in high yields without formation of undesired 1,2-disubstituted and pseudoglycal byproducts. Vanadium(V) is found as active oxidant for the chemical processes which is investigated by UV absorption spectroscopy. Highly ordered one-dimensional low molecular mass organic nanostructured materials are fabricated by nanocrystallization of the chiral nanoscale building blocks. Theoretical calculation by the B3LYP/6-31G** level of theory of the glycal-based chiral benzimidazoles shows out of planar geometry of the 1H-anthra[1,2-d]imidazole-6,11-dione moiety, which is responsible for the strong self-aggregation to generate ultralong nanostructured materials. We have also found nice agreement between the theoretical results with the experimental observation in 2D-NOESY experiments. The photophysical property of the solid nanostructured materials is also reported.