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W3199949059 abstract "A chiral object is one that is non-superimposable on its mirror image by rotational and translational operations. Chirality has unifying fundamental importance for physics, chemistry, astronomy, mathematics, biology, and medicine.1, 2 It is important to emphasize that the discovery of chirality and subsequent proliferation of the concepts of mirror asymmetry in these sciences were prompted by chemical engineering research. Chirality of chemical compounds was discovered by Louis Pasteur (Figure 1), who at that time was investigating the causes of wine spoilage. Pasteur's studies were commissioned by the chemical engineering practitioners —wine and spirit makers of France.3 At age 25, Pasteur discovered the optical implications of chirality in 1848 while analyzing sodium ammonium tartrate crystals under a microscope.4 He observed two types of crystals that were similar but not identical because these crystals were non-superimposable mirror images of each other. A few years later, Pasteur made a conclusion, which became the foundation of stereochemistry: chirality of molecules leads to chirality of their crystals. Solutions of these compounds are optically active, that is, they rotate the plane of linearly polarized light in opposite directions.4 Subsequently, in 1874, van't Hoff5 and Le Bel6 independently proposed that molecular asymmetry could result from the tetrahedral arrangement (configuration) of four different groups on an atom. The formal definition of chirality was given by Lord Kelvin in the Baltimore Lectures on Molecular Dynamics and the Wave Theory of Light in 1904: “I call any geometrical figure, or group of points, chiral, and say that it has chirality if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself”.7 Note that this definition was given many years later after the actual discovery of the role and some practical implications of chirality in chemical processes. Since these times molecules with non-superimposable mirror images are referred to as enantiomers. Among many possible mirror-asymmetrical geometries, chemical technologies are associated with two of them more often than others—tetrahedrons and helices. When an sp3 carbon with tetrahedral geometry has four different substituents, it is chiral and is typically referred to as an optical center, a stereocenter, or a chiral carbon. Simple molecules with one optical center, such as proteinogenic amino acids (AAs), are designated as L- or D-enantiomers according to the Fischer-Rosanoff convention established in 1906.8 Sugars and AAs with the same relative configuration as (+)-glyceraldehyde are assigned D (from the Latin dexter, meaning “right”), and the same as (−)-glyceraldehyde are assigned L (from the Latin laevus, meaning “left”). Because only relative polarization rotation data and chemical configuration of optical centers could be determined at the beginning of the 20th century, limited information was available about the relationship between optical polarization rotation and the chemical structure of the compound giving rise to the rotation. L- or D-notations cannot be used to determine the absolute geometric configuration or the rotation of polarized light, and, thus, a list of chirality descriptors for chemical structures was eventually expanded to include optical properties and other chiral geometries. While D- and L- notations correspond to the spatial arrangement of atoms around the chirality center (or molecule configuration), the prefixes (+) or (−) used above refers to the actual optical activity of the substance: whether it rotates the plane of polarized light clockwise (+) or counterclockwise (−). In 1951, Bijvoet et al.9 established the absolute configuration (the exact spatial arrangement of atoms independent of other molecules) of sodium rubidium (+)-tartrate tetrahydrate using X-ray crystallographic methods. This finding was reconfirmed in 2008 by Lutz and Schreurs using modern XRD techniques combined with the computational processing of the scattering data.10 In 1956, Cahn, Ingold, and Prelog proposed a convention11 (i.e., Cahn-Ingold-Prelog rule) that allowed the designation of the absolute configuration at chiral centers in organic molecules in three-dimensional space. According to a specific set of rules based on atomic numbers, the absolute configuration is designated either as R (from the Latin rectus, meaning “right, in the sense of being correct”) or S (from the Latin sinister, meaning “left, improper”). Note that R/S designations cannot be used to predict the direction of polarization rotation. Axial chirality is a standard geometry of mirror-asymmetric molecules exemplified by allenes and substituted biphenyls.12 These molecules do not have a chiral center but an axis of chirality, about which a set of substituents is held in a spatial arrangement that is not superposable on its mirror image. The enantiomers of axially chiral compounds are usually designated as Ra and Sa, where the asymmetric element is referred to as an improper axis. Molecules may have an improper axis of symmetry but not being chiral, with the best example being the tetrahedral CH4 molecule. Helices represent another standard chiral geometry encountered in biomolecules, biological assemblies, tissues, and organs.13 In organic chemistry, helices are widely presented by helicenes and their derivatives,14 in inorganic chemistry—by multiple inorganic nano- and micro-structures.15, 16 Helices rotating in a counterclockwise manner are typically referred to as left-handed or Λ-enantiomers, whereas those rotating clockwise are referred to as right-handed or Δ-enantiomers. All of these geometries are unified by the possession of a screw axis of symmetry, a typical element of chiral structures at the macro scale. Several extensive reviews on chiral inorganic nanoparticles (NPs) and their assemblies were published over the last few years that described, in various degrees of detail, the chiral nanostructures known today.17-30 Along with their preparation and optical properties, these works also discuss their potential applications which include but not limited to chiral catalysis, enantiospecific separation, biosensing, chiral memory, and chiroptical devices. The most important properties of chiral nanostructures upbearing their applications are (1) strong absorption of circularly polarized light (CPL) and (2) enantioselective interactions with biological objects. For brevity, we will not repeat the common points between these reviews but rather focus on the foundational concepts (Section 2), optical properties (Section 3), and chemical methods (Section 4) being used for engineering their structures and properties. To accomplish this, we must first describe the nature of chirality in inorganic nanostructures and their properties. There are distinct similarities and vast differences in the ways chirality manifests in nanostructures compared to traditional chemical compounds and biomolecules. Mirror asymmetry in NPs and their assemblies is nearly always present at multiple scales and with numerous geometrical elements, making the traditional notations used in organic and inorganic chemistry (R, S) only partially applicable. Except for a few notable cases where the chirality of the nanoscale structures is imparted by illumination with circularly polarized photons,31-34 the primary contribution to the chirality of nanostructures is associated with organic molecules that are chemically bound to their surface. These organic molecules, or surface ligands, represent molecular scale asymmetry embedded in NP structures and are directly relevant to the engineering of all nano- and microstructures. Mirror asymmetries at the same scale can also arise from the binding of achiral molecules to the NPs, creating an optical center that is topologically similar to those in L- and D-AAs.35 The secondary contribution to chirality is related to the NPs organic–inorganic interface that engenders new hybrid electronic states formed between the organic molecules on the surface and the NPs inorganic core. The molecular orbitals describing these electronic states can be strongly asymmetric, which gives rise to the quantum level of chirality. The existence of such orbitals was first established for CdSe NPs,36 revealing the hybridization between the highest occupied molecular orbitals of surface atoms with those of the chiral ligand. This phenomenon was subsequently found in other nanomaterials as well.37-39 As the physical dimensions of chiral geometries are increased, the third contributor to the chirality of nanostructures arises from the surface itself. The difference between the previous case of interfacial states and the surface chirality is that the former one refers to the quantum electronic states formed between organic groups and the inorganic phase, while the latter one refers to the geometry of the surface overall, that is in nanoscale for most cases. The surface distribution of stabilizing ligands is non-random and can be mirror-asymmetric, which becomes especially obvious for small nanoscale clusters.40 It is worth mentioning that individual surface facets of even bulk symmetric solids can be chiral and many complex metal oxides can also exhibit low-energy chiral surfaces.41, 42 The scale of chiral geometries transitions to nanoscale in at least two (curved) dimensions while remaining at the molecular scale in the third. In this sense, surface chirality can be treated as semi-dimensional because it is dominated by planar asymmetry. Applying methods of non-Euclidean space geometry allows for the description of the mirror asymmetry of the NPs in curved space. The next level of chirality, also associated with nanometer-scale geometries, is the crystal lattice of the NPs. Atomic packing in the inorganic phase during the formation of NP cores is affected by the chirality of the surface ligands, which can be particularly strong for small NPs (e.g., 2–3 nm).43, 44 The evidence of chiral distortions of the inorganic lattice of NPs was also observed experimentally for CdS NPs stabilized with penicillamine (Pen),45 cysteine methyl ester hydrochloride-stabilized CdTe NPs,46 and Co3O4 NPs stabilized with cysteine (Cys).43 The chiral distortions of the atomic lattice in small NPs can also be observed computationally (Figure 2A), for example, in Co3O4 and WO3-x NPs.43, 47 The Ramachandran plots, used extensively in structural biology, can be applied to visualize and analyze such distortions (Figure 2B). All of these contributions to mirror asymmetries have characteristic dimensions of these chiral elements, that is, ~1–3 nm. As the NPs become larger, the distortions of the crystal lattice caused by surface ligands disappear for the core, but they may be retained in certain situations. Mirror asymmetry of the surface ligands can also preferentially bias the chiral defects in the inorganic core, which can be exemplified by screw and edge dislocations for various semiconductor NPs (Figure 3A), nanowires, dendrites, carbon nanotubes, and Au nanoplates.48, 50-54 Recent studies revealed that screw dislocations mediate chirality transfer between crystal structure and morphology of model tellurium nanocrystals, and geometrical chirality is not an outcome of the chiral crystal structure or ligands.55 The next level of nanoscale chirality is related to the overall geometry of the NPs that is sometimes referred to as intrinsic or geometrical chirality. Small NPs with zinc-blende crystal lattices were shown to have truncated tetrahedral shapes,31, 35, 56, 57 which is geometrically homologous to an sp3 carbon in optical centers.35 Consequently, mirror-asymmetric tetrahedrons emerge at the nanometer scale when all four corners of the NP tetrahedron are different. For example, the small CdTe NPs can have truncated tetrahedral shapes and have been observed using transmission electron microscopy (TEM, Figure 3B). Considering that non-spherical NPs are commonly found in Nature, the chirality of NPs is not surprising. When the shapes are random, the particle dispersions are racemic.32 The presence of the chiral bias ether due to AAs, circularly polarized photons, or intrinsically chiral crystal lattices changes the NP growth patterns and shifts the distribution of the NP shapes during synthesis toward one or the other chiral geometry.32, 58 For example, this effect can be seen for complex chiral shapes with propeller-like geometries because of the preferred growth on high index planes.59 Mirror-asymmetry of chiral molecules and individual NPs can be transferred into the chirality of their assemblies, which represents the next scale and contribution to chirality in nanostructures. In the simplest case, it leads to mirror-asymmetric superstructures from nanoscale components with the characteristic scale of chirality in between ~20 nm32, 60-63 and ~10 μm.64-66 For example, the observation of twisted nanosheets with a 1300 nm pitch is a clear demonstration how mirror asymmetry at the nanometer and micrometer length scales emerges due to the chirality transfer from molecular components of nanostructures.64 The chirality transfer in NP assemblies can be governed by hydrogen bonding, entropic, and dispersive interactions. The interactions at NP-NP interfaces may result in enantiopure chiral helices67, 68 or twisted ribbons.31 A large class of chiral assemblies of NPs can also be created via templating by biomolecules that could be treated as external source of chirality.17 In this case, the NP building blocks may be racemic or nearly achiral (spheres, rods, triangular nanoplates), and mirror asymmetry emerges during their preferential organization due to geometrically specific interactions with, for example, segments of DNA, proteins, peptides, lipids, sugars, etc. The chiral shapes of these structures can be nanorod pairs,69-71 that can also develop into helical “log-stacks”72 as more nanorods are added. Other geometrical shapes include left-handed (Λ) and right-handed (Δ) helices obtained by folding DNA strands.73 Complex chiral shapes can also be obtained by interconnecting NPs with multiple “springs” represented by DNA.74 Their chirality may not be initially obvious by visual comparison with familiar mirror-asymmetric shapes, such as helices, but they can be identified using chirality measures, for instance, Osipov-Pickup-Dunmur index that changes sign depending on the handedness of the structure.75 Continuing the consideration of scale, one can foresee the translation of the chirality of NPs into sub-millimeter, millimeter, and macroscale.76 Considering the rapid expansion of chiral nanostructures toward biology and medicine, it is instructive and insightful to draw parallels between hierarchical chirality in biological compounds and inorganic nanostructures. As a result of hydrogen bonds and other intermolecular interactions, AAs molecular scale chirality is transferred to the nanoscale chirality of proteins. Proteins are organized into four levels,77 with chiral motifs at different scales (Figure 4A). The further self-assembly and aggregation of proteins can result in complex chiral structures at larger scales, for example, helical protein fibrils.79 These phenomena demonstrate the distinct correlation between biological compounds and chiral inorganic NPs—hierarchical chirality (Figure 4B). However, there are many differences, too, that researchers are in the process of understanding. One of them is the transfer of handedness of building blocks to the higher-order structures. While bioorganic molecules have the apparent stereochemical bias at the secondary structure level (Figure 4A), where right-handed helices are strongly preferred over left-handed helices, the usage of L-Cys as a chiral bias in the self-assembly of chiral inorganic nanostructures can result in both right-handed67, 80 and left-handed67 helical structures.These finding clearly indicate that there is no deterministic transfer of handedness from one scale to another and having molecular components with, say, left symmetry does not mean by any measure that the nanoscale assembly must be also left. To some degree, this is also true in biology because our left and right hands are made from the same L-enantiomers of amino acids. Tracing the bifurcation point and the forces that control is important for both nanoscale inorganic and biological structures. Chirality at multiple scales can be utilized as a metric for engineering NP and their assemblies. Before we discuss the practical approaches to engineering of chiral nanostructures, we need to discuss what the nanoscale design will be for and what are physical, chemical, and biological properties of the nanostructures that we aim to obtain. These properties are derivatives from the chiral geometry at multiple scales and reflect the profound effects of chirality on optical, electrical, and magnetic effects specific to chiral materials. Chemical manifestations of chirality can be observed in catalysis, separations, and polymerization.23 Chirality of NPs is also expected to have multiple implementations in biology that include their effect on drug delivery, cytotoxicity, cell signaling, cell adhesion.81 All of these properties can be related to specific quantitative measures associated with chiral geometries exemplified by the Osipov-Pickup-Dunmur index calculated for the scale(s) characteristic for such properties. Note that additional measures of mirror asymmetry are likely to be needed in the future for establishing effective structure-property relationships this article, we will restrict the discussion to optical properties because they were investigated the best so far. The optical effects of chirality are typically observed by circular dichroism (CD) spectroscopy, which is based on differential absorption of left- and right-hand CPL by chiral compounds.82 State-of-the-art CD spectrophotometers can measure across a wide spectral range, from far-ultraviolet (far-UV) to near-infrared (NIR). Other spectroscopic methods for measuring chirality include optical rotatory dispersion (ORD), Raman optical activity (ROA),83 vibrational circular dichroism (VCD), and more recently, terahertz circular dichroism (TCD).84 These methods cover various ranges of energies, linear and non-linear optical processes, and numerous reviews describe their technical details and various implementations.85-89 CD spectra in the visible (Vis) or NIR range (intensity units are most often expressed in millidegrees) represent the ubiquitous tool to measure chiroptical activity of NP dispersions or thin films. For quantitative analysis of the chiroptical activity, the dissymmetry factor, or g-factor [g = Δε/ε = (εL − εR)/(εL + εR)] is used, where Δε is the molar CD, ε is the molar extinction, and εL and εR are the molar extinction coefficients for left- and right-hand CPL, respectively. As a metric for materials design, g-factor reflects the strength of polarization rotation compared to the strength of light extinction, which is essential for multiple applications because it directly relates to the intensity of transmitted polarized light (rather than being dissipated) for a chiral material. A valid CD experiment for comparing the chiroptical activity of two dispersions of enantiomers involves measuring the amplitudes of spectral bands with nearly equivalent extinction coefficients to avoid multiple concentration-depended processes such as agglomeration and scattering. For thin films, acquiring CD spectra requires even greater care to avoid artifacts from linear dichroism and circular birefringence. These measurements should be performed using Mueller matrix polarimetry90 as it offers an unambiguous and mathematically robust platform for the analysis of light-matter interactions.91 Engineering chiroptical properties of materials includes maximization of CD amplitude and g-factor as well as ‘tuning’ of the chiroptical bands to specific wavelengths. Starting with the latter, the optical properties of nanostructures greatly expand the wavelength/frequency range of chiroptical activity compared to traditional chiral materials. While the typical range for the CD bands of AAs, proteins, and other biomolecules is in the UV region, organic dyes and metal coordination compounds can expand to the Vis and NIR ranges,92-95 typically at the cost of chemical stability. The current palette of chiral inorganic nano- and microstructures includes materials with chiroptical activity in UV,96 Vis,97 NIR,67 medium IR,98 reaching as far as terahertz part of the electromagnetic spectrum.84 Furthermore, the position of the bands and their simultaneous occurrence in different spectral windows can be fine-tuned by all the elements of chirality discussed in Section 2. Besides some nanomanufacturing techniques, resulting nanostructures can be economically produced by self-assembly using methods described in Section 4. Inorganic chiral nanostructures have stronger chiroptical activity than that of similarly sized organic objects because of their high electrical polarizability and magnetic susceptibility.17, 99 While polarizabilities of some organic molecules can be ~2–100 Å3, those of semiconductor NPs are ~104–105 Å3, magnetic and metal NPs have polarizabilities of ~106 Å3. The polarizabilities of NPs increase with their size,99 which results in a marked increase of chiroptical activity in most cases. For example, increase of g-factors observed when transitioning from semiconductor NPs (g = 10−5 − 10−4)100 to semiconductor nanorods (g = 10−4)100 and semiconductor nanoplatelets (g = 10−3).101 This is particularly true for CD amplitudes based on the cumulative effects of light absorption and scattering.31 However, this does not necessarily mean that with NP size increase, the CD strength of chiral NPs will increase. According to the Rosenfeld equation,102 polarizability is only one of the multipliers that could affect the rotatory strength of matter. Therefore, decreasing other parameters while increasing NP size will not result in optical activity increase.36, 100 An increase of optical asymmetry, that is, the ability of the material to rotate the polarization of light without dissipating it, is limited by scattering from nanostructures.103 Most NPs and their assemblies are strong scatterers, leading to low g-factors. Theoretical works demonstrated that, due to the presence of a screw dislocation, nanocrystals of cylindrical shapes, such as semiconductor quantum disks, can have high g-factors up to 0.234,104, 105 which significantly exceed those of chiral molecules (~0.0001)106 and are comparable with those of chiral plasmonic nanostructures (~0.3).107 Besides chiral crystal lattice distortions caused by mirror asymmetry of surface ligands and curved surface of nanostructures (see 2.2, Figure 2), Inorganic crystals can also have chiral crystal lattices. Non-centrosymmetric materials are not “exotic” with the hundreds of known non-centrosymmetric oxides.108 Other bright examples are quartz, β-AgSe, α-HgS, selenium, tellurium. It is expected that NPs based on this arrangement should exhibit much stronger chiroptical activity compared to NPs with achiral space groups. Indeed, it was demonstrated that α-HgS NPs synthesized in the presence of chiral surface ligands, such as Pen, exhibited g-factors up to 0.012.109 At the same time, the chiroptical activities of CdS, CdSe, CdTe, and ZnS NPs with tethered chiral surface ligands were several orders of magnitude smaller, with g-factors less than 0.001.109-111 The combination of chiral lattice and shape can increase the g-factors of nanomaterials even further.112 An increase of g-factors can also be attained by increasing the long-range order in the NPs assemblies.103 At the quantum and molecular levels, the optical effects of chiral inorganic nanostructures observed by CD spectroscopy are weaker than those that arise from other chiral structural components. In other words, the contributions to CD from mirror asymmetries are more significant in scale and more substantial. For instance, individual CdTe NPs typically have weak chiroptical activity and low g-factor values on the order of 10−5.80 CdTe NPs assemble into mesoscale helices that exhibit g-factors up to ~0.01 at λ = 900 nm.80 The presence of chiral centers on the surface of NPs significantly contributes to their light-emitting properties and, in particular, the appearance of circularly polarized luminescence.113 For example, L- and D-Cys ligands induced circularly polarized luminescence in the dispersions of CdSe NPs.36 After emission, the polarization of the photons can be modified by the scattering from the NPs and other particles in the dispersion, as was observed for chiral complex structures.64 In particular, at 650 nm, the polarization rotation of the emission from the quantum states in twisted Au-S nanosheets with L- or D-Cys surface ligands changed its sign but not the wavelength when the dispersed scatterers disassembled from micron-sized chiral complex structures to twisted nanosheets with ~1300 nm pitch. Therefore, along with mirror asymmetry of molecular orbitals giving rise to CPL emission from chiral nanostructures, one should also include practical and theoretical considerations on circularly polarized scattering that may also be referred to as differential scattering.70, 114 Based on the understanding of the multiscale chirality of inorganic nanostructures (Section 2) and the optical characteristics that one wants to attain (Section 3), let us now discuss how to create a wide range of chiral inorganic nanostructures. In this section, we will consider their engineering starting from quantum and molecular scale to nanometer and micrometer scale using established and emerging chemical engineering techniques. Examples of practical implementations for concrete chiral nanostructures will be provided for each method. Note that in many cases, and especially for the processes involving structures with chirality at nano-, meso-, and microscale, the chemical processes are intertwined, and the resulting structures are the products of chiral interactions at multiple scales occurring simultaneously. Among optical characteristics, we will pay special attention to the g-factor because it can be universally applied across different scales, concentration ranges, and has direct technological significance. Chiral NPs are commonly synthesized as dispersions in liquid media in the presence of chiral ligands, which serve as chiral bias. These ligands also act as stabilizers by imparting colloidal and thermodynamic stability to nanoscale colloids. Chirality transfer from chiral ligands to NPs occurs during the nucleation and arrested growth of NPs when chiral molecules attach to the NPs nucleating seeds and surface. Alternatively, this may take place when chiral molecules remain free in solution and generate the chiral environment around growing NPs.115 Chirality of individual NPs can be further translated to complex structures and assemblies based on them. Despite the intensive studies in the field of chiral growth of NPs,17 the correlation between the actual geometrical arrangements of atoms and corresponding optical activity of the resulting NPs is yet to be established. The presence of chiral components during the crystallization process of relatively small NPs (~1–10 nm) leads to their asymmetric growth because various crystalline planes of achiral crystal lattices display different ligand density, ligand-NP bonds, and surface energy. The optical activity of these nanostructures can be tuned by careful adjustment of synthesis conditions, where both types of chiral molecules and their concentration were shown to be critical factors determining the growth of chiral nanocrystals.67, 116, 117 As demonstrated with NPs from tellurium,112 structures with chiral lattice and achiral shape may be chiroptically silent for the spectral range associated with the electronic transitions of the core inorganic material. It directly depends on the quantum nature of electronic transitions associated with such crystal lattice and is a subject to the Brillouin zone selection rules. Te NPs of ~140 nm in length with both chiral lattice and shape have g-factors ~0.003. The significant contribution of the chiral shape rather than chiral lattice toward the chiroptical activity of NPs was supported by simulations of their optical properties. The type of chiral molecules used for the synthesis of tellurium nanostructures and their order of addition was found to affect the final shape of NPs. For instance, the addition of glutathione followed by hydrazine yielded long tellurium nanorods with small g-factors, while hydrazine and glutathione added in a reverse order resulted in tellurium nanocrystals, with higher g-factors. Decoupling of chirality at atomic and nanometer scales in inorganic nanostructures can be achieved through epitaxial growth based on a two-step synthesis (Figure 5A)58 starting with 12 nm α-HgS seeds. These NPs had well-defined crystallographic chirality due to the helical arrangement of Hg and S atoms along the c axis (Figure 5B). Slow, co-addition of Hg and S precursors ensured successive ion layer adsorption and reaction onto the seeds in excess of D- and L-Pen serving as chiral surface ligands. As a result, twisted triangular bipyramid nanostructures with chiral morphologies were obtained (Figure 5C). These structures had an average length and aspect ratio of 76.9 nm and 1.90, respectively. The optical properties of these nanostructures were compared to those of α-HgS nanostructures, which also possessed crystallographic chirality but had achiral morphologies (nanocubes, nano-ellipsoids, nanorods, and nanowires), revealing that the optical activity of α-HgS nanostructures with achiral morphologies originated from the chiral crystal" @default.
W3199949059 created "2021-09-27" @default.
W3199949059 creator A5034588765 @default.
W3199949059 creator A5081872759 @default.
W3199949059 creator A5083242280 @default.
W3199949059 date "2021-10-14" @default.
W3199949059 modified "2023-10-14" @default.
W3199949059 title "Engineering of inorganic nanostructures with hierarchy of chiral geometries at multiple scales" @default.
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