Chiral nanomaterials: theory, synthesis, applications and challenges

Nicholas A. Kotov

Nicholas A. Kotov is a pioneer of theoretical foundations and practical implementations of complex self-assembled nanocomposites being used in various technologies. Examples include nanocomposites from graphite oxide, montmorillonite clay, and cellulose nanofibers. Chiral nanostructures, biomimetic nanomaterials, and graph theoretical representations thereof, are focal points in his work. Nicholas is a recipient of 60+ national and international awards. Nicholas is an advocate for scientists with disabilities.

Jeanne Crassous

Jeanne Crassous (born Costante) is a CNRS Research Director at the Institut des Sciences Chimiques de Rennes (University of Rennes, France) where she is dealing with various fields related to chirality: organometallic and heteroatomic helicenes, fundamental aspects of chirality like parity violation effects, chiroptical activity such as electronic and vibrational circular dichroism, or circularly polarized luminescence, magnetochirality and spintronics.

David B. Amabilino

David Amabilino had his interest in chirality sparked when he moved to the Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) four years before the turn of the century as a postdoc. After seven years at the University of Nottingham, he returned to ICMAB as a research professor. He edited one of the first books on “Chirality at the Nanoscale”, and started the series of conferences on the subject, the next one of which will be in Italy next year. He looks forward to seeing you there!

Pengfei Duan

Pengfei Duan earned his Ph.D. in 2011 from the Institute of Chemistry, Chinese Academy of Sciences, where his doctoral research centered on supramolecular chirality in self-assembled systems and the exploration of chiral nanostructures formed through self-assembly. Following his Ph.D., he pursued postdoctoral research at Kyushu University, focusing on the functionalization of organized excited triplet states via molecular self-assembly. In 2015, he joined the National Center for Nanoscience and Technology. His current research is primarily dedicated to the photochemistry and physics of chiral nanoassemblies, aiming to advance the understanding and application of these systems in nanotechnology and materials science.

Introduction to chirality of nanostructures

In the most general terms, chirality describes the property of a geometric object that is not identical to its mirror image. This seemingly abstract property has profound manifestations in natural sciences: chemistry, biology, physics and astronomy. Better understanding of our World occurred when the most fundamental objects considered by these disciplines were found to be chiral: elementary atomic particles (e.g. electrons), various molecules (e.g. amino acids), electromagnetic waves (e.g. photons), and vector fields (e.g. galaxies). Beyond that, the fact that Life has made its own choice between left and right, remains probably one of the most important scientific questions.

The mathematical definition of chirality does not care about the physical dimensions of the chiral objects, but natural sciences do. While having some geometric similarity, for example the helix is a common shape, the different scales of the varied objects – from 10⁻²² m to 10²¹ m – require different methods of experimental observation of their chiral properties. In fact, the change in scale results in new chemical and physical manifestations of chirality that were not obvious when remaining confined to one discipline and specific dimensions. The match or the mismatch in scales is also crucial in understanding the interplay between the different chiral objects, that can be exemplified by the interactions of helical photons with amino acids.

This is what we currently observe with the advent of chiral nanomaterials, which is presently among the most rapidly developing areas of science. The transition from the angstrom scale of molecules to the nanometre scale of colloidal systems resulted in unexpected and fascinating phenomena, such as chirality-induced spin selectivity (CISS),¹ giant plasmonic circular dichroism (PCD),² circularly polarized light emission,^3,4 electromagnetic superchirality,⁵ supramolecular motors,^6–9 enantioselective crystal growth,¹⁰ and defect-tolerant complexity.¹¹ These research directions not only resulted in new types of chiral materials and applications, such as chiral perovskites,^12,13 thermally stable chiroptical switches,^9,14 chiroplasmonic particles,¹⁵ biosensors,¹⁶ chiral supramolecular polymers,¹⁷ and triplet state catalysts,¹⁸ but are also propelling new implementations in biology, physics, and engineering, exemplified by immunoadjuvants,¹⁹ chiral phonons,²⁰ and on-chip CPL emitters,²¹ respectively.

Themed issue

This selection of articles submitted to Nanoscale, highlight the state-of-the-art of chiral nanostructures addressing the recent development in their theory, synthesis, and implementations. From a theoretical perspective, the works reported here show the origin of magnetic effects in magnetic circular dichroism (https://doi.org/10.1039/D4NR04422C) and chirality transfer in crystalline structures exemplified by chiral perovskites (https://doi.org/10.1039/D4NR03329A). These studies are inherently interconnected because cumulatively, they are strengthening the link between the mathematical definitions of chirality and its physical phenomena associated with tunable nanoscale chirality. They are complemented by state-of-the-art MD simulations of chiral nucleation and growth that reveal the chirality transfer starting from molecular precursors (https://doi.org/10.1039/D4NR04735D).

There is also an expanding understanding of multiple scales of chirality available in nanostructures, as exemplified by the traditional angstrom-scale chirality typical for classical stereogenic centres based on four-substituted sp³ carbon atoms, sub-100 nm propellers in 532 gold helicoids (https://doi.org/10.1039/D4NR03731F) and submicron chirality of the optically active liquid crystals (https://doi.org/10.1039/D4NR03982C) and nematic porous organosilica (https://doi.org/10.1039/D4NR03326D).

We note the greatly expanding chemistries of chiral nanomaterials reported here. Starting from the staples in the field of chiral nanomaterials, such as chiral gold nanoparticles, chiral quantum dots, and chiral nanocarbons, a great diversity of chiral nanoscale segments can be incorporated into supramolecular assemblies and polymers. For instance, helicenic central cores decorated with aggregating units such as siloxane chains (https://doi.org/10.1039/D4NR03389B) or bisamides (https://doi.org/10.1039/D4NR02110J) can self-assemble into chiral supramolecular systems with improved physico-chemical properties such as well-engineered CPL activity. Other illustrative examples include poly(phenylacetylene) functionalized with chiral anilides, which upon coordination to metallic ions, generate deep-blue delocalized radical anions, thus acting as a colorimetric responsive material (https://doi.org/10.1039/D4NR03662J). Colorimetric systems can also be efficiently developed from the supramolecular assembly of a central triarylamine core decorated with chiral urea derivatives and used for fluoride detection (https://doi.org/10.1039/D4NR04175E). Diversification of chemistry of chiral nanomaterials is also obvious considering the reports on emerging chiral sulfur-based nanomaterials (https://doi.org/10.1039/D4NR03736G), zinc orthophosphates (https://doi.org/10.1039/D4NR03809F) and novel perovskites (https://doi.org/10.1039/D4NR03329A; https://doi.org/10.1039/D4NR04735D). The survey of the latest studies on chiral nanomaterials indicates that perovskites of different types represent one of the fastest growing types of chiral nanomaterials.

Considering classical formation of chiral compounds based on coordination complexes, the self-assembly of sulfur-based chiral systems is known to induce chiral discrimination in the solid state. It is being reported that several interactions involving the S atoms can generate chirality-dependent charge transport properties, as nicely illustrated in chiral metal–dithiolene conductors (https://doi.org/10.1039/D4NR04048A). Multiple self-assembly phenomena from molecular scale building blocks is also reported. Also, new evidence of the “sergeants-and-soldiers” principle for nanoscale structures was obtained for the synthesis of intrinsically chiral Au₁₃ clusters (https://doi.org/10.1039/D4NR03810J). Polymerization-induced chiral self-assembly was used to prepare chiral side-chain mesogen block copolymer assemblies, that exhibit odd–even effects in the supramolecular asymmetrical arrangement of the aromatic units in the material (https://doi.org/10.1039/D4NR02532F). Similarly, mirror asymmetric J-aggregates of porphyrin were observed to form in confinement between montmorillonite particles (https://doi.org/10.1039/D4NR03728F). Following the prior studies of plasmonic nanoparticles with PCD,^22,23 co-assembly of gold nanorods with α-synuclein fibrils enables monitoring the handedness and composition of amyloid nanofibers in post-mortem brain (https://doi.org/10.1039/D4NR03002H).

In relation to the chirality-dependent spin-related phenomena or CISS effects, many nanoscale systems can be developed. In this themed issue chiral tetrathiafulvalene enantiomers embedded into CdS/CdSe core–shell nanoparticles are described, displaying efficient spin filtering, as observed by atomic force microscopy (https://doi.org/10.1039/D4NR04574B).

The chirality transfer across the mesoscale in liquid crystalline boundaries not only reveals remarkable supramolecular behaviour but also continuously produce innovative materials beyond conventional design paradigms. Here, molecular imprinting from a liquid crystal was used to transfer the chiral organization to poly(pyrrole) through spark discharge oligomerisation and subsequent electrochemical polymerization, to give materials that act as diffraction gratings and display electrochromic properties (https://doi.org/10.1039/D4NR03982C). Continuing the theme of chiral conducting polymers, a ferrocene-based conjugated polymer was combined with hydroxypropyl cellulose to give helical polymer liquid crystals with asymmetric induction from the carbohydrate to the functional polymer that displays helical magnetism (https://doi.org/10.1039/D4NR04027A).

Building on this foundation, novel material architectures are emerging: non-chiral charge-transfer complexes co-assembled with chiral amino acids yield multicolour CPL with tunable handedness (https://doi.org/10.1039/D4NR04308A), while chiral nematic mesoporous organosilica films, modified by eco-friendly additives, achieve iridescent colour tuning for photonic and decorative uses (https://doi.org/10.1039/D4NR03326D). Innovations in anticounterfeiting integrate upconversion nanoparticles and phosphors into chiral liquid crystals, creating materials with upconverted circularly polarized persistent luminescence that merge three optical phenomena for secure multilevel encryption (https://doi.org/10.1039/D4NR03819C). High-Precision sensing reaches new heights with G-quartet nanofiber-based CPL sensors for ultrasensitive Hg²⁺/I⁻ detection (https://doi.org/10.1039/D4NR03178D).

As the field evolves, challenges in optimizing chiral nanomaterials for 3D displays, spintronics, and solar devices, underscore the need for scalable, sustainable fabrication and multifunctional integration (https://doi.org/10.1039/D4NR03743J). Collectively, these advancements highlight the transformative potential of chiral materials, promising ground-breaking applications in science and technology as the field continues to evolve.

The self-assembly of π-functional materials to give aggregates that show amplification of chirality that can be observed by CD spectroscopy have long inspired studies of chirality at the nanoscale. In this collection, this type of phenomenon has been used to probe the presence of chiral quartz phases in montmorillonite clay, through aggregation and symmetry breaking of porphyrins confined within the acid-activated mineral (https://doi.org/10.1039/D4NR03728F). Silica can also induce chirality into cast films of racemic helicene derivatives, where the molecules adsorb onto the surface of nanometric oxide helices, where the circular dichroism signal is assigned to helical supramolecular assemblies that the racemic mixture of helicenes form on the inorganic material, with no apparent enantiospecific adsorption (https://doi.org/10.1039/D4NR04292A).

Chiral coordination complexes have proved fascinating in a wide variety of scientific areas, and their rich stereochemistry continues to provide materials with interesting properties. A beautiful example of this rich variety is shown here in the preparation of crystalline nanosized chiral coordination cages comprising sulfonylcalix[4]arene tetranuclear M(II) clusters incorporating enantiomerically enriched linkers based on tris(dipyrrinato)cobalt(III) complexes (https://doi.org/10.1039/D4NR03622K).

The imaging of the optical activity of chiral materials is very useful for observing the type and homogeneity of organic thin films. The cheap imaging of films with these characteristics is potentially extremely useful for the growing number of researchers exploring this kind of material, and a low-cost set-up is described here that has a sensitivity down to 250 mdeg and a spatial resolution of 100 μm (https://doi.org/10.1039/D4NR01651C). That kind of measurement can reveal important structural effects in thin organic films, as demonstrated in the observation of film thickness-dependent optical activity in an organic material that in optoelectronic devices gives external quantum efficiency between the two handedness’ of light, correlating with the apparent differential absorbance (g-factor) of the materials (https://doi.org/10.1039/D4NR04269G).

Fundamental challenges and future directions

Despite enormous progress, and obvious vibrancy of the field, future developments on chiral nanostructures face several fundamental challenges. Perhaps one of the largest ones is the full realization that chiral nanostructures have multiple scales of chirality.¹¹ Unlike other chiral objects, for instance neutrinos, electromagnetic waves in vacuum, mattress springs, cyclones, and galaxies, these scales of chirality are strongly interdigitated in nanostructures. In turn, it necessitates a new approach to quantitative description of chirality that should be both gradual and scale sensitive. From the standpoint of a synthetic chemist, the related challenge is precise control of chirality at multiple length scales. This challenge may seem to be quite theoretical, but it is directly related to practicality of the nanostructures and specifically to their use in medicine. Biological nanoscale systems also possess multiscale chirality and their interactions with nanoparticles are governed by the universal ability of nanoscale objects to self-assemble and reconfigure, which directly relates to their chiral geometries. Despite the success of machine learning algorithms for biomolecules, such as AlphaFold²⁴ and ROSETTA,²⁵ these algorithms have large difficulties accounting for D-amino acid-containing peptides, post-translationally inverted chirality, chiral metal center organometallic complexes, and chirality continuum of flexible chiral ligands. All these cases are atypical for biomolecules, but typical for chiral nanostructures. The need for theoretical work on chiral measures describing interdigitated multiscale chirality, manifests vividly in the development of nanoparticle–protein complexes and the use of chiral nanomaterials for adjuvants¹⁹ and immunotherapy agents and the forward-looking work on the latter is reported in this issue (https://doi.org/10.1039/D4NR03542A).

In respect to PCD, reliance on noble metals (Au, Ag), results in limited tunability in optical properties. Expanding PCD effects to other materials, for example ceramics that can also have strong plasmonic and polaronic states, is an emerging challenge. Even more important, is achieving dynamic control of polarization rotation, which is the interplay between the chirality of the nanostructure and chirality of photons through external stimuli like electric fields, temperature, or light, and remains paramount for information technology. Some elements of this effort are evident in the study of the magneto–electric coupling effects reported in this issue (https://doi.org/10.1039/D4NR03329A).

A lot of exciting ideas on the applications of chiral nanostructures were proposed and are being transitioned into practice (https://doi.org/10.1039/D4NR03998J). The area of CPL studies with chiral nanostructures is particularly dynamic: higher and higher CPL metrics, such as dissymmetry factors, g_lum, are being reported nearly every month. The fundamental challenges facing this segment of chiral nanostructures include the intrinsic trade-off between chirality of the emitting states and luminescence efficiency, which makes it difficult to obtain both CPL with high g_lum and brightness. The exact mechanisms behind chirality-dependent radiative decay and chiral exciton interactions remain incompletely understood, particularly for emerging materials like perovskites and quantum dots reported in this issue (https://doi.org/10.1039/D4NR03600J). Additionally, the computational models for predicting true chiral luminescence properties remain computationally expensive, requiring high-level time-dependent density functional theory and many-body quantum approaches.

The multiple scales of chirality present in the chiral nanostructures lead to multiple modes of interactions between light and nanoscale matter. Cumulative light emission from chiral nanostructures, that is often reported as CPL, is comprised of luminescence effects related to atomic scale asymmetry of quantum states, scattering effects related to the chirality of nanostructures overall, and filtering effects related to the submicron scale chirality of thin films.²⁶ All these processes are rarely deciphered. It is indeed difficult, and often tedious but it will be necessary in the future to avoid convoluted sets of metrics, that are often disorienting. As reported in this issue (https://doi.org/10.1039/D4NR04269G), circular polarization of emitted light can also originate not only from classical CD response but also from combined linear dichroism and linear birefringence.²⁷ This effect is often reported as an artefact, but in reality, it can be a powerful tool for polarization control.²⁸ We expect that future studies of chiral nanostructures will rapidly expand in this direction.

Chiral nanostructures research, like many disciplines involving complex interactions, is advancing along several promising directions. Key priorities include electro–optic properties,²⁹ harnessing machine learning approaches for chiral nanostructure prediction/design, leveraging non-equilibrium self-assembly for scalable fabrication, exploring second- and third-order light-emission effects in chiral nanomaterials for next-generation optical and spintronic applications, responsive supramolecular systems³⁰ and stereoselective plasmonic reactivity.³¹ Expanding the understanding of chiral light–matter interactions beyond the visible and infrared range to other parts of the electromagnetic spectrum could hold promise for perception systems and communication devices and is one of the many areas where chiral nanostructures may show new phenomena.

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