Nature-Inspired Chiral Structures

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1		Ju-Hyung Kim	--	2616	2023-11-15 12:04:59	\|
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This entry is adapted from the peer-reviewed paper 10.3390/biomimetics8070527

Diverse chiral structures observed in nature find applications across various domains, including engineering, chemistry, and medicine. Particularly notable is the optical activity inherent in chiral structures, which has emerged prominently in the field of optics. This phenomenon has led to a wide range of applications, encompassing optical components, catalysts, sensors, and therapeutic interventions.

chirality enantiomers mirror images circular polarization

1. Introduction

The diversity of structures found in nature serves as a significant source of inspiration for contemporary research endeavors. Over millions of years, evolution and selective processes have created these structures with optimal designs and functionalities, continually offering researchers novel ideas and unexplored avenues. In particular, innovative solutions are being discovered across diverse fields, such as nanotechnology, materials engineering, medicine, and energy storage ^[1]. Amidst this spectrum of influence, research on chiral structures found in nature is also thriving. Chiral structures possess mirror-image forms in nature, exhibiting asymmetric characteristics akin to left and right hands. These structures exhibit unique physical, chemical, and optical properties, leading to their active exploration in various studies, including their application in nanoscale semiconductors and displays ^[2].

Chiral structures observed in nature come in a variety of shapes and sizes, starting from unique chemical structures identical to the helicene ^[3] and lanthanide complexes ^[4], and can be clearly seen in DNA ^[5], amino acids ^[6], snail shells ^[7], beetle cuticles ^[8]^[9], and even spiral galaxies ^[10]. Researching these natural models enables the design of novel chiral structures tailored for diverse applications. Researching these chiral models enables the design of novel chiral structures tailored for diverse applications. Foremost among these applications are photonics such as photonic crystal ^[11], circularly polarized luminescence dye ^[12], sensors for investigation of selective circularly polarized light ^[13], catalysis for selective activation and treatment ^[14], and biomedical therapy ^[15]. The unique properties of chiral structures are more evident not only in biological and chemical properties but also in optical and electromagnetic properties. The chiral electromagnetic properties of chiral structures, such as electrical magneto-chiral anisotropy dependent on external magnetic fields, current, and the handedness of chiral conductors ^[16], as well as chirality-induced spin selectivity influenced by handedness, are garnering significant attention ^[17]. These properties can be applied and utilized in the fields of spintronics and quantum computing.

Optical activity via the interaction of chiral structures with light refers to the ability of chiral structures to rotate the vibration direction of light passing through them and convert linear polarization to circular polarization ^[18]. Circularly polarized light exhibits a rotating polarization state with a constant magnitude perpendicular to the direction of the electromagnetic field. The clockwise rotation of light from the light source is referred to as right-handed circular polarization (RCP), while the counterclockwise rotation is termed left-handed circular polarization (LCP), denoted as dextrorotatory (+) and levorotatory (−), respectively ^[19]. Exploiting this light rotation property allows for the analysis and utilization of chiral structures’ properties and optical changes. These optical rotation properties can be employed to interpret and utilize the inherent structure and optical activity of chiral materials. To achieve this objective, two analysis methods entail optical rotatory dispersion (ORD), which measures how much chiral molecules rotate plane-polarized light based on the wavelength, and circular dichroism (CD), which assesses the difference in light absorption with respect to left or right circularly polarized light ^[20]^[21]^[22]. Furthermore, various chiroptical properties can be employed to design new functional molecules and structures by optically manipulating chiral structures.

These studies and applications of such structures play a crucial role in developing technologies that manipulate and polarize light, efficient sensor technologies utilizing circularly polarized light, and innovative solutions through medical treatments and reactions. Owing to their fascinating properties, chiral structures are currently attracting significant attention and active research efforts.

2. Chiral Structures Found in Nature

Chiral structures, akin to the uniqueness of human fingerprints, are also discovered throughout nature across various life forms. These structures possess directionality resulting from rotation around a central axis, and the diverse characteristics that emerge from this rotation have inspired numerous studies today. Among the prominent chiral structures found in nature is the chiral nematic arrangement, also known as the cholesteric structure, found in liquid crystals. This structure, alternatively referred to as the Bouligand structure, is prevalent in nature, ranging from microscopic to macroscopic structures, including DNA. This Bouligand structure is not only present in the human body but also commonly observed in various organisms, including insects and plants.

One specific example of a distinctive insect displaying remarkable properties due to such structures is the scarab beetle. This insect forms a Bragg band covering a wide spectral range (340 to 1000 nm) and exhibits LCP reflection ^[8]^[23]. This phenomenon arises from the lamellar structure of chitin in the wing, which leads to an arrangement akin to cholesteric liquid crystals. This unique appearance holds potential applications in diverse optical components, including structural colors. While most species similar to the scarab beetle reflect LCP, Chrysina resplendens exhibits RCP due to distinct birefringence, adding to its intriguing attributes ^[24]. These facts contribute to understanding the characteristics of chiral structures and have sparked heightened interest in the field of optical and optoelectronic device research involving these insects.

Such Bouligand structures can also be found in plants. The Pollia condensata, native to Africa, exhibits a metallic color that maintains vibrancy over time. This phenomenon confirms that Pollia condensata’s color stems from structural colors, a result of the cholesteric arrangement of cellulose layers. Furthermore, this fruit can reflect both LCP and RCP. This polarization behavior can be classified using polarizing filters, demonstrating that the color manifests polarization effects due to the cholesteric arrangement ^[25]. These findings extend to the genetic evolution that allows organisms to thrive in nature. Moreover, optical phenomena arising from cholesteric structures can also be observed in other organisms, such as the Mantis shrimp ^[26]^[27] and the Sheep crab ^[28]. Additionally, in plants, one can observe the asymmetrically rotated structures of flower petals. Most spiraling plants tend to wind around their supporting structures in a right-handed spiral. However, the wild Arabidopsis thaliana grows symmetrically without twisting its petals during normal development ^[29]. Nevertheless, genetic mutations leading to helical growth can cause the entire petal to exhibit a left-handed spiral, similarly observed in snails and marine seashells ^[30]^[31]^[32]. Snails are often determined by maternal genes for their shell coiling. Due to the challenges in reproducing snails with anti-rotational chirality, individuals of the same species typically exhibit chirality in a consistent direction.

In the natural realm, such chiral structures can also manifest even when a species exhibits both left-handed and right-handed morphologies. In Towel Gourd tendrils, the direction of helical growth can be both left-handed and right-handed configurations, attributable to genetic variations within the plant population ^[33]. Additionally, a similar example is found in goat horns ^[34], where a non-superimposable symmetric chiral structure is evident. The array of chiral structures witnessed in the natural realm highlights the fascinating interplay of genetics and form, enriching our understanding of life’s intricate design.

3. Fabrication of Structures Inspired by Nature

The diverse chiral structures found in nature have provided significant inspiration and have been replicated in various forms. These structures share the common characteristic of rotation around a specific axis. This section aims to classify natural chiral structures into three categories and review the major efforts to fabricate them artificially.

3.1. Bouligand Structures

Extensively researched Bouligand structures merit considerable attention. In recent years, advancements in nanofabrication via both bottom-up and top-down approaches have rapidly developed chiral nanomaterials. Notably, these structures are fabricated through various methods, with bottom-up approaches such as the self-assembly of nano cellulose being extensively explored. Cellulose nanocrystal, a natural material, forms Bouligand structures through self-assembly via vapor methods ^[35]. This microstructure, akin to cholesteric liquid crystals, constitutes chiral molecules with spiral pitches used to selectively filter circular polarizations based on Bragg reflection ^[36]^[37]. This structure exhibits varying colors or chiroptical responses depending on the half-pitch while showcasing diverse characteristics based on length and synthesis methods ^[38]^[39]. Materials like cellulose with such attributes can also be combined with top-down approaches (e.g., soft lithography), designing structures that align with Bragg’s theory ^[40]. Y. Zhao et al. demonstrated the fabrication of metallic nanorods into multi-layered structures with controlled twist angles and well-ordered nanorod stacking structures using spacers ^[41]. The computational and experimental results exhibited strong chiroptical effects for RCP and LCP lights due to plasmon coupling as the metallic nanostructures were stacked (at θ = 60°). As more layers were added, the bandwidth gradually broadened in a step-like manner, with seven layers exhibiting chiroptical response characteristics spanning the entire visible range. This confirmed that the optical phenomenon of original uniaxial materials can be produced using layer-by-layer stacking to resemble the optical behavior of conventional cholesteric systems. To address this issue, J. Lv et al. utilized Langmuir–Blodgett technology to align dispersed nanowires and rotated them at appropriate angles to demonstrate cholesteric arrays layer-by-layer ^[42]. The aligned nanowires showed well-oriented structures with deviations of less than 10° in over 90% of cases. Depending on the layers rotated at a 45° angle, this structure exhibited varying uniaxial optical phenomena. In the case of three-layer stacking, it revealed high CD values across a broad bandwidth. Similarly, using a combination of grazing incidence spraying (GIS) methods to facilitate easy layering and highly oriented nanowires, layers stacked at a 60° angle displayed varying CD values across a wide range ^[43]. At the upper part of the stacked structure, it achieved a high CD value of 4000 mdeg (at 380 nm), and the extinction of the metasurface was observed as the number of layers increased.

In addition, to enhance the precision of nanowire alignment, ongoing research has delved into methodologies that leverage external stimuli for orchestrated arrangements. For instance, in the research conducted by H.-Q. Nguyen et al., gold (Au) nanowires sheathed with magnetic oxide nanoparticles underwent alignment along a prescribed axis via the application of a magnetic field ^[44]. Aligned and stacked nanowires were observed to be oriented and layered in a consistent direction, and these aligned nanowires were layered to exhibit distinctive CD responses under varying circularly polarized light conditions. The layers of nanowires, systematically rotated at consistent angles using the magnetic bar, evinced robust chiroptical response characteristics. Notably, as the number of stacked layers increased, this phenomenon intensified. These effects manifested across diverse uniaxial optical phenomena observed at discrete spectral dip positions (420, 512, 565, and 715 nm). At a wavelength of 614 nm, the relative anisotropy ratio demonstrated a pronounced chiral effect with an average magnitude of 0.72. This was in stark contrast to the uniaxial CD exhibited in the absence of such layered alignment. These advancements have markedly streamlined the process of stacking layers based on the chosen nanowire alignment technique and the specific angle of rotation. Furthermore, by exerting control over the particle concentration within the aqueous colloid solution, alongside the size of the nanowires, it becomes feasible to modulate the density of aligned nanowires. This, in turn, facilitates the creation of structures characterized by a spectrum of iridescent hues. Such top-down methodologies are forging novel avenues of technological exploration within the realm of crafting chiral structures employing nanomaterials.

Further advancement in the fabrication of intricate chiral Bouligand structures has been achieved using template-assisted techniques. Instead of controlling the orientation of wire-shaped materials, these techniques employ templates to arrange nanoparticles into arrays and adjust the template’s rotation angle to create mirror-imaged stacked structures. This approach goes beyond the limitations of material shapes with easy orientation, enabling a wider range of materials for chiral structure fabrication by controlling particles. These template-assisted techniques offer ease in modulating desired CD characteristics, encompassing sign, magnitude, and spectral position ^[45].

In this context, further research has been conducted using patterned templates to stack metal patterns, leading to the fabrication of 2D or 3D chiral structures. Primarily, by layering metal patterns onto patterned templates, distinct mirror-image chiral structures are crafted. Taking inspiration from crossed fingers, flexible templates are patterned through buckling. Uniaxial fine wrinkles in the template play the role of anisotropic objects ^[46]. Aligned block copolymer films are positioned at specific angles (±45°) and fabricated over large areas via shear rolling processes. These nanopatterns exhibit chiroptical characteristics. Similarly, studies have progressed using strain to create 3D flexible templates onto which metal layers were adhered via physical vapor deposition ^[47]. The 3D flexible templates fabricated via two cycles of biaxial stretching were able to realize 3D chiral Au microstrip patterns via e-beam evaporation. These enantiomeric (i.e., mirror image) structures demonstrated transmittance signals resembling those of computational simulations and allowed the production of 3D chiral structures without intricate lithography. Recently, research has also explored the stacking of nanopatterns without spacers to enhance strong chiroptical responses by reducing periodicity and amplifying localized plasmonic effects.

3.2. Planar Structures

In contrast to the previously mentioned stacked Bouligand structures, planar structures resembling the flowers of wild Arabidopsis thaliana (e.g., 2D propeller patterns) exhibit exceptional precision and control mainly using precise calculation and lithography techniques. Among various fabrication methods, soft lithography techniques, including nanoimprinting, offer relatively straightforward and cost-effective ways to fabricate patterns, making them favorable choices for creating planar structures. A large-scale chiral perovskite metasurface, produced using pre-patterned elastomeric stamps, can be fabricated into enantiomeric patterns ^[48]. In this work, two types of inks (emitted green/red) were employed to create chiral structures, enabling tunable circularly polarized luminescence. The resulting chiral metasurface exhibited up to 0.16 of photoluminescent dissymmetry factor, demonstrating the ease of controlling the optical properties of nanostructured inks. This technology has diverse applications and possesses high scalability. Further research has been conducted involving the synthesis of elastomer materials and magnetic substances to produce 3D chiral structures through magnetic fields ^[49].

3.3. Helical Structures

Advancing beyond planar structures, the realm of 3D helical structures holds transformative potential. These structures allow the creation of densely packed configurations, leading to enhanced chirality. Furthermore, they capitalize on spatial utilization to incorporate a diverse range of functionalities. However, the fabrication of 3D structures demands increasingly intricate techniques compared to 2D counterparts, necessitating persistent efforts to emulate them. To surmount this challenge, extensive research endeavors are in progress. Currently, studies are exploring techniques such as 3D printing and ion-beam lithography fabrication for producing these intricate structures. Nonetheless, these methods often come with high fabrication costs due to the complexity of generating precise patterns. As a response, research is intensifying its focus on discovering more economical and efficient fabrication methods.

One of the widely used approaches is applying the glancing angle deposition (GLAD) technique to create helical structures by adjusting deposition angles. Recent research reports the successful fabrication of large-scale 3D chiral structures in combination with GLAD and colloid nanohole lithography ^[50]. During the initial stages of metal evaporation, a significant amount of material deposits onto the substrate until the rotation speed gradually increases, controlling the amount of metal reaching the substrate. This process forms spiral-like lamp structures with diminishing diameters. The produced 3D helical Au structures were then cleaned to remove masks and supports, exhibiting a pronounced CD value of up to 13% in the frequency range of 100–400 THz, showcasing large-scale plasmonic chiral structures. Further attempts have been made to use the GLAD deposition technique to create helical structures with various compositions beyond single materials. U. Kilic et al. successfully fabricated a hybrid spiral structure comprising silver (Ag) and silicon (Si) ^[51]. By adjusting evaporation/rotation speeds through e-beam control, a helix structure with a 360 nm pitch was achieved. This result implies the potential to fabricate chiral structures composed of diverse materials, not just single ones. The resulting products displayed enhanced chiral responses and also exhibited different Kuhn’s asymmetric coefficients (g-factor spectra) depending on their compositions.

In addition, recent studies have demonstrated the creation of 3D helical structures with varied handedness via deformation processes by applying pressure to previously fabricated 2D spiral structures ^[52]. The height of the structure varied corresponding to the intensity of N₂ gas pressure applied in the upward and downward directions, which led to strong chiroptical response characteristics. This methodology allows for the facile fabrication of deformable helical structures without using the GLAD technique.

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