Liquid crystal alignment on a chiral surface: interfacial interaction with sheared DNA films

Evaporation-induced self-assembly of liquid crystal biopolymers

Evaporation-induced self-assembly (EISA) is a process that has gained significant attention in recent years due to its fundamental science and potential applications in materials science and nanotechnology. This technique involves controlled drying of a solution or dispersion of materials, forming structures with specific shapes and sizes. In particular, liquid crystal (LC) biopolymers have emerged as promising candidates for EISA due to their highly ordered structures and biocompatible properties after deposition. This review provides an overview of recent progress in the EISA of LC biopolymers, including DNA, nanocellulose, viruses, and other biopolymers. The underlying self-assembly mechanisms, the effects of different processing conditions, and the potential applications of the resulting structures are discussed.

A liquid crystal world for the origins of life

Nucleic acids (NAs) in modern biology accomplish a variety of tasks, and the emergence of primitive nucleic acids is broadly recognized as a crucial step for the emergence of life. While modern NAs have been optimized by evolution to accomplish various biological functions, such as catalysis or transmission of genetic information, primitive NAs could have emerged and been selected based on more rudimental chemical-physical properties, such as their propensity to self-assemble into supramolecular structures. One such supramolecular structure available to primitive NAs are liquid crystal (LC) phases, which are the outcome of the collective behavior of short DNA or RNA oligomers or monomers that self-assemble into linear aggregates by combinations of pairing and stacking. Formation of NA LCs could have provided many essential advantages for a primitive evolving system, including the selection of potential genetic polymers based on structure, protection by compartmentalization, elongation, and recombination by enhanced abiotic ligation. Here, we review recent studies on NA LC assembly, structure, and functions with potential prebiotic relevance. Finally, we discuss environmental or geological conditions on early Earth that could have promoted (or inhibited) primitive NA LC formation and highlight future investigation axes essential to further understanding of how LCs could have contributed to the emergence of life.

Technological Advancements in Bio‐recognition using Liquid Crystals: Techniques, Applications, and Performance

The application of liquid crystal (LC) materials has undergone a modern-day renaissance from its classical use in electronics industry as display devices to new-fangled techniques for optically detecting biological and chemical analytes. This review article deals with the emergence of LC materials as invaluable material for their use as label-free sensing elements in the development of optical, electro-optical and electrochemical biosensors. The property of LC molecules to change their orientation on perturbation by any external stimuli or on interaction with bioanalytes or chemical species has been utilized by many researches for the fabrication of high sensitive LC-biosensors. In this review article we categorized LC-biosensor based on biomolecular reaction mechanism viz. enzymatic, nucleotides and immunoreaction in conjunction with operating principle at different LC interface namely LC-solid, LC-aqueous and LC-droplets. Based on bimolecular reaction mechanism, the application of LC has been delineated with recent progress made in designing of LC-interface for the detection of bio and chemical analytes of proteins, virus, bacteria, clinically relevant compounds, heavy metal ions and environmental pollutants. The review briefly describes the experimental set-ups, sensitivity, specificity, limit of detection and linear range of various viable and conspicuous LC-based biosensor platforms with associated advantages and disadvantages therein.

Mechanism of chiral recognition by enantiomorphous cytosine crystals during enantiomer adsorption

The quest to understand why life exhibits chirality has been far from successful. In the terrestrial theory of chirality emergence in living matter, one of the main possible mechanisms is the chiral recognition of organic molecules by enantiomorphic crystals. In this work, we studied the ability of enantiomorphic cytosine crystals, obtained by Viedma ripening, for chiral recognition by enantiomers adsorption. For this, we used MD calculations, inverse gas chromatography, and adsorption from solutions. The difference between the isotherms of enantiomers was determined using a t-test. We found that cytosine crystals were capable of chiral recognition only when the adsorbate concentration on the surface was sufficient for lateral interactions leading to layer formation. In order to approximate adsorption isotherms, Langmuir, Freundlich, BET, and Fowler-Guggenheim equations were used. The difference in lateral interactions between menthol enantiomers during their adsorption from a solution in n-heptane was established. A mechanism of chiral recognition of the adsorbed substance by cytosine crystals was proposed. The conditions under which chiral recognition could proceed were determined. We observed that, upon adsorption from a solution, chiral recognition manifested itself at higher coverages than in MD simulations. This was caused by the competitive adsorption of the solvent. The results obtained show that adsorption on enantiomorphic crystals could be the source of the first minute enantiomeric excess, providing an opportunity to understand the origin of chiral imbalance.

Generation of 2D DNA Microstructures via Topographic Control and Shearing

2D DNA microstructures are fabricated by applying the shear force to the DNA solution on the microchannels. The "U"-like textures of DNA are clearly observed when the mechanical shearing is applied on the aqueous DNA sample under the topographic confinement, in which the shearing direction is perpendicular to the grooves. The optical textures of U-like microstructures are directly observed by polarized optical microscopy (POM) and laser scanning fluorescent confocal polarizing microscopy (FCPM). The DNA microstructures can be modified by varying the width, showing the multiple U-patterns along with channel direction due to the synergistic interaction between the elastic behavior of DNA chains and topographic boundary condition. The resultant microstructures can be used to align rod-like liquid crystals (LCs) to generate alternatively oriented nematic phase and tilted focal conic domains (FCDs) in the smectic A phase. It is believed that this approach can suggest a hint to use to DNA materials for organizing multiscale hierarchical structures of soft- and biomaterials.

Planar Anchoring of C70 Liquid Crystals Using a Covalent Organic Framework Template

The surface-induced anchoring effect is a well-developed technique to control the growth of liquid crystals (LCs). Nevertheless, a defined nanometer-scale template has never been used to induce the anchored growth of LCs with molecular building units. Scanning tunneling microscopy results at the solid/liquid interface reveal that a 2D covalent organic framework (COF-1) can offer an anchoring effect to template C₇₀ molecules into forming several LC mesophases, which cannot be obtained under other conditions. Through comparison with the C₆₀ system, a stepwise breakdown in ordering of C₇₀ LC is observed. The process is described in terms of the effects of molecular anisotropy on the epitaxial growth of molecular crystals. The results suggest that using a surface-confined template to anchor the initial layer of LC molecules can be a modular and potentially broadly applicable approach for organizing molecular mesogens into LCs.

Microstructure arrays of DNA using topographic control

DNA is a common biomaterial in nature as well as a good building block for producing useful structures, due to its fine feature size and liquid crystalline phase. Here, we demonstrate that a combination of shear-induced flow and microposts can be used to create various kinds of interesting microstructure DNA arrays. Our facile method provides a platform for forming multi-scale hierarchical orientations of soft- and biomaterials, using a process of simple shearing and controlled evaporation on a patterned substrate. This approach enables potential patterning applications using DNA or other anisotropic biomaterials based on their unique structural characteristics.

Metal sensitive and DNA concentration dependent structural rearrangement of short oligonucleotide into large suprastructures

Formation of higher order structures, such as G-quadruplexes and G-quadruplex based large suprastructures into long G-wires and liquid crystals is promising elements for use in healthcare for drug delivery as they are mechanically and thermally stable. In this study, we studied the structures of short 11-mer oligonucleotide 5'-G₂AG₅AG₂-3'(11Pu) which is observed in 3'-UTR region of c-jun protooncogene. We used circular dichroism, UV-thermal melting, native gel electrophoresis and atomic force microscopy to determine the structure of 11Pu. CD results showed that 11Pu formed a mixed G-quadruplex in the presence of Na⁺ with and without Mg²⁺, while it formed a parallel G-quadruplex in the presence of 100 mM K⁺ with or without Mg²⁺. Cation selectivity in inducing the formation of large superstructures was observed in the presence of 100 mM K⁺ with 10 mM Mg²⁺. On the contrary, 10 mM Ca²⁺ did not induce the suprastructures. It was further demonstrated that Mg²⁺ at low concentration induced a parallel G-quadruplex of 11Pu, whereas at 10 mM Mg²⁺ induced a large suprastructure. AFM Images showed that 11Pu formed a G-wire, a liquid crystals and a crystalline lattice depending on the concentration of 11Pu and Mg²⁺. These insights may be employed to design G quadruplex-based nanowires for targeted drug delivery as well as interesting candidates for molecular nanowires. Communicated by Ramaswamy H. Sarma.

Lyotropic Liquid Crystal Phases from Anisotropic Nanomaterials

Liquid crystals are an integral part of a mature display technology, also establishing themselves in other applications, such as spatial light modulators, telecommunication technology, photonics, or sensors, just to name a few of the non-display applications. In recent years, there has been an increasing trend to add various nanomaterials to liquid crystals, which is motivated by several aspects of materials development. (i) addition of nanomaterials can change and thus tune the properties of the liquid crystal; (ii) novel functionalities can be added to the liquid crystal; and (iii) the self-organization of the liquid crystalline state can be exploited to template ordered structures or to transfer order onto dispersed nanomaterials. Much of the research effort has been concentrated on thermotropic systems, which change order as a function of temperature. Here we review the other side of the medal, the formation and properties of ordered, anisotropic fluid phases, liquid crystals, by addition of shape-anisotropic nanomaterials to isotropic liquids. Several classes of materials will be discussed, inorganic and mineral liquid crystals, viruses, nanotubes and nanorods, as well as graphene oxide.

Control of Periodic Zigzag Structures of DNA by a Simple Shearing Method

A periodic zigzag structure of DNA material is successfully fabricated by a simple shearing method. The periodicity of the pattern can be finely controlled by combining the mechanical shearing method with topographic patterns of microchannels. The resultant zigzag patterns can be used as a template to control the alignment of rod-like liquid crystals due to its highly regular periodicity.

Thermotropic liquid crystal films for biosensors and beyond

Recent results on structural properties and possible bio-sensing applications of planar liquid crystal films are reviewed.

Surface Dipole Control of Liquid Crystal Alignment

Detailed understanding and control of the intermolecular forces that govern molecular assembly are necessary to engineer structure and function at the nanoscale. Liquid crystal (LC) assembly is exceptionally sensitive to surface properties, capable of transducing nanoscale intermolecular interactions into a macroscopic optical readout. Self-assembled monolayers (SAMs) modify surface interactions and are known to influence LC alignment. Here, we exploit the different dipole magnitudes and orientations of carboranethiol and -dithiol positional isomers to deconvolve the influence of SAM-LC dipolar coupling from variations in molecular geometry, tilt, and order. Director orientations and anchoring energies are measured for LC cells employing various carboranethiol and -dithiol isomer alignment layers. The normal component of the molecular dipole in the SAM, toward or away from the underlying substrate, was found to determine the in-plane LC director orientation relative to the anisotropy axis of the surface. By using LC alignment as a probe of interaction strength, we elucidate the role of dipolar coupling of molecular monolayers to their environment in determining molecular orientations. We apply this understanding to advance the engineering of molecular interactions at the nanoscale.

Design of Responsive and Active (Soft) Materials Using Liquid Crystals

Liquid crystals (LCs) are widely known for their use in liquid crystal displays (LCDs). Indeed, LCDs represent one of the most successful technologies developed to date using a responsive soft material: An electric field is used to induce a change in ordering of the LC and thus a change in optical appearance. Over the past decade, however, research has revealed the fundamental underpinnings of potentially far broader and more pervasive uses of LCs for the design of responsive soft material systems. These systems involve a delicate interplay of the effects of surface-induced ordering, elastic strain of LCs, and formation of topological defects and are characterized by a chemical complexity and diversity of nano- and micrometer-scale geometry that goes well beyond that previously investigated. As a reflection of this evolution, the community investigating LC-based materials now relies heavily on concepts from colloid and interface science. In this context, this review describes recent advances in colloidal and interfacial phenomena involving LCs that are enabling the design of new classes of soft matter that respond to stimuli as broad as light, airborne pollutants, bacterial toxins in water, mechanical interactions with living cells, molecular chirality, and more. Ongoing efforts hint also that the collective properties of LCs (e.g., LC-dispersed colloids) will, over the coming decade, yield exciting new classes of driven or active soft material systems in which organization (and useful properties) emerges during the dissipation of energy.

Structure-Specific Liquid Crystal Anchoring Induced by the Molecular Combing of Short Oligonucleotides

Surface-immobilized oligonucleotides were "combed" by meniscus motion and exposed to a nematic liquid crystal (LC). Although the oligonucleotides were as short as 16 bases, they were apparently oriented by this process and, in turn, successfully biased the orientation of the adjacent LC material. Single-stranded DNA (ssDNA) induced LC orientation in the combing direction, while hybridized double-stranded DNA (dsDNA) rotated the azimuthal LC orientation by ∼30° from the combing direction. The sensitivity of the chiral response to mixed ssDNA/dsDNA surfaces was characterized by employing complementary DNA that was longer than the immobilized DNA, resulting in single-stranded overhangs of various lengths. A rotated LC orientation was observed even when more than 70% of the DNA was single-stranded, and the transition from the rotated to nonrotated response was apparently discontinuous as a function of ssDNA surface coverage. These phenomena represent a sensitive DNA hybridization detection strategy that can potentially comprise a multiplexed assay.

In-Plane Switching Mode for Liquid Crystal Displays Using a DNA Alignment Layer

We successfully fabricated the in-plane switching mode (IPS) LC display (LCD) based on a double stranded DNA (dsDNA) alignment layer. As widely known, the DNA has the right-handed double helical structure that has naturally grown grooves with a very regular period, which can be used as an alignment layer to control the orientation of liquid crystal (LC) molecules. The LC molecules on this topographical layer of DNA material align obliquely at a specific angle with respect to the direction of DNA chains, providing an instant and convenient tool for the fabrication of the IPS display compared to the conventional ways such as rubbing and mechanical shearing methods. The electro-optical performance and response time of this device were also investigated. Our result will be of great use in further exploration of the electro-optical properties of the other biomaterials.

Liquid crystals anchored on mixed monolayers of chiral versus achiral molecules: continuous change in orientation as a function of enantiomeric excess

The orientations of liquid crystals (LCs) anchored on monolayers formed from mixtures of chiral versus achiral molecules were compared. Changes in the enantiomeric excess of mixed monolayers of chiral dipeptides gave rise to continuous changes in the orientations of nematic LCs, allowing arbitrary tuning of the azimuthal orientations of LCs over a range of ≈100°. In contrast, the same LCs exhibited discontinuous changes in orientation on surfaces presenting mixtures of achiral molecules. These striking differences in the anchoring of LCs on surfaces presenting chiral versus achiral molecules provide insights into the molecular origins of ordering transitions of LCs, and provide new principles based on chiral monolayers for the rational design of surfaces that permit continuous tuning of the orientations of LCs.

Orthogonal orientation of chromonic liquid crystals by rubbed polyamide films

Chromonic liquid crystals (CLCs) have drawn attention for applications to organic electronics and optical films as well as biological materials. Understanding the alignment mechanism of CLCs is important for those applications. Using a polarized transmission optical microscope, we observe the optical texture, dichroism, and birefringence of CLC films of sunset yellow (SSY) confined by polyamide (nylon) films that are rubbed with a brush. The films align with the stacks of SSY molecules oriented, surprisingly, perpendicular to the rubbing direction. We propose that this alignment is stabilized by molecular interaction between the stretched nylon chains and molecular grooves of the SSY stacks rather than elastic energy of the CLCs due to surface topography induced by the rubbing.

Chemical and biological sensing using liquid crystals

The liquid crystalline state of matter arises from orientation-dependent, non-covalent interaction between molecules within condensed phases. Because the balance of intermolecular forces that underlies formation of liquid crystals is delicate, this state of matter can, in general, be easily perturbed by external stimuli (such as an electric field in a display). In this review, we present an overview of recent efforts that have focused on exploiting the responsiveness of liquid crystals as the basis of chemical and biological sensors. In this application of liquid crystals, the challenge is to design liquid crystalline systems that undergo changes in organization when perturbed by targeted chemical and biological species of interest. The approaches described below revolve around the design of interfaces that selectively bind targeted species, thus leading to surface-driven changes in the organization of the liquid crystals. Because liquid crystals possess anisotropic optical and dielectric properties, a range of different methods can be used to read out the changes in organization of liquid crystals that are caused by targeted chemical and biological species. This review focuses on principles for liquid crystal-based sensors that provide an optical output.

Asymmetrically tilted alignment of rigid-rod helical polysilanes on a rubbed polyimide surface

The homogeneous alignments of helical rod-like polysilanes on a rubbed polyimide alignment layer were investigated by polarized optical microscopy (POM) and atomic force microscopy (AFM) analyses. The POM and AFM observations determined that polysilanes with a series of aliphatic side chains helically arranged around the main chains were tilted to the right and left by 33° from the rubbing direction when the handedness of the side-chain helical array is left and right, respectively. It is interesting to note that the side-chain arrays run perpendicular to the rubbing direction on the polyimide surface, which is different from intuitive "knob and hole" packing of the extended polyimide chain and the helical grooves between the side-chain arrays surrounding the polysilane backbone. More surprisingly, both right- and left-tilting smectic domains were simultaneously observed with an equal probability for an achiral polysilane, which apparently has the interconverting right- and left-handed helical segments separated by helical reversals. This might be the first observation of the chiral segregation of dynamic helical polymers.

Enantiomeric interactions between liquid crystals and organized monolayers of tyrosine-containing dipeptides

We have examined the orientational ordering of nematic liquid crystals (LCs) supported on organized monolayers of dipeptides with the goal of understanding how peptide-based interfaces encode intermolecular interactions that are amplified into supramolecular ordering. By characterizing the orientations of nematic LCs (4-cyano-4'-pentylbiphenyl and TL205 (a mixture of mesogens containing cyclohexane-fluorinated biphenyls and fluorinated terphenyls)) on monolayers of l-cysteine-l-tyrosine, l-cysteine-l-phenylalanine, or l-cysteine-l-phosphotyrosine formed on crystallographically textured films of gold, we conclude that patterns of hydrogen bonds generated by the organized monolayers of dipeptides are transduced via macroscopic orientational ordering of the LCs. This conclusion is supported by the observation that the ordering exhibited by the achiral LCs is specific to the enantiomers used to form the dipeptide-based monolayers. The dominant role of the -OH group of tyrosine in dictating the patterns of hydrogen bonds that orient the LCs was also evidenced by the effects of phosphorylation of the tyrosine on the ordering of the LCs. Overall, these results reveal that crystallographic texturing of gold films can direct the formation of monolayers of dipeptides with long-range order, thus unmasking the influence of hydrogen bonding, chirality, and phosphorylation on the macroscopic orientational ordering of LCs supported on these surfaces. These results suggest new approaches based on supramolecular assembly for reporting the chemical functionality and stereochemistry of synthetic and biological peptide-based molecules displayed at surfaces.

Macroscopic liquid crystal response to isolated DNA helices

Nematic liquid crystals (LC) were exposed to isolated DNA molecules extended on a surface that imparted a negligible influence on the LC orientation. Although single-stranded DNA aligned the LC in the extension direction, double-stranded DNA (dsDNA) caused alignment at an oblique angle, providing a characteristic response to the chiral dsDNA helix that was readily observed optically. The intrinsic amplification due to LC orientational correlations enabled a macroscopic visible response to a single molecule of extended dsDNA.

Mechanically generated surface chirality at the nanoscale

A substrate coated with an achiral polyimide alignment layer was scribed bidirectionally with the stylus of an atomic force microscope to create an easy axis for liquid crystal orientation. The resulting noncentrosymmetric topography resulted in a chiral surface that manifests itself at the molecular level. To show this unambiguously, a planar-aligned negative dielectric aniostropy achiral nematic liquid crystal was placed in contact with the surface and subjected to an electric field E. The nematic director was found to undergo an azimuthal rotation approximately linear in E. This so-called "surface electroclinic effect" is a signature of surface chirality and was not observed when the polyimide was treated for a centrosymmetric topography, and therefore was nonchiral.

Polarization-sensitive two-photon microscopy study of the organization of liquid-crystalline DNA

Highly concentrated DNA solutions exhibit self-ordering properties such as the generation of liquid-crystalline phases. Such organized domains may play an important role in the global chromatin topology but can also be used as a simple model for the study of more complex 3D DNA structures. In this work, using polarized two-photon fluorescence microscopy, we report on the orientation of DNA molecules in liquid-crystalline phases. For this purpose, we analyze the signal emitted by fluorophores that are noncovalently bound to DNA strands. In nonlinear processes, excitation occurs exclusively in the focal volume, which offers advantages such as the reduction of photobleaching of out-of-focus molecules and intrinsic 3D sectioning capability. Propidium iodide and Hoechst, two fluorophores with different DNA binding modes, have been considered. Polarimetric measurements show that the dyes follow the alignment with respect to the DNA strands and allow the determination of the angles between the emission dipoles and the longitudinal axis of the DNA double strand. These results provide a useful starting point toward the application of two-photon polarimetry techniques to determine the local orientation of condensed DNA in physiological conditions.

Patterning-induced surface chirality and modulation of director twist in a nematic cell

A substrate coated with a polyimide alignment layer is scribed bidirectionally with the stylus of an atomic force microscope to create an easy axis for liquid-crystal orientation. The resulting noncentrosymmetric topography breaks two-dimensional inversion symmetry and results in a spatial amplitude modulation of an imposed twisted nematic state. This is observed optically as spatially periodic light and dark stripes. When the alignment layer is scribed unidirectionally the centrosymmetric topography maintains inversion symmetry, and no stripes are observed. The appearance of the twist modulation is consistent with a chiral term in the free energy.