Homeotropic Liquid Crystal Alignment Research Articles

Label-free liquid crystal-based optical detection of norfloxacin using an aptamer recognition probe in soil and lake water.

Norfloxacin (NOX), a broad spectrum fluoroquinolone (FQ) antibiotic, is commonly detected in environmental residues, potentially contributing to biological drug resistance. In this paper, an aptamer recognition probe has been used to develop a label-free liquid crystal-based biosensor for simple and robust optical detection of NOX in aqueous solutions. Stimuli-receptive liquid crystals (LCs) have been employed to report aptamer-target binding events at the LC-aqueous interface. The homeotropic alignment of LCs at the aqueous-LC interface is due to the self-assembly of the cationic surfactant cetyltrimethylammonium bromide (CTAB). In the presence of the negatively charged NOX aptamer, the ordering changes to planar/tilted. On addition of NOX, the aptamer-NOX binding causes redistribution of CTAB at the LC-aqueous interface and the homeotropic orientation is restored. This results in a bright-to-dark optical transition under a polarized optical microscope (POM). This optical transition serves as a visual indicator to mark the presence of NOX. The devised aptasensor demonstrates high specificity with a minimum detection limit of 5 nM (1.596 ppb). Moreover, the application of the developed aptasensor for the detection of NOX in freshwater and soil samples underscores its practical utility in environmental monitoring. This proposed LC-based method offers several advantages over conventional detection techniques for a rapid, feasible and convenient way to detect norfloxacin.

A liquid crystal-based sensor exploiting the aptamer-mediated recognition at the aqueous/liquid crystal interface for sensitive detection of serotonin.

We report here a liquid crystal (LC)-based sensor for detecting serotonin (5-HT); the proposed sensor uses target-specific aptamer recognition at a cationic surfactant decorated-aqueous/LC interface. Our detection strategy focuses on the orientational transition of LCs upon biological interactions at the interface. In this sensing system, the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) forms a self-assembled monolayer at the aqueous/LC interface and triggers the homeotropic orientation of LCs. After introducing the 5-HT specific aptamer, an electrostatic attraction occurs between the cationic CTAB and anionic aptamer. This interaction destructs the surfactant monolayer at the interface, inducing reorganization of LC alignment from homeotropic to tilted conditions. In the increasing 5-HT levels, specific binding between 5-HT and the aptamer diminishes the interaction between the aptamer and CTAB, thereby maintaining the homeotropic alignment of LCs. The orientational transition of the LCs was observed under a polarized optical microscope. The developed biosensor has a linear detection range from 1 to 1000nM and a detection limit of 1.68nM. Moreover, the sensor was applied to a human urine sample and a detection limit of 2.25nM was obtained. Overall, the designed LC-based sensor is a sensitive, simple, cost effective, and selective platform for detecting 5-HT in aqueous solutions.

A simple and robust aptasensor assembled on surfactant-mediated liquid crystal interface for ultrasensitive detection of mycotoxin

Here, a simple aptasensing approach is represented to sensitively detect ochratoxin A (OTA) as one of the most perilous mycotoxins with carcinogenic, nephrotoxic, teratogenic, and immunosuppressive sequels on human health. The aptasensor is based on the alteration in the orientational order of liquid crystal (LC) molecules at the surfactant-arranged interface. Homeotropic alignment of LCs is achieved by the interaction of the surfactant tail with LCs. By perturbing the alignment of LCs due to the electrostatic interaction of the aptamer strand with the surfactant head, a colorful polarized view of the aptasensor substrate is induced drastically. While OTA causes the re-orientation of LCs to a vertical state by forming an OTA-aptamer complex that induces darkness of the substrate. This study shows that the length of the aptamer strand impacts the efficiency of the aptasensor; longer strand results in the greater disruption of LCs, and therefore, increases the aptasensor sensitivity. Hence, the aptasensor can determine OTA in the linear concentration range of 0.1 fM-1 pM as low as 0.021 fM. The aptasensor is capable to monitor OTA in grape juice, coffee drink, corn, and human serum real samples. The proposed LC-based aptasensor provides a cost-effective, easy-to-carry, operator-independent, and user-friendly array with great potential to develop portable sensing gadgets for food quality control and health care monitoring.

Fullerene-Based Nanosurfactant for the Automatic Homeotropic Alignment of Liquid Crystals

Fullerene-Based Nanosurfactant for the Automatic Homeotropic Alignment of Liquid Crystals

High-quality deformed lying helix in chiral LC on surface with periodic alignment prepared by two-step optical treatment

ABSTRACT We propose a method for creating a periodic planar-homeotropic alignment (PHA) of liquid crystal (LC) on a film prepared by spin-coating a solution of a mixture of a photosensitive organic azo dye and a surfactant. Irradiation by polarised and non-polarised ultraviolet light provides areas with reliable planar and homeotropic LC alignment, respectively. Using a photomask, gratings with PHA are created, and optical properties of the LC cells based upon them are investigated. For the first time, the characteristic fine features of the field-induced transition of the initially standing chiral LC helix into a Deformed Lying Helix (DLH) configuration with the spiral axis lying in the LC layer plane are visualised and studied. The observed optical images illustrate the high-quality DLH pattern. Numerical simulations of the DLH reveal the peculiarities of the obtained structure and how it is formed.

A self-assembled polyimide layer based on graphene quantum dots for ultrafast switching of liquid crystals

Graphene quantum dots (GQDs) with a large specific surface area, wide emission spectrum, and low toxicity are advantageous for biomedical and optoelectronic applications. Here, we present a facile method to simultaneously provide ultrafast electro-optical switching and strong vertical anchoring of liquid crystals (LCs) using interfacial self-assembly between alkylated GQDs and polyimide (PI) layers in a closed LC device. The newly fabricated self-assembled PI layer with alkylated GQDs were simply fabricated by doping GQDs into the LC medium, and induced a stable and uniform homeotropic alignment of LCs. Moreover, this self-assembled layer provided a superfast switching speed for the device.

Electric Field Dependent Textural Variation inside the Liquid Crystal Droplets with Homeotropic Alignment

The alignment of liquid crystal inside the droplets highly influences the electro-optical behaviour of polymer dispersed liquid crystals (PDLCs). In PDLCs with initial transparent state, LC droplets exhibit homeotropic boundary conditions with darker zone at the centre with ring shaped boundary. In the present work, the textures were observed under parallel and crossed polarizers. The captured information revealed that there are no changes in the central zone of the droplets due to the perfect homeotropic alignment of liquid crystals inside the droplet. The count of the droplets with different ranges was measured using ImageJ software. Further, the effect of electric field on textural variation inside the droplets, measuring the ratio of the size of darker zone to the size of droplet (a/d) was analysed by applying image processing. The response curve was obtained for different range of sizes of droplets from the plot of a/d ratio vs applied voltage and found supportive to the measure of the textural variation inside the LC droplets. Therefore, the a/d ratio can be the valuable parameter for optimizing the parameters such as droplet size, area of darker zone and required voltage for energy efficient PDLC devices.

Versatile homeotropic liquid crystal alignment with tunable functionality prepared by one-step method

Alignment layers are vital to the function of numerous devices based on liquid crystal (LC) materials. The pursue of versatile, effective and even flexible alignment layers, preferably prepared by simple methods, is still actively ongoing. Herein, we propose a facile one-step method by mixing silanes into the starting LC mixtures, which in contact with a glass substrate secede and self-assemble in-situ to form a stable and highly effective homeotropic alignment layer at the interface. Tetradecyldimethyl(3-trimethoxysilylpropyl)ammonium chloride (TDTA) is selected as the example to demonstrate the method, although a number of other silanes can produce similar results. With only 0.05 vol% of TDTA added to a mixture of liquid crystals and reactive mesogens, a uniform monolayer is chemically attached to the substrate, which automatically aligns the LCs homeotropically. Furthermore, by blending the TDTA with acrylate functionalized silanes like 3-(trimethoxysilyl)propyl methacrylate (A174), additional reactive functional groups can be easily introduced into the alignment layer, therefore offering opportunities to adjust the interface properties. An electro-responsive smart window based on the polymer stabilized liquid crystals (PSLCs) is successfully prepared using a one-step method, demonstrating excellent electro-optic performances and notably enhanced adhesion between the substrate and the in-situ formed polymer network. These findings are valuable especially for the development of flexible LC devices.

Template-assisted interfacial self-assembly of amphiphilic poly(ethylene oxide)–poly(propylene oxide)-based triblock copolymers for automatic control of molecular alignment

Hierarchical self-assembled nanoarchitectures based on peculiar self-assembly of block copolymers are advantageous for the nanofabrication of functionally versatile materials with a wide range of potential applications from bioelectronic devices to flexible displays. Herein, we developed a simple yet efficient, cost-effective, scalable strategy for the fabrication of an ultrafast responsive and highly reliable self-constructed polymer nanolayer for automatic control of molecular alignment by using the template-assisted interfacial self-assembly of amphiphilic poly(ethylene oxide)–poly(propylene oxide)-based triblock copolymers. A new type of ultrathin polymer nanolayer was facilely fabricated on polar electrode surface using simple doping in liquid crystal (LC) medium and in situ interfacial self-assembly of a small amount of amphiphilic triblock copolymers in the closed LC cell. A hydrophobic and ultrathin polymer nanolayer with dense nanoneedle arrays formed by interfacial hydrogen bonding between hydrophilic poly(ethylene oxide) block and hydrophilic indium tin oxide electrode induces a spontaneous homeotropic alignment of LCs during programmed self-assembly process. Moreover, this facile approach endows the polymer nanolayer with ultrafast electro-optical switching and high molecular alignment force characteristics, as well as an excellent alignment stability during long-term operation. Compared to commercial polyimide layer, our polymer nanolayer accomplishes the ultrafast falling response time with an improvement of 55.3% and fast rising response time with a reduction of 36.4%.

Hydrogen bond assisted anchoring transitions in nematic liquid crystals at the aqueous interface

Homeotropic alignment of liquid crystals (LCs) at the aqueous interface and their fundamental understanding is still limited known, which is a prerequisite for the development of new kinds of LC-based sensors for the detection of biomolecules in the aqueous phase. Herein, we have investigated the orientational ordering of nematic LC at LC-aqueous interface in contact with the different hydrogen bonding strength of para-substituted phenol derivatives (X; functional group attached to phenol at para position = -NO2, -CN, -Cl, -F, -CHO, -H) intending to understand how the intermolecular hydrogen bonding interactions at LC-aqueous interface amplified into supramolecular ordering in LCs. The hydrogen bonding interactions between -CN group of 5CB and -OH group of phenol derivatives lead to a change in tilt angle from planar to homeotropic ordering of 5CB molecules at LC-aqueous interface. Raman measurement of different mixtures between 5CB and para-substituted phenol derivatives have been recorded to correlate the experimental observations at the LC-aqueous interface as well as direct evidence of hydrogen bonding at the LC-aqueous interface. Density functional theory (DFT) has been used to calculate hydrogen bonding strength and charge transfer mechanism between 5CB and phenol derivatives. The orientation of LC at the aqueous interface depends on the strength of the interfacial hydrogen bonding between phenol derivatives and LC, which varies on substitution of phenol and concentration of the phenol molecules.

A Light‐Emitting Liquid Crystal Display Device without Polarizers and Alignment Layers

AbstractLight‐emitting displays become the future direction of display technology for the radical requirement of reducing energy consumption. Here, an approach of fabricating a luminescent liquid crystal (LE‐LC) display device without polarizers and alignment layers is reported, achieved by using an aggregation‐induced emission (AIE)‐active LE‐LC mixture. Through studying the relationship between emission of the LE‐LC at several orientation modes and the concentration of the AIE‐featured fluorescent dye, the difference of fluorescence at aggregation state is revealed. When the concentration of the fluorescent dye is high enough, the emission of the planar‐aligned LE‐LCs is ≈4 times of the vertical‐aligned LE‐LCs. An electronically controlled LE‐LC cell is thus constructed, in which the homeotropic alignment of liquid crystals is induced by doping of CuInS2 nanoparticles. Therefore, this simple LE‐LC display avoids the use of polarizers and alignment layers, raising the exciting possibility of development of high efficiency and low energy‐consumption LE‐LC devices.

Aptamer Laden Liquid Crystals Biosensing Platform for the Detection of HIV-1 Glycoprotein-120.

We report a label-free and simple approach for the detection of glycoprotein-120 (gp-120) using an aptamer-based liquid crystals (LCs) biosensing platform. The LCs are supported on the surface of a modified glass slide with a suitable amount of B40t77 aptamer, allowing the LCs to be homeotropically aligned. A pronounced topological change was observed on the surface due to a specific interaction between B40t77 and gp-120, which led to the disruption of the homeotropic alignment of LCs. This results in a dark-to-bright transition observed under a polarized optical microscope. With the developed biosensing platform, it was possible to not only identify gp-120, but obtained results were analyzed quantitatively through image analysis. The detection limit of the proposed biosensing platform was investigated to be 0.2 µg/mL of gp-120. Regarding selectivity of the developed platform, no response could be detected when gp-120 was replaced by other proteins, such as bovine serum albumin (BSA), hepatitis A virus capsid protein 1 (Hep A VP1) and immunoglobulin G protein (IgG). Due to attributes such as label-free, high specificity and no need for instrumental read-out, the presented biosensing platform provides the potential to develop a working device for the quick detection of HIV-1 gp-120.

Simple, rapid and sensitive detection of Parkinson's disease related alpha-synuclein using a DNA aptamer assisted liquid crystal biosensor.

Alpha-synuclein (αS) has been proposed as a potential biomarker for the diagnosis of Parkinson's disease (PD). However, the detection of αS using a simple, rapid and sensitive approach is still challenging. Herein, we construct a new type of biosensor for the detection of αS, combining the stimuli-responsiveness of liquid crystals (LCs) and the specific interaction of a DNA aptamer with proteins. In principle, the positively charged surfactant hexadecyltrimethylammonium bromide (CTAB) binds with the negatively charged DNA aptamer via electrostatic interactions; in the presence of αS, the DNA aptamer specifically binds with αS and releases CTAB, which is an amphiphilic molecule and subsequently assembles at the LC-aqueous interface, resulting in a homeotropic alignment of LCs with a dark optical signal. In the absence of αS, CTAB binds with the DNA aptamer without affecting the alignment of LCs, which shows planar anchoring with a bright optical signal. The response time of LCs towards αS is rapid and can be down to minutes. The LC biosensor established here has a good specificity for αS and can recognize αS even from a mixture of proteins. The LC biosensor also exhibits high sensitivity with a limit of detection of αS as low as 10 pM, which is comparable to that of the enzyme-linked immunosorbent assay. This work provides a new strategy for the detection of αS in a simple, rapid and sensitive manner, possessing promising potentials towards early diagnosis and clinical applications.

Manipulation of liquid crystal alignment through interfacial modification of photoresponsive polyimides remotely actuated by dual wavelength linearly polarized light

High-quality alignment of liquid crystals (LCs) for large area display has been highly demanding for display market. To achieve a uniform and stable LC alignment, a photoresponsive azo-polyimide with two different substitution ratio of azo-dye moiety in the side chain of the polyimide has been successfully synthesized and characterized. Depending on the ratio of azo-dye substitutions, the solubility and coating nature of polyimides vary greatly and initially show random planar or homeotropic alignment of LCs. In situ photoalignment process has been performed to achieve a homogeneous planar alignment of LCs from the initial random planar state and a uniform pretilt state from the initial homeotropic cells using linearly polarized visible light (LPVL) irradiation in the isotropic phase of LCs. Furthermore, obtained alignment states have been stabilized using unpolarized UV light for RM polymerization in the nematic phase of LCs. Hence, the dual wavelength photoalignment approach has been successfully implemented to obtain the different stable alignments of LCs. Also, the results conclude that the tilt angle varies by dose of LPVL irradiation and gets significantly altered due to change in reactive mesogen (RM) concentrations. The surface investigations reveal a contrasting nature for the planar and vertically aligned substrates before and after RM stabilization.

Vertical Alignment of Nematic Liquid Crystals Based on Spontaneous Alignment Layer Formation between Silver Nanowire Networks and Nonionic Amphiphiles

The vertical arrangement of nematic liquid crystals (LCs) in displays can be generally achieved by introducing a polyimide material onto indium tin oxide electrodes. However, this method requires multiple coating and deposition processes as well as high curing temperature, restricting the potential applicability to flexible displays. Thus, we herein propose the facile approach for homeotropic alignment of nematic LCs based on spontaneous alignment layer formation between silver nanowire networks and nonionic amphiphiles. The silver nanowires as transparent electrode materials were spin-coated on glass substrate and 4-(4-heptylphenyl)benzoic acids as nonionic amphiphiles were doped into the LC medium. The nonionic amphiphiles were spontaneously bonded to the polyvinylpyrrolidone capping layer of silver nanowire networks through polar interactions, creating the self-assembled alignment layer of nonionic amphiphiles on silver nanowire electrodes. In addition, the alkyl chains of the amphiphiles interacted with the LC molecules, leading to stable directional LC alignment along vertical direction. The electro-optical characteristics of the manufactured LC cell were comparable to those of conventional device including polyimide layer and indium tin oxide electrode. Overall, the combination of silver nanowire electrode and nonionic amphiphiles presents a new way to achieve the vertical alignment of nematic LCs without polyimide layer.

Characterization of surface anchoring energy of nematic liquid crystals via electrohydrodynamic instability.

Herein, a method is proposed to determine the azimuthal anchoring energies of surface liquid crystals (LCs), as they gradually change orientation from a vertical to a horizontal state owing to an increase in the voltage applied to each LC cell. The LC cells are characterized using the direction of the Williams roll pattern related to the midplane LC director of the conduction regime of the electrohydrodynamic convection patterns of LCs. The application of the midplane LC directions, obtained from the direction of the roll patterns, to the Ericksen-Leslie equation produces the precise values of the surface anchoring strength. The hybrid type 90°-twisted nematic LC cell, composed of homeotropic and homogeneous LC alignment layers on the top and bottom substrates, respectively, was used to find the azimuthal anchoring energy of the surface LCs, indicated by voltages at the initially vertically aligned LC state. It was observed that the surface azimuthal anchoring energy on the homeotropic layer increased with an increase in voltage. We expect that the proposed technique may be excellent in terms of ease of use, simplicity, and accuracy because the azimuthal anchoring energy can be visually evaluated through the roll pattern.

Highly Stable, Pretilted Homeotropic Alignment of Liquid Crystals Enabled by In Situ Self-Assembled, Dual-Wavelength Photoalignment

Homeotropic alignment of liquid crystals (LCs) is one of the key modes used in commercial liquid crystal displays (LCDs) due to its advantages, such as high contrast ratios and fast response speed....

Preparation and electrooptic study of reverse mode polymer dispersed liquid crystal: Performance augmentation with the doping of nanoparticles and dichroic dye

ABSTRACTReported herein, the preparation, morphological, and electrooptic (EO) characterization of reverse mode polymer dispersed liquid crystals (PDLCs) with nematic liquid crystal (LC) and UV curable polymer optical adhesive using polymerization induced phase separation method. The PDLCs are switchable between transparent and opaque states due to the homeotropic and planar alignment of LC in their OFF and ON states of applied voltage, respectively. Further, effect of doping of a fixed concentration of azo dye and silica nanoparticles (NPs) on morphological, EO and response characteristics of same PDLC sample was also analyzed. Experimental results showed that doped reverse mode PDLCs have the higher OFF state optical transmission and boost up in the scattering ON state compared with pristine reverse mode PDLC. The phenomenon is also supported by extinction coefficient and absorption study based on Beer's law. The threshold and operating voltages were found reduced ~1.56 and ~1.73 times for NPs and (NPs + dye) doped reverse mode PDLCs, respectively, along with better contrasts to the pristine reverse mode PDLC. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48745.

Titania Laden Liquid Crystal Based Sensor as Photocatalyst.

The present research work elucidates the photocatalytic effect of titania on liquid crystal based sensor used to detect sodium arsenate in water. The titania nanoparticles with an average size of 18-20 nm were synthesized by hydrothermal method and calcined up to 500 °C to obtain pure anatase phase. The catalytic activity of titania nanoparticles was enhanced by surface modification with (3-aminopropyl) trimethoxysilane. The sensing experiment was performed under visible light irradiation while keeping the concentration of substrate, analyte, the thickness of the liquid crystal layer, temperature and pH values constant. In addition, span 80 (a non-ionic surfactant) was used to obtain homeotropic alignment of liquid crystals. The TiO₂ nanoparticles gave dark to bright texture within 53 min while the functionalized titania responded in less than 25 min under similar sensing condition. The detection results showed that the functionalized titania nanoparticles possess greater photocatalytic activity under visible light and enhanced the sensing response than unfunctionalized titania nanoparticles.

Photo‐Tunable Supramolecular Ultrathin Surfaces for Simultaneous Homeotropic Anchoring and Superfast Switching of Liquid Crystals

AbstractPhoto‐tunable supramolecular ultrathin surfaces functionalized with a soft ultraviolet (UV) treatment of star‐shaped reactive amphiphiles are fabricated to simultaneously afford homeotropic anchoring of liquid crystals (LCs) and improve the LC switching performance. Photochemically functionalized ultrathin surfaces are generated by cross‐linking star‐shaped amphiphilic monomers in the self‐assembled monolayers with retention of cluster structure under electric field. Since these star‐shaped amphiphiles can construct the close‐packed self‐organized molecular layer with an alkyl brush at the interface between indium tin oxide and LCs through intermolecular hydrogen bonding, they might make elegant alignment surfaces for the homeotropic anchoring of LCs to provide a new type of vertical alignment layer. It is experimentally revealed that supramolecular ultrathin layers with vertical nanopillars form hydrophobic surface, and afford a stable and uniform homeotropic alignment of LCs even in high‐temperature environment. Furthermore, a soft UV irradiation on the self‐assembled ultrathin surfaces under electric field yields superfast responsive cross‐linked ultrathin surfaces with low power consumption compared to conventional polyimide layer. Notably, this controlled methodology offers advantages of simplicity, low‐cost, and scalability in the fabrication of functional alignment surfaces without the need of complex manufacturing steps.