Optical Response Of Liquid Crystals Research Articles

Sensitive detection of 17β-estradiol at a picomolar level using an aptamer-assisted liquid crystal-based optical sensor.

A liquid crystal (LC)-based aptasensor was developed that can detect 17β-estradiol (E2) at the picomolar level. This aptasensor is based on competitive reactions of the aptamer that interacts with cetyl trimethyl ammonium bromide (CTAB) and E2 at the aqueous/LC interface. The long alkyl chain of CTAB anchored the 4-cyano-4'-pentylbiphenyl (5CB) to a homeotropic state and controls the local anchoring depending on the extent of electrostatic interaction with the aptamer. Upon addition of the aptamer solution to the CTAB-saturated LC layer, LCs change from dark to bright optical response. This is due to the perturbed orientation of 5CB at the aqueous/LC interface as a result of electrostatic attraction of the cationic group of CTAB and the phosphate group of the aptamer. The conformational change of the aptamer due to specific binding with E2 weakens the electrostatic attraction between CTAB and aptamer. When specific binding becomes relatively dominant, CTAB induces the orientation of LCs to the homeotropic state, resulting in a dark optical image observed. We also analyzed the change in the optical response of LCs according to the interfacial events and compared the grayscale values of the optical image for each concentration of E2 to determine the detection limit. Accordingly, the detection limit of the E2 sensor was found to be 3.1pM (0.8pg/ml) in Tris-buffered saline (TBS), and 6.8pM (1.9pg/ml) in human urine. The LC-based optical aptasensor was thus shown to be highly sensitive and selective with no requirement for complex analysis equipment.

Optical and dielectric realisation of biomolecular detection using gold nanoparticles bio-conjugate with liquid crystal

Dielectric biosensor based on nematic liquid crystal platform has been fabricated using sandwich immunoassay technique for the detection of a biomolecule, human cardiac troponin I (cTnI). The optical response of Liquid crystal (LC) biosensor and its corresponding frequency-dependent dielectric properties have been investigated as a function of cTnI concentration. Gold nanoparticles conjugate with detection antibody (AB-GNP) is used for the enhancement of both the optical and dielectric signal of the biosensor. AB-GNP binding with cTnI alter the surface energy of the substrate causing disturbances in LC alignment resulting in a change in transmission. The transmission of the LC cell has been found to be varying from dark to bright texture under polarized microscope as a function of cTnI concentration. A change in low-frequency permittivity with increasing cTnI concentration that forms an increasing order of sandwich immunocomplex with AB-GNP revealed an interfacial surface phenomenon. A positive shift in the onset frequency of permittivity is observed due accumulation of immunocomplex with increasing concentration of cTnI at electrode/LC interface. The dielectric response ε′ of the biosensor is fitted with Hill-Langmuir equation over a concentration range of 0.01 to 10 ng mL−1 cTnI. The dissociation constant ‘Kd’ of 1.02 ± 0.24 ng/mL−1 indicates strong cTnI binding affinity for substrate antibody. A nearby value of 1, calculated for Hill coefficient ‘n’ revealed no co-operativity characteristic of cTnI binding to the primary capture antibody, wherein each binding site of the antibody functions independently at the electrode surface, causing accumulation of charged species revealing LC interfacial surface electrical property.

Convolutional Network Analysis of Optical Micrographs for Liquid Crystal Sensors

We provide an in-depth convolutional neural network (CNN) analysis of optical responses of liquid crystals (LCs) when exposed to different chemical environments. Our aim is to identify informative ...

In situ calibration for a phase-only spatial light modulator based on digital holography

Reliable phase-only spatial light modulators (SLMs) are in demand for accurate phase modulation in a wide range of fields. Due to the nonlinear optical response of liquid crystals and the limited manufacturing process available, the spatial nonuniformity of the phase modulation by the pixels should be measured and/or calibrated. We propose an in situ calibration method based on digital holography to calibrate the spatial nonuniformity of phase modulation of the SLM. The SLM panel is divided into blocks composed of pixels. The differential phase on hundreds of blocks can be reconstructed through the holograms. The distribution of modulated phase can then be derived after eliminating statics phase anomalies. The spatial nonuniformity of the panel can be measured for calibration with high efficiency. A modulated phase step on the SLM was calibrated to increase linearly. The spatial nonuniformity was calibrated to decrease by more than 75% using only a beam splitter and an imaging sensor. The in situ strategy for low cost and efficient calibration was demonstrated with optical experiments using a 4K (3840 × 2160 pixels) phase-only SLM.

Construction of liquid crystal droplet-based sensing platform for sensitive detection of organophosphate pesticide

Alkaline phosphatase (ALP), as an essential disease biomarker, widely distributes in various tissues, and plays a significant role in diagnosis and treatment of some diseases (e.g. cancers and liver dysfunction). Dichlorvos (DDVP), as one of typical organophosphate pesticide, could be hydrolyzed by ALP. In this work, we constructed a simple but robust 4-cyano-4′-pentylbiphenyl (5CB) droplet sensing platform for sensitive and convenient detection of DDVP based on its hydrolysis by ALP. The optical responses of liquid crystals (LCs) originated from controlling orientations of 5CB by using an ALP cleavable surfactant, sodium monododecyl phosphate (SMP). A dark crossed optical image of 5CB was observed due to formation of the SMP monolayer at the aqueous/LC droplet interface. Subsequently, 5CB turned to bright fan-shaped optical appearance upon addition of ALP and SMP mixture. Interestingly, LCs remained dark crossed optical images in contact with the pre-incubated mixture of DDVP and ALP. The detection limit of DDVP reached ~0.1 ng/mL. Isothermal titration calorimetry (ITC) and humam ALP ELISA kit provided strong evidence for the detection mechanism. This strategy was also used to evaluate whether the maximum pesticide residue limits of tomatoes conform to the relevant national standard requirement or not. Our study holds promise for real-time and label-free detection of pesticides with high sensitivity by employing portable LC-based sensing platform.

Liquid crystal/polymer fiber mats as sensitive chemical sensors

We present quantitative measurements of the optical response of liquid crystal (LC)/polymer composite fiber mats to toluene and acetone vapors. Our analyses in comparison with control measurements of pure LC film and polymer fiber mats show that the chemicals can pass through the polymer sheath of the fibers and be absorbed by the LC in the core. This absorption changes the optical properties of the fiber mats which can be used to produce sensitive and reversible detection. The sensitive response at low concentrations of both acetone and toluene demonstrates the feasibility of using these fibers for highly sensitive and specific sensors for volatile organic compound detection.

Colossal nonlinear optical response of liquid crystals

We briefly review the main steps related to the discovery of the extraordinarily large nonlinear optical response of nematic liquid crystals doped by Methyl Red leading to detection of n2>103 cm2/W in 1μm thin films, that is the strongest nonlinear response ever reported in liquid crystals. This investigation started from the researches of Professor Yuri Reznikov on light-induced anchoring effects in these materials.

Detection of mRNA from Escherichia coli in drinking water on nanostructured polymeric surfaces using liquid crystals

In this study, we demonstrate the detection of mRNA from Escherichia coli in drinking water using thermotropic liquid crystals (LCs). After hybridization of complementary mRNA with the single-stranded DNA immobilized on a polymer substrate containing periodic sinusoidal wave patterns, the orientation of LCs transits from a uniform to a non-uniform state, thereby inducing a change in the optical response of LCs. The periodic sinusoidal features of the polymer substrate are obtained through buckling the poly-(dimethylsiloxane) slide on a cylindrical surface, followed by replicating the associated relief structures on a poly-(urethaneacrylate) surface, where a film of gold was deposited. Then, thiol-modified single-stranded DNA was functionalized on the gold film as an mRNA receptor. The formation of mRNA–single-stranded DNA complex, which covers the sinusoidal nanostructures on the surface, induces the orientational transition of LCs. This result indicates that LCs can be used to report the specific hybridization of mRNA with single-stranded DNA, which holds promise for the sensitive and label-free detection of viable bacterial pathogens in drinking water.

A liquid crystal display with consistent moving image quality regardless of viewing angles

This paper proposes a new overdrive (OD) technology to precisely compensate for the viewing angle dependent characteristics of LCDs. This paper reports that optical response of liquid crystal displays (LCDs) is considerably dependent on viewing angles for the first time. The new OD technology applies different OD look-up tables (LUTs) depending on the viewing angles. In addition, we combine a new OD technology with an eye tracker that is usually adopted for autostereoscopic 3D LCD systems. The application results show that a new OD technology improves the motion image quality perfectly regardless of viewing angles. We expect that our proposed method will definitely enable the LCD products to have consistent motion image quality regardless of viewing angles. • This paper reports that optical response of liquid crystal displays (LCDs) are considerably dependent on viewing angles for the first time. • We propose a new overdrive (OD) technology to precisely compensate for the viewing angle dependent characteristics of LCDs. • The new OD technology applies different OD look-up tables (LUTs) depending on the viewing angles. • We also combine a new OD technology with an eye tracker that is usually adopted for autostereoscopic 3D LCD systems. • Application results show that the new OD technology improves the motion image quality perfectly regardless of viewing angles.

Label-free detection of viruses on a polymeric surface using liquid crystals

In this study, we demonstrated a label-free detection of viruses using liquid crystals (LCs) on a polymeric surface with periodic nanostructures. The polymeric nanostructures, which hold sinusoidal anisotropic patterns, were created by a sequential process of poly-(dimethylsiloxane) buckling and replication of the patterns on a poly-(urethane acrylate) surface containing a film of gold. After immobilization of human cytomegalovirus- and adenovirus-antibodies onto the polymeric surface treated with a mixed self-assembled monolayer, a uniform appearance reflecting the uniform orientation of 4-cyano-4′-pentylbiphenyl (5CB) was observed. Conversely, binding of viruses to their antibody decorated surface induced a random appearance of 5CB from the random orientation of 5CB. The uniform to random orientational transition of 5CB indicates that the anisotropic topography of the polymeric surface was masked after specific binding of viruses to the antibody decorated surface. We evaluated the specificity of the binding events by confirming topographical changes and optical thickness using atomic force microscopy and ellipsometry, respectively. These results demonstrate that polymeric surfaces with continuous anisotropic patterns can be used to amplify the presence of nanoscopic virions into an optical response of LC, as well as expand the scope of LC-based biological detection on polymeric solid surfaces.

Experimental study of a textural transition in liquid crystals under shear stress induced by surface friction

Results of a study of the optical response of liquid crystals with helical supramolecular structure to surfacefriction-induced shear stress in aerodynamic experiments are reported. The effect of re-orientation of molecules from confocal (light-scattering) to planar (selectively reflecting light) texture is considered. The textural transition is examined by the spectral and colorimetric methods. Dependences of the reflection spectra of the formed planar texture on time and on the mean shear-stress level were obtained. It is shown that the dependence of the texture transition time on the shear-stress level can be approximated, in a logarithmic scale, with a linear dependence; this allows using the effect for flow diagnostics.

High-Contrast Imaging of Biomolecular Interactions Using Liquid Crystals Supported on Roller Printed Protein Surfaces

In this study, we report a new method for the high contrast imaging of biomolecular interactions at roller printed protein surfaces using thermotropic liquid crystals (LCs). Avidin was roller printed and covalently immobilized onto the obliquely deposited gold surface that was decorated with carboxylic acid-terminated self-assembled monolayers (SAMs). The optical response of LCs on the roller printed film of avidin contrasted sharply with that on the obliquely deposited gold surface. The binding of biotin-peroxidase to the roller printed avidin was then investigated on the obliquely deposited gold substrate. LCs exhibited a non-uniform and random orientation on the roller printed area decorated with the complex of avidin and biotin-peroxidase, while LCs displayed a uniform and planar orientation on the area without roller printed proteins. The orientational transition of LCs from uniform to non-uniform state was triggered by the erasion of nanometer-scale topographies on the roller printed surface after the binding of biotin-peroxidase to the surface-immobilized avidin. The specific binding events of protein-receptor interactions were also confirmed by atomic force microscopy and ellipsometry. These results demonstrate that the roller printing of proteins on obliquely deposited gold substrates could provide a high contrast signal for imaging biomolecular interactions using LC-based sensors.

Dynamic imaging of enzymatic events at polyelectrolyte-disrupted phospholipid membranes using liquid crystals

In this study, we employed a simple liquid crystal (LC)-based system to dynamically image enzymatic events at the aqueous/LC interface decorated with polyelectrolyte-disrupted phospholipid membranes. Since polyelectrolytes were shown to disrupt the arrangement of the self-assembled phospholipid monolayer and induced a dark-to-bright shift in the optical response of LCs that support the phospholipid membrane, we observed that the transfer of an aqueous solution of protease onto the polyelectrolyte-disrupted phospholipid membrane resulted in a gradual recovery of the optical response of LCs from bright to dark appearance. Due to the enzymatic event that occurs at the aqueous/LC interface, the generated polyelectrolyte fragments desorbed from the interface to the bulk solution. This led to the restoration of the disrupted phospholipid monolayer, which resulted in recovery of the optical response. These results suggest that the polyelectrolyte-decorated membrane-supported LCs could be potentially used to examine a range of biological interactions that involve polyelectrolytes. Furthermore, the LC-based system holds great promise for label-free and real-time investigation and detection of biomolecular interactions coupled to membrane disruption and restoration, which might have potential utility in the clinical diagnosis and treatment of membrane-associated disease.

Theoretical Study of the Effect of Light on Nematic to Isotropic Phase Transition

Optical response of liquid crystals is very important from the point of view of applications of liquid crystals in display and data storage devices. Experiments have shown that UV rays incident on nematic liquid crystals doped with photoactive azo compounds lower the nematic isotropic transition temperature (TNI). This is attributed to the transformation of photoactive trans isomer to cis isomer due to UV rays. We have earlier developed a mean field model involving molecular explanation for two lengths in which the mutual orientation of near neighbor molecules changes from an antiparallel to a parallel configuration as the temperature lowered. In this paper, this model is extended to include the length change from trans to cis isomer. The calculated values of change in TNI qualitatively agree with experimental trends.

Impact of dyes on the nonlinear optical response of liquid crystals implementing the Z-scan technique

The study of the nonlinear refractive index response γ of several organic dyes and their impact on the nonlinear optical (NLO) properties of nematic liquid crystals (LC) was performed via Z-scan measurements. For his purpose, a low power CW He-Ne laser system (λ ≈ 633 nm) was implemented. Studies were carried out at the low absorption spectroscopic region of the implemented samples (dyes, liquid crystals and mixtures at different ratios of these materials). Samples were prepared at 1% weight of the used solvent (THF) and were sandwiched in glass cells with a gap thickness of ~100 μm. The implemented dyes have shown the largest optical nonlinearities and represent the main contributors to the cubic NLO-properties of the LC:Dye mixtures. In our particular studies, 5CB liquid crystal doped with DR1 azo-dye, resulted in the simultaneous positive and negative exhibition of nonlinear refractive indexes γ, depending on the polarization state of the excitation laser beam. Experimental conditions and results are described in detail.

A Principle of Detecting and Differentiating Dialdehydes from Monoaldehydes by using Surface Reactions and Liquid Crystals

In this Letter, we report a principle of discriminating dialdehydes from monoaldehydes by using optical responses of liquid crystals (LCs) triggered by a surface reaction between aldehydes and surface amine groups. When a solid surface is decorated with a mixture of long hydrocarbons and primary amines, a surface reaction between dialdehydes and the surface amines generates an aldehyde-terminated surface (which can be tested by using a triglycine-copper complex as an aldehyde marker). This type of surface is able to trigger an ordering transition of 5CB supported on the surface. In contrast, the surface reaction between monoaldehydes and the surface amines leads to a hydrocarbon-terminated surface which does not influence the orientations of 5CB on the surface. Because LCs are birefringent materials, the changes of orientations can cause distinct optical textures visible to the naked eye. The methodology reported herein provides a simple optical method of detecting and distinguishing dialdehydes from monoaldehydes with high sensitivity (∼10 μM for glutaraldehyde and phthaldialdehyde) and specificity.

Dark-to-Bright Optical Responses of Liquid Crystals Supported on Solid Surfaces Decorated with Proteins

Protein assays are critical analytical tools performed in various biochemical laboratories to quantify the concentration of proteins. In this study, we report the optical responses of a thin layer of liquid crystals supported on glass slides decorated with proteins and the utility of this phenomenon as a new "all-or-nothing" type of protein assay. It was found that the orientations of liquid crystals are very sensitive to the concentration of protein solution applied to the surface. When the protein concentration exceeds a critical value (IgG 5.0 microg/mL, BSA 6.0 microg/mL, FTIC-anti-biotin 0.40 microg/mL, and FITC-anti-IgG 0.37 microg/mL), the thin layer of liquid crystals gives a very sharp dark-to-bright optical response within a small concentration range. This characteristic is not observed in any traditional protein assays, which are based on the adsorption of UV or visible light. The optical response is also very precise and reproducible. It is not affected by the thickness of the liquid crystal cell or the amount of organosilanes coated on the glass slides. The liquid crystal-based protein assay may be very useful for screening purposes, especially when a simple positive or negative answer is desired.

Poling of liquid crystals in thin films.

We examine the effect of a strong DC electric field on the molecular orientational order and the nonlinear optical response of liquid crystals in thin films. We compare the results of second-harmonic generation measurements with the predictions of two models, one assuming that the dipoles carried by the molecules have no interactions (the isotropic model), and the other assuming that the dipoles evolve in a Maier-Saupe orienting field responsible for the liquid-crystalline order (the Maier-Saupe model). In both cases, we take into account the effect of surfaces and confinement on the behavior of the molecules. We find that the molecular dipoles behave as predicted by the isotropic model, but that their reorientation is correlated in such a way that the apparent dipole moment of the reorienting units is one order of magnitude larger than the molecular dipole moment.