Chang-Hyun Jang

Spontaneous formation of micrometer-scale liquid crystal droplet patterns on solid surfaces and their sensing applications

In this study, we report spontaneous formation of two characteristic micrometer-scale liquid crystal (LC) droplet patterns on solid surfaces and also demonstrate the application of the designed LC platform to construct new types of LC-based sensors. By simply spreading LCs dissolved in organic solvents on glass microscope slides, we observed one- and two-dimensional LC droplet patterns with distinctive optical textures that represent different orientations of LCs under a polarized microscope. LC droplets supported on surfaces exhibited high stability during temperature-induced phase transitions of LCs. In addition, based on the distinguishable optical appearance of the LC droplet patterns, their applications in monitoring the presence of water vapors, amphiphiles, and vapors of volatile organic compounds (VOCs) were demonstrated. These results indicate that the surface-anchored LC droplets show high promise for the development of simple, robust, and versatile LC pattern-based sensing devices for real-time and label-free detection of chemical and biological events.

Orientational behaviors of liquid crystals coupled to chitosan-disrupted phospholipid membranes at the aqueous–liquid crystal interface

In this study, we investigated the orientational behavior of liquid crystals (LCs) which is associated with the chitosan-disrupted phospholipid membrane at the aqueous/LC interface. The optical response of LCs changed from dark to bright after the transfer of an aqueous solution of chitosan onto the LC interface decorated with self-assembled monolayers of a negatively charged phospholipid, dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DOPG). The chitosan-lipid interactions induced a rearrangement of the membrane, and thus, resulted in an orientational transition of LCs from a homeotropic to a planar state, thereby triggering a dark-to-bright shift in the optical response. We observed that LCs exhibited a bright-to-dark shift after an aqueous solution of lysozyme was transferred onto the chitosan-disrupted membrane, which implied that an enzymatic reaction between lysozyme and chitosan took place. We found that the addition of bovine serum album (BSA) induced a bright-to-dark change in the optical response; while LCs remained to appear bright after the transfer of chymotrypsin onto the aqueous/LC interface. We then further examined the interactions between other polyelectrolytes and phospholipid membranes.

Real-time and sensitive detection of lipase using liquid crystal droplet patterns supported on solid surfaces

In this study, we demonstrate a real-time and sensitive strategy for monitoring of lipase activity using liquid crystal (LC) droplets with micrometre scales supported on solid surfaces. The LC droplet pattern was formed by evaporating a solution of nematic LC, 4-cyano-4′-pentylbiphenyl (5CB) dissolved in heptane. The surface-anchored LC droplets displayed a bright fan-shaped appearance in buffer solution, while they exhibited a dark cross appearance when they were in contact with an aqueous mixture of glycerol trioleate (GT) and lipase. Due to adsorption of the released oleic acid generated from the enzymatic reaction between GT and lipase, LCs adopted a perpendicular orientation instead of planar orientation at the aqueous/LC droplet interface, resulting in contrasting optical response. Using this approach, the presence of 0.1 μg/mL lipase in the aqueous solution could be detected within 6 min. When compared to previously reported LC-based sensors for the detection of lipase, this simple and convenient method has the advantage of real-time monitoring of lipase activity with high sensitivity. In addition, this strategy also has high potential for application of the LC droplet pattern for sensing applications.

Nematic liquid crystal micro-droplets on solid surfaces and their ordering transition in bulk aqueous solution

Here, we report the establishment of an experimental system based on liquid crystal (LC) micro-droplets on solid surfaces. These micrometre-sized LC droplets were formed by the dispersion of 0.5% (v/v) 4-cyano-4ʹ-pentylbiphenyl (5CB) dissolved in n-heptane onto an octadecyltrichlorosilane (OTS)-treated glass. Polarised microscopy revealed a dandelion shape or a double-rod shape when LC micro-droplets were immersed in 4 mm in the depth of non-surfactant solution, while distinct diamond-shaped textures were obtained when contacted with the bulk surfactant solution. Different types of optical textures were observed due to various configurations of the local director n. In non-surfactant solution, a parallel configuration led to a bright white-coloured dandelion shape or double-rod shape, while in surfactant solution the colourful diamond-shaped textures were caused by normal configuration. The unprecedented observations we report in this paper will lead to further insight into the properties of sessile LC micro-droplets on solid surfaces. The experimental system established in this paper can be broadly applied to report the liquid–liquid interfacial phenomena, and provide important guidance to the design of surface-anchored LC micro-droplets as chemo-responsive materials for use in optical sensors.

Sensitive Detection of Trypsin using Liquid-Crystal Droplet Patterns Modulated by Interactions between Poly-L-Lysine and a Phospholipid Monolayer

Liquid-crystal (LC) droplet patterns are formed on a glass slide by evaporating a solution of nematic LC dissolved in heptane. In the presence of an anionic phospholipid, 1,2-dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) (DOPG), the LCs display a dark cross pattern, indicating a homeotropic orientation. When LC patterns are incubated with an aqueous mixture of DOPG and poly-L-lysine (PLL), there is a transition in the LC pattern from a dark cross to a bright fan shape due to the electrostatic interaction between DOPG and PLL. Known to catalyze the hydrolysis of PLL into oligopeptide fragments, trypsin is preincubated with PLL, significantly decreasing the interactions between PLL and DOPG. LCs adopt a perpendicular orientation at the water-LC droplet interface, which gives rise to a dark cross pattern. This optical response of LC droplets is the basis for a quick and sensitive biosensor for trypsin.

Diagnosis of tuberculosis using a liquid crystal-based optical sensor

In this study, we developed an advanced liquid crystal (LC)-based biosensor using polymeric surfaces to detect anti-tuberculosis antibodies, specifically in solution. To manufacture the anisotropic nanostructures, poly (dimethylsiloxane) was stretched and subsequently relieved. Before the deposition of a gold film, the nanostructures were replicated on poly(urethane acrylate)-coated silica substrates. Then, tuberculous antigens (ESAT-6) were immobilized onto the surfaces previously treated viaN-hydroxysuccinimide (NHS)/N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC) chemistry. The LCs (4-cyano-4′-pentylbiphenyl, 5CB), which aligned in parallel with the plane of the antigen-immobilized surface, showed a uniform appearance. However, after incubation of the anti-tuberculosis antibody (anti-ESAT-6) on the antigen-immobilized surface, 5CB showed a random appearance that corresponded to its random orientation. In control experiments, the tuberculous antigen-immobilized surfaces retained a uniform appearance after incubation with another anti-tuberculosis antibody, human serum albumin, and phosphate-buffered saline. These results verified that the tuberculous antigen-antibody complexes on the surface masked the anisotropic surface topography, which led to the transition of 5CB from a uniform orientation to a random one. Consequently, LC sensing systems based on nanostructured polymeric surfaces could be used in the labelfree detection of anti-tuberculosis antibodies, and they could be expanded as a feasible application in the immunoassay of tuberculosis.

Imaging catalase reactions through interactions between liquid crystals and oil-in-water emulsions

In this study, a simple label-free technique for imaging catalase reactions through interactions between liquid crystals (LCs) and oil-in-water emulsions was developed. We prepared a dodecanal-in-water emulsion, and mixed the emulsion with 4-cyano-4′-pentylbi-phenyl (5CB) droplets. In the absence of hydrogen peroxide, a dark image was observed between crossed polarisers because the 5CB and dodecanal droplets coalesced resulting in the formation of a population of droplets comprised of 5CB and dodecanal that are isotropic. In contrast, if hydrogen peroxide was present, a bright appearance was observed, because some fraction of the dodecanal was converted to dodecanoic acid, which prevented coalescence of 5CB and dodecanal, and thus the 5CB droplets retained their nematic order. Since catalase could decompose hydrogen peroxide, we observed a dark image after LC made contact with a dodecanal–hydrogen peroxide mixture that had been pre-incubated with catalase. Control experiments were conducted to confirm the specificity of the enzymatic reaction. The detection limit of catalase in our system was 0.1 μg/mL. These results demonstrate that LC with oil-in-water emulsion can be used for the detection of relevant enzymatic reactions.

Real-space observation of internal ordering configurations of sessile liquid crystal micro-droplets

ABSTRACT In this study, we report a simple method to determine the internal ordering configurations of sessile liquid crystal (LC) micro-droplets. We studied the ordering of LC micro-droplets under perpendicular, tangential and hybrid boundary conditions, using a polarised microscope equipped with a full-wave retardation plate. Four types of internal ordering configurations were found in sessile LC micro-droplets: two different types of dark cross configuration, a four-petal configuration and a three-petal configuration. These findings provide valuable information for further characterisation and analysis of the optical images of LC droplets and their corresponding configurations. GRAPHICAL ABSTRACT

Optical imaging of cholylglycine by using liquid crystal droplet patterns on solid surfaces

Determining cholylglycine (CG) levels is of great importance in the detection of liver abnormalities. In this study, we present a novel liquid–crystal (LC)-based method assisted with cholylglycine hydrolase (CGH) for the optical detection of CG levels. The detection method is based on the disruption of the orientations of a nematic LC, 4-cyano-4′-pentylbiphenyl (5CB), doped with dodecyl aldehyde. Aldehyde-doped 5CB droplets were placed on pre-treated glass slides and patterned with solutions of interest. When in contact with a small drop of a CG aqueous solution, bright fan-shaped LC droplet patterns were observed under polarizing optical microscopy, indicating a planar orientation of LC at the aqueous/LC interface. However, aldehyde with short-alkyl chain couples with glycine released from the enzymatic reaction between CG and CGH forming amphiphilic Schiff bases, which display dark cross patterns of LC, suggesting a homeotropic orientation of the LC. This system may offer a highly sensitive and methodologically simple approach to determine CG levels for clinical diagnostics and commercial applications.

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

AbstractIn 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. Figureᅟ