Orientational Transition Of Liquid Crystals Research Articles

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.

Highly sensitive label-free liquid crystal-based aptasensor to detect alpha-fetoprotein

ABSTRACT In this study, a novel label-free optical aptasensor based on liquid crystals (LCs) for sensitive and specific alpha-fetoprotein (AFP) detection was developed. The modification parameters were optimised to limit the effect of the background signal and increase the sensitivity of the sensor. The AFP-specific aptamer was immobilised on the substrates using an APTES + GA bridge. The binding of AFP to its aptamer causes changes in the surface topography and disrupts the LC alignment. The orientational transition of LC could be observed, from dark to bright, using POM. The changes in the topography of the surfaces were confirmed by AFM, ellipsometry, and water contact angle measurements. Under optimal conditions, the proposed LC-based aptasensor exhibited optimal performance in AFP detection. The detection limit for AFP reached 12.62 pg mL−1 in HEPES buffer and 18.65 pg mL−1 in human serum, with the detection range from 20–800 mL−1 for both the conditions. Owing to advantages such as simplicity, speed, and high sensitivity and specificity, the proposed label-free LC-based aptasensor has potential applications in the clinical diagnosis of various tumour markers.

A stable liquid crystals sensing platform decorated with cationic surfactant for detecting thrombin

A liquid crystals (LCs) sensing platform to detect thrombin was demonstrated based on high specificity of aptamer recognition. Initially, LCs doped with a cationic surfactant, octadecyl-trimethylammonium bromide (OTAB), adopted homeotropic alignment corresponding to dark optical image. Upon introduction of thrombin aptamer, the orientational transition of LCs from homeotropic to tilted state occurred due to the electrostatic interaction between it and OTAB. And accordingly a visual dark-to-bright alteration was observed for the polarized light microscope (POM) image of LCs. In the presence of thrombin with which the aptamer preferred to bind, OTAB released could induce the vertical arrangement of LCs and a dark optical image was regained. Therefore, by observing the changes of bright and dark optical appearances for LCs, thrombin could be analyzed with a detection limit of ~ 136 nM. The LCs sensing system, assisted with the specific combination of target molecule and its corresponding aptamer, reveals a simple and effective way to achieve label-free and low-cost detection of biomolecules in related clinic diagnosis.

Effect of Single/Mixed Surfactant Systems on Orientations of Liquid Crystals and Interaction of Proteins with Surfactants at Fluid Interfaces

A series of single surfactant systems, i.e, quaternary ammonium-based gemini surfactants with different spacers and alkyl chain lengths (m-n-m; m = 12, n = 2, 3, 4, 6; n = 3, m = 12, 14, 16), halogen-free surface-active ionic liquid (HF-SAILs) with different symmetries ([Cnmim][C12H25SO4]; n = 6, 8, 10, 12), and single-chain cationic surfactants including 1-dodecyl-3-methylimidazolium bromide ([C12mim]Br) and dodecyltrimethylammonium bromide (DTAB), along with certain combinations of different surfactants (12-3-12/[C12mim]Br and 12-3-12/DTAB) were applied to an aqueous/liquid crystal interface (ALI). All the surfactants could induce an orientational transition of liquid crystals (LCs) from a planar to homeotropic state, which caused a bright-to-dark optical shift. It was proved that double-chain surfactants and the mixed surfactants inclined to adsorb at the ALI triggering the orientational transition. Inspiringly, a quicker and more sensitive dark-to-bright optical response was observed for mixed surfactant system-decorated interfaces in contact with proteins (such as bovine serum albumin (BSA), lysozyme, and trypsin) as opposed to the single surfactant systems. The ALI decorated by the 12-3-12/[C12mim]Br system was particularly efficient and exhibited the most sensitive optical response for BSA (0.01 ng mL−1). The order parameters (SCD) of surfactants tails at the interface and the free energy of proteins with 12-3-12 and [C12mim]Br were calculated, respectively. The results explain that the 12-3-12/[C12mim]Br-laden ALI shows a quicker and more sensitive optical response for BSA. This work inspired us to study mixed surfactant systems-decorated LC interfaces and further provides new insights for different chemical and biological applications.

Liquid crystals-based sensor for the detection of lithocholic acid coupled with competitive host-guest inclusion.

A simple, low-cost and label-free strategy for detecting lithocholic acid (LCA) was designed at the liquid crystals (LCs)/aqueous interface via competitive host-guest inclusion. In this method, sodium dodecyl sulfate (SDS) was initially adsorbed on the fluid interface and induced LCs to adopt the homeotropic ordering. Inclusion complexation of SDS and β-cyclodextrin (β-CD) disturbed interaction between LCs and SDS and evoked LCs to keep a tilted alignment. When injecting LCA into the mixed solution of SDS and β-CD, SDS excluded from the cavity of β-CD by competitive host-guest inclusion and could be re-adsorbed at the LCs/aqueous interface, resulting in the orientational transition of LCs from tilted to homeotropic state. Correspondingly, a bright-to-dark optical response was observed under polarized optical microscope (POM). The as-prepared LCs-based sensor could detect LCA as low as about 2 μM in aqueous solution. Moreover, the practicability of the approach was validated by monitoring the known amount of LCA in human urine. This work offers an appealing approach for the detection of LCA which has a great potentiality in clinical diagnosis.

Sensitive Liquid Crystal‐based Sensor for Monitoring the Enzymatic Activities of Trypsin

In this study, a highly sensitive and label‐free method was developed to monitor the enzymatic activities of trypsin via orientational transition of liquid crystals (LCs) coupled to the interactions between the polyelectrolyte and phospholipid monolayer. Generally, the positively charged polyelectrolyte interacted with the negatively charged phospholipid monolayer by electrostatic interaction, which caused reorganization of the phospholipid membrane and induced a homeotropic to planar orientational transition of LCs. Enzymatic cleavage of the polyelectrolyte, which was caused by trypsin, eliminated the electrostatic interaction that occurred at the aqueous/LC interface and restored the LC alignment. The optical response of the LC changed in a way that corresponded with the LC molecular arrangement, which enables naked‐eye detection under polarized optical microscopy. A rather low detection limit, down to 10 ng/mL, was achieved for trypsin activity detection by applying the proposed method.

Liquid Crystal Based Optical Sensor for Imaging Trypsin Activity at Interfaces Between Aqueous Phases and Thermotropic Liquid Crystals

In this study, we developed a liquid crystal (LC)-based optical sensor for monitoring enzymatic activity through orientational changes in liquid crystals (LCs) coupled to the properties of a poly--lysine (PLL)-based polymeric membrane. We prepared a PLL-based polymeric membrane at the planar interface between the thermotropic liquid crystal and aqueous phases. The PLL-based polymeric membrane was obtained by contacting the PLL solution with water immiscible LCs, 4-cyano-4'-pentyl-biphenyl (5CB) doped with adipoyl chloride. We then investigated the membrane properties by examining the permeability of the membrane to phospholipids, 1,2-didodecanoyl-rac-glycero-3-phosphocholine (DLPC). The permeability of the membrane to transport phospholipids was monitored through the orientational transition of 5CB in contact with the dispersions of DLPC. Since trypsin can enzymatically catalyze the hydrolysis of PLL, we incubated an aqueous trypsin solution with the membrane for 2 h at room temperature to cause an increase in the permeability of the polymeric membrane to DLPC. As a result, a bright to dark optical shift of LCs was observed, which implied that an enzymatic reaction between trypsin and PLL-based membrane occurred. Two control experiments using chymotrypsin and bovine serum albumin (BSA) revealed no sign of improved permeability based on the orientational transition of LCs.

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.

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.

Gravitational field-induced orientational transition of aligned nematic liquid crystals

We demonstrate induced orientational transition of liquid crystal (LC) E7 between two optical fibre tips in a gravitational field as a result of minimising the free energy. The LC orientational transition is from a homeotropic state, with respect to fibre tip, to a tilted state as the spacing between two fibre tips is increased. The orientational distortion introduces birefringence in the optical fibre that changes the polarised direction of the output light. At short spacing, the undistorted homeotropic orientation of LC is preferred. However, at longer spacing, a distorted orientation is preferred. Once the LC director profile is known, a Finite Difference Time Domain method is used to calculate the optical properties, which agree well with the experimental results.