Nematic liquid crystals confined in microcapillaries for imaging phenomena at liquid-liquid interfaces

Liquid crystal microcapillary-based sensors for affordable analytical applications

Stimuli-responsive properties of liquid crystals (LCs), when combined with their optical properties, offer sensitive and rapid sensing applications. Here, we propose and demonstrate a microcapillary-based method to be applied for the online detection of amphiphilic species, which can be further used for tracking biological and chemical species in aqueous media. Specifically, we used compartments (300-1400 μm) of nematic 4-cyano-4'-pentylbiphenyl (5CB) that were positioned into cylindrical glass microcapillaries that promote homeotropic anchoring. The flat surfaces of the cylindrical LC compartments were in contact with an aqueous media. We characterized the equilibrium and nonequilibrium response of LCs upon a change in their anchoring at the aqueous interfaces. Upon anchoring transition, we observed the formation of a positively charged defect at the proximity of the interface that moved to the center of the LC compartment and reached equilibrium, a four-petal configuration. This transition was observed to take an average of 41 ± 19 min., which we related to the motion of the defect due to the imbalance of the elastic forces. During the transition, we observed metastable states which could be removed via thermal treatment. We showed the capillary sensors to be useful considering their ease of additional quantification. We also show that the sensors are reversible that facilitate temporal and cumulative quantification. The findings reported in this study can further be used to develop sensors for specific purposes that require continuous tracking of the chemical and biological species that is critical for the health and safety of the individuals and society.

An acetylcholinesterase-based biosensor for the detection of pesticides using liquid crystals confined in microcapillaries

Here, we demonstrate a capillary-sensing platform based on liquid crystals (LCs) confined in microcapillaries for simple and sensitive detection of acetylcholinesterase (AChE) and its inhibitors. LC droplets were formed through sequential injection of LCs and an aqueous solution into trichloro(octyl)silane (OTS)-treated microcapillaries. When the confined LC droplets make contact with a cationic surfactant solution, myristoylcholine chloride (Myr), the formation of a Myr monolayer at the aqueous/LC interface induces a horizontal orientation of the LCs at the interface along the microcapillary, producing an optical LC droplet texture of a four-petal shape. On the other hand, AChE can catalyze the hydrolysis of Myr into choline and myristic acid. The hydrolyzed Myr is unable to form a monolayer at the aqueous/LC interface, and therefore the confined LC droplets exhibit two bright-lined optical images when in contact with the pre-incubated mixture of Myr and AChE, corresponding to the homeotropic orientation of LCs at the interface. However, in the presence of AChE-inhibiting pesticides, such as fenobucarb and malathion, the activity of AChE is inhibited, and thus, the enzymatic hydrolysis of Myr cannot occur. As a result, the confined LC droplets present the four petal-shaped optical images when in contact with the pre-incubated mixture of Myr, AChE, and pesticides. Based on this principle, an LC-based microcapillary sensor was developed and utilized for the detection of pesticides. Using this sensing platform, fenobucarb and malathion were detected at limits of 5 pg/mL and 2.5 pg/mL, respectively. Moreover, the proposed biosensor was successfully applied to the determination of pesticides in real river water. Therefore, this LC-based microcapillary sensor is a promising platform for simple, rapid, and label-free detection of pesticides with very high sensitivity.

Temperature-Driven Anchoring Transitions at Liquid Crystal/Water Interfaces

Controlling the anchoring of liquid crystal molecules at an interface with a water solution influences the entire organization of the underlying liquid crystal phase, which is crucial for many applications. The simplest way to stabilize such interfaces is by fabricating liquid crystal droplets in water; however, a greater sensitivity to interfacial effects can be achieved using liquid crystal shells, that is, spherical films of liquid crystal suspended in water. Anchoring transitions on those systems are traditionally triggered by the adsorption of surfactant molecules onto the interface, which is neither an instantaneous nor a reversible process. In this study, we report the ability to change the anchoring of 4-cyano-4'-pentylbiphenyl (5CB), one of the most widely used liquid crystals, at the interface with dilute water solutions of polyvinyl alcohol (PVA), a polymer commonly used for stabilizing liquid crystal shells, simply by controlling the temperature in the close vicinity of the liquid crystal clearing point. A quasi-static increase in temperature triggers an instantaneous reorientation of the molecules from parallel to perpendicular to the interfaces, owing to the local disordering effect of PVA on 5CB, prior to the phase transition of the bulk 5CB. We study this anchoring transition on both flat suspended films and spherical shells of liquid crystals. Switching anchoring entails a series of structural transformations involving the formation of transient structures in which topological defects are stabilized. The type of defect structure depends on the topology of the film. This method has the ability to influence both interfaces of the film nearly at the same time and can be applied to transform an initially polydisperse group of nematic shells into a monodisperse population of bivalent shells.

Cholesteric Liquid Crystal Droplets for Biosensors

By utilizing a microfluidics approach, we prepared uniformly sized cholesteric liquid crystal (CLC) droplets from MLC-2132 doped with a chiral dopant (S)-4-cyano-4'-(2-methylbutyl)biphenyl (CB15). We studied the helical structures and reflecting color patterns of high- and low-dopant CLC droplets coated with poly(vinyl alcohol) (PVA) and sodium dodecyl sulfate (SDS). One central large spot with reflecting color in the CLC droplets (initially coated with PVA for planar anchoring) changed to many small spots with the same reflecting color (chicken-skin pattern) when an SDS aqueous solution was introduced to increase the homeotropic anchoring power. These small spots subsequently merged into several spots (flashlight pattern) with time. The CLC droplets coated with poly(acrylic acid)-b-poly(4-cyanobiphenyl-4'-oxyundecyl acrylate) (PAA-b-LCP) (CLC_PAA droplets) were pH-responsive. Their helical structure and the reflecting color pattern changed because of protonation (at low pH) and deprotonation (at high pH) of the carboxylic group of PAA, which causes the planar (tangential) and perpendicular (homeotropic) orientations, respectively. The CLC_PAA droplets immobilized with glucose oxidase (GOx) and cholesterol oxidase (ChO) (CLC_PAA-GOx and CLC_PAA-ChO droplets, respectively), for glucose and cholesterol detection, exhibited high sensitivity (0.5 and 2.5 μM for the CLC_PAA-GOx and CLC_PAA-ChO droplets, respectively), good selectivity, and fast response (≤4 s). Further optimization will enhance their performance as biosensors. With this novel approach, detection is possible by observing the coloring pattern of CLC droplets, without the crossed polarizers that are necessary for nematic LC biosensor systems.