Qiong-Zheng Hu

Imaging trypsin activity through changes in the orientation of liquid crystals coupled to the interactions between a polyelectrolyte and a phospholipid layer.

In this study, we developed a new type of liquid crystal (LC)-based sensor for the real-time and label-free monitoring of enzymatic activity through changes in the orientation of LCs coupled to the interactions between polyelectrolyte and phospholipid. The LCs changed from dark to bright after an aqueous solution of poly-l-lysine (PLL) was transferred onto a self-assembled monolayer of the phospholipid, dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DOPG), at the aqueous/LC interface. Interactions between the positively charged PLL and the negatively charged DOPG drove the reorganization of the phospholipid membrane, which induced an orientational transition in the LCs from a homeotropic to planar state. Since the serine endopeptidase trypsin can enzymatically catalyze the hydrolysis of PLL, the dark-to-bright shift in the optical response was not observed after transferring a mixed solution of PLL and trypsin onto the DOPG-decorated LC interface, indicating that no orientational transitions in the LCs occurred. However, the optical response from dark to bright was observed when the mixture in the optical cell was replaced by an aqueous solution of PLL. Control experiments with trypsin or an aqueous mixture of PLL and deactivated trypsin further confirmed the feasibility of this approach. The detection limit of trypsin was determined to be ~1 μg/mL. This approach holds great promise for use in the development of LC-based sensors for the detection of enzymatic reactions in cases where the biological polyelectrolyte substrates of enzymes could disrupt the organization of the membrane and induce orientational transitions of LCs at the aqueous/LC interface.

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.

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

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.