Aqueous Liquid Crystal Interfaces Research Articles

Ultrasensitive Detection of Lipid-Induced Misfolding of the Prion Protein at the Aqueous-Liquid Crystal Interface.

The misfolding of the α-helical cellular prion protein into a self-propagating β-rich aggregated form is a key pathogenic event in fatal and transmissible neurodegenerative diseases collectively known as prion diseases. Herein, we utilize the interfacial properties of liquid crystals (LCs) to monitor the lipid-membrane-induced conformational switching of prion protein (PrP) into β-rich amyloid fibrils. The lipid-induced conformational switching resulting in aggregation occurs at the nanomolar protein concentration and is primarily mediated by electrostatic interactions between PrP and lipid headgroups. Our LC-based methodology offers a potent and sensitive tool to detect and delineate molecular mechanisms of PrP misfolding mediated by lipid-protein interactions at the aqueous interface under physiological conditions.

A liquid crystal-based biosensor for detection of insulin driven by conformational change of an aptamer at aqueous-liquid crystal interface

Insulin is a critical predictor for the function of pancreatic islet beta cells, which plays a crucial role in diagnosing diabetes and diabetes-related disorders. Herein, we propose and validate a label-free and cost-effective aptamer-based optical LC biosensor for detection of insulin based on the directional recognition of biomolecular binding events at a responsive aqueous-liquid crystal (LC) interface. The binding of insulin and aptamer adsorbed on CTAB triggers a conformational change of the aptamer from G-quadruplex to stretched structure, inducing homeotropic to planar alignment and correspondingly dark to bright optical image change of the LC films. The molecular dynamic (MD) simulation validates that the orientational transition is associated with the interaction energy changes at the interface, which is in coordination with the optical observation. This LC biosensor takes advantages of simple preparation, easy operation, rapid sensing, high specificity for insulin determination in the range of 0.1–1.0 nM within 5 min. This sensor is also applicable for insulin detection in diluted human urine and serum. Additionally, the optical cell arrays allow to detection multiple samples of the same/different biomarkers at the same time. Such a strategy offers a potential basis for monitoring other clinical biomarkers, and for point-of-care testing (POCT) as well.

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.

A New Strategy for Reporting Specific Protein Binding Events at Aqueous-Liquid Crystal Interfaces in the Presence of Non-Specific Proteins.

Aqueous-liquid crystal (LC) interfaces offer promise as responsive interfaces at which biomolecular recognition events can be amplified into macroscopic signals. However, the design of LC interfaces that distinguish between specific and non-specific protein interactions remains an unresolved challenge. Herein, we report the synthesis of amphiphilic monomers, dimers, and trimers conjugated to sulfonamide ligands via triazole rings, their assembly at aqueous-LC interfaces, and the orientational response of LCs to the interactions of carbonic anhydrase II (CAII) and serum albumin with the oligomer-decorated LC interfaces. Of six oligomers synthesized, only dimers without amide methylation were found to assemble at aqueous interfaces of nematic 4-cyano-4'-pentylbiphenyl (5CB) to induce perpendicular LC orientations. At dimer-decorated LC interfaces, we found that concentrations of CAII less than 4 μM did not measurably perturb the LC but prevented non-specific adsorption and penetration of serum albumin into the dimer-decorated interface that otherwise triggered bright, globular LC optical domains. These experiments and others (including competitive adsorption of CAII, BSA, and lysozyme) support our hypothesis that specific binding of CAII to the dimer prevents LC anchoring transitions triggered by non-specific adsorption of serum albumin. We illustrate the utility of the approach by reporting (i) the relative activity of two small-molecule inhibitors (6-ethoxy-2-benzothiazolesulfonamide and benzenesulfonamide) of CAII to sulfonamide and (ii) proteolytic digestion of a protein (CAII) by thermolysin. Overall, the results in this paper provide new insight into the interactions of proteins at aqueous-LC interfaces and fresh ideas for either blocking non-specific interactions of proteins at surfaces or reporting specific binding events at LC interfaces in the presence of non-specific proteins.

Surfactin-Laden Aqueous–Liquid Crystal Interface Enabled Identification of Secondary Structure of Proteins

The development of stimuli-responsive biomimetic systems to understand interactions between proteins and surfaces is of an increasingly important scientific interest due to its potential applicatio...

Correction to “Design of Aqueous-Liquid Crystal Interfaces To Monitor Protein Aggregation at Nanomolar Concentrations”

Correction to “Design of Aqueous-Liquid Crystal Interfaces To Monitor Protein Aggregation at Nanomolar Concentrations”

Gold nanoparticle-mediated signal amplification of liquid crystal biosensors for dopamine.

A unique design strategy was developed for the detection of dopamine using a newly synthesized amphiphile containing boronic acid head group at the aqueous-liquid crystal (LC) interface. The optical signal of LC for the detection of dopamine was highly amplified in the presence of functionalized gold nanoparticles.

Design of Aqueous-Liquid Crystal Interfaces To Monitor Protein Aggregation at Nanomolar Concentrations

Amyloids are proteinaceous aggregates, the deposition of which is associated with neurodegenerative diseases, such as Alzheimer’s disease. In vitro protein aggregation requires high protein concentration, which is generally far from physiological concentration. Here, we utilize the interfacial properties of liquid crystals (LCs) to monitor the membrane-induced aggregation of a bacterial functional amyloid, curli, at nanomolar concentration. The binding event triggers an orientational transition of the LC, which is accompanied by the appearance of dynamic spatial patterns enabling sensitive detection of lipopolysaccharide (LPS)-mediated protein aggregation. Quantification of LC response shows a sigmoidal time profile, typical of a protein fibrillation assay. Curli is composed of two subunits (CsgA and CsgB) and is expressed on the outer membrane of Gram-negative bacteria containing LPSs endotoxin. CsgA forms the major subunit of curli, which is nucleated by the membrane-tethered minor subunit CsgB. Using a...

Alignment of nematic liquid crystals decorated with gemini surfactants and interaction of proteins with gemini surfactants at fluid interfaces

A series of cationic gemini surfactants with diverse chemical structures, that is, imidazolium-based gemini surface active ionic liquids (gemini IM-SAILs) with different alkyl chain length or spacer length, viz. 1,s-bis(3-alkylimidazolium-1-yl) ethane bromide ([Cn-s-Cnim]Br2; s = 2, n = 6, 8, 10, 12, 16; n = 12, s = 2, 4, 6, 10), and quaternary ammonium-based gemini surfactants (gemini QASaa) with different symmetries, viz. 1,2-bisalkylquaternary ammonium bromide (m-2-n; m = 12, 14, 16, n = 8, 10, 12, m + n = 24), were synthesized and utilized to decorate aqueous/liquid crystal interfaces (ALI). Initially, the optical response of the LCs changed from bright to dark after incubation with gemini IM-SAILs (except [C6-2-C6im]Br2) or gemini QASaa aqueous solutions, due to the formation of stable surfactant monolayers at the ALI. We verify that gemini IM-SAILs with shorter spacer or longer hydrophobic chains are more conducive to adsorption onto the interface, and gemini IM-SAILs form monolayers more easily than the corresponding monomers or gemini QASaa. Interestingly, a dark-to-bright shift in the optical image of the LCs subsequently occurred after the fluid interface decorated with the gemini surfactants came into contact with Bovine serum albumin (BSA), a negatively charged protein in neutral environments, whereas the optical appearance of LCs did not change upon addition of two other proteins with positive charge (viz. lysozyme and trypsin). Therefore, based on the different action mechanisms, a low-cost, label-free, and convenient LC-based sensing platform using the gemini surfactant-decorated LC interface was constructed for identification of the proteins with opposite charges.

Mesoscopic simulations of temperature-dependent anchoring and wetting behavior at aqueous-liquid crystal interfaces in the presence of a rod-coil amphiphilic monolayer.

Dissipative particle dynamics simulations have been applied to study the temperature dependent anchoring and wetting behavior of thermotropic liquid crystals (LCs) in the presence of a rod–coil amphiphilic monolayer at the aqueous–LC interface. Upon cooling in the nematic phase, a thermally-induced anchoring transition from homeotropic through tilted to planar has been observed. The growth and propagation of smectic order from the interfaces to the bulk nematic LCs are demonstrated to be mainly responsible for this novel transition sequence. In particular, when a complete smectic layer in the amphiphile monolayer is induced around the bulk transition of nematic–smectic-A, the propagation of homeotropic alignment fails instantly and a unique planar anchoring configuration is formed instead. While heating towards the isotropic phase, simulation results show that the nematic–isotropic transition of confined LCs is slightly shifted to a higher temperature, and a nematic wetting layer with homeotropic alignment appears in the rod block monolayer when the bulk LCs is isotropic. Our systematic simulations throughout the whole phase regimes of LCs provide important molecular-level insight into how the coupling between the ordering of LCs and adsorbents and their temperature dependencies affect the anchoring behavior in this complex system, which should be instrumental in the rational design and application of advanced LC-based biosensors with optimal operating temperature range.

Liquid Crystal Unveiled Interactions between Melittin and Phospholipids at Aqueous-Liquid Crystal Interface

The interaction of honeybee apitoxin (melittin) with phospholipid membrane is very crucial to unveil the mechanistic details of the interaction of peptide with phospholipid membrane. In this regard, herein, our paper advances the design of a liquid crystal (LC)-based sensor for precise and quantitative imaging of melittin–phospholipid interaction through interfacial ordering transition. Micrometer-thick films of nematic 4-cyano-4’-pentylbiphenyl (5CB) LC undergo easily visualized ordering transitions in response to melittin at 1,2-didodecanoyl-sn-glycero-3-phosphocholine (DLPC) laden aqueous-LC interfaces. This observation implies strong interaction of DLPC with melittin at those interfaces. Our experiments also reveal that Ca2+ plays a crucial role for faster dynamic response of the LC during the interaction. The presence of Ca2+ towards kinetics of melittin-DLPC interaction has been found to be highly specific compared to other alkaline earth metal ions. In addition, these interactions were conveniently quantified by measurement of both light intensity (transmitted through the LC under crossed polars) and optical retardance of the LC. Finally, we have investigated the conformational behavior of the melittin in solution as well as in presence of DLPC vesicles using circular dichroism (CD) and vibrational circular dichroism (VCD) spectroscopy. This sensing system significantly outweighs the conventional biological and sepctrscopic assays in terms of robustness, sensitivity, easy sample preparation and clear optical read out. Overall, this paper demonstrates a promising approach in designing a simple LC based biosensor to study and quantify the interaction of melittin with phospholipid membrane at aqueous-LC interfaces.

Self-Assembly of Bioconjugated Amphiphilic Mesogens Having Specific Binding Moieties at Aqueous–Liquid Crystal Interfaces

New bioconjugated amphiphilic mesogens with recognition groups have been synthesized, and their self-assembly behavior has been characterized at aqueous–liquid crystal (LC) interfaces. Specifically, the rod-shaped 2,3-difluoro-4′-(4-trans-pentylcyclohexyl)biphenyl-based mesogen was conjugated with either biotin or the arginine–glycine–aspartic acid (RGD) peptide sequence through a tetraethylene glycol chain. Langmuir film measurements revealed that the two biorecognition moieties lead to very different surface pressure–area isotherms, indicating that biotin and RGD have distinct effects on self-assembled monolayers formed by these bioconjugated mesogens at aqueous surfaces. Measurements of the surface-induced orientations of LCs exhibited by biotin-conjugated mesogens mixed with the room-temperature nematic LC 4-cyano-4′-pentylbiphenyl (5CB) revealed formation of cholesteric phases (consistent with the chiral nature of biotin) and evidence of the presence of the conjugated mesogen at aqueous interfaces. P...

DNA Hybridization-Mediated Liposome Fusion at the Aqueous Liquid Crystal Interface.

The prominence of receptor-mediated bilayer fusion in cellular biology motivates development of biomimetic strategies for studying fusogenic mechanisms. An approach is reported here for monitoring receptor-mediated fusion that exploits the unique physical and optical properties of liquid crystals (LC). PEG-functionalized lipids are used to create an interfacial environment capable of inhibiting spontaneous liposome fusion with an aqueous/LC interface. Then, DNA hybridization between oligonucleotides within bulk phase liposomes and a PEG-lipid monolayer at an aqueous/LC interface is exploited to induce receptor-mediated liposome fusion. These hybridization events induce strain within the liposome bilayer, promote lipid mixing with the LC interface, and consequently create an interfacial environment favoring re-orientation of the LC to a homeotropic (perpendicular) state. Furthermore, the bi-functionality of aptamers is exploited to modulate DNA hybridization-mediated liposome fusion by regulating the availability of the appropriate ligand (i.e., thrombin). Here, a LC-based approach for monitoring receptor (i.e., DNA hybridization)-mediated liposome fusion is demonstrated, liposome properties that dictate fusion dynamics are explored, and an example of how this approach may be used in a biosensing scheme is provided.

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.

Surfactant–DNA interactions at the liquid crystal–aqueous interface

The presence of single-stranded (ssDNA) vs. double-stranded (dsDNA) DNA at a surfactant-laden aqueous–nematic liquid crystal (LC) interface results in distinctly different orientations of the LC molecular axis; this is of practical interest as a method to detect DNA hybridization. Results presented here provide new insights into the molecular-level mechanisms of these phenomena. The adsorption of ssDNA to a cationic surfactant-laden aqueous–LC interface caused LC reorientation, leading to coexistence between homeotropic and planar (birefringent) oriented regions. Fluorescence microscopy revealed that ssDNA preferentially partitioned into the birefingent regions, presumably causing a decreased surface coverage of surfactant and the resultant planar LC orientation. Both electrostatic and hydrophobic effects were found to be critical to inducing LC reorientation. In particular, insufficient ssDNA adsorption occurred in the absence of a cationic surfactant (e.g. with no surfactant or with a non-ionic surfactant), demonstrating the importance of electrostatic interactions with the polyanionic ssDNA. Even in the presence of a cationic surfactant, however, polyanions without hydrophobic side-group moieties (poly[acrylic acid] and dsDNA) caused no LC reorientation, while polyanions with hydrophobic side groups (polystyrene sulfonate and ssDNA) initiated the desired LC reorientation. These observations are consistent with the fact that interfacial hybridization of adsorbed probe ssDNA to complementary target ssDNA caused a reorientation from planar back to homeotropic. We propose that ssDNA forms an electrostatic interfacial complex with cationic surfactant where the hydrophobic nucleobases associate directly with the LC phase, effectively competing with surfactant molecules for interfacial sites. Upon hybridization, the hydrophobic character of the ssDNA is lost and the nucleobases no longer associate directly with the LC phase, allowing the surfactant molecules to pack more closely at the interface.

Liquid Crystal-based Imaging of Enzymatic Reactions at Aqueous-liquid Crystal Interfaces Decorated with Oligopeptide Amphiphiles

In this study, we investigated the use of liquid crystals to selectively detect the activity of enzymes at interfaces decorated with oligopeptide-based membranes. We prepared a mixed monolayer of tetra(ethylene glycol)-terminated lipids and carboxylic acid-terminated lipids at the aqueous-liquid crystal (LC) interface. The 17 amino-acid oligopeptide SNFKTIYDEANQFATYK was then immobilized onto this mixed monolayer through N-hydroxysuccinimide-activation of the carboxylic acid groups. We examined the orientational behavior of nematic 4-cyano-4'-pentylbiphenyl (5CB) after conjugation of the 17 amino-acid oligopeptide with the mixed monolayer assembled at the interface. Immobilization of the oligopeptide caused orientational transitions in 5CB, with a change from homeotropic (perpendicular) to tilted alignment, which was primarily due to the reorganization of the monolayer. The orientation of the 5CB molecules returned to its homeotropic state after contacting the interface containing <TEX>${\alpha}$</TEX>-chymotrypsin, which can cleave the immobilized oligopeptide. Control experiments confirmed that the enzymatic activity of <TEX>${\alpha}$</TEX>-chymotrypsin triggered the ordering transitions in the LC. These results suggest that the LC can provide a facile method for selective detection of enzymatic activity.

The effect of cholesterol on protein-coated gold nanoparticle binding to liquid crystal-supported models of cell membranes

We report an easily visualized liquid crystal (LC)-based system to study the biophysical interactions between protein-coated gold nanoparticles (AuNPs) and LC-supported cell membrane model. The model consists of mixed phospholipid/cholesterol monolayer self-assembled at aqueous–LC interface. Protein-coated AuNPs were found to disrupt the mixed phospholipid/cholesterol monolayer. As a result, orientational transitions of LCs were triggered and optical responses of LCs from dark to bright were observed. The mixed monolayers with higher cholesterol contents were found to be more susceptible to the disruption by protein-coated AuNPs, and hydrophobic interaction played a major role in the monolayer disruption. We also found that the time for non-specific binding of fibrinogen-coated AuNPs to the mixed phospholipid/cholesterol monolayer was similar to that of specific binding of neutravidin-coated AuNPs to the mixed phospholipid/biotin-capped phospholipid monolayer. Results obtained from this study may offer new understanding in the potential nanotoxicity pathway, where the biophysical interaction between nanomaterials and cell membrane is an important step.

Nematic ordering drives the phase separation of mixed monolayers containing phospholipids modified with poly(ethylene glycol) at aqueous–liquid crystal interfaces

We report that homogeneous Langmuir monolayers formed from mixtures of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine conjugated to poly(ethylene glycol) (Mw = 2000 g mol−1, DPPE-PEG2000), when transferred onto interfaces formed between aqueous phases and nematic liquid crystals (LCs), phase separate in a manner that leads to patterned orientations of the LCs. Quantitative fluorescence microscopy establishes that the patterned orientations of the LCs correspond to coexisting interfacial phases that differ in total lipid density as well as lipid composition. Subsequent investigation determined that the interfacial phase separation of lipid is driven by the combined influence of the nematic ordering of the LC and the interactions between the PEG chains of DPPE-PEG2000. The latter effects impact the packing of the lipid monolayer and thus its interaction with the LC. In contrast to the phase separation of PEG-lipids within Langmuir films, where the repulsive interactions between PEG chains suppress phase separation at high interfacial densities of lipid, the results of our study indicate that repulsive interactions between PEG chains promote the phase separation of lipid at the aqueous–LC interface. Our results reveal a fundamentally new mechanism leading to control of the interfacial phase behavior of PEG-lipids, and suggest new principles for manipulation of the organization of multi-component lipid assemblies at interfaces.

A liquid crystal-based sensor for real-time and label-free identification of phospholipase-like toxins and their inhibitors

We report a liquid crystal (LC)-based sensor for real-time and label-free identification of phospholipase-like toxins. Beta-bungarotoxin exhibits Ca2+-dependent phospholipase A2 activity whereas alpha-bungarotoxin and myotoxin II do not exhibit any phospholipase activity. The sensor can selectively identify beta-bungarotoxin, when it hydrolyzes a phospholipid monolayer self-assembled at aqueous–LC interface, through orientational responses of LCs. As a result, optical signals that reflect the spatial and temporal distribution of phospholipids during the hydrolysis can therefore be generated in a real-time manner. The sensor is very sensitive and requires less than 5pg of beta-bungarotoxin for the detection. When phospholipase A2 inhibitors are introduced together with beta-bungarotoxin, no orientational response of LCs can be observed. In addition, the regeneration of the sensor can be done without affecting the sensing performance. This work demonstrates a simple and cost-effective LC-based sensor for identifying phospholipase-like toxins and for screening compound libraries to find potential toxin inhibitors.