The experimental setup was recently described in detail Fig

The experimental setup was recently described in detail [14]. Fig. 2(a) shows schematics of the setup. The liquid crystal sensor is placed between either crossed linear or left- and right-handed circular polarizers and imaged with a CCD camera mounted on an inverted microscope. The liquid crystal is smeared uniformly in one half of a Veco folding nickel 50/100mesh 20-μm thick TEM grids with cell diameters in the range of 0.2–0.4mm. The liquid crystal forms stable freestanding films in each grid cell. Tweezers hold this grid parallel to the bottom of the dish for the aqueous material. When both sides of the film are in contact with air, the average orientation of the liquid crystal purchase pitavastatin (“director”) aligns perpendicular to the interfaces (“homeotropic” alignment), which appears dark between crossed linear or left- and right-handed circular polarizers. The dish is then filled with water to make contact with the bottom of the grid. Pure water promotes a director alignment parallel to the interface (“planar” alignment). Since the alignment is homeotropic on the air side, hybrid alignment results. Biologically relevant materials are then added to the water, which may change the alignment and the optical properties of the liquid crystal film, providing the basis of sensing. A close-up view of the liquid crystal between air and aqueous interfaces is shown schematically in Fig. 2(b). Since the aqueous solution is optically isotropic, the picture observed in the inverted polarizing microscope depends only on the alignment of the liquid crystal. With hybrid alignment, the texture appears uniformly bright and colored between circular polarizers and usually inhomogeneous between linear crossed polarizers because that configuration is sensitive to the lateral distribution of the optical axis [14].
Results When amphiphilic phospholipids, such as 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) are added to water, they migrate to the LC/water interface with tails embedded in the LC orthogonal to the interface while exposing their headgroups to the aqueous medium. At sufficiently high (>1mM) concentrations of phospholipid in the medium, this process rapidly leads to homeotropic LC alignment both on the air and liquid side. Once the lipid is adsorbed to the surface, the TEM grid is removed from contact with the dispersion and placed on top of a drop of deionized water for five or more minutes to rinse away any unadsorbed lipid. DLPC monolayers covering the LC/water interface remain stable, as indicated by a dark texture showing homeotropic alignment at both the air and water interfaces, even after rinsing the film with pure water, indicating that the DLPC molecules bound to the liquid crystal interface do not desorb into pure water. However, when the DLPC-decorated LC surface is put in contact with the protein apoLp-III which binds to phospholipids [22], the texture brightens, thus indicating a change of the alignment from the aqueous side [23]. This reveals that a binding process of the apolipoprotein with DLPC can be visualized by liquid crystals and encouraged us to test the response of an antibody-decorated LC surface to its antigen. To construct a decorated liquid crystal interface to respond to specific antigens, we employed biotin–avidin binding, an extremely strong interaction used in labeling of antibodies [24]. Fig. 2(c) illustrates the cascade of biotinylated lipid, avidin, biotinylated anti-goat (targeting) IgG and goat (targeted) IgG adsorbed at the liquid crystal–water interface described below. At each step of fabricating the sensor and testing it against different antibodies, the liquid-crystal filled grid was rinsed with ultrapure deionized water, and then gently lowered onto a 0.5ml droplet of the desired dispersion. First, we prepared vesicles containing 2wt.% Biotin-X-DHPE in DLPC and added water to create a vesicle suspension with a 1mM lipid concentration. Unlike in other experiments where sensing the presence of the lipid is the primary aim [11,14,19], the goal here was merely to produce a dense lipid layer adsorbed to the surface. Fig. 3(a–c) shows the typical progression of film appearance upon addition of the lipid vesicles. Before the lipid reaches the LC interface, the texture is uniformly bright between circular polarizers. As the lipid vesicles open and attach to the liquid crystal interface, dark patches appear and grow; however, even after a long time (several hours), metastable birefringent domains persist, a characteristic feature of lipid-nematic liquid crystal interactions [13,25,26]. The formation of a stable uniform homeotropic texture can be facilitated by heating the liquid crystal to its isotropic phase and cooling back to the nematic phase, as seen in Fig. 3(c). Such a homeotropic texture remained stable after rinsing the film with water and thus removing any unbound lipid.