Endotoxin-induced structural transformations in liquid crystalline droplets

Abstract

The ordering of liquid crystals (LCs) is known to be influenced by surfaces and contaminants. Here, we report that picogram per milliliter concentrations of endotoxin in water trigger ordering transitions in micrometer-size LC droplets. The ordering transitions, which occur at surface concentrations of endotoxin that are less than 10(-5) Langmuir, are not due to adsorbate-induced changes in the interfacial energy of the LC. The sensitivity of the LC to endotoxin was measured to change by six orders of magnitude with the geometry of the LC (droplet versus slab), supporting the hypothesis that interactions of endotoxin with topological defects in the LC mediate the response of the droplets. The LC ordering transitions depend strongly on glycophospholipid structure and provide new designs for responsive soft matter.

Fig. 1

(A) Lipid A portion of endotoxin. (B) DLPC. (C) DOPC. (D) SDS. (E) 5CB. (F and G) Bright-field (F) and polarized light (G, crossed-polars) micrographs of an LC droplet in endotoxin-free water (15). The red arrows indicate boojums at the aqueous-LC interface of the droplet. (H) Schematic illustration of the bipolar configuration of the LC droplet corresponding to (F) and (G). (I and J) Bright-field (I) and polarized light (J, crossed-polars) micrographs of an LC droplet after exposure to endotoxin from E. coli (O127:B8; 1 μg/mL) in water. (K) Schematic illustration of the radial configuration of the LC droplet corresponding to (I) and (J). (L) Percentage of 8300 LC droplets in 40 μL of water that exhibited a radial configuration, plotted as a function of endotoxin concentration. The inset shows the response of the LC droplets when 94,000 droplets were used. The LC-in-water emulsion droplets (diameters of ~4 to 8 μm) were prepared by sonication of nematic 5CB in water that was free of endotoxin (15). The droplet numbers were determined using flow cytometry (15). Endotoxin concentrations were validated using an independently experiments, and n indicates the total number of LC emulsion droplets that were analyzed. Scale bars, 5 μm.

Fig. 2

(A) Bulk concentrations of lipids or surfactants in solution (40 μL) required to cause at least 50% of 8300 LC droplets added to each solution to adopt a radial configuration. (B) Schematic illustration of the experimental apparatus used to transfer Langmuir monolayers of lipid A at prescribed surface densities from an air-water interface onto a planar aqueous-LC interface (13). In brief, micrometer-thick films of nematic 5CB (with planar interfaces) were prepared within the pores of Au grids supported on glass slides treated with octadecyltrichlorosilane (to cause perpendicular ordering of the LC). The supported LC was then immersed downward through the lipid A monolayer at the surface of the water (15) to transfer the monolayer onto the planar aqueous-LC interface. (C) Surface pressure-area isotherm measured for a Langmuir monolayer of lipid A and the optical appearance (between crossed polars) of films of 5CB with planar interfaces (hosted within the metallic grids) after transfer of Langmuir films of lipid A at the indicated interfacial density onto the 5CB-aqueous interface [see (B) for details]. The control corresponds to LC passed into endotoxin-free water. The bright optical appearance of the control indicates an orientation of the LC that is parallel to the aqueous-LC interface. The optical appearance of the sample prepared at an area per molecule of lipid A of 148 Ų/molecule indicates a tilted state of the LC. The dark optical appearance of the sample prepared at 115 Ų/molecule indicates a perpendicular orientation of the LC at the aqueous interface. Scale bars, 300 μm.

Fig. 3

(A) Confocal fluorescent micrograph of an LC droplet in contact with a solution of BODIPY-labeled endotoxin. The point defect at the center of the droplet with a radial configuration exhibits a strong fluorescence signal. The concentration of endotoxin in solution was 20 μg/ml in order to achieve a sufficient signal intensity to image the droplets. Imaging was performed using an argon laser at an excitation wavelength of 488, and the detector collected the emission with wavelengths from 499 nm to 634 nm (15). Scale bar, 5 μm. (B) Photobleaching of the BODIPY-labeled endotoxin at the center of the LC droplet. The upper panel shows the intensity of the excitation laser, and the lower panel shows the corresponding fluorescence intensity. Loss of fluorescence signal is apparent during the periods of sample illumination (e.g., 0 to 230 s), and recovery of fluorescence is apparent in the periods of time when the laser incident on the sample was blocked (e.g., 230 to 350 s); A.U. indicates arbitrary units. The incident light intensity is indicated as a fraction of the maximum laser power. (C) Time taken for LC droplets to exhibit radial ordering after a thermal quench from the isotropic to nematic phase, plotted as a function of the droplet radius. The experiment was performed with 21,500 LC droplets dispersed in 100 μL of solution containing 10 pg/mL endotoxin. The line shown in the figure has a slope of 2.