The spontaneous assembly of phospholipids at planar interfaces between thermotropic liquid crystals and aqueous phases gives rise to patterned orientations of the liquid crystals that reflect the spatial and temporal organization of the phospholipids. Strong and weak specific-binding events involving proteins at these interfaces drive the reorganization of the phospholipids and trigger orientational transitions in the liquid crystals. Because these interfaces are fluid, processes involving the lateral organization of proteins (such as the formation of protein- and phospholipid-rich domains) are also readily imaged by the orientational response of the liquid crystal, as are stereospecific enzymatic events. These results provide principles for label-free monitoring of aqueous streams for molecular and biomolecular species without the need for complex instrumentation.

We report an experimental system that permits optical imaging of the reversible adsorption of amphiphiles at stable, planar interfaces formed between aqueous phases and immiscible thermotropic liquid crystals.Copper grids (hole sizes of 19-292 µm and thicknesses of 18-20 µm) supported on glass surfaces treated using octadecyltrichlorosilane were impregnated with nematic 4-cyano-4′-pentylbiphenyl (5CB). Films of 5CB confined within the grids were stable (did not dewet the grid) to immersion into either water, aqueous solutions of sodium dodecyl sulfate (SDS), or aqueous solutions of SDS containing NaCl. Whereas the anchoring of 5CB on the copper grid dominated the appearance of the 5CB when using grids with hole sizes of 19 µm, reversible changes in the orientation of the liquid crystal (observed using polarized light) caused by adsorption of SDS at the liquid crystal-aqueous interface were readily observed when using grids with hole sizes larger than 19 µm. With increasing concentrations of SDS in the aqueous phase, a series of reproducible and distinct surface-driven distortions were observed within the 5CB confined to the grid. We also observed the effects of added NaCl on the distortions induced within the liquid crystal to be consistent with increased adsorption of SDS caused by screening of the electrostatic interactions between the SDS adsorbed at the interface. This result suggests that the orientation of the 5CB is influenced by the areal density of SDS molecules at the liquid crystal-aqueous interface (probably through steric interactions). Because the aqueous phase contacting the liquid crystal can be exchanged (thus permitting the addition and removal of reactants), this experimental system is a simple and broadly useful one for investigations in which liquid crystals are used to amplify interfacial phenomena at fluid interfaces into optical images.

It is known that the orientations assumed by thermotropic liquid crystals (LCs) in contact with water are sensitive to the types and concentrations of surfactants and/or polymers present in the aqueous phase. This work expands upon these past observations by developing criteria for surfactants that give rise to a particular orientation of a contacting nematic LC formed formed from 4′-pentyl-4-cyanobiphenyl (5CB). We observe surfactants that have a bolaform structure ((11-hydroxyundecyl)trimethylammonium bromide (HTAB), dodecyl-1,12-bis(trimethylammonium bromide) (DBTAB), 11-(ferrocenylundecyl)trimethylammonium bromide (FTMA)) and which adopt looped configurations at air-water/oil-water interfaces cause planar anchoring of 5CB. In contrast, classical surfactants (alkyltrimethylammonium halides (CnTABs, n > 8), sodium dodecyl sulfate (SDS), and N,N-dimethylferrocenylalkylammonium bromides (FCnABs, n > 12)) that assume tilted orientations at air-water/oil-water interfaces can give rise to a homeotropic orientation of 5CB. By comparing SDS, dodecyl trimethylammonium halide (DTAB), and tetra(ethylene glycol) monododecyl ether (C12E4), we conclude that the nature of these headgroups does not measurably influence the orientation of the LC. However, the orientation of the LC is found to depend on the aliphatic chain length and the areal density of the adsorbed surfactant. When using surfactants with short alkyl chain lengths (n ) 8 for C nTAB and n ) 7 and 12 for FCnAB), we observe the orientation of 5CB to remain parallel to the interface up to concentrations at which the 5CB begins to be solubilized by the surfactant. These results, when combined, lead us to conclude that interactions between the aliphatic chains of the surfactant and 5CB, which are influenced by the conformation of the surfactant, largely dictate the orientation of the 5CB.

This paper reports an experimental investigation of the self-assembly of phospholipids (l-alpha-phosphatidylcholine-beta-oleoyl-gamma-palmitoyl (l-POPC), dipalmitoyl phosphatidylcholine (DPPC), and l-alpha-dilauroyl phosphatidylcholine (l-DLPC)) at interfaces between aqueous phases and the nematic liquid crystal (LC) 4'-pentyl-4-cyanobiphenyl. Stable planar interfaces between the aqueous phases and LCs were created by hosting the LCs within gold grids (square pores with widths of 283 microm and depths of 20 microm). At these interfaces, the presence and lateral organization of the phospholipids leads to interface-driven orientational transitions within the LC. By doping the phospholipids with a fluorescently labeled lipid (Texas Red-1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (TR-DPPE)), quantitative epifluorescence microscopy revealed the saturation coverage of phospholipid at the interface to be that of a monolayer with an areal density of approximately 49 +/- 8% relative to hydrated lipid bilayers. By adsorbing phospholipids to the aqueous-LC interface from either vesicles or mixed micelles of dodecyltrimethylammonium and phospholipid, control of the areal density of phospholipid from 42 +/- 10 to 102 +/-18% of saturation monolayer coverage was demonstrated. Fluorescence recovery after photobleaching (FRAP) experiments performed by using laser scanning confocal microscopy (LSCM) revealed the lateral mobility of fluorescently labeled DPPE in l-DLPC assembled at the interface with the liquid crystal to be (6 +/- 1) x 10(-12) m(2)/s for densely packed monolayers. Variation of the surface coverage and composition of phospholipid led to changes in lateral diffusivity between (0.2 +/- 0.1) x 10(-12) and (15 +/- 2) x 10(-12) m(2)/s. We also observed the phospholipid-laden interface to be compartmentalized by the gold grid, thus allowing for the creation of patterned arrays of phospholipids at the LC-aqueous interface.

We report a study of the interactions of proteins with monolayers of phospholipids (D/L-alpha-dipalmitoyl phosphatidylcholine and L-alpha-dilauroyl phosphatidylcholine) spontaneously assembled at an interface between an aqueous phase and a 20-microm-thick film of a nematic liquid crystal (4'-pentyl-4-cyanobiphenyl). Because the orientation of the liquid crystal is coupled to the organization of the lipids, specific interactions between phospholipase A2 and the lipids (binding and/or hydrolysis) that lead to reorganization of the lipids are optically reported (using polarized light) as dynamic orientational transitions in the liquid crystal. In contrast, nonspecific interactions between proteins such as albumin, lysozyme, and cytochrome-c and the lipid-laden interface of the liquid crystal are not reported as orientational transitions in the liquid crystals. Concurrent epifluorescence and polarized light imaging of labeled lipids and proteins at the aqueous-liquid crystal interface demonstrate that spatially patterned orientations of the liquid crystals observed during specific binding of phospholipase A2 to the interface, as well as during the subsequent hydrolysis of lipids by phospholipase A2, reflect the lateral organization (micrometer-sized domains) of the proteins and lipids, respectively, at the aqueous-liquid crystal interface.