Interactions between DNA and adsorbed poly(l-lysine) (PLL) on liquid crystal (LC) droplets were investigated using polarizing optical microcopy and epi-fluorescence microscopy. Earlier, we demonstrated that adsorption of PLL to the LC/aqueous interface resulted in homeotropic orientation of the LC and thus exhibited a radial configuration of the LC confined within the droplets. Subsequent adsorption of DNA (single-stranded DNA/double-stranded DNA) at PLL-coated LC droplets was found to trigger an LC reorientation within the droplets, leading to preradial/bipolar configuration of those droplets. To our surprise, subsequent exposure of complementary ssDNA to ssDNA/adsorbed PLL-modified LC droplets did not cause the LC reorientation. This is likely due to the formation of polyplexes (DNA–PLL complex) as confirmed by fluorescence microscopy and atomic force microscopy. In addition, dsDNA-adsorbed PLL droplets have been found to be effectively useful to displace (controlled release) propidium iodide (a model drug) encapsulated within dsDNA over time. These observations suggest the potential for a label-free droplet-based LC detection system that can respond to DNA and may provide a simple method to develop DNA-based drug nanocarriers.

Bovine serum albumin (BSA), concanavalin A from Canavalia ensiformis (Jack bean) (Con A), cathepsin D from bovine spleen (CathD), lysozyme from chicken egg white (Lyz), human haemoglobin, fluorescein conjugated BSA (FITC-BSA), FITC labelled ConA (FITC-ConA), "poly-Llysine (PLL) solution 0.1 % (w/v) in H2O, 4-Cyano-4-pentylbiphenyl (5CB LC) and 5mM tris buffered saline (TBS) (pH 7.4) was obtained from Sigma-Aldrich (St. Louis,MO)" as mentioned in our earlier report. [1] "Deionization of a distilled water (DI water) source was performed using a Milli-Q-system (Millipore, Bedford, MA)" as reported earlier.[1] Glass microscopic slides (Fisher's Finest Premium grade) were purchased from Fischer Scientific (Pittsburgh, PA).

We report a new method for label-free, sensitive, and facile detection of lead(II) ions (Pb 2+ ) based on an aptamer−target binding event, which is recognized by orientations of liquid crystals (LCs) at aqueous interfaces. The LC film suspended in the aqueous phase demonstrated a homeotropic orientation in contact with a cationic surfactant cetyltrimethylammonium bromide (CTAB) due to selfassembly of CTAB molecules at the aqueous−LC interface. The ordering of LC subsequently changed to planar in the presence of the spinach RNA aptamer (SRNA) due to interactions between CTAB and SRNA. In the presence of the Pb 2+ ion, the ordering of LC changed to homeotropic caused by reorganization of CTAB at the LC−aqueous interface. This is due to formation of more stable quadruplex structures of SRNA with Pb 2+ ions in comparison to the CTAB-SRNA complex. The sensor exhibited a detection limit of 3 nM, which is well below the permissible limit of Pb 2+ in drinking water. Our experiments establish that addition of Pb 2+ leads to (i) the formation of Pb 2+ -SRNA complexes and (ii) a decrease in density of SRNA on the LC interface, but additional studies are required to determine which of these processes underlie the response of the LCs to the Pb 2+ . We have also demonstrated the potential application of the LC sensor for detection of Pb 2+ in tap water. Unlike current laboratory-based heavy-metal-ion assays, this method is comparatively simple in terms of instrumentation, operation, and optical readout.

We report an investigation of interfacial phenomena occurring at aqueous-liquid crystal (LC) interfaces that triggers an orientational ordering transition of the LC in the presence of cardiolipin (CL) by varying pH, salt concentration and valence. In particular, the effects of three different conformational isomeric forms of the CL are observed to cause the response of the LC ordering to vary significantly from one to another at those interfaces. An ordering transition of the LC was observed when the CL is mostly in undissociated (at pH 2) and/or in bicyclic (at pH 4) conformation in which LC shows changes in the optical appearance from bright to dark. By contrast, no change in the optical appearance of the LC was observed when the pH of the system increases to 8 or higher in which the CL mostly exists in the open conformation. Fluorescence microscopy measurements further suggest that pH-dependent conformational forms of the CL have different ability to self-assemble (thus different packing efficiency) at aqueous-LC interfaces leading to dissimilar orientational behavior of the LC. Specifically, we found that change in headgroup-headgroup repulsion of the central phosphatidyl groups of the CL plays a key role in tuning the lipid packing efficiency and thus responses to interfacial phenomena. Orientational ordering transition of the LC was also observed as a function of increasing the ionic strength (buffer capacity) and strongly influenced in the presence of mono and divalent cations. Langmuir-Blodgett (LB) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements provide further insight in modulation of the lipid packing efficiency and alkyl chain conformation of the CL at different pH and ionic conditions. Overall, the results presented in this paper establish that LCs offer a promising approach to differentiate different conformations (label free detection) of the CL through ordering transition of the LC at aqueous-LC interfaces.

We report the orientational behavior of nematic liquid crystals (LCs) influenced by a cyclic lipopeptide, polymyxin B (PmB). It was found that PmB can spontaneously self-assemble at aqueous-LC interfaces and induce a homeotropic ordering of the LCs at those interfaces, thus resulting in dark optical appearance of the LC under cross polarizers. Density functional theory studies substantiate the experimental findings that the stability of homeotropic anchoring of the LC is strongly influenced by the hydrophobic interactions between aliphatic tails of PmB and LC molecules along with the additional supramolecular interactions between their head groups. Interestingly, exposure of the PmB-laden aqueous-LC interface to anionic serum proteins such as bovine serum albumin (BSA) and human hemoglobin triggered a planar reorientation of the LC, leading to a bright optical state of the LC. This allows label-free characterization of the biomolecular interactions between proteins and antibiotics (i.e., PmB) in vitro at those interfaces. Such peptidic (PmB)-based LC interfaces can also distinctly amplify the adsorption of β-sheet-rich proteins (fibronectin and concanavalin A) through appearances of fibril-like spatial patterns which are, however, not observed in the presence of α-helix-rich proteins (BSA). Such changes in the optical patterns of the LC in contact with β-sheet-rich proteins occur at nanomolar concentrations at those interfaces, and thus the method could be useful to detect toxic amyloids at a low concentration regime. We envision that our simple label-free optical system may open a wide avenue to detect an extensive assortment of interfacial biochemical events occurring at the aqueous-LC interfaces.