Influence of 4-cyano-4'-biphenylcarboxylic acid on the orientational ordering of cyanobiphenyl liquid crystals at chemically functionalized surfaces.

We report two methods that involve tailoring of the chemical composition of the nematic liquid crystal 4-cyano-4'-pentylbiphenyl to achieve control over the orientational ordering of the liquid crystal on chemically functionalized surfaces. The first method involves the direct addition of 4-cyano-4'-biphenylcarboxylic acid to 4-cyano-4'-pentylbiphenyl. The second method involves exposure of 4-cyano-4'-pentylbiphenyl to ultraviolet light and photochemical generation of a range of products, including 4-cyano-4'-biphenylcarboxylic acid. The addition of the acid or exposure to ultraviolet light accelerated the rate at which the liquid crystal exhibited an orientational transition from planar to perpendicular (homeotropic) alignment on surfaces presenting ammonium groups. The appearance of the homeotropic orientation of the UV-treated 4-cyano-4'-pentylbiphenyl on ammonium-terminated surfaces was dependent on the thickness of the film of liquid crystal (13-50 mum), consistent with a dipolar coupling between the liquid crystal and the electric field associated with an electrical double layer generated at the ammonium surface. Although the addition of 4-cyano-4'-biphenylcarboxylic acid or UV treatment of the liquid crystal also promoted homeotropic orientations on surfaces presenting hydroxyl groups, the orientations of the UV-treated liquid crystal on the hydroxyl-terminated surface did not change with thickness of the film of liquid crystal in the manner observed on the ammonium-terminated surfaces. The latter result indicates that the mechanism leading to homeotropic anchoring on hydroxyl-terminated surfaces is distinct from that on ammonium-terminated surfaces. Measurements performed using polarization modulation infrared reflection-absorption spectroscopy suggest that hydrogen bonding between the 4-cyano-4'-biphenylcarboxylic acid and the hydroxyl-terminated surface is responsible for the homeotropic anchoring on the surface. Finally, the orientation of the liquid crystal on methyl-terminated surfaces was not influenced by the addition of 4-cyano-4'-biphenylcarboxylic acid nor UV treatment. These results illustrate how the chemical composition of liquid crystals can be manipulated to achieve control over their ordering on surfaces that possess chemical functionality relevant to the development of liquid crystal-based sensors and diagnostic tools. We illustrate the utility of this approach by using the tailored liquid crystal to amplify and optically transduce the presence of proteins arrayed on ammonium-terminated surfaces.

Using liquid crystals to report membrane proteins captured by affinity microcontact printing from cell lysates and membrane extracts.

The chemical heterogeneity of proteins makes development of general and facile surface-based methods for protein analysis a substantial challenge, particularly when analyzing transmembrane proteins. Here, we report a simple surface-based procedure that permits detection of transmembrane proteins from crude cell lysates and cell membrane extracts. The method relies on the use of thermotropic liquid crystals to amplify and report the presence of the transmembrane proteins captured by an affinity ligand on the surface of an elastomeric stamp. A merit of this approach is that the proteins can be imaged on surfaces without requiring the use of matched pairs of antibodies, labels, or complex instrumentation. Detection of epidermal growth factor receptor, a transmembrane glycoprotein, is demonstrated.

Imaging the binding ability of proteins immobilized on surfaces with different orientations by using liquid crystals.

We report an investigation of the binding ability of a protein immobilized on surfaces with different orientations but in identical interfacial microenvironments. The surfaces present mixed self-assembled monolayers (SAMs) of 11-[19-carboxymethylhexa(ethylene glycol)]undecyl-1-thiol, 1, and 11-tetra(ethylene glycol) undecyl-1-thiol, 2. Whereas 2 is used to define an interfacial microenvironment that prevents nonspecific adsorption of proteins, 1 was activated by two different schemes to immobilize ribonuclease A (RNase A) in either a preferred orientation or random orientations. The binding of the ribonuclease inhibitor protein (RI) to RNase A on these surfaces was characterized by using ellipsometry and the orientational behavior of liquid crystals. Ellipsometric measurements indicate identical extents of immobilization of RNase A via the two schemes. Following incubation of both surfaces with RI, however, ellipsometric measurements indicate a 4-fold higher binding ability of the RNase A immobilized with a preferred orientation over RNase A immobilized with a random orientation. The higher binding ability of the oriented RNase A over the randomly oriented RNase A was also apparent in the orientational behavior of nematic liquid crystals of 4-cyano-4'-pentylcyanobiphenyl (5CB) overlayed on these surfaces. These results demonstrate that the orientations of proteins covalently immobilized in controlled interfacial microenvironments can influence the binding activities of the immobilized proteins. Results reported in this article also demonstrate that the orientational states of proteins immobilized at surfaces can be distinguished by examining the optical appearances of liquid crystals.

Imaging of affinity microcontact printed proteins by using liquid crystals.

This paper reports the design of surfaces on which thermotropic liquid crystals can be used to image affinity microcontact printed proteins. The surfaces comprise gold films deposited onto silica substrates at an oblique angle of incidence and then functionalized with a monolayer formed from 2-mercaptoethylamine. Ellipsometric measurements confirm the transfer of anti-biotin IgG to these surfaces from affinity stamps functionalized with biotinylated bovine serum albumin (BSA), while control experiments performed using anti-goat IgG confirmed the specificity of the IgG capture on the stamp. On these surfaces, anti-biotin IgG caused nematic phases of 4-cyano-4'-pentylbiphenyl (5CB, Delta epsilon = epsilon(parallel) - epsilon(perpendicular) > 0) to assume orientations that were parallel to the surfaces (planar anchoring) but with azimuthal orientations that were distinct from those assumed by the liquid crystals on the amine-terminated surfaces not supporting IgGs. Following incubation of these samples for >8 h at 36 degrees C, we observed that the amine-terminated regions of the surface not supporting IgG cause 5CB to undergo a transition from planar to perpendicular (homeotropic). Because N-(4-methoxybenzylidene)-4-butylaniline (MBBA) (Delta epsilon < 0) does not undergo a similar transition in orientation, this transition is consistent with the effects of an electrical double layer formed at the amine-terminated surface on the liquid crystal. Following the transition to homeotropic anchoring, the liquid crystals provide high optical contrast between regions of the surface supporting and not supporting IgG. We conclude that amine-terminated surfaces (I) uniformly align liquid crystals when not supporting proteins and (II) have sufficiently high surface free energy to capture proteins delivered to the surface from an affinity stamp, and thus they form the basis of a useful class of surfaces on which affinity microcontact printed proteins can be imaged using liquid crystals.