Using liquid crystals to amplify protein-receptor interactions: Design of surfaces with nanometer-scale topography that present histidine-tagged protein receptors

This paper reports a strategy for the oriented immobilization of protein receptors on gold films possessing nanometer-scale topographies and the detection of protein binding events to these receptors by using liquid crystals. The approach revolves around the use of self-assembled monolayers (SAMs) formed from nitrilotriacetic acid (NTA)-terminated alkanethiols, 1, and tri(ethylene glycol)-terminated alkanethiols, 2. The SAMs are formed on ultrathin gold films that are deposited from a vapor onto silica substrates oriented at an oblique angle of incidence. Single-component SAMs formed from 2 on these gold films resist nonspecific protein adsorption (using cell lysates) and promote uniform planar anchoring of the nematic liquid crystal, 4-cyano-4'-pentylbiphenyl (5CB). Surprisingly, the azimuthal orientation of nematic 5CB is parallel to the direction of maximum roughness within the gold film when using SAMs formed from 2 but perpendicular to the direction of maximum roughness when tetra(ethylene glycol)-terminated SAMs are formed on the gold films. Mixed SAMs formed from 1 and 2 bind the hexahistidine-tagged protein MEK via specific complexation of the hexahistidine tags of MEK to the Ni-II-NTA complexes on the surface. When gold films are prepared by oblique deposition at an angle of 30degrees from the normal, we measure bound MEK to disrupt the uniform orientation of 5CB, thus leading to an easily visualized change in the optical appearance of the liquid crystal. However, by using gold films deposited at an angle of 40degrees from the normal, we report that bound MEK does not disrupt the alignment of the liquid crystal whereas anti-MEK IgG bound to the MEK does lead to a nonuniform alignment. These results, when combined with appropriate control experiments, suggest that nanostructured surfaces presenting NTA and ethylene glycol terminated SAMs form a useful interface for imaging proteins bound to histidine-tagged, surface-immobilized receptors.