Measurement of the Azimuthal Anchoring Energy of Liquid Crystals in Contact with Oligo(ethylene glycol)-Terminated Self-Assembled Monolayers Supported on Obliquely Deposited Gold Films

Abstract

We report measurements of the orientations and azimuthal anchoring energies of the nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) on polycrystalline gold films that are deposited from a vapor at an oblique angle of incidence and subsequently decorated with organized monolayers of oligomers of ethylene glycol. Whereas the gold films covered with monolayers presenting tetra(ethylene glycol) (EG4) lead to orientations of 5CB that are perpendicular to the plane of incidence of the gold, monolayers presenting tri(ethylene glycol) (EG3) direct 5CB to orient parallel to the plane of incidence of the gold during deposition of the gold film. We also measure the azimuthal anchoring energy of the 5CB to be smaller on the surfaces presenting EG3 (3.2 +/- 0.8 microJ/m2) as compared to EG4 (5.5 +/- 0.9 microJ/m2). These measurements, when combined with other results presented in this paper, are consistent with a physical model in which the orientation and anchoring energies of LCs on these surfaces are influenced by both (i) short-range interactions of 5CB with organized oligomers of ethylene glycol at these surfaces and (ii) long-range interactions of 5CB with the nanometer-scale topography of the obliquely deposited films. For surfaces presenting EG3, these short- and long-range interactions oppose each other, leading to small net values of anchoring energies that we predict are dependent on the level of order in the EG3 SAM. These measurements provide insights into the balance of interactions that control the orientational response of LCs to biological species (proteins, viruses, cells) on these surfaces.

Quantitative Methods Based on Twisted Nematic Liquid Crystals for Mapping Surfaces Patterned with Bio/Chemical Functionality Relevant to Bioanalytical Assays

“…31,32 Single component monolayers of EG4 are known to orient nematic 5CB with an easy axis that is perpendicular to the in-plane direction of gold deposition, evident in Figure 5 (top squares) as a twist angle of 87.9 ° ± 1.0 °. 13 The orientation of the 5CB on the EG4 monolayer deviated from the orientation of the easy axis by an angle of φ = 0.2 ° ± 1.2 °, which indicates that the monolayer of EG4 caused strong anchoring of 5CB. A previous study concluded that SAMs of EG4 formed from aqueous solutions are more ordered than SAMs of EG4 deposited from ethanolic solutions.…”

“…[12][13][14] In this approach, a nematic LC such as 4'-pentyl-4-cyanobiphenyl (5CB, Figure 1A) is confined between two surfaces that are structured so as to promote the LC to adopt mutually orthogonal orientations at the two confining surfaces ( Figure 1B). The interactions of the LC with the two confining surfaces cause a twist distortion to form within the LC.…”

UV Polymerisation of Surfactants Adsorbed at the Nematic Liquid Crystal–Water Interface Produces an Optical Response

“…[29][30][31][32][33][34][35][36][37][38] In addition to the surfactant studies described above, the optical response triggered by surface anchoring changes of LC systems has been used to investigate many different surface processes. These include the patterning of solid surfaces, [1,4,6,9,14,23,24] surface re-organisation of adsorbed phospholipids, [12,20] protein and ligand binding, [2,5,8,10,16,17,18,22,21] bound virus imaging, [15] chemical [3] and enzyme catalysed reactions, [13] the pH response of adsorbed polymers, [26] layer-by-layer polymer film assembly [25] and DNA hybridisation. [28] Herein we show that the planar-homeotropic transition can be induced by the adsorption of various UV polymerisable surfactants and that, in some cases, UV polymerisation of the adsorbed film triggers a homeotropic-planar transition.…”

Enantiomeric Interactions between Liquid Crystals and Organized Monolayers of Tyrosine-Containing Dipeptides

“…This result contrasts to all past studies of ω-terminated alkanethiols on gold films in which the easy axes of smectic and nematic LCs have been measured to be either parallel or perpendicular to the direction of deposition of the gold film. 18,19,27,31,38,62,63 This point, which is related to the chirality of the adsorbates, is addressed in detail in the Discussion. Third, we note that there is a substantial difference between the orientations assumed by nematic 5CB on the gold films decorated with either L-C-L-Y or L-C-L-[p]Y.…”

“…25,38 Semitransparent gold films (used for optical measurements of LC orientations) were prepared by sequentially depositing 8 nm of Ti and 20 nm of Au. The films were deposited at an angle of incidence of 35° ± 1°.…”