Design of Surfaces for Liquid Crystal-Based Bioanalytical Assays

Abstract

Surface-induced ordering of liquid crystals (LCs) offers the basis of a label-free analytical technique for the detection of surface-bound biomolecules. The orientation-dependent energy of interaction of a LC with a surface (anchoring energy of LC), in particular, is both sensitive to the presence of surface-bound molecules and easily quantified. Herein we report a study that analyzes a simple model of twisted nematic LC systems and thereby identifies surfaces with LC anchoring energies in the range of 0.5 μJ/m 2 to 2.0 μJ/m 2 to be optimal for use with LC-based analytical methods. Guided by these predictions, we demonstrate that analytic surfaces possessing anchoring energies within this range can be fabricated with a high level of precision (< 0.1 μJ/m 2 ) through formation of monolayers of organothiols (with ω-functional groups corresponding to oligoethyleneglycols and amines) on gold films deposited by physical vapor deposition at oblique angles of incidence. Finally, by using the human epidermal growth factor receptor (EGFR) as a model protein analyte, we have characterized the influence of the anchoring energies of the surfaces on the response of the LC to the presence of surface-bound EGFR. These results, when combined with 32 P-radiolabeling of the EGFR to independently quantify the surface concentration of EGFR, permit identification of surfaces that allow use of LCs to report surface densities of EGFR of 70-90 pg/mm 2 . Overall, the results reported in this paper guide the design of surfaces for use in LC-based analytical systems.

Chemical and biological sensing using liquid crystals

“…As an example of this approach, Lowe et al [48] covalently functionalized polydimethylsiloxane (PDMS) stamps with an antibody to the trans-membrane protein called epidermal growth factor receptor (EGFR). The anti-EGFR functionalized PDMS stamps were then used to capture EGFR and transfer it to amino-tetra(ethylene glycol)-terminated alkanethiol (EG4N) functionalized gold films by affinity contact printing.…”

“…These effects were proposed to be the result of the presence of EGFR on the surface masking interactions between the EG4N SAM and the 5CB. The researchers demonstrated that mass densities of EGFR as low as 30-40 pg/mm 2 could be detected using the above mentioned method [48, 49]. …”

Recent Advances in Colloidal and Interfacial Phenomena Involving Liquid Crystals

“…The azimuthal anchoring energy W az can be calculated as: $$W_{\mathit{az}}=\frac{2K_{22}\mathit{\psi }}{d\sin 2\mathit{\phi }}$$where ψ is the total twist angle of the liquid crystal, d is the thickness of the film of LC, and φ is the magnitude of the angular displacement of the director from the easy axis. This so-called “torque-balance” method has been used to measure the anchoring energies of LCs on a variety of SAMs formed on obliquely deposited gold films 19, 23-25, 36. As shown in Figure 2E for the case of SAMs formed from EG4 and the corresponding amine-terminated alkanethiol (EG4N, Figure 2A), the magnitudes of anchoring energies on these SAMs typically range from 1-10μJ/m 2 .…”

“…The presence or absence of biological molecules immobilized on surfaces has been shown to lead to large changes in the anchoring energies of LCs 19, 24, 36 Figure 3A. shows how the presence of proteins printed onto surfaces using affinity microcontact printing 117-122 influence the ordering of LCs 25, 47. In this example, polydimethylsiloxane (PDMS) stamps were covalently functionalized with an antibody to epidermal growth factor receptor (EGFR).…”

“…In this example, polydimethylsiloxane (PDMS) stamps were covalently functionalized with an antibody to epidermal growth factor receptor (EGFR). The anti-EGFR-functionalized PDMS stamps were used to capture EGFR and transfer it to EG4N-functionalized gold films by mechanical contact (contact printing) 25 Figure 3A. shows polarized light micrographs of thin films of nematic 5CB in contact with either (i) a SAM formed from EG4N, or (ii) a SAM formed from EG4N onto which EGFR was transferred by affinity contact printing 25 Figure 3B.…”