Liquid crystal-based biosensing: exploiting the electrical and optical properties of various liquid crystals in quantitative bioassays

Studies of the interaction between biomolecules and liquid crystals (LCs) opened up a promising new field of biosensing technologies. It is established that biomolecules can be detected and analyzed through the disruption of the regular alignment of LCs at an LC-glass or LC-water interface, thus enabling the development of biosensing platforms for DNA, lipids, and proteins, as well as larger biological entities such as pathogens and viruses. Potential clinical application of LC-based bioassays has been demonstrated in the immunodetection of cancer biomarkers, hepatitis-B antibody titering, and early detection of Alzheimer’s disease. While conventional LC-based biodetection relies primarily on optical anisotropy in 4-cyano-4'-pentylbiphenyl (5CB), a common and historically significant nematic compound, it has been brought to our attention that a wide variety of novel LC materials, especially those currently investigated in the design of LC display devices, present intriguing electrical and optical properties that are applicable to the detection and quantitation of biomolecules. In this chapter, we focus on the advantages and importance of widening the variety of biosensing mesogens. The biosensing application of LCs other than 5CB, such as nematic LCs of large birefringence, cholesteric LC, blue-phase LC, dye-associated LCs including dye LC and dye-doped LC, and dual-frequency LC at the LC-glass interface, is introduced. Not only do these LCs exhibit birefringent characteristics and biosensing capabilities similar to 5CB when interfaced with biomolecules, but their unique electrical and optical properties contribute to further enhancement in detection sensitivity and, in particular, quantitative analysis. Because one of the technical drawbacks in current LC-based biosensing techniques is the lack of quantitative methods, we elaborate on the innovative approaches of transmission spectrometry, capacitance, electro-optical, and dielectric measurements toward the development of quantitative bioassays. The unique features of LCs as flexible biosensing materials enable the development of unconventional label-free bioassays for cost-effective, fast-screening, and even color-indicating biomedical and biophotonic devices for clinical and point-of-care diagnostics.