Liquid crystal-based microcapillary sensors

Kurt, Elif
Stimuli-responsive properties of liquid crystals (LCs), when combined with their optical properties, offer sensitive and rapid sensing applications. In the first part of this thesis study, we propose and demonstrate a microcapillary-based method to be applied for the online detection of amphiphilic species, which can be further used for biological and chemical species in aqueous media. Specifically, we used compartments (300-1400 µm) of nematic 4-cyano-4’-pentylbiphenyl (5CB) that is positioned into cylindrical shaped glass microcapillaries that promote homeotropic anchoring. The flat surfaces of the cylindrical LC compartments were in contact with an aqueous media. We characterized the equilibrium and nonequilibrium response of LC upon a change in the anchoring of LCs at the LC-aqueous interfaces. Upon anchoring transition, we observed a formation of a positively charged defect at the proximity of the interface that moved to the center of the LC compartment and reached equilibrium, four-petal configuration. This transition was observed to take an average of 41±19 min., which was significantly larger than the time that would take for diffusion and adsorption of the surface-active species at LC-water interfaces. We concluded this to relate to the motion of the defect due to the imbalance of the elastic forces. During the transition, we observed metastable states which could be removed via thermal treatment. We showed that the capillary sensors to be more advantageous over the flat film counterparts in terms of the ease of quantification. We also show that the sensors are reversible that facilitates temporal and cumulative quantification. In the second part of this thesis study, we developed aptasensors to be used in both flat and microcapillary geometries. We sequentially functionalized the glass surfaces first with mixed monolayer of (3-Aminopropyl)triethoxysilane (APTES) and dimethyl octadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (DMOAP), then glutaraldehyde solution (GA) to immobilize neomycin aptamer via glutaraldehyde linkage method. Through the binding of aptamer to its target molecule, the conformation of aptamer was changed resulting in the LC configuration transition from homeotropic to degenerate planar/tilted which we visualized under polarized optical microscope. The detection limit for both geometries was found to be in between 50-500 nM neomycin B concentrations. We exposed sensors to different target solutions consisting of neomycin B and ampicillin, and carried out qualitative and quantitative analyzes. The findings reported in this thesis can further be used to develop sensors for specific purposes that require continuous tracking of the chemical and biological species that is critical for the health and safety of the individuals and society.


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Citation Formats
E. Kurt, “Liquid crystal-based microcapillary sensors,” M.S. - Master of Science, Middle East Technical University, 2022.