Liquid crystal-based label-free low-cost sensing platform: Engineering design based on interfacial interaction and transport phenomena

Abstract

Typically, the detection methods of chemical species in sensing platform involves the use of labelling agents. Even though it is selective, the process is complex and time consuming. Liquid crystals (LC) are sensitive and rapidly responsive to the presence of any foreign species. When a LC layer interacts with aqueous (ionic surfactant) solution, there is homeotropic alignment of the director orientation due to the anisotropic interactions at the interface. This leads to the dark birefringence patterns observed in polarised microscope under cross polarisation. In the presence of H+ in the aqueous layer, the LC orientation is distorted resulting in a change of the birefringence patterns in the polarised micrographs. We have prepared an optical cell where the aqueous solution can interact with the LC layer confined within micrometre sized grids. The flow arrangement and the associated mass transfer process is optimised for enhanced interaction. The elastic interactions of the LC layer with H+ at the interface distorts the director orientation, and the birefringence pattern is captured with polarised microscope (cross-polarisation). There is a correlation between the polarised micrographs and the concentration of the H+, which forms the basis of detection principle. The detection of H+ has no interference with salts. The present method can be used for recognition of (bio-) chemical reactions releasing H+. The developed optical flow cell is inexpensive, less than $1. The outcome of the present work may be useful for technological adaption and prospective use in point-of-care devices and field test kits.