Water pollutants detection using liquid crystal emulsions

Abstract

Liquid crystals are a state of matter with properties between liquids and solids, specifically sharing fluidity with liquids and anisotropy with crystalline solids. Anisotropy refers to the different behaviour of physicochemical properties in different directions of space. Optical anisotropy, also known as birefringence, is the property from which the application of liquid crystals as biosensors arises. This property is found in materials that have two refractive indices, and therefore, produce double refraction of light. A beam of unpolarized monochromatic light, when passing through a birefringent material such as liquid crystals, is refracted at two different angles, resulting in two beams of polarized light: the ordinary beam and the extra-ordinary beam. The refractive index of the extra-ordinary beam depends on the orientation of the molecules within the liquid crystal. Since the orientation of liquid crystal molecules is highly sensitive to the physicochemical conditions of the surrounding environment, this property can be used to detect changes in the medium surrounding the water-liquid crystal interface. Given that a sphere has a higher surface-to-volume ratio than a flat interface separating two phases, an emulsion would be a more sensitive detection system for changes occurring at the surface of liquid crystal droplets. To create emulsions that are as monodisperse as possible, we will use a microfluidic device, the FemtoJet4i (purchased from Eppendorf). We will study the effects of the surrounding conditions and the internal structure of liquid crystal droplets depending on the surfactant adsorbed to the surface. The boundary conditions can favour the alignment of liquid crystal molecules in parallel (planar anchoring) or perpendicular (homeotropic anchoring) to the surface. The changes in orientation that result in different refraction of light can be detected using polarized optical microscopy, an easy technique for characterizing liquid crystal droplets. An emulsion of liquid crystal in water induces planar anchoring at the interface due to the polarity of water. In contact with a water pollutant, depending on its structure and characteristics, it will alter the orientation of the liquid crystal molecules within the droplets. We will study whether this change occurs and if it can be detected with polarized light microscopy. Therefore, we will assess whether a liquid crystal emulsion could be used to detect pollutants dispersed in water