Fabrication of liquid-core fiber-optic structure for large-area CO₂ sensing using ionic liquids

Abstract

Among the ionic liquids (ILs) that are known for their CO₂ absorption properties, the optical properties of 1-ethyl-3-methylimidazolium acetate ([emim][Ac]) and 1-butyl-3-methylimidazolium acetate ([bmim][Ac]) have been investigated with the aim of assessing their suitability for use in a CO₂ sensor with a liquid-core fiber-optic structure. Fiber-optic sensors offer multiple benefits, including a large-area sensing capability and immunity to electromagnetic interference. In these two ILs with their different cation alkyl chain extensions, similar levels of change in the refractive index were observed for both [emim][Ac] and [bmim][Ac]; this change was demonstrated to lead to a change in the numerical aperture of a waveguide equipped with an [emim][Ac] core with a maximum value of 0.017787. Waveguide samples were fabricated using both [emim][Ac] and [bmim][Ac] and the output spectra of these samples were compared in terms of their liquid absorption characteristics, which were measured before the samples were packed in a gas-permeable Teflon^®AF cladding tube. The liquid-core waveguides demonstrated successful light transmission over a length of 10 cm that agreed with the absorption characteristics of each of the core liquids. The CO₂ concentration level inside the core liquid was believed to cause the transparency of the waveguide to deteriorate as a result of bubble formation. The growth of the CO₂ bubbles is irreversible and is assumed to be promoted by a kinetic stimulus and some other factors. The ILs comparison considered in this study will be useful for further development of the liquid-core waveguide-structured CO₂ sensor. The transmission length of the sensor could be elongated by optimizing both the waveguide and the core IL.