In this paper, we propose a hetero-core optical fiber with a high refractive index material, TiO2, deposited on the inner layer of the Au island films, which is fabricated by sputtering method and annealing. It was confirmed that this structure of TiO2 deposited on the inner of the Au island films strongly excites the absorption peak in the visible light region and detects the refractive index change in the near-infrared region compared to the conventional hetero-core optical fiber sensor using Au island structure.
We have proposed and developed a CO2 sensor by coating hetero-core optical fiber surface plasmon resonance (SPR) sensors with ionic liquid (IL) gel coating. The hetero-core optical fiber has no electrical contact in the sensor portion as a feature of the optical fiber. In our previous study, it was confirmed that the proposed hetero-core fiber CO2 sensor using 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) as an IL can successfully detect CO2 gas. However, so far, ILs other than [EMIM][BF4] for the sensor has not been tested yet. Since it is known that the CO2 solubility in ILs changes by changing cations and/or anions, CO2 sensing characteristics of the proposed sensor could be affected by the used IL. In the present paper, we examined how the change in cation and/or anion in the IL affects the CO2 sensing characteristics of the proposed sensor using four different ILs. 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][Tf2N]), 1-butyl-3-methylimidazolium trifluoroacetate ([BMIM][TFA]), 1- butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), having different CO2 solubility in descending order. By measuring the transmitted spectra and, optical loss change, we evaluated the sensitivity and responsiveness of the fabricated CO2 sensors. The results showed that for all four IL coatings, the SPR spectra was observed using the fabricated sensors. Further, All the CO2 sensors with different ILs showed response to CO2 gas. Especially, the sensor prepared using the IL of [BMIM][TFA] exhibited the highest optical loss change in response to CO2 concentration change.
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