A Highly Sensitive Ethanol Liquid Sensor Based on No-Core Fiber Coated With Metal-Organic Framework

Integration of optical fiber with functionalized nanofilms offers great potential for the realization of highly sensitive sensor. In this article, a highly sensitive ethanol liquid sensor based on no-core fiber (NCF) coated with porous metal-organic framework nanofilms is proposed, which consists of two segments of NCF and one segment of single mode fiber (SMF), named as NSN structure. The combination of the multimode interferences (MMIs) effect in NCF and the preconcentration effect in the porous zeolitic imidazolate framework (ZIF-8) enables the detection of low-concentration ethanol aqueous solution. The NSN sensing structure comprises two cascaded MMIs and compact ZIF-8 nanofilms are uniformly formed on the surface of NCFs by using in situ crystallization technology. Due to the coreless structure of NCF, the fraction of power in the evanescent wave is enhanced, which makes the MMI effect in NCFs highly sensitive to the surrounding refractive index. Furthermore, the ethanol molecules can be adsorbed and concentrated in ZIF-8 nanofilms. This leads to an amplification of the refractive index variation, which is caused by the presence of ethanol molecules. The experimental results show the proposed sensor has a high sensitivity of 1.522 nm%
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, and the limit of detection (LOD) of 0.90% (ethanol/aqueous solution and volume fraction).