a b s t r a c tAn optical fibre long period grating (LPG) modified with a thin film of ZIF-8, a zeolitic immidazol framework material and a subgroup of metal organic framework family, was employed for the detection of organic vapours. ZIF-8 films were deposited onto the surface of the LPG using an in-situ crystallization technique by mixing freshly prepared 12.5 mM zinc nitrate hexahydrate and 25 mM 2-metyl-imidazole solutions in methanol. The ZIF-8 film was characterized by scanning electron microscopy. The thickness and refractive index (RI) of films deposited after 1,2,3,5 and 10 growth cycles were determined by using an ellipsometer. The crystallinity of the films was confirmed by X-ray diffraction pattern. The LPG was designed to operate at the phase matching turning point to provide the highest sensitivity. The sensing mechanism is based on the measurement of the change in the RI of the film induced by the penetration of the chemical molecules into the ZIF-8 pores. The responses of LPGs modified with 2 and 5 growth cycles of ZIF-8 to exposure to methanol, ethanol, 2-propanol and acetone were characterised. The sensitivity of the measurements to humidity as an interfering parameter was also investigated.

a b s t r a c tAn optical fibre long period grating (LPG) based volatile organic compound (VOC) sensor coated with ZIF-8, a material from the zeolite imidazolate framework (ZIF) family, functional coating is presented. ZIF-8 film was deposited onto the surface of the LPG using an in-situ crystallization technique by mixing freshly prepared 12.5 mM zinc nitrate hexahydrate and 25 mM 2-metyl-imidazole solutions in methanol. A concentration specific response to acetone, ethanol and methanol vapour was obtained over the high concentration range up to tens thousands ppm. The LPG based sensor works at phase-matching turning point (PMTP) and such operates at the highest sensitivity. A novel approach for the data analysis based on the measurement of the bandwidth of the U-shaped attenuation band of the LPG is introduced. The optimized sensor shows a sensitivity of 0.015 ± 0.001 and 0.018 ± 0.0015 nm/ppm and limit of detection (LOD) of 6.67 and 5.56 ppm for acetone and ethanol respectively.

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An optical fibre long period grating (LPG) based carbon dioxide (CO2) sensor coated with HKUST-1, a material from the metal organic framework family, functional coating is presented. In-situ crystallization and layer by layer (LbL) techniques of HKUST-1 thin film synthesis are compared in terms of the feasibility of the deposition procedure (time and cost efficiency) and the sensitivity of the film to carbon dioxide. The sensing mechanism is based on the measurement of the change of the refractive index (RI) of the coating that is induced by the penetration of CO2 molecules into the HKUST-1 pores. The HKUST-1 film was characterized by scanning electron microscopy (SEM). The thickness and refractive index (RI) of the 10, 20 and 40 layers thick films were determined using ellipsoetry. The crystallinity of the films was examined by X-ray diffraction pattern (XRD). While no response to CO2 was observed for the sensor coated using the in-situ crystallization technique, an LPG modified with 10, 20 and 40 layers of HKUST-1 films using LbL method upon exposure to CO2 in the range of 500 ppm to 40,000 ppm showed good sensitivity. The film containing 40 layers exhibited the highest sensitivity to CO2 with an obtained detection limit of 401 ppm.

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