Fig. 1. Principle of the sensor. (a) Cross section of the optical-fiber; (b) principle of the sensing system

Fig. 2. XRD pattern. (a) XRD patterns of Ni-MOF-74 and Co/Ni-MOF-74; (b) Co/NiO pattern and standard XRD patterns of NiO

Fig. 3. Element distribution patterns of the material. (a) Ni element in Co/Ni-MOF-74; (b) Co element in Co/Ni-MOF-74; (c) Co element in Co/NiO; (d) Ni element in Co/NiO; (e) O element in Co/NiO

Fig. 4. TGA curve of Co/Ni-MOF-74 powders

Fig. 5. SEM images. (a) Ni-MOF-74; (b) Co/Ni-MOF-74; (c) Co/NiO

Fig. 6. SEM image of the FCF. (a) Outside surface; (b) cross section

Fig. 7. Sensitivity and response-recovery curve of the sensor. (a) Wavelength drift under different volume fractions; (b) response-recovery curve

Fig. 8. Selectivity and temperature sensitivity of the sensor. (a) Selectivity; (b) wavelength drift at different temperatures

Fig. 9. Transmission spectra at different time