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Nickel ions (Ni²⁺) probe with enhanced sensitivity is experimentally demonstrated by using a normal fiber Bragg grating (FBG) cascaded with functionalized fiber tip. Bilayers of chitosan/poly acrylic acid is functionalized directly onto the cleaved fiber tip via layer by layer assembly. When the Ni²⁺ is absorbed onto the bilayers, the Fresnel reflectivity of the fiber tip, which forms the noise in the background of FBG’s reflection spectrum, will be modulated due to Ni²⁺-caused refractive index variation of the bilayers. By demodulating the optical reflectivity ratio between FBG peak and background noise of the reflection spectrum, Ni²⁺ concentration can be measured successfully with a maximum sensitivity of 40.52 dB/mM. The relative method not only eliminates the influence caused by power fluctuation of the light source on measurement accuracy, but also allows independent temperature measurement through the shift …