A ZIF-8-Functionalized Evanescent Wave Coupled Optical Fiber Bragg Grating for CO ₂ Detection

The detection of carbon dioxide (CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>) is important for environmental protection. This paper proposes a CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sensor based on ZIF-8 functionalized evanescent wave coupled fiber Bragg grating (FBG). The sensor is constructed by coupling a multimode fiber (MMF) with a single-mode FBG, resulting in a comb - like structure spectrum and generating a strong evanescent field in the FBG region. A ZIF-8 thin film with a thickness of approximately 132 nm is coated. After ZIF-8 absorbs CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>, its refractive index increases, the intensity of the evanescent wave in the grating region is enhanced, leading to a decrease in the effective refractive index of the optical fiber. The FBG transmission spectrum produces a sensitive blue shift, and the wavelength variation has a power - function relationship with the CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration, thus realizing CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> detection. A rapid detection method is proposed, which can quickly determine within 20 seconds in the range of 0 - 20% CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration. In addition, the sensor has good selectivity for CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> when applied to automobile exhaust detection, it accurately judges that the CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration in the exhaust of a 15 - year - old car exceeds the standard, while the other vehicles meet the standard.The sensor has the advantages of high sensitivity, fast response and simple preparation, providing an effective solution for the rapid monitoring of CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in automobile exhaust and the environment.