Research On Fiber Optic Gas Sensing Based On Metal Organic Frameworks

As the issues of environment pollution have drawn more and more attention for civilization,monitoring and sensing of toxic and harmful gases is becoming increasingly important.Metal-organic frameworks(MOFs)are emerging porous materials,which show broad application in gas storage and environmental monitoring.They can be used in the absorption of harmful gas such as sulfide,volatile organic compounds or in separation of waste water.Meanwhile,owing to the small size and immunity to electro-magnetic field,optical fiber devices are widely used for sensing and monitoring.In this thesis,we propose and fabricate a cascaded long period fiber grating device with MOF ZIF-8 materials for gas sensing and absorption/desorption monitoring.The research of this dissertation is outlined as follows:(1)Firstly,the current research status of optical sensors,the basic principles and mechanisms of several common optical sensors with the emphasis of optical fiber sensors,and the research status of metal organic framework materials are introduced.Based on this,a sensor using metal organic framework materials and cascaded long period fiber gratings is proposed.(2)The basic theory of long period fiber grating and cascaded long period fiber grating is introduced.Based on the basic theory of optical fiber,the dispersion and the effective refractive index of the fundamental mode and the cladding mode in single mode fiber are calculated.The parameters of the grating were simulated and the adsorption theory of metal organic framework materials is introduced.These theoretical analyses pave the way for the subsequent experiments.(3)After the simulation,the parameters of the device were determined.The fabrication method of the long period fiber grating in this dissertation is introduced.The physical and chemical properties of porous MOF ZIF-8 used in this dissertation are then outlined and the in-situ growth of ZIF-8 film on fiber is investigated A scanning electron microscope is used to determine the surface and the thickness of the ZIF-8 film that needs to be grown on the cascaded long-period fiber gratings.(4)Six linear organic gas(methanol,ethanol,ethyl ether,acetone,ethyl acetate and dichloromethane),the sizes of which are smaller than the aperture of ZIF-8(3.4 ?),are used in gas sensing in ZIF-8 coated cascaded long period fiber grating.The resultsshows that those small molecules can easily immerse into the chanel of ZIF-8 and then change the effective index of cladding mode to make the drift of the centra wavelength of interferometer.The gas absorption and desorption of small molecules are relatively fast and repeatable.The results indicate that our fabricated fiber device can be used for these small molecules sensing.Four moleucles(toluene,cyclopentanone,tetrahydrofuran and cycylohexane)with larger sizes than the aperature of ZIF-8 are used to investigate the gas desorption.The results show that ZIF-8 can absorb the large moleucles and the molecules trapped in the channel.By the replacement of methanol gas,those trapped gas can be released.The phenomenon indicates that our fiber device can be used as an instrument to dynamically monitor the absorption and desorption of large molecules in the porous ZIF-8.In conclusion,a cascade of long period fiber grating gas sensing and monitoring devices based on a new metal organic framework materials is fabricated.The experimental results show that the device has potential application as a gas sensing and monitoring instrument for new material research.