Study On Graphene Oxide Functionalized Tilted Fiber Bragg Grating For Heavy Metal Ions Sensing And Its Sensitization Mechanism

Heavy metals（such as lead,mercury,copper,and cadmium）are a class of environmental pollutants that have toxic effects on humans,animals and plants.Since they cannot be degraded or destroyed,heavy metals can exist for a long time in the natural environment.Water containing heavy metal ions can cause various diseases and functional disorders in humans,and high concentration of heavy metals can even cause cancer.At the same time,the development of the analysis field in recent years has increasingly attracted people’s attention to environmental health,which requires qualitative and quantitative analysis of the concentration of pollutants.Meantime,optical fiber sensor is gradually being used in environmental detection due to their high sensitivity,small size,and immunity to electromagnetic interference.Therefore,in this paper,an ultra-sensitive Graphene Oxide functionalized tilted fiber Bragg grating（GO-TFBG）sensor is designed to detect trace heavy metal ions in aqueous solutions and some methods are explored to further improve the detection sensitivity.The main research contents are as followed:First,a GO-TFBG heavy metal ions sensor was investigated.The traditional wet film formation will lead to the stacking of GO nanosheets on the surface of the substrate,and the film presents a large number of agglomerations and wrinkles.The appearance of agglomeration will cause the carboxyl groups on the surface of GO to be wrapped in the agglomerates,reducing the sensing efficiency.In this paper,the film-forming process of GO was improved to avoid the agglomeration and stacking of GO caused by the"coffee ring"effect,therefore,the carboxyl groups on the exposed surface were fully exposed,and the adsorption efficiency of GO was improved.The specific measures were as followed:（1）Purifing GO to obtain size Uniform GO nanosheets;（2）using anhydrous ethanol with a smaller surface tension as a solvent;（3）treating the substrate with oxygen plasma to improve surface wettability Through proper surface energy matching,the GO thin film was flat and free of agglomeration,thereby increasing the accessibility of the carboxyl groups on the GO surface and improving the detection limit and sensitivity,while avoiding randomness caused by agglomeration and stacking,ensuring the consistency of performance between devices.The detection results of different ions showed that the response of the GO-TFBG to Cu²⁺was relatively weak,while the detection limits of Pb²⁺and Cd²⁺could reach 10^-10 mol/L（ppt level）,and the corresponding sensitivities were 0.426 and 0.385 d B/(nmol·L^–1)（2.06 and 3.43 d B/ppb）,respectively.The sensor showed excellent inter-device consistency,and the performances of the 5groups of devices were very stable.The results of this study showed that improving the assembly method of sensitive materials could effectively improve the sensitivity of the sensor,lower the detection limit,and achieve excellent device consistency,which is a useful reference for other sensor research based on 2D materials.Secondly,TiC_xN_1-x/N-TiO₂ heteroepitaxial nanoparticles with visible-near-infrared plasmon resonance effect have been investigated.Due to Pauli blocking,the inter-band transition of GO in the near-infrared band is very weak.In order to explore the way to improve the sensitivity of optical fiber sensors,TiC_xN_1-x/N-TiO₂heteroepitaxial nanoparticles were prepared by plasma-induced method.High-angle annular dark-field microscopy revealed that metallic TiC_xN_1-x nanoparticles smaller than 5 nm in size were epitaxially grown on the surface of TiO₂ nanoparticles,and formed near-ideal ohmic contacts.The metallic TiC_xN_1-x nanoparticles had strong extinction properties due to the localized surface plasmon resonance（LSPR）effect.The chemical interface damping effect of TiC_xN_1-x/N-TiO₂ introduced the hot electrons excited in TiC_xN_1-x transfer to the side of TiO₂,which led to the dephasing of the LSPR of TiC_xN_1-x,and the resonance peak was broadened to near infrared band,while the ultraviolet absorption of TiO₂ was bleached.Therefore,using TiC_xN_1-x/N-TiO₂ heteroepitaxial nanoparticles to replace GO,which absorbs weaker near-infrared light,is expected to improve the sensitivity of optical sensors.In summary,a GO-TFBG heavy metal ions sensor was designed in this paper.By optimizing the GO film-forming process,the agglomeration-free coating of the thin film on the surface of the fiber was realized,which increased and the accessibility of the carboxyl groups at the edge of the GO nanosheet.The sensing results of different heavy metal ions showed that the sensor after optimized film formation had high sensitivity,and the detection limit was as low as 10^-10 mol/L.In addition,in order to further improve the sensitivity of the sensor,TiC_xN_1-x/N-TiO₂ heteroepitaxial nanoparticles with visible-near-infrared plasmon resonance effect were prepared by plasma-induced method,and the hot electron transfer mechanism in the heteroepitaxial structure was elucidated,providing an efficient preparation scheme for the synthesis of metal-semiconductor heteroepitaxial structures,and provided theoretical guidance for the application of heterostructures in the fields of photocatalysis and optoelectronic devices.