Development Of The Fiber Optic Sulfate Ion Sensor And Its Application In The Concrete Environment

Sulfate ion erosion is one of the main factors affecting the durability of concrete structures.When the sulfate ion transported in the concrete pore system reaches a higher concentration,the erosion and deterioration of the concrete is manifested in the following three aspects: the sulfate ions will chemically react with cement hydration products,precipitate crystals of swelling erosion products and corrode steel bars in concrete structures.This will cause the concrete structure to expand,crack,peel,and lose strength,which will seriously affect the service life of the concrete structure and cause major safety accidents and economic losses.By placing a sensor in the concrete structure,it can monitor the sulfate ion erosion at multiple points in real time,thereby realizing the early warning of the durability of the concrete structure.At the beginning of the sulfate ion erosion,corresponding protective measures should be taken to avoid or reduce its more serious erosion damage to the concrete structure.The optical fiber sensor has the advantages of fast response speed,high sensitivity,small size and easy in-line detection inside materials,and is widely used in chemical analysis.Fluorescence analysis is characterized by high analytical sensitivity,strong selectivity,and ease of use.And the fluorescence analysis based on the fluorescence complexes has brought some organic and inorganic substances which do not emit fluorescence into the scope of fluorescence analysis.This thesis proposes to combine the fluorescence analysis method with the optical fiber sensor to develop an optical fiber sensor based on the fluorescent complexes for monitoring sulfate ion concentration in a concrete environment,which has important engineering and economic significance for the durability warning of concrete structures.The main research contents include the following aspects:（1）The scandium-morin fluorescent complex was synthesized by double decomposition reaction and organic complexation reaction.The scandium-morin fluorescent complex was used as a fluorescent probe of sulfate ion,and the structure and properties of the fluorescent complex were characterized.The results show that the scandium ion and morin form fluorescent complex through coordination bonds at a molar ratio of 1:2.The optimal excitation wavelength and the optimal emission wavelength of the fluorescent complex are Ex=420 nm and Em=500 nm,respectively.Based on the change of the binding energy of scandium during the reaction of fluorescent complexes with sulfate ions,the reaction mechanism of quenching the fluorescence of fluorescent complexes by sulfate ions was explored.（2）Fluorescent complexes are particularly sensitive to sulfate ions.The calibration equation for the calibration in a sulfate ion standard solution is: I₀/I =0.35095 + 0.00382 ,and the correlation coefficient is = 0.9986.This is almost the same as the slope of the standard curve of the fluorescent complex in the alkaline concrete simulation solution with a p H range of 7-12.6.The mechanism of selective detection of sulfate ions by fluorescent complexes was investigated from the perspective of ion space configuration.（3）The sulfate ion cellulose acetate sensitive membrane and the sulfate ion polyvinyl butyral sensitive membrane were prepared by embedding method,and the surface morphology and hydrophilicity of the two sensitive membranes were characterized respectively.The fluorescence quenching effect of sulfate ion on the solid-state sensitive membrane was investigated.The results show that the fluorescent complexes in the solid-state sensitive membrane can still be fluorescence quenched by sulfate ion,and the higher the sulfate ion concentration,the stronger the fluorescence quenching effect on the sensitive membrane.（4）Two kinds of fiber optic sulfate ion sensors were prepared.The standard equation for the calibration of the fiber optic sulfate ion sensor based on the CA sensitive film in the standard solution of sulfate ion is: I₀/I = 0.21043 +0.01093 ,and the correlation coefficient is R²= 0.97859.While the calibration of sulfate ion in alkaline concrete simulation solution is affected by p H value,the slope of the standard equation will increase with the increase of p H value.The standard equation for the calibration of the fiber optic sulfate ion sensor based on the PVB sensitive film in the standard solution of sulfate ion is: I₀/I = 0.86829 +0.00498 ,and the correlation coefficient is R²= 0.99412.And it is not affected by p H value when sulfate ion is calibrated in alkaline concrete simulation solution.