Fluorescence Quenching–Based Optical Fiber Sensor for Monitoring Chloride Ion Concentration in Pore Solution of Soils

Model tests for investigating chemical transport through a complex geologic and/or hydrogeological site are still challenging owing to the lack of minor-sized sensors for real-time monitoring of the chemical concentrations exclusively. This paper presents a novel optical fiber sensor to monitor the concentration of chloride ion in soil pore solutions based on fluorescence quenching. Lucigenin is coated at the end of core optical fiber and is excited to generate fluorescence by the evanescent wave of the light propagating along the optical fiber. The quenching characteristic of lucigenin fluorescence by chloride ion is utilized to measure the chloride ion concentration in soil pores. The intermediate product generated in the fluorescent coating material due to light exposure can automatically decompose, which facilitates subsequent repeated monitoring. A sand box with the pore solution displaced by higher NaCl-concentration solutions in a staircase order was used to validate the performance of the presented optical fiber sensor. The developed optical fiber sensor can lower the effect of preferential flows induced by the buried sensors and is promising for the applications in model tests and in situ conditions.