A fiber-optic fluorescence sensor for lithium ion in acetonitrile

A novel fiber-optic fluorescence sensor based on a controlled-release reagent for the determination of lithium ion in organic solvents is proposed. The fluorogenic indicator 2-(2-hydroxyphenyl)benzoxazole is contained in a minipolyethylene tube as the reagent reservoir and is brought into contact with the analyte solution by diffusion across a poly(vinyl chloride) (PVC) membrane to form a strongly fluorescent complex at the membrane/solution interface. The fluorescence signals produced are measured via two joined optical fibers positioned closely to the backside of the PVC membrane for light illumination and collection. The sensor is useful for measuring Li⁺ at concentrations in acetonitrile ranging from 1.0 × 10-6 to 1.0 × 10^-2 M with a detection limit of 3.0 × 10^-7 M. The steady-state response can be reached within seconds, and the signal changes are fully reversible. The sensor shows minimal interference effects from other alkali metal and alkaline earth metal cations and has good stability and durability when stored in acetonitrile solutions.