Silver Ion Selective Optodes Based on Novel Thia Ether Compounds

Flow-through type fiber-optic silver ion selective optodes based on novel cyclic and noncyclic thia crown ether compounds and a lipophilic anionic indicator dye have been developed. Pellicular octadecylsilica beads were coated with a lipophilic organic liquid containing the thia ether compound as the silver ion selective ionophore and LAD-3 as the color changeable indicator dye. The optode system was constructed by packing these color changeable ion sensing beads in a flow-through optical cell (cell volume, 7 μL) having a quartz window attached directly to the distal end of a bifurcated optical fiber. The optode based on a lipophilic thia crown ether that has two sulfur atoms (tetradecyl-14-dithiacrown-4) exhibited the most sensitive response to silver ion of all the optodes investigated. This optode could detect silver ion in concentrations ranging from 10^-6 to 10^-2 M at pH 7.0 with 0.05 M Tris-H₂SO₄ buffer as the flowing solution by measuring the absorbance change in the sensing beads at 516 nm. This optode showed a highly selective response to Ag⁺ without any interference from other transition metal ions, except for Hg²⁺, up to 10^-3 M, as well as alkali metal and alkaline earth metal ions up to 1 M. Optodes employing highly lipophilic ionophores showed good reproducible response to Ag⁺ for at least 2 weeks.