Fibre-optic potassium ion sensors based on a neutral ionophore and a novel lipophilic anionic dye

A novel lipophilic anionic dye, N-2,4-dinitro-6-octyloxyphenyl-2′,4′-dinitro-6′-trifluoromethylphenylamine (LAD), was synthesized. On deprotonation at neutral Ph it forms anions and brings about a change in the absorption spectrum in the visible region. This anionic dye was incorporated into a poly(vinyl chloride) matrix membrane with dibenzo-18-crown-6 (DB18C6) or valinomycin, which are K⁺-selective neutral ionophores. An optical K⁺ sensor was prepared using the polymeric membrane set 1 mm apart from the tip of a bifurcated optical fibre with a cylindrical plastic support. This sensor could detect a wide range of K⁺ concentrations (10^-6-1 M K⁺ for the DB18C6-based sensor, 10^-8-10^-2 M K⁺ for the valinomycin-based sensor) at Ph 7.0 by measuring the absorbance change at 513 nm. The factors that influence the response sensitivity of the sensor are discussed theoretically.