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.
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