Optical sensors: An ion-selective optrode for potassium

A new type of sensor (called ion-selective optrode) for the continuous determination of electrolytes is presented that can be exploited in optical and fibre-optic sensors. It is based on the ability of certain fluorescent dyes to respond to an electrical potential at the interface between a lipid phase and an aqueous phase. The potential is created by addition of a neutral ion-carrier, and its magnitude is measured with an appropriate potential-sensitive dye. An opto-sensor for potassium is described. A lipid bilayer is formed on a glass support by applying the Langmuir-Blodgett film technique. A lipid-soluble rhodamine dye is incorporated into this layer together with valinomycin as the ion-carrier. When exposed to potassium ion solutions in the 0.01–100 mM range, fluorescence intensity is continuously diminished. It is shown that two kinds of response towards potassium solutions occur: one is selective for this ion, but the other is unselective, being evident with other metal ions. the unselective response can be compensated for by a reference optrode. The signal change of the ion-selective optrode depends on the nature of the layer and the concentration of dye and valinomycin. The relationship between ΔI/I (i.e., the reduction in intensity relative to the total signal) and the logarithmic analyte concentration is linear over a wide range, typically 0.01–10 mM. The selectively factor over sodium varies from 2.5 to 5.