Thin Layer Ionophore-Based Membrane for Multianalyte Ion Activity Detection

A concept is introduced that allows one to detect the activity of multiple ions simultaneously and selectively with a single ion-selective membrane. This is demonstrated with ∼300 nm thin plasticized PVC membranes containing up to two ionophores in addition to a lipophilic cation-exchanger, overlaid on an electropolymerized poly-3- octylthiophene (POT) film as the electron to ion transducer. The ion-selective membranes are formulated under ionophore depleted conditions (avoiding excess of ionophore over ionexchanger), which is purposely different from common practice with ion-selective electrodes. Cyclic voltammetry is used to interrogate the films. An anodic scan partially oxidizes the POT underlayer, which results in the expulsion of cations from the membrane at an appropriate potential. During the scan of a membrane containing multiple ionophores, the least bound ion is expelled first, giving distinct Gaussian peak shaped ion transfer voltammetric waves that are analyzed in terms of their peak potential. These potentials are found to change with the logarithm of the ion activity, in complete analogy to ion-selective electrodes, and multiple such waves are observed with multiple ionophores that exhibit no obvious interference from the other ionophores present in the membrane. The concept is established with lithium and calcium ionophores and accompanied by a response model that assumes complete equilibration of the membrane at every applied potential. On the basis of the model, diffusion coefficients in the membrane or aqueous phase bear no influence on the peak potentials as long as thin layer behavior is observed, further confirming the analogy to a potentiometric experiment. Idealized ion transfer waves are narrower than experimental findings, which is explained by a broader than expected anodic peak for the oxidation of conducting polymer. The correspondence between experiment and theory is otherwise excellent in terms of thin layer behavior and Nernstian shift of the peaks with analyte concentration