Enhancing EMF stability of solid-state ion-selective sensors by incorporating lipophilic silver-ligand complexes within polymeric films

The EMF stabilities of microfabricated ammonium and sodium ion-selective sensors prepared by incorporating a lipophilic silver-ligand complex and free ligand (either 1,3-(bis-phenylphosphine) propane (DPP) or thioether derivatized calix[4]arene (TDC)) along with appropriate ion-selective ionophores (nonactin or methylmonensin) within polymer films are examined. The silver-ligand complex and free ligand are added to function as a reversible electron redox pair at a polymer film/conductor (Ag0-epoxy) interface. The resulting solid-state sensors exhibit improved EMF stability during the first 20 h of soaking in a solution of analyte ion as well as over an extended 20-day period. Further, it is shown that the remaining initial EMF drift of the solid-state sensors, which results from water uptake by the film, can be further reduced by optimizing film plasticizers. With optimized plasticizer composition and a well-established electron transfer pair at the conductor/membrane interface, more stable potentiometric responses are achieved. Impedance studies of the solid-state ion sensors support the existence of a reversible redox couple at the Ag0-epoxy/membrane interface.