Redox-active self-assembled monolayers for solid-contact polymeric membrane ion-selective electrodes

With a view to the miniaturization of ion-selective electrodes (ISEs), thin (10-20 μm) polymer membranes are directly contacted to Au covered with a redox-active, lipophilic self-assembled monolayer (SAM). Several homogeneous and mixed monolayers are characterized by reflection-absorption infrared spectroscopy, ellipsometry, scanning tunneling microscopy, cyclic voltammetry, and contact angle measurements. These Au/thiol surfaces are combined with different K⁺-selective sensing membranes based on poly(vinyl chloride) (PVC), polyurethane (PUR), or PVC/PUR blends as a matrix and valinomycin as an ionophore. The sensors are investigated with regard to their potential stability in the presence of O₂ and redox-active species. The occurrence of potential drifts upon changing the conditioning KCl solution to a NaCl solution is used as an indicator for the formation of an aqueous film between the membrane and Au/SAM. Stable systems are obtained with mixed monolayers (advancing contact angle θ_a ≈ 83°) and PVC membranes with a lower than usual plasticizer content (33 wt %), the ternary systems PVC/PUR/plasticizer (1:1:1), and PUR with 33 wt % plasticizer. On the other hand, a water film is formed between Au/SAM and conventional PVC membranes having 66% plasticizer and with less lipophilic monolayers uniquely based on a redox-active compound (θ_a ≈ 70°). The new solid-contact ISEs are promising both for miniaturization and for improving lower detection limits.