Covalent Attachment of Molecularly Thin PVC Membrane by Click Chemistry for Ionophore-Based Ion Sensors

Solid-contact ion-selective electrodes (SC-ISEs) are widely used in clinical diagnostics devices and are expanding into other fields including environmental and wearable sensors. Despite significant materials research efforts, the most widely used polymer for membrane preparation remains plasticized poly(vinyl chloride) (PVC). Owing to its nonreactive nature, it is normally applied onto the electrode substrate by evaporative solvent casting. Unfortunately, the resulting films tend to adhere poorly onto the electrode substrate and it is difficult to fabricate robust, well-defined sensing films with controlled thickness down to the nanoscale. This work introduces, for the first time, a PVC polymer membrane that is chemically bonded to the electrode substrate by Cu(I)-catalyzed azide-alkyne cycloaddition (click chemistry). A molecularly thin PVC membrane in which a fraction of the chlorine atoms of the PVC are replaced by azide functionalities is covalently attached to an electropolymerized PEDOT substrate containing alkyne groups by click chemistry. Electrochemical quartz crystal microbalance studies during the electropolymerization and membrane click reaction procedures suggest layer thicknesses of 776 and 42 nm of the conducting polymer and PVC membrane layer, respectively. Once doped with plasticizer and ion sensing components, the ultrathin PVC membrane exhibited a near-Nernstian response slope and excellent ion selectivity. Analytical properties were further improved by overcoating the PVC film with solvent cast PVC, resulting in a standard deviation of the E⁰ value for potassium-selective electrodes of 1.60 mV.