Electrochemical Visualization of an Ion-Selective Membrane Using a Carbon Nanoelectrode

Molecular and physical probes have been widely employedto investigatephysicochemical properties and mechanisms of interfaces due to theirability to provide accurate measurements with temporal and spatialresolution. However, the direct measurement of electroactive speciesdiffusion in ion-selective electrode (ISE) membranes and quantificationof the water layer have been challenging due to the high impedanceand optical opacity of polymer membranes. In the present work, carbonnanoelectrodes with ultrathin insulating encapsulation and good geometricalstructure are reported as physical probes for direct electrochemicalmeasurement of the water layer. The scanning electrochemical microscopyexperiment exhibits positive feedback at the interface of the freshISE, and negative feedback after conditioning for 3 h. The thicknessof the water layer was estimated to be ca. 13 nm. For the first time,we provide direct evidence that, during conditioning, the water moleculesdiffuse through the chloride ion selective membrane (Cl-ISM) untila water layer establishes at almost 3 h. Furthermore, the diffusioncoefficient and concentration of oxygen molecules in the Cl-ISM arealso directly electrochemical measured by introducing ferrocene (Fc)as a redox molecule probe. The oxygen concentration in the Cl-ISMdecreases during conditioning, suggesting the diffusion of oxygenfrom ISM to the water layer. The proposed method can be used for theelectrochemical measurement of solid contact, providing theoreticalguidance and advice for the performance optimization of ISEs.

Automated summary

In this study, carbon nanoelectrodes were used as physical probes to directly measure the water layer in ion-selective electrode membranes. It was found that water molecules diffuse through the chloride ion selective membrane until a water layer is formed. Additionally, the diffusion coefficient and concentration of oxygen molecules in the membrane were directly measured using a redox molecule probe.