Visualization of water and ion transport in ion-selective membranes

An approach allowing visualization of processes occurring during pretreatment of ion-selective membranes is proposed. This method does not interfere with potentiometric performance of sensors, thus it can be used to follow pretreatment of sensors intended for electrochemical studies. The approach uses neutral dye Nile red added to the membrane in minute amounts. Due to unique properties of the dye, water partition to the membrane can be monitored as decrease of emission characteristic for the dye dispersed in the lipophilic phase (plasticizer) and formation of a new emission peak at longer wavelengths. The latter is characteristic for Nile red present in the aqueous phase dispersed in the lipophilic liquid, its formation is related to dye interactions with cation exchanger. Visually this process can be observed as change in the cation selective membrane color from red to violet. This process is reversible – drying of the membrane or incorporation of primary ions to the phase results in increase of the emission at wavelength characteristic for Nile red dispersed in the plasticizer phase, and change of the membrane color back to red. Thus, the proposed approach allows insight into the membrane pretreatment process: equilibration with water, drying or incorporation of ions and ultimately results in better control of this process. • An simple visualization of ion-selective membrane pretreatment process is proposed. • Equilibration of the membrane with water, drying or incorporation of ions results in color and emission spectra change. • Required incorporation of Nile red to the membrane is not affecting potentiometric performance. • The role of ion-exchanger in water incorporation to the membrane phase is highlighted.