Adding Colour to Ion-selective Sensors

The composition of classical ion-selective potentiometric sensors receptors requires the presence of ionophore and ion-exchanger within plasticized polymeric matrix. Typically, none of these compounds is active in optical mode, thus typically electrochemical, potentiometric, ion-selective sensors are colorless.
Adding dyes to ion-selective membranes is attractive challenge that can extend application of highly selective sensors beyond the current paradigm, benefiting from absorption (colorimetry) or emission (fluorimetry) modes. Even more challenging it is to achieve these using dyes that do not perturb electrochemical performance of the sensors, and do not induce receptor sensitivity to pH changes. This can be accomplished using Nile blue or Nile red dyes, that are useful for ion-selective sensing [1].
The presence of electrically neutral dye, such as Nile red, in the ion-selective membrane intended for potentiometric sensing offers the possibility of colorimetric/ fluorometric or even visual monitoring ions and water transport within the phase during pretreatment – to optimize pretreatment time, to track membrane history [2].
Another possibilities arise from using cationic dyes as cation-exchanger counter ions, necessary additive while preparing neutral ionophores containing receptors. Thus ionophore driven ion-exchange between sample and the membrane during conditioning gives rise to possibility of monitoring this process. It is shown that analytical information can be obtained from the conditioning process. It is also shown that colorimetric, analytically useful signal obtained is especially attractive for analysis of samples where vast majority of interfering ions prevails.
1. A. Konefał, P. Piątek, K. Maksymiuk, A. Michalska Sens. Actuators: B. Chemical 391 (2023) 134022
2. E. Stelmach, K. Węgrzyn, P. Piątek, K. Maksymiuk, A. Michalska Sens. Actuators: B. Chemical 422 (2025) 136653