Unusual response behavior of ion exchange-based potassium ion-selective nano-optodes based on bis(crown ether) neutral ionophores and a cationic solvatochromic dye

Abstract

In this study, we used three different potassium ionophores developed for ion-selective electrodes to fabricate nanoemulsion-type ion-selective optodes (NE-ISOs) with a pH-independent response mechanism based on cation exchange between a polarity-responsive cationic dye in the organic phase and potassium ions in the aqueous phase and compared their responses. As a result, the NE-ISO prepared with valinomycin as the typical ionophore showed an ideal response in which the initial fluorescence intensity decreased with increasing ion concentration, as the cationic dye was extruded out into the aqueous phase following extraction of potassium ions by valinomycin. On the other hand, when the same experiment was performed using a bis-crown-type ionophore with a long alkyl chain, the initial fluorescence intensity became extremely small and increased with increasing potassium ion concentration, indicating a completely opposite response behavior that cannot be explained by the conventional ion exchange model. As a result of extraction experiments of hydrophobic organic cations and investigation of ionophore concentrations in nanodroplets, we obtained some results that suggest that the interaction between the ionophore molecule and the dye molecule at the organic–aqueous interface and the accumulation behavior of the ion–ionophore complex at the organic–aqueous interface greatly affect the response behavior to ions, depending on the molecular structure of the ionophore. These results contradict the conventional perception in the development of ion-selective optodes that the same principle can be applied to measure different ions by changing the ionophore. It clearly demonstrates the importance of considering the chemical structures of the ionophores and dye molecules and their interaction, in addition to the response mechanism when designing NE-ISOs.