Ionophore-Based Ion-Selective Optodes Using Hydrocarbons as Ultralow-Polarity Media

Ionophore-based ion-selective optodes, together with ion-selective electrodes, constitute a highly selective class of chemical sensors for detecting ionic species such as electrolytes. The choice of water-immiscible matrix in these sensors critically affects their analytical performance such as sensitivity and selectivity. In this study, we investigate ion-selective optodes that employ high-molecular-weight hydrocarbons as a new class of solvents. Compared with conventional plasticizers, hydrocarbons such as hexadecane dramatically enhance the sensitivity of ion-selective optodes by up to 4 orders of magnitude and substantially improve their selectivity. This enhancement arises from the stronger ionophore-ion binding affinity in the ultralow-polarity medium, leading to nearly complete extraction of the analyte ions and an exhaustive response with high sensitivity. The total color change is also expanded because the pH-sensitive chromoionophore exhibits a higher pK_a in hexadecane, ensuring its full protonation in the absence of the analyte ions. For sensing components that are insoluble in hydrocarbons, binary solvent mixtures consisting of a hydrocarbon and a plasticizer can be used while maintaining the enhanced response. This hydrocarbon-induced improvement in response is observed across a wide range of optodes incorporating various optical reporters (e.g., pH indicators and ionic dyes) and ionophores for different target ions (e.g., calcium and potassium ions). Two optode formats including liquid ion-selective optodes and paper-based ion-selective optodes are demonstrated using hexadecane as the medium. Collectively, these findings establish hydrocarbons as a new class of solvents that can significantly boost the analytical performance of ionophore-based ion sensors.