Optical Ion Sensing Platform Based on Potential-Modulated Release of Enzyme

We report here on an optical ion sensing platform, in which a polymeric membrane ion-selective electrode (ISE) serves as not only a potentiometric transducer for ion activities in the sample solution but also a reference electrode for the potential-modulated release of enzyme from an iron-alginate-horseradish peroxidase (HRP) thin film modified working electrode. The ISE and working electrode are physically separated by a salt bridge. The dissolution of the HRP-embedded thin film can be triggered by the reduction of Fe³⁺, which is modulated by the potential response of the ISE to the target ion in the sample. The released enzyme induces the oxidation of its substrate mediated by H₂O₂ to produce a visual color change. With this setup, an optical ion sensing platform for both cations (e.g., NH₄⁺) and anions (e.g., Cl^-) can be obtained. The proposed platform provides a general and versatile visual-sensing strategy for ions and allows optical ion sensing in colored and turbid solutions.