Selective ion-ionophore complexation in a polymeric membrane is crucial to various sensing applications. In this work, we report on a novel voltammetric approach based on a thin polymeric membrane to determine the stoichiometry and overall formation constant of an ion-ionophore complex. With this approach, a ∼1.6 μm thick ionophore-doped membrane contacts an aqueous solution containing an excess amount of a target ion to facilitate voltammetric ion transfer across the membrane/water interface. Advantageously, the resultant thin-layer voltammogram shows no diffusional effect, which simplifies the theoretical modeling and quantitative analysis of the voltammogram. We predict theoretically that the complexation stoichiometry affects not only the peak current and peak potential of the thin-layer voltammogram, but also the symmetry of the peak shape with respect to the peak potential. Experimentally, a symmetric voltammogram ensures the formation of a 1:1 complex for a Na(+)-selective ionophore. By contrast, the asymmetric shape and peak current of voltammograms are used to demonstrate that a Ca(2+)-selective ionophore forms 1:3 and 1:2 complexes with calcium and magnesium ions, respectively. The complexation stoichiometry is needed to yield the formation constants that are consistent with those determined previously by potentiometry. In addition, both 1:2 and 1:1 complexes are voltammetrically observed with another Na(+)-selective ionophore, which was assumed to form only a 1:2 complex in previous potentiometric studies. The formation constants of both complexes are determined from a single voltammogram to reveal that the preceding formation of a 1:2 complex thermodynamically hampers the voltammetric observation of a 1:1 complex.

The capability to detect multianalyte ions in their mixed solution is an important advantage of voltammetry with an ionophore-based polymeric membrane against the potentiometric and optical counterparts. This advanced capability is highly attractive for the analysis of physiological ions at millimolar concentrations in biological and biomedical samples. Herein, we report on the comprehensive response mechanisms based on the voltammetric exchange and transfer of millimolar multiions at a thin polymeric membrane, where an ionophore is exhaustively depleted upon the transfer of the most favorable primary ion, IzI. With a new volt-ammetric ion-exchange mechanism, the primary ion is exchanged with the secondary favorable ion, JzJ, at more extreme potentials to transfer a net charge of |zJ|/nJ – |zI|/nI for each ionophore molecule, which forms 1:nI and 1:nJ complexes with the respective ions. Alternatively, an ion-transfer mechanism utilizes the second ionophore that independently transfers the secondary ion without ion exchange. Experimentally, a membrane is doped with a Na+- or Li+-selective ionophore to detect not only the primary ion, but also the secondary alkaline earth ion based on the ion-exchange mechanism, where both ions form 1:1 complexes with the ionophores to transfer a net charge of +1. Interestingly, the resultant peak potentials of the secondary divalent ion vary with its sample activity to yield an apparently super-Nernstian slope as predicted theoretically. By contrast, the voltammetric exchange of calcium ion (nI = 3) with lithium ion (nJ = 1) by a Ca2+-selective ionophore is thermodynamically unfavorable, thereby requiring a Li+-selective ionophore for the ion-transfer mechanism.