Potentiometric Selectivity of Potassium Ion-Selective Electrodes Based on Silane- and Disiloxane-Bridged Bis(15-crown-5) Ionophores

Potassium-selective ionophores with high selectivity and chemical stability are essential for electrochemical sensing in clinical, biological, and environmental analysis. Here, three new silane- and disiloxane-bridged bis(15-crown-5) derivatives—two silane-bridged isomers bearing two octyl groups (1) or two 2-ethylhexyl groups (2), and a disiloxane-bridged derivative bearing four isopropyl groups (3)—were synthesized and evaluated as ionophores for poly(vinyl chloride) membrane ion-selective electrodes (ISEs). The ISEs based on 1–3 exhibited near-Nernstian responses to K+. Electrodes based on silane-bridged 1 and 2 showed higher K+ selectivity over Li+, Na+, Mg2+, and Ca2+ than the commercially available pimelate-bridged bis(benzo-15-crown-5) ionophore 4, and their selectivity was comparable to that of valinomycin for most of the metal cations examined. Notably, compound 2, bearing bulky 2-ethylhexyl groups, showed exceptional tolerance in strongly acidic media and maintained a nearly constant K+/Na+ selectivity (potentiometric selectivity coefficient, log kK,Na » −3.4) for more than 200 days under intermittent dry-storage conditions, whereas the ISE based on 4 showed a marked loss of selectivity after approximately 100 days. Disiloxane-bridged 3 also exhibited high K+ selectivity and acid tolerance. Overall, sterically optimized silane/disiloxane bridging units offer an effective strategy for designing durable, high-performance synthetic K+ ionophores.