Potentiometric Solid-Contact K⁺ Ion-Selective Electrodes Based on the KMnFe(CN)₆ Transducer

Solid-contact ion-selective electrodes (SC-ISEs) are typically constructed using ion-selective membrane (ISM)-based configurations. However, such structures often suffer from water-layer formation and the weak mechanical stability of the ISM. Herein, we report an ISM-free K⁺-SC-ISE based on a Prussian blue analogue transducer, KMnFe(CN)₆, eliminating the need for a conventional ionophore-based ISM layer. KMnFe(CN)₆ was synthesized via a one-step citrate-assisted co-precipitation method. The material functions as a bifunctional transducer, in which the open framework structure with ion-transport channels enables selective K⁺ recognition, while the redox-active Mn centers facilitate ion-to-electron transduction. The fabricated KMnFe(CN)₆-based K⁺ sensor exhibits a near-Nernstian response with a sensitivity of 52.3 ± 1.0 mV dec^-1 and a rapid response time of 25 s. The linear range and limit of detection were determined to 10^-4 to 10^-1 M and 5.8 × 10^-5 M, respectively. The sensor also demonstrates selectivity to representative interfering ions, with log K_ij of -2.39 ± 0.12 (Na⁺), -2.86 ± 0.09 (Li⁺), -3.06 ± 0.09 (Ca²⁺), -2.74 ± 0.12 (Mg²⁺) and -0.95 ± 0.08 (NH₄⁺). By eliminating the ISM layer, the water-layer effect is effectively avoided, resulting in excellent long-term stability with a potential drift of 57.2 ± 6.1 μV h^-1 over 7 days. The sensor was further applied to the analysis of K⁺ in real lake water samples, where the measured concentration showed good agreement with ion chromatography results. This work provides an ISM-free SC-ISE strategy for ion analysis in water environments.