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 Kij 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.