Ion-selective-membrane-free high-pressure potentiometric ammonium ion sensing

The state-of-the-art solid-contact ion-selective electrodes (SC-ISEs) for NH₄⁺ primarily utilize organic carrier-based ion-selective membranes (ISM). However, they face challenges such as the water-layer effect at the SC/ISM interface and the weak mechanical strength of the ISM. In this work, we present an ISM-free, high-pressure potentiometric NH₄⁺ sensor based on a bifunctional transducer, specifically a framework of copper hexacyanoferrate (CuHCF). CuHCF serves as both an ion-to-electron transducer and an NH₄⁺ recognition element. The sensing mechanism involves electron transfer from the Fe redox center coupled with the ion transfer of NH₄⁺ within its framework channels. To further develop an all-solid-state sensor, we integrated a solid-contact reference electrode of silver/silver tetraphenylborate electrode. This all-solid-state NH₄⁺ sensor demonstrates Nernstian response sensitivity and comparable selectivity under 1 MPa pressure. Importantly, it avoids the generation of a water layer and exhibits long-term stability. This work highlights a concept for ISM-free high-pressure potentiometric NH₄⁺ sensing.