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

The state-of-the-art solid-contact ion-selective electrodes (SC-ISEs) for NH 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 NH sensor based on a bifunctional transducer, specifically a framework of copper hexacyanoferrate (CuHCF). CuHCF serves as both an ion-to-electron transducer and an NH recognition element. The sensing mechanism involves electron transfer from the Fe redox center coupled with the ion transfer of NH 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 NH 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 NH sensing.