Beyond Nonactin: Potentiometric Ammonium Ion Sensing Based on Ion-selective Membrane-free Prussian Blue Analogue Transducers.

The determination of ammonium ions (NH) is of significance to environmental, agriculture, and human health. Potentiometric NH sensors based on solid-contact ion selective electrodes (SC-ISEs) feature point-of-care testing and miniaturization. However, the state-of-the-art SC-ISEs of NH during the past 20 years strongly rely on the organic ammonium ionophore-based ion selective membrane (ISM), typically by nonactin for the NH recognition. Herein, we report a Prussian blue analogue of copper(II)-hexacyanoferrate (CuHCF) for an ISM-free potentiometric NH sensor without using the ionophores. CuHCF works as a bifunctional transducer that could realize the ion-to-electron transduction and NH recognition. CuHCF exhibits competitive analytical performances regarding traditional nonactin-based SC-ISEs of NH, particularly for the selectivity toward K. The cost and preparation process have been remarkably reduced. The theoretical calculation combined with electrochemical tests further demonstrate that relatively easier intercalation of NH into the lattices of CuHCF determines its selectivity. This work provides a concept of the ISM-free potentiometric NH sensor beyond the nonactin ionophore through a CuHCF bifunctional transducer.