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

Abstract

The determination of ammonium ions (NH₄⁺) is of significance to environmental, agriculture, and human health. Potentiometric NH₄⁺ 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 NH₄⁺ during the past 20 years strongly rely on the organic ammonium ionophore-based ion selective membrane (ISM), typically by nonactin for the NH₄⁺ recognition. Herein, we report a Prussian blue analogue of copper(II)-hexacyanoferrate (CuHCF) for an ISM-free potentiometric NH₄⁺ sensor without using the ionophores. CuHCF works as a bifunctional transducer that could realize the ion-to-electron transduction and NH₄⁺ recognition. CuHCF exhibits competitive analytical performances regarding traditional nonactin-based SC-ISEs of NH₄⁺, 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 NH₄⁺ into the lattices of CuHCF determines its selectivity. This work provides a concept of the ISM-free potentiometric NH₄⁺ sensor beyond the nonactin ionophore through a CuHCF bifunctional transducer.