Metal oxide nanomaterial-modified ion-selective electrodes for detection of NH₄⁺ in aquaponic systems: electrochemical analyses, characterization, and sensing mechanism

Potentiometric NH₄⁺ sensors were developed by modification of ion-selective electrodes with different metal oxide nanomaterials forming solid-contact layers, including titania (TiO₂), manganese dioxide (MnO₂), and stannic oxide (SnO₂). A potentiometric method was used to determine the basic analytical parameters of the sensors. Furthermore, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and water layer tests were used to evaluate the electrochemical features of the electrodes. The results for the modified metal oxides and unmodified electrodes were compared, and the effectiveness of each material as a solid-contact layer on the electrode was evaluated. The best results were found for an electrode with a solid-contact layer of SnO₂ nanomaterial (GCE/SnO₂/NH₄⁺-ISM). The GCE/SnO₂/NH₄⁺-ISM electrode displayed a selective and fast response to NH₄⁺ ions (4.8 s) during the potential measurements. In this case, a slope of 47.17 mV/decade (R = 0.99), a linearity range of 1 × 10⁻⁷–1 × 10⁻² M, and a limit of detection of 1.18 × 10⁻⁸ M were obtained. This electrode exhibited good reproducibility, a high potential response, and stability, making it an attractive alternative for the development of effective SC-ISEs to detect NH₄⁺ in aquaponic nutrient solutions.