Ultra-wide range dual-mode sensor based on ternary phase diagram guided ZIF-8 for ammonium assay

A dual-mode sensor with a wide concentration range for NH₄⁺ detection was presented based on the confined effect of zeolitic imidazole framework-8 (ZIF-8) combined with o-phthalaldehyde (OPA). OPA@ZIF-8 was ingeniously designed by the ternary phase diagram to obtain the best detection performance. Benefiting from the confined effect of ZIF-8, OPA actively collided and reacted with NH₄⁺. This sensor can realize the wide concentration ranging over four orders of magnitudes for NH₄⁺ determination. The low concentrations of NH₄⁺ triggered a significant fluorescent response with a detection limit as low as 0.054 µM, while the high concentrations can produce a colorimetric signal from colorless to blue. The mechanism study showed that the confined effect of ZIF-8 effectively accelerates the reaction to reach equilibrium, addressing the issue of poor stability of reaction between OPA and NH₄⁺ reaction product. In the actual sample detection, the recovery reached 91.1%-97.9%, verifying the reliability of this method. This study not only provides a feasible method for detecting NH₄⁺ in various environments, but also offers insights into the research of sensing systems under the confinement effect regulation of nanomaterials.