A ternary reaction triggering fluorescence/colorimetric dual-mode signals for visual detection of ammonium nitrogen...

Ammonium (NH₄⁺) is one of the most commonly used nitrogen fertilizer in agricultural production and plays an important role in plant growth and ecological nitrogen cycling. Therefore, it is crucial to develop a real-time and accurate method for determining the content of NH₄⁺. Herein, we reported a dual-mode sensor for the visual and quantitative assay of NH₄⁺ by utilizing a ternary reaction of ammonium with 3,4-Thiophenedicarboxaldehyde (TDA) and sulfite. Specifically, the colorimetric mode of TDA/Na₂SO₃ based sensor for high NH₄⁺ concentrations (0.25-5.0 mM) displayed a color change from colorless to orange-yellow under natural light as the ammonium concentration increased. Subsequently, the addition of red emission CdTe QDs (R-QDs) constructed a ratiometric fluorescent probe for determination of low-concentration ammonium (from 0 to 0.75 mM), accompanied by the fluorescence color change from red to orange and then to yellow under UV light. Furthermore, the proposed probe solution and the corresponding test paper utilizing colorimetric and fluorescence modes have good anti-interference and high sensitivity, and the limit of detection (LOD) value is down to 24 nM in ratiometric fluorescence mode. Particularly, the test paper combined with the smartphone color recognizer was developed for on-site and real-time detection of NH₄⁺. The assay results were consistent with those obtained using the standard method, exhibiting the potential for visual and on-site detection of NH₄⁺ at different concentration ranges in environmental water and soil samples.