Portable smartphone platform utilizing dual-sensing signals for visual determination of wide concentration ammonium in real samples

Ammonium (NH4+) is crucial for plant growth as well as various industrial processes, the accurate and field measurement of the NH4+ level is of great significance for agricultural precision fertilization and water quality monitoring, but excessive amounts of ammonium can lead to eutrophication of aqueous environments. Herein, the ability of the proposed probe solution and test paper to colorimetric change towards high-concentration NH4+ and ratiometric fluorescence response to low-concentration NH4+ has been developed for visual detection of ammonium in real samples. Specifically, high concentration of NH4+ (0.5-7 mM) could interact with the o-phthalaldehyde (OPA) in presence of K2SO3, resulting in the colorimetric transition from colorless to wine-red with increasing the NH4+ concentration. Subsequently, the addition of the red-emitting CdTe QDs (R-QDs) con-structed the OPA/K2SO3/R-QDs ratiometric fluorescent probes for low-concentration NH4+ (less than 1 mM) sensing with a limit of detection (LOD) down to 18 nM, accompanied by a distinct fluorescence color shift from red to blue under UV light. Furthermore, the visualization detection integrated with smartphone was converted to data information (RGB value) through a Color Recognizer APP and successfully used for visual quantitative identification of NH4+. Actually, the proposed probe solution and test paper have been applied to the determi-nation of NH4+ in soil extracts and wastewater samples. Compared with the standard methods, this established strategy can supply stable and reliable detection results, and allow fast sensing at room temperature, exhibiting great application potential for visual on-site detection of NH4+ at different concentration ranges in environmental samples.

Automated summary

A new probe solution and test paper have been developed for visual detection of ammonium, which can perform colorimetric change for high-concentration ammonium and ratiometric fluorescence response for low-concentration ammonium. It has great application potential for on-site detection of ammonium in environmental samples.