Carbon Dots-Based Fluorescent Sensor for Glyphosate Detection Via Fe³⁺-Mediated Fluorescence Quenching and Recovery

In this work, four different carbon dots (CDs) were synthesized by hydrothermal carbonization, each from a distinct low molar mass carbon source (ascorbic, citric, maleic, or succinic acid) in combination with the same nitrogen source (ammonium citrate). The carbon nanoparticles were thoroughly characterized using a range of techniques, including fluorescence spectroscopy, UV-Vis spectroscopy, Raman spectroscopy, transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The optical and structural characterization revealed that the choice of precursors influences the quantum yield of fluorescence (QY) and emission behavior, while yielding nanoparticles of similar sizes (1.7-2.8 nm). The CDs exhibited excitation-dependent emission, which is attributed to distinct energy levels arising from the presence of various surface functional groups, including hydroxyl, carboxyl, amine, and amide groups. Among the synthesized CDs, those prepared from succinic acid and ammonium citrate (SAC) exhibited the highest selectivity and sensitivity toward Fe³⁺ ions, with a detection limit (LOD) of 0.37 μM. This property was exploited to design an "off-on" sensor for the herbicide glyphosate. After a detailed optimization of parameters for glyphosate detection, which included the incubation time of ferric ions and glyphosate as well as the solutions pH, a LOD value of 0.59 μM was obtained, corresponding to concentrations significantly lower than the maximum allowable concentration by international standards. Finally, the developed sensor was applied to real tap water and urban stream samples, and the results showed its potential to be used in rapid, low-cost, and environmentally friendly tests, making it a promising alternative for monitoring the herbicide in aquatic environments.