Development of a pH-Responsive, SO₄^2--loaded Fe and N co-doped carbon quantum dots-based fluorescent method for...

A novel sulfate-loaded iron-nitrogen co-doped carbon quantum dots (SO₄^2--CQDs)-based fluorescent method was synthesized by the facile and environmentally friendly pyrolysis of persimmon frost (carbon source) and (NH₄)₂Fe(SO₄)₂·6H₂O. After SMMC-7721 cells were incubated with the SO₄^2--CQDs for 24 h, more than 95% of the cells remained viable, even at a high concentration of the SO₄^2--CQDs, indicating excellent biocompatibility and low toxicity. In addition, it was able to be taken up by the cells to emit their bright blue fluorescence after excitation at 365 nm, indicating suitable cell permeability. The SO₄^2--CQDs also exhibited a unique response to changes in pH, which was applied in the detection of OPs by relying on the production of acetic acid from the hydrolysis of acetylcholine (ACh) by acetylcholinesterase (AChE), which decreased the pH and engendered an increase in the fluorescence of the SO₄^2--CQDs; however, the inhibition of AChE by glyphosate resulted in little influence on fluorescence intensity due to the lack of acetic acid produced. This mechanism was the basis for the development of a sensitive assay for the detection of glyphosate. The resulting assay had a limit of detection of 0.066 ng/mL. Furthermore, the method was successfully applied for the precise and accurate monitoring of the concentration, distribution, and variation of glyphosate residues in chives and cultivated soil. Therefore, the proposed method was anticipated to provide a promising alternative for other detection methods to enable the reliable analysis of OPs in food products.