A highly sensitive dual-read assay using nitrogen-doped carbon dots for the quantitation of uric acid in human serum and urine samples.

A simple dual-read assay for uric acid (UA) was developed based on a combined ratiometric fluorescent and colorimetric strategy using nitrogen-doped carbon dots (N-CDs). The biosensor relies on the oxidation of UA by uricase to produce H2O2, which was then converted to •OH radicals by I-, resulting in the oxidation of o-phenylenediamine (OPD) to 2,3-diaminophenazine (DAP). In the presence of UA, the colorless biosensor system changed to yellow. Furthermore, the presence of DAP quenched the fluorescence emission of the N-CDs at 427 nm based on the inner filter effect (IFE). With increasing UA concentrations, the fluorescence intensity of the biosensor at 427 nm decreased but increased at 580 nm, demonstrating the ratiometric response. A strong linearity was observed between the fluorescence intensity ratio of DAP to N-CDs (I580/I427) and the corresponding UA concentration over the range 0.5-150 µM, and a limit of detection (S/N ratio of 3) of 0.06 µM was calculated. The dual-read assay was successfully employed in the quantitation of UA in human serum and urine samples, revealing its potential for measuring UA in clinical samples.

Development of a pH-Responsive, SO42--loaded Fe and N co-doped carbon quantum dots-based fluorescent method for highly sensitive detection of glyphosate.

A novel sulfate-loaded iron-nitrogen co-doped carbon quantum dots (SO42--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 SO42--CQDs for 24 h, more than 95% of the cells remained viable, even at a high concentration of the SO42--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 SO42--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 SO42--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.