Nitrogen-doped carbon quantum dots for sequential "ON-OFF-ON" fluorescence detection of Ag⁺ and L-cysteine in...

The construction of sensitive and selective analytical platforms for metal ions and biomarkers continues to be a prominent focus within analytical chemistry. In this work, nitrogen-doped carbon quantum dots (N-CQDs) were prepared and utilized as a fluorescent probe for the sequential determination of silver ions (Ag⁺) and L-cysteine (L-Cys). Under ultraviolet excitation, the as-prepared N-CQDs emitted stable blue fluorescence, which was markedly quenched upon the introduction of Ag⁺ due to the formation of a ground-state complex, thereby yielding an "ON-OFF" response. Subsequent addition of L-Cys led to the restoration of fluorescence intensity, attributed to the competitive interaction between L-Cys and Ag⁺, thus realizing an "OFF-ON" detection mode. The developed sensing system exhibited a linear relationship with Ag⁺ concentrations ranging from 0 to 300 µM, with a detection limit of 0.12 µM, and with L-Cys concentrations from 0 to 150 µM, giving a detection limit of 0.35 µM. Moreover, the platform demonstrated outstanding selectivity toward Ag⁺ and L-Cys over common interfering ions and amino acids, respectively. Successful application of the sensor in human serum samples further confirmed its practical feasibility, highlighting its potential utility in bioanalytical and environmental applications.