A nitrogen-doped carbon quantum dots fluoroprobe for sequential detection of Hg2+ and biologically important biothiols: Its practical applications in biological, food and environmental samples.

A new N-doped carbon quantum dots (N-CQDs) fluorescent probe was synthesized via a solvent-free solid-phase carbonization method. The sensing material was characterized using various instrumental techniques, including Fourier Transform Infrared (FTIR), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Dynamic Light Scattering (DLS). The designed probe selectively detected Hg2+ ions in an ON-OFF manner and biothiols [Cysteine (Cys), Homocysteine (Hcy) and Glutathione (GSH)] in an OFF-ON mode in pure water, emitting a distinct blue-green fluorescence colour. The N-CQDs probe sensitively and selectively detected Hg2+ ions in ultrapure water as the sensing medium. The detection and quantification limits for Hg2+ ions were calculated from the standard calibration graph as 19.16 and 63.87 nM, respectively. The LOD/LOQ for Cys, Hcy and GSH were 34.81/116.35 nM, 29.67/98.91 nM, and 29.67/98.91 nM, respectively. The validation studies (selectivity, sensitivity, linearity range, precision, repeatability, intermediate precision and robustness) were performed for Hg2+ ions sensing. Finally, the N-CQDs sensor was successfully applied to real sample applications in water, food, living cells and fish samples with satisfactory results. These results demonstrated that the N-CQDs-based fluorescent sensor is well-suited for real samples with minimal interference from coexisting metal ions. In addition, the N-CQDs probe has been effectively employed in the development of cotton swabs, paper-based test kits, and a smartphone-integrated platform for detecting these analytes in real samples.