Synthesis of sulfur-rich nitrogen dots from a single source precursor and its application in dual-mode sensing

In this work, a strategy of sulfur-rich doped nitrogen dots (S-Ndots) based on a single-source precursor (SSP) was developed and applied to sensing of Hg²⁺. Fluorescent S-Ndots were synthesized by microwave-assisted method using 2-azidothiazole as a single-source precursor, which served as carbon, nitrogen and sulfur source simultaneously. This SSP approach guarantee efficient core doping and improve the effect of doping in a molecular level, which is more efficient and simpler than the doping strategy with multiple precursors. The results showed that the as-synthesized S-Ndots have high sulfur component up to 24.6%, and their specific structure effectively promoted artificial peroxidase activity and coordination interaction. Interestingly, the fluorescence intensity could be quenched obviously and the peroxidase activity of the S-Ndots were selectively inhibited by Hg²⁺, then the S-Ndots were applied to the dual-mode sensing of Hg²⁺ with both fluorometric and colorimetric readout. The S-Ndots-based sensing assay with simplicity and rapidity showed high selectivity to Hg²⁺ with the detection limit of 0.1 μmol/L by fluorescence spectroscopy and a distinguishable change in the color was observed by the naked eye. In addition, the accuracy and precision were evaluated based on the quantitative detection of Hg²⁺ in industrial waste water samples with satisfactory results. Moreover, the S-Ndots with good biocompatibility were further explored for imaging of Hg²⁺ in A549 cells. As a novel member of nitrogen quantum dots family, the S-Ndots hold great promise to broaden its applications in environmental and biological sensing.