Design and fabrication of dual functional sulfur nanodots with reducibility and fluorescence properties for sensitive and selective analysis of metal ions in environmental water samples

Reduced-state sulfur nanodots (r-SDs) with reducibility, satisfactory fluorescence properties and high stability were synthesized for the sensitive and selective analysis of metal ions, including Hg2+, Fe3+ and Cu2+. The reduction property of SDs was discovered for the first time, and the detailed reduction mechanisms of r-SDs were investigated. The reducibility of r-SDs exhibits the in situ reduction of Au3+ to Au nanoparticles (AuNPs/r-SDs) and also the reduction of Fe3+ and Cu2+ to Fe2+ and Cu+, respectively. On the one hand, selective analysis of Hg2+ can be achieved through dual-mode fluorescence and colorimetric probes of thymine-modified AuNPs/r-SDs (T-AuNPs/r-SDs) on account of the special bonding of T-Hg2+-T, as verified by density functional theory simulations. Furthermore, the fluorescent probes r-SDs-1, 10-Phenanthroline (r-SDs-Phen) and r-SDs-neocuproine (r-SDs-Nc) were applied to the quantitative analysis of Fe3+ and Cu2+ by the specific recognition of Phen-Fe2+ and Nc-Cu+ after the reduction of Fe3+ and Cu2+ to Fe2+ and Cu+ by r-SDs. The linear ranges of T-AuNPs/r-SDs, r-SDs-Phen, and r-SDs-Nc for Hg2+, Fe3+, and Cu2+ were 5.0–70.0 nmol/L (fluorescence), 10.0–500.0 nmol/L (colorimetric), 1.0–100.0 μmol/L, and 1.0–50.0 μmol/L, with limits of detection of 0.82 nmol/L, 4.05 nmol/L, 0.36 μmol/L, and 0.56 μmol/L, respectively. In addition, good recoveries for practical applications in environmental water samples can be obtained to confirm the analysis accuracy. The successful results provided a new idea for the application of nanomaterials with reducibility and fluorescence properties in analytical chemistry.