N,S-co-doped carbon dots (N,S-CDs) were synthesized via a single-step solvothermal process by using sodium lignosulfonate and p-phenylenediamine as carbon/nitrogen/sulfur sources. The N,S-CDs have an average diameter of 2.02 ± 1 nm and display green fluorescence, with excitation/emission peak wavelengths at 380/540 nm for optimal fluorescence. Fluorescence is excitation wavelength-dependent and stable in aqueous salt solutions. The fluorescence of the N,S-CDs is selectively quenched by Fe(III) and Ag(I) ions. These ions can be quantified by fluorometry with a limit of detection of 1.7 μM for Fe(III) ions and 11.6 μM for Ag(I) ions. The N,S-CDs also undergo solvatochromism in that emission is green in water solution but blue in polar organic solvents such as ethanol or N,N-dimethylformamide. The color of fluorescence gradually shifts from green to blue when continuously increasing the fraction of organic solvent in water. Graphical abstract N,S-co-doped carbon dots (N,S-CDs) are synthesized by using sodium lignosulfonate and p-phenylenediamine as C/N/S sources. The N,S-CDs can sensitively detect Fe(III) and Ag(I) ions based on fluorometry, and can be used as a solvatochromic probe.
Keywords: Carbon dots; Color change; Dual ions detection; Fluorescent probe; Green emission; Metal ions; N,S-co-doping; Photoluminescence; Solvents; Solvothermal method.