Green emitting N,S-co-doped carbon dots for sensitive fluorometric determination of Fe(III) and Ag(I) ions, and as a solvatochromic probe

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.