Carbon dots doped with nitrogen and sulfur and loaded with copper(II) as a turn-on fluorescent probe for cystein, glutathione and homocysteine

We report on the synthesis of fluorescent carbon dots doped with nitrogen and sulfur (N,S-CDs) via a hydrothermal process in pure water and starting from a mixture of alfalfa and garlic. Compared to N-doped CDs, the N,S-CDs exhibit a high fluorescence quantum yield (10 %) and a more longwave emission maximum (at 481 nm). The results also pave the way to tune the luminescence of CDs. The fluorescence of the N,S-CDs is quenched by Cu(II) ions but is recovered by addition of any of the biothiols cystein, glutathione or homocysteine. This turn-on effect was exploited to design a method for the quantification of these thiols in concentrations as low as 86 nM. The method was successfully applied to the determination of Cys in (spiked) human serum samples.