Optical properties of N- and S-doped carbon dots based on citric acid and L-cysteine

A comparative study of the absorption and fluorescent properties of carbon dots (CDs) obtained by microwave synthesis from citric acid and L-cysteine has been carried out. The sizes of synthesized CDs vary from 2 to 8 nm, with an average size of 3.5 nm. FTIR spectra confirm the presence of oxygen-containing (-OH and С=О), as well as -CN, -NH and -SH groups. It has been shown that for L-cysteine-doped CDs, the absorption spectrum additionally exhibits a band with a maximum at 350 nm, whereas for undoped CDs it was absent. For all investigated CDs, increase in the excitation wavelength led to the “red” shift of the fluorescence spectrum. The maximum fluorescence intensity is observed upon excitation at 350 nm. The highest fluorescence quantum yield (68%) was obtained for CDs with a citric acid and L-cysteine ratio of 1:0.5 and 1:1. The fluorescence lifetime of doped CDs also increases from 6.6 ns to 7.8 ns. It is shown that doping of CDs with N- and S-containing groups makes it possible to control their absorption and fluorescent properties.