Photoluminescence properties of L-cysteine-derived carbon dots prepared in non-aqueous and aqueous solvents

Fluorescent carbon dots (CDs) can be produced from amino acids such as L-cysteine by an intermolecular dehydration reaction between NH₂ and COOH groups, followed by carbonization. This dehydration reaction may be hindered in aqueous media because the amino acid molecules are surrounded by water; therefore, we used a non-aqueous reaction media to facilitate the dehydration reaction. In the present study, L-cysteine was heated at 230 °C for 30 min in a non-aqueous solvent, diphenyl ether, which has a high boiling point of 258 °C, to yield CDs-NA. For comparison, we also hydrothermally prepared CDs-A from L-cysteine dissolved in water, with the use of an autoclave and microwave heating system under the same temperature and time conditions. The maximum photoluminescence (PL) intensity of the water dispersion of CDs-NA was 1.5 times as high as that of the CDs-A. The absolute PL quantum yield (QY) of CDs-NA was 4.2%, which was 1.4 times as high as that of CDs-A, 3.0%. Characterization of the elemental compositions, particulate properties, and chemical bonding states confirmed that the non-aqueous solvent not only facilitated CD formation via the dehydration reaction but also contributed to the formation of sulfonic acid groups by oxidation of thiol groups. The rapid particle growth and formation of the additional functional groups might contribute to the increased PL intensity and absolute PLQY of CDs-NA. Through the optimization of the synthesis conditions in a non-aqueous solvent, the absolute PLQY of CDs-NA was increased from 4.2% to 10.2%.