Optimizing the microwave-assisted hydrothermal synthesis of blue-emitting L-cysteine-derived carbon dots

We synthesized carbon dots (CDs)from biocompatible L-cysteine, containing N and S, through an eco-friendly, facile hydrothermal reaction. These L-cysteine-derived CDs are produced by pyrolysis and carbonization after dehydration between NH ₂ and COOH groups of L-cysteine. To optimize the microwave-assisted hydrothermal synthesis of the blue-emitting L-cysteine-derived CDs, we varied the hydrothermal temperature, hydrothermal duration, synthesis pH, and L-cysteine concentration. We synthesized CDs from 17 to 133 mmol L ⁻¹ aqueous solutions of L-cysteine whose pH values were 1, 7, or 13 through hydrothermal treatment at 110–230 °C for 5–120 min using a microwave heater. The absorbance of transitions related to C[dbnd]C, C[dbnd]O, and C[dbnd]N bonds increased through hydrothermal treatment at increasing temperature and for increasing duration. At the same time, the photoluminescence (PL)intensity and PL quantum yield (QY)of blue emission from the CDs under near-UV excitation increased. We propose that higher temperature and longer duration promote the carbonization, oxidation, and doping of nitrogen into CDs, resulting in the formation of π levels, surface state levels related to functional groups, and N-doped levels, respectively. The absorbance and PL intensity of the CDs prepared at pH 7 were the highest among the pH values 1, 7, and 13. The protonation of NH ₂ groups at pH 1 and the deprotonation of COOH groups at pH 13 suppressed dehydration between the NH ₂ and COOH groups of L-cysteine. In contrast, rapid dehydration at pH 7 induced carbonization, contributing to the maximized absorbance and PL intensity. The absorbance, PL intensity, and PL QY increased through hydrothermal treatment at increasing L-cysteine concentration. Higher L-cysteine concentration promoted dehydration between molecules and the subsequent carbonization, improving the optical properties.