Synthesis of nitrogen-doped carbon dots from aromatic amino acids

Nitrogen-doped carbon dots (NCDs) were synthesized from amino acids via the hydrothermal method under an acidic condition. L-histidine, L-phenylalanine, L-tyrosine, L-tryptophan, and glycine were used as a precursor which NCDs synthesized from those precursors were labeled as His-NCDs, Phe-NCDs, Tyr-NCDs, Trp-NCDs, and Gly-NCDs, respectively. Spectroscopic and microscopic characterizations were operated to investigate optical properties, chemical structures, and morphologies of NCDs. The results show that all NCDs show a spherical shape with an average diameter of 4.11±1.20 nm. Their energy gap between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) obtained from fluorescence spectroscopy can be used to classify NCDs into two groups. The first group is Gly-NCDs and Phe-NCDs which their HOMO-LUMO energy gap is ⁓5 nm. Furthermore, the other group is His-NCDs, Tyr-NCDs, and Trp-NCDs which their HOMO-LUMO energy gap is ⁓4 nm. Gly-NCDs show lower content of surface functional groups than other NCDs. From the HOMO-LUMO energy gap and the low content of surface functional groups of Gly-NCDs, their fluorescence emission center might be a core state. The fluorescence emission center of Phe-NCDs was assumed to be like Gly-NCDs due to their similar HOMO-LUMO energy gap. His-NCDs, Tyr-NCDs, and Trp-NCDs show higher content of surface functional groups. Hence, the fluorescence emission center of His-NCDs, Tyr-NCDs, and Trp-NCDs was assumed to be a surface state. ID/IG values of NCDs were obtained from Raman spectra. The results show that NCDs synthesized from aromatic amino acids, except Tyr-NCDs, show significantly lower ID/IG than Gly-NCDs. The aromatic sidechain of precursors might cause higher graphitic content in the carbon core of NCDs. However, Tyr-NCDs show the highest ID/IG. A larger defect area in the graphitic carbon core of Tyr-NCDs might be altered by the steric effect of a hydroxyl group during polymerization and carbonization.