Decoding Carbon Dot Purity by Nuclear Magnetic Resonance

Carbon dots (CDs) have attracted increasing attention in recent years and have been widely explored in many fields. However, several challenges still limit their further development, particularly the unclear atomic structure and fluorescent mechanisms. Addressing these issues requires a thorough purification of crude CD products followed by reliable purity assessment. In this work, we combine dialysis and NMR analysis to evaluate the effectiveness of purification and consequent purity of CDs. Two representative CDs, citric acid-ethylenediamine-derived CDs and citric acid-p-phenylenediamine-derived CDs were synthesized via hydrothermal treatments. The crude products were first filtered and aliquoted for dialysis. Collected retentates were characterized by microscopy and spectroscopy techniques. With increasing dialysis time, the mass of retentates gradually decreased reaching a steady state. In the NMR spectra, sharps peaks gradually attenuated and eventually disappeared, indicating progressive removal of impurities. Meanwhile, the ratio of integrated broad peaks to sharp peaks increased and reached a plateau with prolonged dialysis. A linear correlation was observed between CD purity and the integrated peak ratios. This work establishes a standardized and (semi-)quantitative NMR-based methodology for CD purity assessment, enabling a systematic differentiation and relative quantification of CDs and coexisting small molecular species, thereby addressing a key limitation of previous analyzes.