Abstract:

Blue luminescent carbon quantum dots(B-CQDs) were synthesized by a simple one-step hydrothermal method using lignin and m-phenylenediamine as precursors. Green luminescent carbon quantum dots(G-CQDs) were synthesized by nitric acid oxidation. The optical properties and structures characteristics of these two CQDs were characterized by UV absorption spectroscopy, fluorescence spectroscopy, TEM, FT-IR and XPS. The cytotoxicity and cell imaging properties of G-CQDs were also tested. The results showed that nitric acid played an important role in the synthesis of G-CQDs. The oxidation of nitric acid increased the graphite N content, deepened the graphitization, passivated the surface state, and red-shifted the fluorescence emission wavelength. The results of structural characterization showed that the prepared B-CQDs and G-CQDs were mainly composed of C, N and O elements. All of them had abundant hydrophilic groups such as —OH, —NH, C—O and —COOH on the surface, which were monodisperse in water with the average particle size were 1.3 nm and 2.5 nm, respectively. The results of optical property analysis showed that the excitation wavelengths of B-CQDs and G-CQDs were Ex=392 nm and Ex=446 nm, and the corresponding emission wavelengths were 488 nm and 514 nm, respectively. They exhibited excitation-dependent fluorescence emission behavior and excitation-independent emission behavior, respectively. The results revealed that the possible emission mechanism of G-CQDs belonged to the bandgap fluorescence emissions based on conjugated π-domains. The synthesized G-CQDs with excellent photoluminescence, stable fluorescence and low cytotoxicity could be applied to bioimaging of HeLa cells.

Key words: lignin, green carbon quantum dots, bandgap emission, cell imaging