Synthesis Of Fluorescent Carbon Dots And Study On Theiroptical Properties

Fluorescent carbon dot(CDs) is a novel functional material because of their unique photoluminescence(PL) from near UV to near-infrared, good biocompatibility and outstanding physicochemical properties. They are used as a candidate for a wide range of applications, including biological and chemical sensing, bioimaging, drug delivery, light-emitting diodes(LEDs), peroxidase-like catalysts, sensitizers for solar cells, photocatalysts, etc. PL emission of CDs involve differents mechanisms, amone them, the surface structures and compositions of CDs are very important to ture their complicated luminescence mechanisms. Heteroatom doping is an effective method to adjust the structure and composition of CDs and thus enhances their optical properties. Single doped N-CDs and co-doped N/Si-CDs and N/S-CDs with excellent luminescent properties and good water solubility were synthesized via a one-step hydrothermal method using green raw materials at 200℃ for 24 h, in which folic acid is carbon and nitrogen sources, 3- aminopropyl trimethoxy silane(APTMS) is silicon source, sulfur powder is sulfur source, glycerol is active substance. The synthesized involves one-step condensation, polymerization and carbonization reaction.The as-synthesized N-CDs with the nitrogen content of 3.59% have good dispersion with an average size of 2.2 nm and exhibit strong violet PL with a quantum yield(QY) of 38%. In addition, the N-CDs show excitation-dependent PL behavior. The co-doped N/Si-CDs are nearly monodisperse with an average size of 2.45 nm and exhibit intense violet PL with a quantum yield(QY) of 46%. The nitrogen and silicon contents of N/Si-CDs are 10.4% and 4.6%, respectively. The other co-doped N/S-CDs with an average size of 2.75 nm present intense blue PL with a quantum yield(QY) of 43%. The nitrogen and sulfur contents of N/S-CDs are 9.6% and 5.4%, respectively. The doping amount of Si and S sources are a key factor affecting the PL properties of CDs. In comparison to N-CDs, the co-doped N/Si-CDs and N/S-CDs present better PL properties, excitation-independent PL behavior and increased quantum yields due to their uniform sizes and improved surface states due to the synergistic effects of dopants. At the same time, the functionalized surfaces of the co-doped CDs also render them outstanding stability and water-solubility.The N-CDs demonstrate a sensitive response to Cu2+ and Hg2+, and their readily enter the cytoplasm of Hela cells and exhibited no significant cytotoxicity. Both N/Si-CDs and N/S-CDs exhibit highly selective and sensitive for Fe3+, but the mechanism of the such quenching are different. It could be explained by the efficient PL quenching effects of Fe3+ on the N/Si-CDs, owing to the higher specific thermodynamic affinity of Fe3+ to surface N. The quenching mechanism of Fe3+ on the N/S-CDs may be attributed to a special synergistic reaction between Fe3+ and phenolic hydroxyl. The as-synthesized N/Si-CDs show a sensitive response to Fe3+ with a wide linear range of 0.01-45mM and a low detection limit is 3.8nm. The N/S-CDs present a wide linear range of 0.01-50mM with a detection limit as low as 3.5nm. Both co-doped CDs as a novel fluorescence probe present greatly potential in environmental and clinical Fe3+ monitoring and beyond.