Synthesis Of Heteroatom-co-doped Carbon Dots And Their Applications In Detection Of Metal Ions

Carbon dots(CDs) are an emerging new class of fluorescent nanomaterials with a size of generally less than 10 nm. They have excellent optical performance, the tunable excitation and emission behavior, high fluorescent stability, low toxicity and good biocompatibility, and have been widely used in the more and more fields, becoming a rising star of nano carbon materials. At present, one are committed to the study of the preparation methods of high quantum yield fluorescent CDs. Ionic liquids(ILs) generally possess organic nitrogen-containing bulky cation, not only as carbon source, but also as nitrogen source, and can also be used as good precursors for the preparation of N-doped photoluminescent carbon dots(N-CDs). Given all this, the preliminary explorations were carried on the preparation of co-doped fluorescent carbon dots with ionic liquids as raw materials in this thesis. The main research contents and innovations are as follows:1. A facile approach was reported to prepare nitrogen- and sulfur-co-doped fluorescent carbon dots(N/S-CDs) with sulfuric acid carbonization of 1-butyl-3-methylimidazolium 2-amino-3-mercaptopropionic acid(L-cysteine) salt([C4mim][Cys]) ionic liquid as a precursor. The optical properties of the aqueous N/S-CDs dispersion were analyzed with the UV-vis absorption and fluorescence spectra. The as-prepared N/S-CDs exhibited good solubility, high resistance to photobleaching, and excitation-dependent emission behavior. It was found that sulfur in the as-prepared N/S-CDs exists in two forms: thiophene sulphur and oxide-sulphur. The former comes from the precursor and the latter comes from sulfuric acid. Furthermore, the N/S-CDs were employed as a fluorescent probe for detecting Cu2+ in real water samples with satisfactory recovery.2. A simple approach was developed to prepare nitrogen-, and sulfur-co-doped fluorescent carbon dots(N/S-CDs) by one step hydrothermal treatment of [C4mim][Cys] and polyethylene glycol PEG2000 N. It was observed that the size of carbon dots became smaller with the extension of reaction time, and the amorphous carbon was gradually transformed into a crystal structure of carbon dots. Besides, a higher reaction temperature favors the formation of carbon dots with higher quantum yields. Meanwhile, the content of water also affected the quantum yield of CDs. The role of the doped sulfur atom on the optical performance was explored by comparing the optical properties and structures between two kinds of carbon dots(N-CDs and N/S-CDs). It was found that the highest quantum yield is owing to the synergistic effect of nitrogen–sulfur co-doping. Furthermore, the N/S-CDs were employed as a fluorescent probe for detecting Hg2+ in aqueous solutions, with high selectivity and sensitivity.3. A simple and green approach was developed to prepare water-soluble, nitrogen-, bromine-co-doped fluorescent carbon dots(N/Br-CDs) by one step hydrothermal treatment of 1-butyl-3-methylimidazolium bromine salt([C4mim][Br]) ionic liquid and polyethylene glycol PEG2000 N. The as-prepared N/Br-CDs exhibited high resistance to photobleaching, excitation-dependent emission behavior as well as the up-conversion photoluminescence performance. The quantum yield of N/Br-CDs reached to 51.7 %. Compared with the N/S-CDs(13.71 %) prepared using the same method, the quantum yield of N/Br-CDs was improved dramatically. It was found that the bromine atoms in the N/Br-CDs change the surface structure and promote the formation of the chromophore to some extent, and the functional modifications by PEG also play a promoting role. The developed approach would potentially open up a simple and environment-friendly route to prepare heteroatom-co-doped, high quantum yield carbon dots.