Study On Synthesis Of Fluorescent Carbon Dots And Their New Applications

Carbon dots (CDs), a next generation green nanomaterial, are a promising alternative to fluorescent inorganic semiconductor nanocrystals and organic dyes due to their extraordinary optical, electronic, and biochemical properties, such as green synthesis, chemical stability, good dispersibility in water, low photobleaching, easy functionalization, good biocompability and low cytotoxicity. The outstanding property of CDs make them have great potential for bioimaging, photocatalysis, energy conversion, drug delivery, fluorescent ink and sensing applications. The research results on their synthesis, characterization, and applications have experienced exponential growth since 2004. However, as a new type of fluorescent carbonnanomaterials, there is a need for rich in-depth look at the following aspects: enhancing the quantum yields, looking insight into the origin of the photoluminescence and looking for the new applications.In this dissertation, we aim to propose the green synthsis of high quantum yield (QY) CDs based on different carbon sources and try to find out their new applications. The main contents are as follows:(1) A simple and straightforward synthetic approach for CDs with high QY of 87.6% is proposed by a one-pot hydrothermal chemical reduction with thiourea and urea. The obtained CDs are well dispersed in water, have excellent fluorescence property, present a uniform size and a graphite-like structure. It is shown that they have potential for applications as inks, as humidity sensors and as probes for the detection of inorganic salts in pure water. A synergistic reduction mechanism was also investigated using Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The findings show that using both thiourea and urea during the one pot synthesis enhances the luminescence of the generated carbon nanodots.(2) The nitrogen-doped CDs (NCDs) were synthetized by one-step hydrothermal carbonization of chitosan at a mild temperature and exhibit a strong fluorescence. The obtained NCDs show good water solubility, pH stability, and excellent fluorescence property. Moreover, the fluorescence of the solution can be quenched effectively by Fe3+ and then be sensitively turned on in the presence of AA via an "off-on" fluorescence response through the oxidation-reduction between Fe3+ and AA. A wide detection linear range for AA was found to be from 1μM to 200 μM with a detection limit of 0.18 μM. The switch sensor was also successfully applied to detect vitamin C tablets with satisfactory recovery ranging from 96.9 to 106%, exhibiting great opportunities for practical application in food and clinical system.(3) At ambient pressure and low temperature, CDs were synthesized via a simply solid-phase reaction of calcium gluconate without introducing any other additives. The obtained CDs show good water solubility, excellent chemical stability, salinity tolerance and excellent fluorescence property. The obtained CDs were used as biocompatible fluorescent probes for imaging of bacterial blight of soybean. This result offers a new strategy to fabricate monodispersed CDs with well-defined morphology and excitation-independent emissions properties for bacteria imaging.