Research On Synthesis And Application Of Biomass Carbon Quantum Dots

Carbon Quantum Dots(CQDs) are emerging “zero-dimension” carbon-based materials, which are generally referred to spherical carbon nanoparticles smaller than 10 nm with unique fluorescence, photostability, water-solubility and biocompatibility. Compared withtraditional semiconductor QDs and organic dyes, CQDs not only possess high fluorescence, but also overcome their intrinsic drawbackssuch as poorphotostability and high toxicity, and thus have attracted intense interests in recent years. Specially, “green” CQDs synthesized from biomass materials are considered as ideal alternative of semiconductor QDs and organic dyes due to their abundance, simple and green synthesis process, and excellent optical properties.CQDs holdgreatpotential for various applications including bioimaging, disease therapy, photonics, catalysis, sensing and inks. This dissertationis focusing on the optical properties and applications of biomass CQDs synthesized via one-step hydrothermal carbonization. The obtainedbiomass CQDs are nanosized(<10nm), strongly fluorescent, water-soluble, photostable, low-toxic and biocompatible. Application of these biomass CQDs in cellular imaging, sensing and nanocomposites were explored. The effect of nitrogen-doping on the optical properties and the fluorescence quenching mechanism are also deeply discussed:1. Biomass CQDs were synthesized via one-step hydrothermal carbonization method frombiomass materials with different molecular structure, composition and size, including polypyranose derivatives, polyfuranoses and polyaminosaccharides, for investigatingthe structure-opticalproperties correlations. We found that the quantum yields(QY) of nitrogen-free biomas CQDs are less than 5%, whilst those fromnitrogen-containingbiomass(e. g. aminoglucose, oligochitosan and chitosan) are all more than 10%, the maximum is up to 16%. These results indicated that small amount of nitrogen-doping can significantly enhance the optical properties of biomass CQDs.In further investigation, multicolor CQDs were obtained by acidic oxidationthe sediment byproducts. By changing the acidic oxidation condition, blue to yellow light-emittingCQDs were obtained and their fluorescence properties were preliminarily discussed.2.To enhance the fluorescence properties of nitrogen-free biomass CQDs, alkali-soluble xylan was selected as starting materials and ammonia was chosen as nitrogen-doping agent in hydrothermal synthesisto investigate the effect of nitrogen-doping on morphology, composition and fluorescence properties. The results showed that nitrogen-doping can induce decrease of particle size(from 19 nm to 7 nm), formation of amide groups and enhancement of QY(from 2% to 13%). We also investigated their fluorescence properties with different ammonia concentrations, pH, solvents and doping agents, as well as their photoluminescence mechanism. Cellular toxicity experiment was used to test the biocompatibilityof biomass CQDs before/after nitrogen-doping. Biomass CQDs after nitrogen-doping were successfully applied in vitro bioimaging, performing good potentialin biomedicine application.3. Natural polyaminosaccharides chitosan was chosenas starting materials to synthesize fluorescent biomass CQDs with 15.26% quantum yield and their morphology, composition and fluorescence properties were investigated in detail. Fluorescence sensing systems in aqueoussolution were constructed by using chitosan CQDs based on their strong quenching responses for high valence metal ions including Fe3+,Mn7+ and Cr6+(in the form of CrO42- and Cr2O72-, respectively), in which their corresponding quenching constants are 5530, 20275, 5080 and 7270 M-1, respectively. Further “off-on” sensing system based on the reducibilityof Fe3+and selective quenching of chitosan CQDs was constructed for quantitative analysis ofreducing agentslike ascorbic acid, Ti3+ and glutathione. These results provided great potential for quantitative analysis of water pollutants and anti-oxidants.4. Nitroaromatic explosives with different substitutes wereselected as quenching agents to investigate the quenching mechanism of biomass CQDs. It was found that biomass CQDs have strong fluorescence response for nitrophenolsand the quenching behaviors are not influenced by the number of nitro groups, whilst weak response for nitrotoluenes but the quenching behaviors are inversely proportional to the number of nitro groups. The analysis on energy levels, spectral overlaps, fluorescence lifetimes and donor-acceptor distances deduced that the quenching behaviors should follow energy transfer mechanism but not electron transfer reported in most of literatures. Researches on quenching by organic dye molecules and spectroelectrochemistry further support the predominant role of energy transfer in fluorescence quenching. This work provides important evidences for research on photoluminescence mechanismof CQDs.5. As biomass CQDs possess abundant surface functional groups, they were applied to form fluorescent composite hydrogels with alginate and nanocellulose. Monosaccharides with different structures, including glucose, D-xylose and glucosamine, are selected as starting materials to synthesize biomass CQDs for investigating their enhancement effect on the two hydrogel systems.It is found that biomass CQDs not only maintain original fluorescence properties, but also effectively enhance the mechanical properties of hydrogels.These hydrogel systems were applied in Fe3+ and AuNPs sensing. The results demonstrated that hydrogel can serve as an excellent carrier of biomass CQDs, which is vital for the potential application of CQDs in drug delivery and disease diagnose.