Study In Rapid Synthesis Of Carbon Dots And Their Applications In Pollutants Detection

Carbon dots(CDs) were a new class of carbon nanomaterials with sizes below 10 nm.Owing to their excellent fluorescence, low toxicity, good solubility, high photostability and superior chemical stability, CDs have widely been employed as fluorescence probes for determination of analytes. Up to now, tremendous effort has been made to synthesize CDs with excellent characters through different ways or materials. However, many of them suffered from various drawbacks such as time-consuming procedures, low quantum yields,addition of strong acid, strong alkali or other toxic reagents,which limited the practical applications of CDs in analytical detection. Therefore, it is very necessary to develop simple, fast, green and low-cost method for preparation of CDs. Meanwhile, fewer studies have been published in which CDs-based fluorescence probes were used for determination of pollutants. The aim of this theis is to develop fast and environment-friendly method for preparation of CDs and apply the CDs to design novel fluorescence probes for determination of pollutants. Furthermore, a Fe3O4–graphite composite was obtained in trying to synthesize magnetic CDs by using graphite and we found that the composite can be used for degradation of levofloxacin.The main contents of this thesis are as follows:1. Nitrogen-doped carbon dots(N-CDs) with a quantum yield of 23.6% were synthesized by microwave pyrolysis of urea using diethylene glycol as the high boiling point reaction medium. The N-CDs were spherical and monodisperse with a size distribution between 1.5 to 5.5 nm. The N-CDs exhibited excellent water-soluble property and remarkable stability under extreme ionic strengths and light illumination. Thefluorescence of the N-CDs could be quenched by Fe3+ through the static quenching mechanism, but not by other common metal ions. On this basis, the N-CDs can be used as a facile sensing platform for label-free sensitive and selective detection of Fe3+ in a linear range of 1.6-333.3 μmol L-1, and the limit of detection was 0.45 μmol L-1 obtained at a signal-to-noise ratio of 3. Importantly, the N-CDs-based fluorescence probe was applied to the analysis of Fe3+ in drinking water and human serum samples and obtained satisfied results.2. A simple, fast and low-cost pyrolysis method was developed to prepare carbon dots(CDs) by using polyethyleneimine(PEI) as the precursor. The CDs have an average size about 2.1 nm. The fluorescence of CDs can be effectively and selectively quenched by metronidazole through the static quenching mechanism and inner filter effect. Based on this phenomenon, the CDs were employed as a highly sensitive and selective fluorescence probe for metronidazole detection. Under the optimal conditions, the linear range for metronidazole was 0.06-15 μg m L-1 with a detection limit of 20 ng m L-1. Finally, the fluorescence probe was applied for determination of metronidazole in milk samples. In all spiked levels, the recoveries of metronidazole are in the range of 96.7-102.2%.3. A facile and green route has been developed for the fabrication of fluorescent carbon dots(CDs). Wool and hair were chosen as the precursors to prepare CDs via a one-step microwave-assisted pyrolysis process and the obtained CDs were denoted as w-CDs and h-CDs, respectively. This method was much simpler and faster than previously reported methods for preparation of CDs based on biomass materials. The sole reagent used in the preparation process was distilled water, which avoid the complicated post-treatment process to purify the CDs. The fluorescence of the w-CDs could be quenched by silver nanoparticles(Ag NPs) based on inner filter effect(IFE). The presence of glyphosate could induce the aggregation of Ag NPs and thus result in the fluorescence recovery of the quenched CDs. Based on this phenomenon, we constructed a fluorescence system(CDs/Ag NPs) for determination of glyphosate. Under the optimized conditions, the linear range of this method was 0.025-2.5 μg m L-1 and the limit of detection was 12 ng m L-1.Furthermore, the established method was used for glyphosate detection in cereal samples and the recoveries were in the range of 89.5%-110.0%.4. Fluorescent CDs were prepared by using thiourea as the carbon source and diethylene glycol as the reaction medium. The CDs were obtained only within three minutes under microwave irradiation and mainly distributed in the range of 1.5-3.5 nm with an average size of 2.3 nm. The fluorescence of the CDs could be quenched by Cu2+ through the static quenching mechanism. On this basis, the CDs can be used as a fluorescence probe for detection of Cu2+ in a linear range of 0.2-25 μmol L-1, and the limit of detection was0.05 μmol L-1. The quenched fluorescence of CDs could be recovered with the addition of glyphosate based on a competitive affinity of Cu2+ between glyphosate and the functional groups on the surface of CDs, which provides a turn-on sensing strategy for determination of glyphosate. Under the optimal conditions, the linear range of the method was 0.03-10 μg m L-1 with a detection limit of 16 ng m L-1. Finally, this fluorescence probe was employed for the determination of glyphosate in environmental water samples with recoveries in the range of 93.3%-106.7%.5. The Fe3O4–graphite composite was prepared based on a one-step solvothermal process. The composite was used as a heterogeneous Fenton-like catalyst for the degradation of levofloxacin(LEV) in an aqueous solution. The results revealed that the composite exhibited excellent properties for the degradation and mineralization of LEV,achieving a nearly complete degradation of 50 mg L-1 LEV in 15 min and 48% of total organic carbon removal in 60 min under optimal conditions. The synergistic results of the graphite structure and Fe3O4 MNPs may contribute to the high catalytic activity of the composite. Compared with pure Fe3O4 MNPs, the composite has lesser iron leaching during the degradation of LEV and the degradation efficiency for LEV remained approximately 80% at the fifth recycling run.