Preparation And Application Of Highly Fluorescent Carbon Dots And Their Composites

Carbon dots?CDs?were accidentally discovered by Xu et al.in 2004.Owing to their small particle size,good aqueous dispersibility,multicolour wavelength tuned emission,photostability,low toxicity and high biocompatibility,they have been applied in many fields,including imaging,sensing,catalysis and photoelectronic devices,etc.There are still some unsolved problems that limit their development.First,the theory of the formation of CDs and the mechanism of their fluorescence are still unclear;second,it is difficult to effectively improve the quantum yield of CDs and regulate their properties;third,the applications of CDs need to be further explored.The present thesis focuses on the preparation and application of CDs for various purposes.First,different carbon sources and doping agents were explored to improve the quantum yield and tune the properties of CDs.Then a series of fluorescence sensors for particular substances were constructed.Finally,the CDs were combined with TiO₂ or SiO₂ to develop multi-functional materials.The details are as follows:1.CDs were firstly prepared with Hongcaitai as carbon source by hydrothermal method achieving quantum yield of 21% under optimal synthesis conditions which was much higher than other reported CDs prepared with natural precursors.The obtained CDs were divided into two parts based on their different solubilities in ethanol,CDs-A?ethanol soluble?and CDs-B?ethanol insoluble?.CDs-A containing more reductive hydroxyl groups were more sensitive towards ClO^-;CDs-B containing sulfur were more sensitive towards Hg²⁺.Thus,CDs-A and CDs-B were successfully applied to determine ClO^- in tap water and Hg²⁺ in river water,respectively.The results demonstrated that the purification of CDs on their different solubilities is necessary for better sensing performance.2.Highly fluorescent nitrogen doped carbon dots were prepared by using citric acid as the carbon source with the binary dopants,i.e.melamine and formaldehyde,named as MF-CDs.The co-dopants played a crucial role to enhance the fluorescence quantum yield of the MF-CDs,reaching 48%,in which melamine was the major nitrogen source,while formaldehyde was the co-dopant to regulate the state of nitrogen in the MF-CDs.The fluorescence of the MF-CDs was found to be quenched by Hg²⁺,and then recovered by halogen ions?X^-,X=Cl/Br/I?.Correspondingly,the “turn off” and “turn off-on” sensing systems were separately established to detect Hg²⁺ and X^-.In addition,the “turn off” ClO^- sensor and the “turn off-on” Fe²⁺ sensor were constructed in a similar way.This work provided a multi-functional carbon dots.3.Nitrogen and sulfur co-doped carbon dots?TF-CDs?were fabricated by hydrothermal method with citric acid as carbon source,thiourea and formaldehyde as doping agents.TF-CDs exhibited the quantum yield of 45%,much higher than 4% and 3% of carbon dots with only thiourea or formaldehyde doping.It was found that formaldehyde co-doping played an important role in improving quantum yield and regulating the properties of CDs such as the composition of surface functional groups and the fluorescent response to pH.The as-prepared TF-CDs demonstrated high selectivity and sensitivity towards tetracycline antibiotics.Using doxycycline as a representative target analyte,the fluorescence sensor based on TF-CDs provided the detection limit of 0.06 ?g/m L with good recovery of 97%-110% for milk samples.This work provided a new method for rapid detection of tetracycline antibiotics residues.4.The carbon dots??-CD-CDs?were prepared by hydrothermal method with citric acid and ?-cyclodextrin mixed solution as carbon source.The fluorescence of the as-prepared ?-CD-CDs could be selectively quenched by isoniazid,which was used to detect isoniazid in urine samples.The detection limit was 0.14 ?g/m L and the recovery was satisfactory?91%-109%?.5.N,S and P tri-doped carbon dots?NSP-CDs?were prepared by hydrothermal method with sulfonamide and phosphoric acid as precursors.The fluorescence of NSP-CDs was sensitive to pH and the concentration of Cl^-.NSP-CDs were successfully applied as efficient fluorescent probes for the detection of Cl^-in simulated sweat,with the recovery of 94%-110%,showing their potential in diseases screening and diagnosis.In addition,TiO₂-NSP-CDs composites were prepared.It was found that the NSP-CDs can enhance the photocatalytic activity of TiO₂ for the degradation of methylene blue under UV irradiation.The material acted as a promising candidate for sewage treatment and environmental remediation.6.Novel highly fluorescent carbon dots?Si CDs?were prepared with 3-aminopropyl trimethoxysilane as the carbon source and boric acid as the doping agent by hydrothermal method and were successfully decorated on the surface of mesoporous silica microspheres?SiO₂?,obtaining SiO₂-Si CDs.Depending on its high selective affinity to Au³⁺ and the relatively large surface area and pores,the SiO₂-Si CDs exhibited not only selective fluorescence detection of Au³⁺ in water matrix but also efficient removal of Au³⁺.In addition,the Au³⁺ adsorbed on the SiO₂-Si CDs can be in-situ reduced into small size and evenly dispersed gold nanoparticles by Na BH₄,showing high catalytic efficiency for reduction of 4-nitrophenol.The remarkable performance of SiO₂-Si CDs in detection,adsorption and catalysis demonstrated it as an efficient approach to develop the multifunctional materials.7.The contents of the first six chapters were summarized,and the limits and prospects were presented.8.The working principles and the selectivity tuning methods of CDs based sensors were summarized and prospected.