New Strategy For The Development Of Fluorescent Carbon Dots And Their Applications

Objectives:（1）To design and development of a purity method which can greatly increase the quantum yield（QY）and the purity of aggregation-caused quenching blue carbon dots（NCDs）.NCDs with simple preparation method and high QY were developed.NCDs was effectively used for the highly sensitive detection of tetracycline antibiotics in milk and urine,and for the determination of trace Cu²⁺in tap water.（2）To design and peparation of aggregation-caused quenching sulfur doped orange carbon dots with double emission（O-FCDs）and non-sulfur doped aggregation quenching blue fluorescent carbon dots with single emission（B-FCDs）.The mechanism of fluorescence color change caused by sulfur doping was studied and the O-FCDs were applied to residual methyl blue sensing in fish and cell imaging.（3）To design and preparation of aggregation-induced emission fluorescent carbon dots（AIE-CDs）in blue,green,yellow and orange,and the mechanism of fluorescence tunability of AIE-CDs was studied in detail.AIE-CDs were used for double anti-counterfeiting and information encryption of acid control switch.The endocytosis mechanism into the cytoplasm and the application for rapid cytoplasmic imaging of AIE-CDs were studies.Methods:（1）Blue fluorescent NCDs was prepared by a simple top-down solvothermal method by using oil red O as carbon and nitrogen sources,and formamide as reaction solvent.NCDs with high quantum yield（QY）and high purity were obtained by optimizing various factors affecting QY（including the amount of precursors,reaction temperature and reaction time,etc.）,combined with liquid-liquid extraction and dialysis,Transmission electron microscopy（TEM）,high-resolution transmission emission microscopy（HRTEM）,Fourier transform infrared spectroscopy（FT-IR）,X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）were used to study the external morphology,internal structure,the types and contents of the elements,and the types of functional groups of NCDs.The optical properties of NCDs were studied by using UV-vis and fluorescence analysis.Then,NCDs was used in the sensitive detection of Cu²⁺and tetracycline antibiotics.The quenching mechanism of NCDs was evaluated by UV-vis detection,fluorescence lifetime measurement and Stern-Volmer equation analysis.Finally,NCDs gel was prepared for the visual detection of trace Cu²⁺in tap water.（2）Orange fluorescent carbon dots（O-FCDs）with double emission wavelength and blue fluorescent carbon dots B-FCDs with single emission wavelength were synthesized by using 1,3-dihydroxynaphthalene which only containing C,H and O elements as precursor and sulfuric acid（S-doped）-ethanol solution or ethanol as solvent.In order to obtain FCDs fluorescence powders with high QY and high yield,two kinds of FCDs were purified by ethanol extraction-water precipitation method.The structures and optical properties of two kinds of FCDs were characterized by TEM,HRTEM,FT-IR,UV,FL,XPS and atomic force microscopy（AFM）.By comparing the structure,functional groups and optical properties of O-FCDs and B-FCDs,the effect of sulfur doping on the fluorescence color of FCDs was predicted.In addition,O-FCDs can be used for methyl blue sensing.The sensitivity and stability of the sensor and the fluorescence quenching mechanism of methyl blue on O-FCDs were studied systematically.Besides,application of O-FCDs based sensor in fish samples was studied systematically.Finally,the toxicity and biocompatibility of the two FCDs were investigated,and the cell imaging on cholangiocarcinoma cells was performed.（3）By using the spiral crystal violet with high symmetry as carbon and nitrogen source,sulfuric acid-aqueous solution as the reaction solvent,blue,green,yellow and orange AIE-CDs were prepared by controlling the content of sulfuric acid in solvent,and regulating the reaction temperature and reaction time.By means of TEM,HRTEM,laser Raman spectroscopy（Raman）,AFM,FT-IR,XPS,UV and fluorescence analysis,the external morphology,internal structure,types and contents of elements,and the types of functional groups of AIE-CDs were studied.By comparing the characterization results of four-color AIE-CDs,the effects of sulfuric acid dosage,reaction temperature and reaction time on the structure,functional group properties and fluorescence properties of AIE-CDs were studied systematically.Ascribed to the special aggregation luminescence and p H response properties of four-color AIE-CDs,they were further used for anti-counterfeiting and information encryption.Besiedes,because of the high QY of blue AIE-BCDs,they were used for the imaging of tumor cells.The imaging site of AIE-BCDs was studied by DAPI colocalization experiment with nuclear imaging.By optimizing the imaging conditions,the optimal imaging time and the concentration of AIE-BCDs for cell imaging were studied.Finally,the entosis mechanism of AIE-BCDs in melanoma cells was investigated by the control variable method.Results:（1）Blue fluorescent NCDs were successfully prepared by using oil red O as aprecursor through a simple one-step solvothermal method for the first time.By liquid-liquid extraction and dialysis,the QYs of the purified NCDs were increased from 33%to 68%for the first time.The purification was based on the solubility of NCDs and over-carbonized substances,small molecular fluorescent substances and oil red O（raw materials）in formamide and dichloromethane.The characterization results showed that the NCDs was a granular material with obvious graphene lattice stripes with average size of 4.5 nm.The surface of NCDs was rich in various functional groups such as N-H,O-H,-CO-NH-and C-O.The existance of these functional groups can not only improve the luminescence performance,but also increase the hydrophilicity and stability of NCDs.Furthermore,NCDs can be used as a multifunctional sensing platform for Cu²⁺and tetracycline antibiotics.Compared with other co-existing metal ions and antibiotics,the platform showed a sensitive responses to Cu²⁺and tetracycline antibiotics.In detail,NCDs showed high selectivity and sensitivity to the fluorescence response of Cu²⁺in the concentration range of 10 n M-50μM,and the limit of detection（LOD）of Cu²⁺was 4 n M.NCDs also showed high sensitivity to tetracycline antibiotics with LODs ranging from 50 p M-100 n M.In addition,the fluorescence quenching mechanisms of Cu²⁺and tetracycline antibiotics on NCDs were IFE and static quenching.Finally,the fluorescence sensing platform based on NCDs was successfully applied to the detection of Cu²⁺and tetracycline antibiotics in actual samples,with satisfactory results.（2）Orange fluorescent carbon dots（O-FCDs）with dual emission wavelength and blue fluorescent carbon dots（B-FCDs）with single emission wavelength were successfully prepared by using 1,3-dihydroxynaphthalene as the precursor through adjusting the content of sulfuric acid in ethanol.Combined with ethanol extraction-water precipitation method,B-FCDs and O-FCDs with high yields（9.0%and21.3%）and high quantum yield（QYs,43.0%and 13.7%）were obtained.The structure and optical properties of O-FCDs and B-FCDs were studied by TEM,AFM,Raman spectroscopy,FT-IR,XPS,UV-vis and fluorescence analysis.The results showed that sulfuric acid promoted the carbonization and oxidation of the precursors during the reaction.Compared with B-FCDs,O-FCDs had a narrower lattice spacing and band gap,indicating that sulfur doping played an important role in fluorescence tuning.Moreover,O-FCDs had good sensitivity to methyl blue,and the linear response range was 0.0 5-100μM with a LOD of 20 n M.These results were satisfactory for the detection of methyl blue in real fish samples.In addition,two FCDs showed good biocompatibility and negligible cytotoxicity,while O-FCDs showed better cell imaging effect than B-FCDs.Therefore,as a sensor platform,O-FCDs has broad application prospects in methyl blue detection and biological imaging.（3）Blue,green,yellow and orange fluorescent AIE-CDs were successfully prepared by adjusting the content of sulfuric acid in the reaction solvent,reaction temperature and reaction time.Various characterization and analysis results showed that sulfuric acid mainly played the role of dehydration,carbonization and S-doping in the reaction.When sulfuric acid was added,the carbonization degree of the precursor and the size of AIE-CDs increased,while the slit decreased,resulting in the redshift of the fluorescence.With the increase of the content of sulfuric acid,pyrrole was decomposed into amino nitrogen.The content of pyrrole was decreased,the content of amino nitrogen was increased,the band gap was decreased,resulting in the continuely redshift of the fluorescence of AIE-CDs.As a conquesce,AIE-CDs with green,yellow and orange fluorescence were obtained.In addition,with the increase of reaction time and reaction temperature,the carbonization degree and the surface defect states of increased,and the fluorescence of AIE-CDs gradually blueshifted,and thus Blue-emitting AIE-BCDs with the QY up to 43%were gained.By using the acid sensitivity,adjustability and AIE characteristics of AIE-CDs,the anti-counterfeiting and information encryption systems developed.These systems had high security and commercial value.Cell imaging studies showed that the concentration of blue AIE-BCDs showed little cytotoxicity in the concentration range of 2.5 to 1000μg·m L^-1.Besides,the results of cell imaging showed that the optimal imaging time of cell imaging was 5min and the optimal imaging concentration of AIE-BCDs was 50μg·m L^-1.AIE-BCDs entered the cell mainly through reticular protein-mediated endocytosis and distributed in the cytoplasm.Conclusions:（1）By using oil red O as a precursor,NCDs was successfully prepared by a simple one-step solvothermal method.After liquid-liquid extraction,the QY of NCDs reached to 68%.The blue fluorescent NCDs can be used as a fluorescence sensor with high selectivity,high sensitivity,wide detection range and low LODs for tetracycline antibiotics and Cu²⁺.In addition,the gel which was made up of NCDs can be used for the on-site visual qualitative analysis of Cu²⁺in water by using a portable ultraviolet lamp.The NCDs based sensing platform had been successfully applied to the detection of Cu²⁺and tetracycline antibiotics in actual samples with satisfactory results.These all results indicated that NCDs has great potential in the detection of Cu2+ and tetracycline antibiotics.（2）Using 1,3-dihydroxynaphthalene as the precursor and sulfuric acid as the single variable,two kinds of high-yield and high-QY FCDs（B-FCDs and O-FCDs）were successfully prepared by one-step solvothermal method combined with ethanol extraction-water precipitation method.The introduction of sulfuric acid significantly increased the oxidation degree and graphitization degree of O-FCDs,reduced the lattice spacing and narrowed the band gap,and thus the fluorescence color of O-FCDs changed from blue（B-FCDs）to orange.O-FCDs can be used for proportional ratio fluorescence detection of methyl blue.In addition,OFCDs also has great potential applications in cell imaging.（3）AIE-CDs with blue,green,yellow and orange fluorescence were prepared by using crystal violet as the precursor and aqueous solution with different concentration of sulfuric acid as solvent at different temperatures and reaction times.The mechanisms of the influence of sulfuric acid concentration,reaction time and temperature on the change of the fluorescence color were analyzed through various characterization.The discovery of these fluorescence mechanisms provided a reasonable explanation for the change of fluorescence behavior of AIE-CDs.The development of this synthesis strategy provides a new way to prepare other AIE-CDs with long emission wavelength and high QY.In addition,the successful preparation of polychromatic acid-sensitive fluorescent AIECDs is of great significance for anti-counterfeiting,information encryption and rapid cytoplasmic imaging.