Preparation,Properties And Application Of Multicolor Carbon Dots

Carbon dots,nearly quasi-spherical zero-dimensional emerging carbon-based nano-functional materials,were first discovered in 2004 during the process of purifying single-walled carbon nanotubes by arc discharge.Due to its heterogeneous atoms,which can be doped,surface functional groups can be modified,wide UV absorption band,low toxicity,biocompatibility,particle surface effects,tunneling effects and other excellent optical and electrical properties,it had a wide range of applications in metal ion detection,solar cells,biomarkers,cell imaging,optoelectronic devices and other fields.After more than ten years of development,many of these areas had achieved many breakthroughs and innovations.However,most carbon dots were limited to a short wavelength range and quantum yield were low.This thesis was based on improving synthesis and purification processes,increasing the production rate and quantum yield of carbon dots,and preparing multicolor carbon dots.Through the doping of different heteroatoms,the regulation of thermal solvents and the optimization of synthesis conditions,carbon dots of different wavelengths included red,yellow,green and blue were prepared,and finally realized their applications of anti-counterfeiting labels,Fe³⁺detection and photoluminescent light-emitting devices.The details were as follows:?1?Using malic acid as a carbon source to synthesize blue carbon dots and their application of anti-counterfeiting labels:a blue fluorescent carbon dots with a quantum efficiency of 25%were prepared by microwave digestion using malic acid as a carbon source.Through the injection of acetone,the polarity of the precursor changed,and the target products were largely sedimented.The yield of carbon dots was as high as 65%.XPS and FTIR characterization results showed that the surfaces of the synthesized carbon dots were coated with a large number of oxygen-containing functional groups,which determine that the prepared carbon dots had good water solubility and alcohol solubility.At the same time,the carbonization process of dehydration condensation was qualitatively analyzed and reaction mechanism was also theoretically explained.Excitation and emission spectra showed that there are two energy-transfer peaks with equivalent intensity on the excitation spectrum,corresponding to the emitted blue and yellow light.This special excitation-dependent property can realize their application of anti-counterfeiting labels.?2?Using p-phenylenediamine as a carbon source to synthesize pH-sensitive green fluorescent carbon dots and their application of pH and Fe³⁺detection:Using p-phenylenediamine as carbon source and phosphoric acid as hot solvent,pH-sensitive green carbon dots were synthesized by hydrothermal method.When the excitation wavelengths were increased from 370 nm to 450 nm,the emission wavelength was fixed at 510 nm,showing a good excitation-independent feature.The pH value was adjusted by mixing NaOH with phosphoric acid solution.When the pH value decreased,the fluorescence intensity increased.However,when the pH value increased,the fluorescence intensity decreased until quenching.This fluorescence quenching/recovery phenomenon of the carbon dots was reversible and reproducible,which provided a theoretical basis for the application of fluorescent probes.At the same time,the carbon dots showed unique detection performance for Fe³⁺.Through the response law of fluorescence intensity,the limit concentration of Fe³⁺were calculated,and the purpose in detecting the ferric ion of the carbon dots was finally realized.?3?Using neutral red as a carbon source and regulating solvents to synthesize red,yellow and blue carbon dots and their application of photoluminescent devices:According to mastery of solvent and reaction temperature,using central red as a carbon source,red,yellow,and blue dots with the quantum yields of 34%,53%and 70%were achieved by solvothermal method.Structural and optical characterizations showed that the different emission wavelengths were due to the different amounts of C=N and=O in the respective surface states.From blue,yellow to red carbon dots,as the content of C=N and C=O in the surface state increased,the n-?*transitions produced by conjugated absorption bands of the two chemical functional groups will accumulate,which caused red shift in the absorption band,and the overlap of absorption spectrum and the excitation spectrum will be red-shifted,eventually resulting in different fluorescence emissions;The difference in effective energy transfer,the difference in the surface epoxy functional content,and the the difference in self-absorption caused by the overlap of the emission peak and the absorption peak were the root causes of different quantum yields of the three carbon dots;The red and yellow carbon dots were excitation-independent because their surfaces were sufficiently passivated and had a uniform fixed emission state.Blue carbon dots were excitation-dependent because their surfaces were partially passivated and there were many different n-?*transition levels.Through the doping of the polyvinylpyrrolidone dispersion matrix,the solid carbon nanophosphor had overcame the self-organized quenching and exhibited almost the same quantum yield as the liquid carbon dots.The above results indicated that these carbon dots not only had fluorescence stability,but also a considerable material compatibility and a good dispersibility,which provided conditions for the realization of photoluminescent devices.Finally,the composite solid carbon nano-fluorescent materials were combined with the blue chips that prepared red,yellow and blue LED devices with color coordinates of?0.66,0.32?,?0.42,0.52?and?0.14,0.06?.