Synthesis,Luminescent Mechanism And Analytical Application Of Multi-Emission Carbon Dots

In recent years,many photoluminescent nanomaterials such as carbon dots,graphene quantum dots,metal-organic frameworks,semiconductor quantum dots,perovskite quantum dots and others have been widely used in biochemical analysis,disease treatment and other fields for their unique luminescent properties.As new fluorescent nanomaterials,carbon dots have shown great advantages in biology,clinical medicine,photodynamic therapy,and other fields due to their good biocompatibility.Furthermore,due to their excellent fluorescence properties,cheap raw materials,easy synthesis,adjustable emission wavelength and other advantages,they have great potential in optoelectronic devices,photocatalysis,optical anti-counterfeiting.The unique fluorescence property of carbon dots is one of the reasons why carbon dots are more widely used than other nanomaterials.At present,most of the reported carbon dots only have single emission which only have one emission at a single excitation wavelength.Although the excitation-dependent carbon dots have different emissions under different excitation wavelengths,they are still single emission carbon dots.The established fluorescent probes based on the single emission carbon dots only have fluorescence intensity change,and it is difficult to determine the concentration of analytes by the naked eye directly.In the preparation of LED lamps,carbon dots often have been mixed with the other emissive carbon dots or considered the commercial blue light-emitting as the substrate,and the operation is complex.In the field of optical anti-counterfeiting,the variability is too small,so it is easy to be copied and replaced.Therefore,it is of great significance to develop carbon dots with multiple emission peaks at a single excitation wavelength.Although there are many reports about how to synthesize carbon dots,there are a few methods for synthesizing multi-emission carbon dots.Besides,the research on the specific luminescent mechanism of multi-emission carbon dots is insufficient,and the development of the luminescent mechanism is helpful for the controllable preparation of carbon dots.Based on the above problems,the purpose of this study is to synthesize multi-emission carbon dots,and study the internal changes in the synthesis process according to their raw materials,understand the origin of luminescence centers,provide ideas for the controllable synthesis of multi-emission carbon dots,and explore the application of multi-emission carbon dots.The specific contents are as follows:（1）The nitrogen and phosphorus codoped dual-emission carbon dots（N,P-CDs）are synthesized successfully and applied in the analysis of hypochlorite（Cl O^-）.N,P-CDs with blue and red emissions have been synthesized by one-step hydrothermal method using Jianna Green B which is rich in nitrogen as raw materials,and（NH₄）₂HPO₄ as the dopant.It is easy to operate,synthesize,and can be applied to practical analysis and detection without complicated pretreatment.According to their preparation process,it can be found that the nitrogen-related surface state is the main origin of red emission.The doped phosphorus can improve the fluorescence intensity of carbon dots.In this study,the detection of Cl O^-by N,P-CDs must be carried out in the presence of 2-（4-Morpholino）Ethanesulfonic Acid（MES）.The red fluorescence of N,P-CDs is quenched while the blue fluorescence remained unchanged with the increase of Cl O^-concentration.In addition,the addition of Cl O^-can make the absorption of N,P-CDs from rose red to colorless,which provides a dual-channel of ratio fluorescence and colorimetry,and has good practical value.The results show that Cl O^-can react with intermediate formed by MES and N,P-CDs to change the surface state of N,P-CDs,thus changing the absorption.And the electron exchange energy transfer occurs between the intermediate and Cl O^-,leading to the fluorescence quenching.（2）At present,dual-emission carbon dots are the most reported multi-emission carbon dots,such as our previous work.Therefore,carbon dots with more emission peaks may have more advantages in the preparation of optical devices and sensors.To solve this problem,we using 1-Pyridylazo-2-naphthol（PAN）and tris（2,2’-bipyridyl）ruthenium（II）chloride hexahydrate（Ru-bpy）as the carbon source to synthesis the multi-emission carbon dots（M-CDs）with concentration-dependent property by one-pot solvothermal method.The M-CDs have special concentration dependence luminescence properties.There are four luminescent centers in the high concentration of M-CDs,located at 385nm,423 nm,493 nm and 586 nm,and can be attributed to the molecular state,surface state,edge state and carbon core luminescence,respectively.While in the low concentration of M-CDs,there are only red and blue emissions in the visible region.Through the extraction and separation of M-CDs,it is found that M-CDs are composed of red and blue carbon dots（B-CDs and R-CDs）,and both of them only show single luminescence property even in the high concentration solution.The experiment shows that in the high concentration of M-CDs,B-CDs and R-CDs aggregate based on hydrogen bond forces.The fluorescence resonance energy transfer of B-CDs and R-CDs induces the edge state luminescence.The emission peaks of M-CDs at high concentrations show the potential in the photoelectric field and can be used in sensor Morin by ratio fluorescent method in low concentration of M-CDs.In addition,during the preparation of M-CDs,the solid carbon dots（S-CDs）with solid room temperature phosphorescence have been obtained,which can be successfully applied to optical anti-counterfeiting.In the same synthesis process,different materials with different properties are obtained simultaneously,and the efficient utilization of raw materials was realized.This study focuses on the synthesis of multi-emission carbon dots,discusses the influence of raw materials on synthesizing multi-emission carbon dots,and describes their luminescent mechanism,analysis,and application in detail.Take visualize sensor as the analysis goal,the multi-emission carbon dots have been successfully used in visual analysis.