Spectral Properties And Applications Of High-efficiency Red-emission Carbon Dots

Carbon dots are a new type of nanomaterials whose size is generally below 10 nm.Due to many unique and fascinating physical and chemical properties like adjustable fluorescence spectrum characteristics,pre-designed functionality,good biocompatibility,low raw material costs and green pollution-free,carbon dot has attracted the attention of many researchers and has been developed.With the improvement and development of the preparation process,the measure of preparing carbon dots has been improved.Moreover,the physical and chemical properties of carbon dots have become better and its scope of application has expanded.At present,carbon dots are mainly used in detection,LED,drug delivery and other fields.Among them,detection,especially the detection of substances in organisms is of great significance.However,in vivo detection has some special requirements for the performance of carbon dots,including higher quantum yield,long-wavelength emission light,etc.In order to prepare red carbon dots with high quantum yield,this paper summarizes the reported preparation methods and luminescence principles of red carbon dots.On this basis,10 kinds of unreported precursor molecules were selected for experiment and analysis.In this process,we focused on analyzing the relationship between the luminescence of the product and the structure of the precursor.For further experiments,3,4 diamino benzonitrile/3,5 diamino benzonitrile with symmetric structure and specific functional groups was selected.Finally,high-efficiency red-light carbon dots with emission wavelengths at 625 nm are prepared.The specific work of this paper is as follows:(1)Through the analysis of the reported raw materials,the general conditions of the red carbon dot precursor are summarized,like containing a benzene ring or a naphthalene ring,the polymerization of several molecules or itself has symmetry.On this basis,we selected 10 materials that have not been reported to be used for the preparation of red carbon dots for experiments,and selected two precursors that can synthesize carbon dots with high quantum yield without optimizing the experimental conditions.Subsequently,Gaussian software was used to compare these two molecules with similar molecules reported to analyze their bond energies and determine the order of the study.The characteristics of the carbon dots of3,5diaminobenzonitrile was studied first.(2)Using 3,5diaminobenzonitrile as the precursor,pH-sensitive N-CDs with high brightness and adjustable emission wavelength were prepared by a one-step hydrothermal method.Carbon dots have a relatively high quantum yield,and the QY in aqueous solution and ethanol solution can reach 41.3% and 67.0%,respectively.After studying the fluorescence spectra of carbon dots,it is found that the emission wavelength of carbon dots does not depend on excitation,but has visible and diverse responses to the pH of the environment.Therefore,we use carbon dots for pH sensing in water bodies.In addition,in the experiment to explore the influence of small molecules on the luminescence of carbon dots,we found that carbon dots also have a response to tetracycline and have a lower limit of detection(LOD)of 3.5(?)g/L.Moreover,the structure of the 3,5 diamino carbon dots was analyzed and it was concluded that the cyano-group on the benzene ring of the precursor molecule hindered the polymerization of the molecule.(3)Using 3,4diaminobenzonitrile as carbon source,a high-quantum yield red carbon dot HR-CDs was prepared.In some solvents,the quantum yield of HR-CDs is as high as 87.4%.Experiments show that the emission wavelength of HR-CDs is hardly affected by the solvent,and the fluorescence intensity is responsive to water content.Further research found that in a certain range of water content(0-15%),HR-CDs has a similar response to water in different solvents,so it can be used to detect the water content in mixed organic solvents.In addition,HR-CDs have high sensitivity to water and good resistance to solvents,so they can be used for the detection of water content in ethanol gasoline(the detection range is 0-16%,and the detection limit is 0.023%).Based on the above results,this work has certain significance for the preparation of high-luminescence red-emitting carbon dots and the detection of water content.