Synthesis And Properties Of Fluorescent Carbon Dots Using Uracil Derivatives As Precursors

Carbon dots(CDs)are fluorescent carbon materials with a diameter less than 10 nm.They have received extensive attention from researchers since they were discovered in 2004.Because of its good fluorescence characteristics,special excitation dependence,good water solubility,biocompatibility and low toxicity,carbon dots are widely used in many fields such as biological imaging,fluorescent probes,gene delivery,photocatalysis and environmental detection.Due to the fact that the luminescent mechanism of carbon dots is not uniform and the fluorescence is weak in solid state,it is very important to realize controllable synthesis of carbon dots and solve the phenomenon of fluorescence quenching induced by aggregation in solid or concentrated solution.In this thesis,different types of functional groups were derived from the framework of natural metabolite uracil.Three types of carbon dots were synthesized by one-step or step-by-step hydrothermal method.The solid-state luminescence of carbon dots was improved by matrix assisted dispersion,and ammonia sensing was realized.The main research contents are as follows:(1)In order to explore the relationship between the functional group structure of precursors and the function of materials,blue fluorescent CDYX and blue-green fluorescent CDHB with particle sizes of 1.7 nm and 3 nm were synthesized by hydrothermal method with nitroso substituted uracil without oxidative polymerization characteristics and diamino substituted uracil with typical oxidative self polymerization characteristics as raw materials.The optimal reaction temperature and time were 160℃ and 6 h,and the highest quantum yields were 64%and 58%,respectively.By means of TEM,FT-IR and XPS,the results showed that the two carbon dots contained uracil residues,which reflected the detection of mercury and silver ions.However,due to the oxidative self polymerization of diaminouracil,the synthesized CDHB belongs to the hydrogen bonded nitrogen doped carbon dots,which is easy to form H-type aggregates,showing a concentration dependent red shift emission spectrum,while CDHB does not.It is confirmed that the structure of precursor is related to the construction of carbon nucleus.(2)Based on the difference of group reaction selectivity,CDHB was transformed into a green fluorescent CDzw with a particle size of 2.7 nm and good water solubility by one pot series reaction.The structure characterization showed that the uracil group in the CDHB structure was hydrolyzed to carboxyl group and amino group.The conversion of ligand reduced the hydrogen bonding between carbon dots,which made CDzw not show obvious concentration dependence and showed recognition for nickel and cobalt ions.The detection limits were 0.34 μM and 1.18 μM,respectively.The absolute quantum yield of CDzw is 7.2%.It has good cell compatibility and has been successfully applied to HeLa cell imaging.(3)Based on the matrix-assisted dispersion method,the economical β-cyclodextrin-CDHB binary composite luminescent material was screened through the solid-state luminescence characteristics of carbon dot CDHB in different substrates against aggregation-induced fluorescence quenching.Based on the CDHB concentration-dependent emission characteristics,a two-component solid colorimetric sensor array with the carbon dot as the sensing core was assembled,which can detect ammonia at the ppm level through emission spectra and CIE chromaticity diagrams.The channel of ammonia response mainly occurs at the emission peak of carbon dot H-type aggregates at 520 nm-530 nm.The principle is that ammonia molecules are alkaline and have strong hydrogen bonding,which can effectively quench carbon dot fluorescence.The repeatability experiments showed that the sensor still maintains a good ammonia response after 10 alternations.