Investigation On A Typical Carbon Dots:Structural Composition Analysis,Fluorescence Mechanism Exploration And Application Expansion

Carbon dots(CDs)generally refer to nanoparticles,at least one dimension having a size of 10 nm or less,possessing inherent fluorescence properties and having a core,in which carbon is the main element.They have the advantages of low preparation cost,good water solubility,excellent photoluminescence(PL)performance and good biocompatibility.According to the structural composition,the CDs material can be summarized as graphene quantum dots,carbon nanodots and polymer dots.The graphene quantum dots generally refer to one or more layers of graphene or graphene oxide sheets with modified groups attached to the edges.Carbon nanodots are typically spherical nanoparticles that can be divided into carbon nanodots with a lattice structure and the ones without lattice structure.Polymer dots refer to polymer nanoclusters that are physically or chemically crosslinked.The synthesis strategy of CDs is generally divided into two methods: top-down and bottom-up.Top-down refers to the formation of zero-dimensional CDs by cutting three-dimensional or two-dimensional carbon materials.The bottom-up method mainly obtains CDs by the condensation and carbonization of small molecules or polymers.CDs materials often have inherent fluorescence properties,and the explanation of the fluorescence mechanism includes carbon core state,surface state,molecular state,and crosslink enhanced emission(CEE)effect.CDs can be widely used in fluorescence imaging,biomedical,optoelectronic,catalysis,sensing and other fields.In the year of 2013,Dr.Zhu et al.prepared a type of CDs by hydrothermal treatment of citric acid and ethylenediamine,obtaining blue fluorescent emission CDs with quantum yield up to 80%.The obtained aqueous solution of CDs had good fluorescent properties and can be used alone or in combination with other materials to develop more applications.This work has received a lot of attention(the literature has been cited more than 1,100 times so far).Other research groups have developed CDs similar to this type of CDs,and their raw materials,synthetic methods,etc.are similar,such as replacing ethylenediamine with mercaptoethylamine,cysteine,ethanolamine and diethylenetriamine.In this thesis,this typical CDs prepared by hydrothermal treatment of citric acid/ethylenediamine was chosen as a research example to study the controversial scientific problems of the fluorescence mechanism,composition structure and formation process of CDs and to expand the application of CDs.Firstly,we started from investigating the PL quenching phenomenon of the CDs.We studied the PL quenching phenomenon of ferric ions and ferrous ion/hydrogen peroxide mixture for CDs,and speculated the quenching mechanism.We have found that the PL quenching of CDs by ferric ions mainly resulted from dynamic quenching.The quenching mechanism of ferrous ion/hydrogen peroxide was due to the generation of a hydroxyl radical,which had high oxidation activity and resulted in oxidative damage to the fluorescence center of the CDs.Based on the PL quenching phenomenon of CDs fluorescence,we confirmed the feasibility of the PL quenching detection method of iron ions,Hemin or hydrogen peroxide,in order to achieve efficient,rapid and sensitive detection.More importantly,we studied the fluorescence mechanism of CDs by quenching,assuming the existence of dual fluorescence centers of molecular state and carbon core state.Secondly,we separated the typical CDs by column chromatography,analyzed the structural composition of the CDs and found a molecule,imidazo[1,2-a]pyridine-7-carboxylic acid,1,2,3,5-tetrahydro-5-oxo-,(IPCA).By studying the properties of IPCA,we determined that IPCA was the molecular fluorescence center in the CDs.Furthermore,we designed and prepared a variety of IPCA derivatives;in the case of Et-IPCA,it also has a bright fluorescent emission like IPCA,which has higher yield,is more convenient to purify and has better light stability.Furthermore,we proposed the hypothesis that citric acid could react with many types of diamines or their analogs to form molecules with IPCA-like structure and strong PL emission properties.Through systematic toxicological experiments,we verified that the right concentration of CDs and IPCA materials had neither acute toxicity nor significant long-term toxicity;this provided a reference for the safe application of CDs in the medical field.Furthermore,we developed the application of CDs/IPCA in the fields of neural tracing,stem cell imaging and regulation of human islet amyloid polypeptide(hIAPP)aggregation.Both nanomaterial CDs and fluorescent molecule materials IPCA can be applied to the field of fluorescent probes.They have their own characteristics.IPCA materials have clear structure,strong fluorescence emission,and have the property of inhibiting hIAPP self-aggregation.As a nanocomposite,CDs have better water solubility,lower possibility to cause hemolysis after injection and longer fluorescence tracking time.Thirdly,we analyzed the main components in the CDs,and speculated that it was most likely to be a crosslinked polymer structure,indicating that this type of CDs we studied are actually polymer carbon dots(PCDs).We adjusted the chemical composition and optical properties of the PCDs by controlling the hydrothermal temperature,initial pH value,and starting material ratio during the synthesis.We also analyzed the fluorescence properties of different components in the PCDs to explore the effects of molecular states and CEE effect emission on the PL of the materials.We have summarized the relationship among the components in the PCDs,proposed the chemical equilibrium among the raw material monomers,IPCA small molecules,crosslinked polymer structure and carbonized structure during the synthesis,and discussed the hydrothermal temperature as an important factor on the equilibrium.Finally,we explored the formation of typical PCDs from the perspective of condensation polymerization of water-soluble monomers to form crosslinked polymers.We proposed a special water-dispersed nano-crosslinking polymerization model to explain the formation of PCDs that are not applicable to the traditional 2-3 crosslinking polymerization theory.We summarized the properties of water-dispersed nano-crosslinking polymerization,analyzed the experimental methods and reaction processes,and discussed its core principle.Using the dissipative particle dynamics-stochastic chemical reaction computer simulation method,we simulated the hydrothermal synthesis process of PCDs.We have confirmed that crosslinking condensation polymerization can induce hydrophilic/hydrophobic nanophase separation;condensation polymerization can produce nanoparticles,which are well dispersed in water.