Synthesis Of Nitrogen Doped Carbon Dots Based On Ionic Liquids And Their Application In Detection Of Hg²⁺

Carbon quantum dots(carbon dots,CDs)are carbon-based nanomaterials with a size less than 10 nm.The CDs have been exploited in many potential applications including bioimaging,biosensor,catalytic and energy transfer/storage and so on,because of their unique properties,such as low toxicity,good biocompatibility,excellent chemical stability and solubility,and wide excitation/emission spectra.Although the studies of CDs have attracted many scientists' attention,and made great progress,there are still some problems existing in the research such as low quantum yields(QYs),easily quenched at solid state,and ambiguous luminescent/formation mechanism.In terms of the above issues,we will design and prepare a series of novel N-doped carbon dots(N-CDs)using various kind of ionic liquids as precursors or passivation agents,and then carry on the preliminary exploration around these problems.The work was supported financially by the National Science Foundation(Nos.21573058,21303044,21173070),and the major contents and innovations are as follows:1.Novel N-CDs with high QYs(20.9%)were prepared using 1-butyl-3-methylimidazolium chlorine([C4mim]Cl)ionic liquid and asparagine as nitrogen and carbon source by the pyrolytic method.The CDs show blue-green fluorescence under ultraviolet light when they are dispersed in water,while show yellow fluorescence when the N-CDs suspension are dried.The obtained CDs can successfully be used as fluorescence probe to detect Hg2+ with high selectivity,and the detection limitation of the N-CDs is 3.2nM.2.Novel N-CDs with high QYs(52%)and good solubility were prepared using guanidine carbonate and citric acid as precursors by the hydrothermal method.The as-prepared N-CDs exhibit high resistance to photobleaching,strong salt resistance,and pH-dependent behaviors.The QYs of N-CDs increase with the prolongation of the reaction time during the preparation process.In addition,the obtained CDs can successfully be used to detect Hg2+ as fluorescent probes.3.A novel approach was developed to prepare N-CDs,whose relative QYs are up to 43%,with polyethylene glycol(PEG2000N)as carbon source,1-butyl-3-methylimidazolium bromine([C4mim]Br)ionic liquid as passivation agent,inorganic acids accelerator by the hydrothermal method.The obtained CDs notonly emitted bright blue fluorescence in water but also showed green fluorescence in common organic solvents.Meanwhile,they both have down-conversion photoluminescence property and up-conversion photoluminescence behavior.Noticeably,the CDs can still maintain high photoluminescence intensity even at low pH.It was found that the intensity and QYs of the N-CDs are more sensitive to the concentration of H+ during the synthesis progress.Through the analysis and characterization of the production,we deduced that H+ accelerates the formation of the –COOH/–OH groups existing on the CDs,and then accelerates their combination with imidazolium rings of ionic liquids,which improves subsequently the photoluminescence performance of the CDs.This work carried out a preliminary exploration around the preparation,application,and formation mechanism of CDs based on ionic liquids.The obtained CDs overcome the shortcomings such as low QYs,and easily quenched at solid state.It provides new research approaches for synthesizing CDs with high performance.