Identification Of Fluorophores In Carbon Dots And The Performances Of The Synthesized Surfactant Derived From Carbon Dots

Carbon dots（CDs）or carbon nano dots derived from carbonous organic compounds attracted extensive interests due to the luminescence.They were applied in many domains depending on several distinguishing features such as inexpensive raw materials,convenient synthesis,wonderful fluorescence and non-toxicity.These had been reported in many reports.However,the accurate structural information and the relationship between the luminescence and the chemical structure were obscure since the produced CDs were mixtures.Therefore,the understanding on the fluorescent mechanism was based only on the theoretic hypothesis.Herein,we focused on identification of fluorophores in CDs based on principles of fluorescent quenching and demonstrated the fluorescent functional groups in CDs with experimental facts.Additionally,two CDs cationic surfactants were synthesized derived from the CDs containing tertiary amine.The measurements show that they both possessed well surface activity,fluorescence and antibiosis.The detailed contents were depicted as following.（1）Many reports about preparing CDs with blue fluorescence which derived from small molecules of carbohydrate had been published.For instance,citric acid could be as a raw material commonly in preparing several CDs.However,which functional groups and what kinds of structure in CDs were responsible for the luminescence?Most of the explanations for these according to current references were based on some reasonable assumptions.The accurate information was rare to be disclosed by experimental facts.Herein,we focused on seeking fluorophores of the CDs prepared by citric acid and glycine via fluorescent quenching,whose emission was 400 nm.The fluorescent quenching accompanied by the UV-visible analyses demonstrated that essence of fluorescence,which was manipulated by fluorophores of conjugated C=C bonds and the aldehyde groups.The erythrine precipitate produced by reaction between Fehling’s solution and CDs demonstrated the aldehyde group in CDs.TThhiiss was the fiirrsstt report abboouutt the aldehyde group in the CDs to the best of our knowledge.Destructing of aldehyde groups or C=C bonds in the CDs leaded to complete decay of luminescence.Additionally,such experimental results were similar to those of the CDs prepared by the sole citric acid which predicted the analogy of the CDs in type of fluorophore and chemical structure.（2）Mixture of polyethylene glycol（MW=300）and hydrogen peroxide was suffered from hydrothermal treatment at 180°C for 1h in the first place for obtaining the precursor with blue fluorescence,whose emission was 450 nm.It could be quenched by aniline,sodium hypochlorite,potassium hypermanganate and Fehling’s solutions respectively.The reaction between the product and Fehling’s solution produced large lots of erythrine precipitate.These results were similar to the ones obtained from the CDs made by citric acid and glycine,which predicted that the both fluorophores were identical completely.Such acidic product could be modified by m-phenylenediamine,which leaded to obvious red-shift of the emission,which shifted to 520 nm.And the fluorescence quantum yield approached as high as 36.71%.It performed wonderful in fluorescent staining for MCF7.（3）A novel cationic surfactant was prepared by quaternization on the CDs with1-chlorododecane.Firstly,the CDs with blue emission were prepared by means of hydrothermal treatment on mixture of ethylenediamine tetraacetic acid,ethylenediamine and hydrogen peroxide in aqueous medium at 180°C for 1 h.The hydrophilic groups such as carboxyl,tertiary and primary amino groups in the CDs ensured the hydrophilicity.Meanwhile,the tertiary amino group could be quaternized by 1-chlorododecane in very mild conditions.Thus the amphiphilic CDs were obtained,which performed wonderfully in declining surface tension of water and reducing contact angle of water.The value of surface tension of the dilute solution reached 26.7 m N m^-1,which was superior to several new types of Gemini cationic surfactants.The fluorescence quantum yield approached 6.44%.Moreover,the quaternization improved the antibiosis greatly foorr Esscchheerriicchhiiaa coli.When the concentration was diluted to 0.41 mg m L^-1the CDs solution still remained almost a hundred percentage of antibacterial activity.Calculation results exhibited that 1.223alkyl chains were attached to a CD averagely.（4）Another CDs cationic surfactant was synthesized.The CDs were prepared by hydrothermal treatment on mixture of citric acid,N,N’-dimethylethylenediamine and hydrogen peroxide.Successively,quaternization of the CDs with 1-chlorododecane was executed in ethanol medium for preparing the novel cationic surfactant.The measurement showed that the fluorescent quantum yield of OX-CDs-C₁₂H₂₅approached 16.21%.Surface tension of the dilute solution reached a minimum of 30.0m N m^-1,which was comparable to some cationic Gemini surfactants.Additionally,it performed synergistic surface effect when OX-CDs-C₁₂H₂₅was mixed with sodium dodecylbenzene sulfonate.This predicted that the CDs surfactant could be as a component of compounded detergent.Also,it performed well in inhibiting growth of Escherichia coli when the solution was diluted to 0.21 mg m L^-1.The calculated results showed that 2.608 alkyl chains were bonded to the CDs averagely.