Preparation Of Nitrogen-doped Carbon Dots And Its Application In The Detection Of Food Additives

In recent years,the problem of food safety has aroused people’s close attention.The abuse of food additives has seriously harmed people’s health,so it is very important to develop a simple and rapid method for the detection of food additives.Carbon dots（abbreviated CDs）are fluorescent carbon nanomaterials that are less than 10 nm in size,spheroidal in shape,and contain many functional groups on the surface.Due to the advantages of good biocompatibility,easy preparation,low toxicity or non-toxicity and excellent fluorescence characteristics,carbon dots are widely used in biochemical analysis,biosensing,photocatalysis and biological imaging.Due to the simple structure of the original carbon dots,which only contains C,H and O elements,low fluorescence quantum yield,bad selectivity and sensitivity limits its application in analytical detection as a fluorescent probe.In this paper,we studied the preparation method and fluorescence properties of doped carbon dots and the preparation conditions of nitro-doped carbon dotts with high fluorescence quantum yield were optimized.A new method for the detection of nitrite,ascorbic acid,Cu²⁺and sulfite was established based on nitro-doped carbon dots.The main research contents are as follows:1.Optimization of preparation conditions for nitrogen-doped carbon points.Firstly,the carbon points doped by various non-metallic elements（B,N,P,S,Cl）were prepared by one-step hydrothermal method.It is found that N-CDs has the highest QY（quantum yield）.Secondly,the effects of different nitrogen sources and carbon sources on the fluorescence intensity of N-CDs were studied.The results showed that the carbon dots prepared with citric acid（CA）as carbon source,diethylenetriamine（DETA）and Tris（hydroxymethyl）aminomethane as nitrogen source all had higher fluorescence quantum yield（QY）.Then,the effects of synthesis temperature and time and nitrogen doping content on the QY of carbon dots were further studied.The results show that the reaction temperature has has a very significant effect on the QY of carbon dots.Finally,the optimum preparation condition is to control the molar ratio of carbon source and nitrogen source to1:1,and heating at 200℃for 8 hours.2.N-CDs₁ as a fluorescent probe for the detection of nitrite and ascorbic acid.N-CDs₁with fluorescence quantum yield up to 42.5%was synthesized from citric acid and Tris under optimal conditions.The quenching effect of Fe³⁺on N-CDs₁ fluorescence and the strong oxidation property of nitrite under acidic conditions can oxidize Fe²⁺into Fe³⁺,thus turn off the fluorescence of N-CDs₁.Ascorbic acid is reductive and can reduce Fe³⁺to Fe²⁺to turn on the fluorescence of N-CDs₁.Therefore,the"on-off"and"on-off-off"fluorescence sensors based on carbon dots were proposed for the detection of nitrite and ascorbic acid,respectively.The linear ranges of nitrite and ascorbic acid were 0-400μM and 100-250μM,respectively.The detection limits were 0.42μM and 0.34μM,respectively.The standard recoveries were 97.58%-104.7%and the standard deviations were less than 5%.Therefore this method is suitable for the determination of nitrite and ascorbic acid in food.3.Determination of Cu²⁺and sulfite by ratio fluorescence probe based on N-CDs₂.N-CDs₂,with fluorescence quantum yield of 39.7%,was synthesized from CA and DETA under optimal conditions.The emission wavelength of N-CDs is 455 nm at the excitation wavelength of 360 nm.Cu²⁺can selectively oxidize non-fluorescent o-phenylenediamine（OPD）to 2,3-diaminophenazine（DAP）,which can quench the fluorescence of N-CDs₂.The fluorescence emission peak of DAP was located at 575 nm at 360 nm excitation wavelength.With the increase of DAP content,the fluorescence intensity at 575 nm increased,and the fluorescence of N-CDs₂ at 455 nm decreased gradually.The fluorescence intensity ratio of bimodal has a good linear relationship with the concentration of Cu²⁺.The linear range of Cu²⁺detection is 0-100μM and 100-600μM,and the detection limit is 0.15μM.In addition,sulfites are reductive and can consume Cu²⁺,thus hindering the oxidation of OPD by Cu²⁺.After adding sulfite,the fluorescence intensity at 575 nm was weakened,and the fluorescence of N-CDs₂ at 455 nm was gradually enhanced.The fluorescence intensity ratio of bimodal has a good linear relationship with the concentration of sulfite.The linear range of sulfite detection was 25-500μM,and the detection limit was 0.54μM.This method was applied to the determination of Cu²⁺in water samples and sulfite in white sugar and rock sugar with satisfactory results.