Graphene Quantum Dots For The Detection Of Mercury,Iodine,Oganic Molecules In Aqueous Solution

Graphene quantum dots(GQDs)is a two-dimensional carbon material with only one atom thick.GQDs have excellent performances such asoutstanding electronic,mechanical properties and good chemical stabilities.Besides,GQDs show fairly low toxicity,high solubility,excellent stability,stable photoluminescence,large surface area,thus making them good candidatesfor application in the fields of fluorescent probe,electro-optical equipments,biosensors,cell imaging,drug delivery,solar cell,etc.Enlightened by the above characteristics,the research of GQDs mainly focuses on fluorescencesensors,cell imaging and drug loading.There are various methods for preparing GQDs,such as hydrothermal method,concentrated acid oxidation,solvothermal method,UV-Fenton oxidation,electrochemistry oxidation,organic molecular synthesis,micro-wave assisted oxidation.The carbon source forpreparing GQDs involves graphene films,carbon fibers,graphene oxide,graphite rod,carbon nanotubes,carbon black,glucose,citric acid,leaves,etc.Current researches mainly focus on chemical preparation and optical properties of GQDs.In our work,we use carbon fiber as the carbon source to prepare the GQDs through the chemical stripping of carbon fiber,and then the GQDs were modified with thymine.The resulting thymine-modified GQDs can be used to detect mercury ions,and importantly,they can be further used for the detection of iodide ions and thiol-containing amino acids.The sensors have the advantages of high sensitivity,good selectivity,simple operation,rapid response,etc.This thesis includes the following four chapters.In the first chapter of this paper,we briefly introduce the preparation methods,properties and applications of GQDs.Based on the fluorescence properties of GQDs,we designed the experimental scheme for this thesis.In second chapter,we introduce the preparation and characterization of thymine-modified GQDs,and the application of this material for the detection of mercury ions,iodide ions and mercapto-containing amino acids.Herein,we used chemical stripping of carbon fiber to prepare GQDs.The GQDs prepared by this method have the advantages of uniform size,good dispersion,high quantum yield and fine photo-stability.In addition,thymine-modified GQDs have a very fine sensitivity and selectivity for mercury ions.The LOD for mercury ions obtained by this method is 211 nM.Furthermore,based on the "T-Hg2+-T" structure,a simple "turn on" fluorescence probe is developed.The probe can be used to detect iodide ions and thiol-containing amino acids(such as cysteine,gluthathione and homocysteine).At first,the mercury ions were added into the material solution,and the formation of "T-Hg2+-T" structure leaded to the aggregation of GQDs,which resulted in the decrease of fluorescence.Whenthe iodide ions were added to the solution,the "T-Hg2+-T" structure is opened due to the formation of a more stable complex between the mercury ions and iodide ions,and the fluorescence of GQDs is restored.The LOD for iodide detection is 2.80 ?M.Moreover,according to the HSAB theory,the binding ability between mercury ions and thiol groups is very strong.Therefore,the thymidine-modified GQDs can also be used to detect the amino acids with thiol groups.Through this method,the LOD for cysteine,gluthathione and homocysteine aredetermined to be 4.01 ?M,1.92 ?M,2.00 ?M,respectively.In the third chapter of this thesis,we investigate the detection of ferric ions and ascorbic acid by the GQDs.Due to the oxygen-containing function groups on GQDs and the good affinity of ferric ions with oxygen-containing groups,ferric ions can cause the fluorescence decrease of GQDs.By this way,the LOD for ferric ions is 5.7 nM.Besides,ascorbic acid have strong reducibility,and ferric ions have strong oxidizability,so oxidation-reduction reaction can occur between them.During the reaction,the ferric ions are reduced to ferrous ions,and the coordination between ferric ions and oxygen-containing functional groups is destroyed,leading to the recovery of fluorescence.The LOD for ascorbic acid detection is 536 nM.This method has good selectivity and high sensitivity for ascorbic acid.In the final chapter,we summary the experimental results of this paper and predict the prospect of GQDs in the optical sensor field.