Studies On Electrochemical Chiral Sensors Based On Graphene-based Composites

Chirality is the basic attribute of natural world,and chiral recognition has great research value in various fields.The amino acids are the basic structural unit of life and precursors for chiral pharmaceuticals,so chiral recognition of amino acids is conducive not only to basic research,but also to the development of medical science.Meanwhile electrochemical chiral sensors have been attracting increasing attention due to their operation easy,real-time,economy,environment,high sensitivity and so on.The reduced graphene oxide with excellent electrochemical properties has been widely applied not only for providing an effective complex substrates but also for amplifying the steric hindrance and ability of chiral recognition.In this doctoral thesis,electrochemical chiral sensors of tryptophan,lysine and tyrosine enantiomers are based on graphene-based composites.The main contents are as follows:1.Firstly,the 1,10-phenanthroline copper(II)with steric hindrance was synthesized,and RGO/PhenCu composites was formed by π-π non-covalent interactions between RGO and 1,10-phenanthroline.The morphology and structure of composites were determined by SEM,FT-IR,UV,and XRD.This needle-like structure gives two advantages: one is increasing the contact area between the composites and enantiomers and the other is increasing the steric hindrance during the chiral recognition.The composites dispersed in anhydrous ethanol were droped on GCE and then characterized by CV,EIS,galvanostatic charge–discharge tests.The results indicated that the composites have good electrochemical performance,capacitive performance and chiral recognition ability.The peak currents were decreased with obviously different after the improved ligand exchange reaction between composites and tryptophan,so it could identify the tryptophan enantiomers by electrochemical method.Then we continued to discuss the influence of the concentration of tryptophan,testing time and pH.The peak current decreased linearly with the increase of the L-tryptophan,when the electrochemical chiral sensor was applied to detect and analyze tryptophan enantiomers.Many test results showed that this sensor with high sensitivity and rapid detection has been developed to recognize Trp enantiomers and could be used to detect the contents of one type in enantiomers mixture.2.To improve the efficiency of electrochemical chiral recognition,we chose chiral L-Trp as ligand to synthesize L-TrpCu(II).The RGO/LysCu composites were formed by π-π interactions between RGO and L-Trp.The morphology and structure of composites were determined by SEM,FT-IR,UV,and XRD.The composites dispersed in anhydrous ethanol were droped on GCE and then characterized by CV and EIS.The results indicated that the composites have good electrochemical performance,which could apply to electrochemical chiral recognition.The change of peak current and peak potential were obviously different after the ligand exchange reaction between composites and lysine,so it could identify the lysine enantiomers by electrochemical method.Then the influence of the concentration of lysine,testing time and pH were further discussed.When the electrochemical chiral sensor was applied to detect and analyze lysine enantiomers,the peak current decreased with the increase of the D-Lys,but it is not linear.However,the shift of peak potential were linearly with the increase of the D-Lys,this showed that the sensor could be used to detect the contents of one type in enantiomers mixture by the shift of peak potential.And this sensor depending on changes of potential is more stable and reliable,simple and convenient,quick and efficient.The improved ligand exchange reaction was further confirmed by this experiment.3.Firstly,L-Lys Cu(II)was synthesized with L-lysine and divalent copper ions.Next,the nucleophilic substitution has occurred between the oxygen containing groups on the surface of GO and the terminal amino of L-LysCu(II)with smaller steric hindrance.This reaction would controllable synthesis a self-assembly RGO/L-LysCu(II)composites.Finally,the RGO/Lys chiral composites will be obtained through eliminating copper ion with EDTA.This synthetic method can effectively protect the chiral recognition site of L-lysine.The character and structure of composites were determined by SEM,FT-IR,XPS,XRD,water soluble test.The RGO/Lys retained carboxyls and aminos on chiral carbon atom of L-Lys,so it possessed good water solubility,biocompatibility and chirality.The chiral carbon atom of L-lysine can expose to graphene surface due to the intermolecular forces,which are breaked after the forming of composites.After the composites dispersed in anhydrous ethanol were droped on GCE,it could form diastereoisomer with D-Trp through three point interaction principle,but L-Trp cannot.The cyclic voltammetry could convert this discrepancy into electrochemical signals in order to achieve chiral recognition.Meanwhile,the influence of the concentration of tryptophan,testing time and pH were further discussed.After we got the optimal conditional experiment,this sensor could be used to detect the contents of one type in tryptophan enantiomers mixture and has a short detection time,high recognition efficiency.Through this experiment,we would also put forward the three point interaction principle,which is applied to electrochemical chiral recognition,and it is beneficial to explain further the theory of electrochemical chiral recognition.4.In order to increase the number of chiral sites,improving the ability of chiral recognition,we had synthesized RGO/CS chiral composites.The character and structure of composites were determined by SEM,FT-IR,XPS.The chiral composites have good electrochemical performance by CV and EIS,and it could chiral recognize tyrosine enantiomers with low concentration in the appropriate electrolyte by DPV.The RGO/CS chiral composites could form diastereoisomer with L-Tyr through three point interaction principle by using double helix structure and hydroxide radicals on chiral carbon atom of composites,which cause the reduction of peak current.But the D-Tyr can not form this strong intermolecular forces.Meanwhile,we continued to discuss the percentage between Tyr and PBS,testing time and pH.After we got the optimal conditional experiment,this sensor could be used to detect the contents of one type in tyrosine enantiomers mixture and has a short detection time,high recognition efficiency.