Application Of The Graphene-based Composites In The Construction Of Electrochemical Chiral Sensors

Chirality is a very common phenomenon in nature.The amino acids that make up the human body are basically D-type.Enantiomers with different configurations in different fields may have different effects,so the recognition of chiral molecules is particularly important.Compared with traditional methods,electrochemical recognition methods have the advantages of fast and sensitive detection,simple operation,and low price,have been widely used in chiral recognition.Graphene has a high specific surface area,good mechanical strength,and excellent thermal and electrical conductivity,which can provide a sensing platform for the poorly conductive polysaccharide-based chiral selector,thereby improving recognition efficiency.In this paper,graphene was used as the base material,and modified graphene was used to deload chiral selectors（polysaccharides and small amino acid molecules）to synthesize chiral materials and identify amino acid enantiomers via electrochemical method.It is mainly composed of the following three parts:1、The chiral composites was prepared by Michael addition（or Schiff base reaction）between polydopamine-functionalized graphene（rGO-PDA）and L-lysine（L-Lys）for the construction of electrochemical sensing platform.First,L-Lys copper（II）（L-Lys-Cu）complex was synthesized to protect the chiral site on L-Lys,and rGO-PDA was synthesized by using dopamine to reduce and physically modify graphene oxide（GO）.The catechol functional group（nucleophilic anion）on rGO-PDA and the terminal amino group（electrophilic conjugated system）on L-Lys-Cu were used for Michael addition（or Schiff base reaction）to synthesize rGO-PDA-L-Lys-Cu.Then the copper ion in rGO-PDA-L-Lys-Cu was removed to prepare rGO-PDA-L-Lys chiral composite to fabricate an electrochemical chiral recognition platform for the tryptophan（Trp）enantiomers.Scanning electron microscopy,infrared spectroscopy,XPS and other characterizations indicated that the composites were synthesized successfully.Differential pulse voltammetry can reflect the difference in recognition of L-Trp and D-Trp of chiral interfaces on electrochemical signals.The results showed that rGO-PDA-L-Lys had a stronger effect on D-Trp than L-Trp.This was because diastereomers were formed between D-Trp and L-Lys.UV-visible spectroscopy was further confirmed this conclusion.In addition,the stability and anti-interference ability of the sensing platform were also studied.rGO-PDA-L-Lys/GCE can also be used for the detection of real samples.The effects of concentration,incubation time,pH,and recognition temperature on the recognition effect were also explored.2、In the previous chapter,small amino acid molecules were selected as chiral selectors.Although L-Lys had certain chiral sites and chiral centers,when small molecules were used as chiral selectors,there were fewer chiral sites.Besides,It were not very stable to use graphene functionalized by physical methods as the base material.Therefore,in this chapter,chitosan（CS）with more chiral sites was selected as the chiral selector.And 3,4,9,10-perylene tetracarboxylic acid was intercalated into GO throughπ-πinteraction,and rGO-PTCA with carboxyl group was obtained after reduction,which had a three-dimensional porous structure,realizing the transition of base material from two-dimensional to three-dimensional.Chiral composites（rGO-PTCA-CS）can be obtained by the amidation reaction between the carboxyl group on rGO-PTCA and the amino group of chitosan.SEM,FT-IR,XPS and electrochemical approaches were used to characterize the composites.The graphene-based chiral material with enantioselectivity was constructed by combining the chiral properties of chitosan and the excellent electrochemical properties of rGO-PTCA for the electrochemical chiral recognition of Trp enantiomers.rGO-PTCA-CS/GCE had higher recognition ability on L-Trp than D-Trp with an enantioselectivity coefficient of 3.0.The sensor had a linear response in the concentration range of 1 mM to 10 mM for Trp.The detection limit for L-Trp and D-Trp were 1.2μM and 3.0μM（S/N=3）,respectively.In addition,the electrochemical chiral sensing platform can be used for the detection of real samples,while exploring the detailed recognition mechanism.3、In the above two chapters,graphene was used as the base material,and graphene was modified by coating or non-covalent force to prevent agglomeration caused byπ-πinteraction.However,there are fewer active sites for graphene modified by these two methods.so in this chapter,graphene was modified by heteroatom doping.Heteroatom doping can solve the agglomeration to a certain extent and increase the active sites to improve the recognition efficiency.In addition,although single polysaccharide had more chiral sites,it will aggregate due to intermolecular forces.Taking the above factors into consideration,using carboxymethylβ-cyclodextrin（β-CD-COOH）and chitosan（CS）as raw materials,a chiral selector based on intigrated polysaccharides was prepared via the amidation reaction of-COOH onβ-CD-COOH and-NH₂ on CS.The electrochemical chiral sensor was constructed by combining the advantages of polysaccharide CS-β-CD and three-dimensional nitrogen and sulfur co-doped graphene（NSG）.CS-β-CD as a chiral selector can provide a large number of chiral sites,while NSG as a substrate material can amplify the electrochemical signal.The chiral recognition of Trp enantiomers by chiral sensing platforms was studied using differential pulse voltammetry.NSG/CS-β-CD modified GCE had higher electrochemical signal to L-Trp than D-Trp.When the Trp isomer and NSG/CS-β-CD form a diastereomeric-selector complex,it had different steric hindrances,which made it easier for L-Trp to penetrate the modified electrode membrane and reach the electrode surface,thus resulting in larger peak currents.UV-Vis spectroscopy further proved that CS-β-CD had higher binding energy to D-Trp.In addition,the electrochemical sensor can be used for the detection of the real samples.