Construction Of Electrochemical Sensor Based On Overoxidized PEDOT And Overoxidized PPy Composite Films And Electrocatalytic Mechanism

As common conductive polymers,PEDOT and PPy have some commonalities,such as five-membered heterocyclic rings in the structure,similar polymerization mechanisms,similar dopant types and polymerization potential range matching,etc.,so they are easy to combine.However,PEDOT and PPy are susceptible to overoxidation reactions under certain conditions(such as high potential,strong oxidants,etc.),showing shorter conjugated chains of polymers and decreased conductivity.Therefore,in this dissertation,suitable methods have been explored to improve the electrochemical ability of overoxidized PEDOT(OPEDOT)and overoxidized PPy(OPPy)and applied to the construction of electrochemical sensors based on the two overoxidized polymers for highly sensitive and selective detection of rutin and luteolin.Meanwhile,the mechanism of the enhanced electrocatalytic ability of OPEDOT-OPPy composite films have been discussed in detail.(1)Fabrication of the electrochemical sensor based on GQDs/PEDOT composite electrode material for the determination of rutinCombined with the large specific surface area of graphene quantum dots(GQDs)and the high conductivity of PEDOT,GQDs/PEDOT/GCE were prepared by two-step CV electropolymerization method for rutin detection.Compared with GQDs/GCE and PEDOT/GCE alone,the current responses of GQDs/PEDOT/GCE to rutin were increased by approximately 10 times and 3 times,respectively.The sensor had a wide linear range(0.05 to10?mol/L),low detection limits(11 nmol/L),and a high recovery rate(>90%)for reliable analysis of rutin in rutin tablets.(2)Electrochemical sensors based on OPPy/OPEDOT/GQDs-CS composite films for the extraction process optimization of luteolin in peanut shellsGQDs and CS were combined as stable conductive substrates for the composite electrode.OPPy/OPEDOT/GQDs-CS/GCE was prepared by layer by layer assembly technology and CV polymerization overoxidation methods.However,the composite electrode had no current response to luteolin.After incubating in Na OH-Et OH solution for 30 min,the current on the composite electrode was increased to approximately 30?A,the R_ct value of the composite electrode decreased by approximately 54 times,and the specific surface area increased by approximately 7 times.By BBD method,the optimum preparation condition of the composite electrode was as follows:the mass ratio of CS to GQDs of 5,the number of polymerization cycles of EDOT and PPy of 4,respectively.The peak current of luteolin on the optimized composite electrode increased to 43.21?A,thus improving the sensitivity of the sensor.This sensor had good the linear relationship in the concentration range of 0.01?10?mol/L with the low detection limit of 7.5 nmol/L(S/N=3).By XPS and FT-IR analysis,it had been shown that both PPy and PEDOT were overoxidized before and after incubation,and the content of-C=N-and-SO₂/-SO groups in composite films increased after incubation,indicating the further overoxidation of the composite films.Next,this fabricated electrode was applied to optimize the extraction process of luteolin in peanut shells.By BBD method,the optimum extraction process was as follows:the liquid-to-feed ratio of 120 m L/g,the extraction temperature of 40°C,the extraction time of 20 min,the ultrasonic power of 90%,and the extraction rate was 0.769%(RSD=2.676%),which was close to the predicted value of 0.810%.The extraction rate of luteolin in the peanut shell produced in Shangqiu City,Henan Province was highest,followed by Yuncheng City,Shanxi Province,and finally Longyan City,Jiangsu Province,which was close to the results measured by the HPLC method,indicating that the constructed electrochemical sensor can be used for the accurate detection of luteolin in peanut shell extract.(3)Construction of rutin electrochemical sensor based on OPEDOT-OPPy molecularly imprinted polymerization filmThe molecularly imprinted electrochemical sensor based on OPEDOT-OPPy composite film with a three-dimensional worm-like nanorod structure was constructed by CV copolymerization overoxidation methods with EDOT and Py as bifunctional monomers and rutin as template molecule.The preparation conditions of MIP were optimized by orthogonal experimental design method as follows:the number of polymerization cycle of 2,the molar ratio of EDOT to Py of 8:4,the polymerization potential range of 0?1.7 V,and the scanning rate of 0.08 V/s.Compared to MIP/1.5 and MIP/1.9,MIP/1.7 had better properties such as higher currents to rutin,less charge transfer resistance,faster charge transfer rates,and higher N⁺and S⁺doping ratio according to the electrochemical and XPS analysis.The constructed sensor had a wide detection range(0.5 nmol/L?1.5?mol/L and 5?50?mol/L)and low detection limit(0.24 nmol/L).The peak current of MIP/1.7 for rutin was 3 times that of NIP/1.7,and much higher than that of other flavonoids with similar structures,exhibiting high sensitivity and selectivity.The measured content of rutin in Flos Sophorae Immaturus based on MIP/1.7sensor was 25.36%(RSD=3.11%),which was close to 27.24%(RSD=2.16%)measured by the HPLC method,showing the accuracy of the established method.(4)Discussion on the overoxidation behavior of PEDOT-PPy composite filmWith the increase of positive end potential(E_up),the current response of the composite electrodes prepared in Li Cl O₄-water,Li Cl O₄-acetonitrile,TEAP-acetonitrile and SDS-water to rutin and luteolin gradually decreased or disappeared.The XPS analysis showed the-C=O group content of the composite film increased as the E_up increased.With Li Cl O₄ as a dopant,the composite polymer film prepared in acetonitrile had higher doping ratio and lower-C=O group content than that prepared in water.In the same solvent,the composite film prepared with macromolecular TEAP and SDS had a higher doping ratio and lower-C=O group content compared with small molecule Li Cl O₄.Therefore,the composite film prepared in Li Cl O₄-water had the largest degree of overoxidation and the weakest electrocatalytic ability,while the composite film prepared in TEAP-acetonitrile solution with the potential range of 0?1.7 V maintained relatively high electrocatalytic ability.(5)Electrocatalytic mechanism discussion of OPEDOT-OPPy composite filmThe electrocatalytic behavior of OPEDOT-OPPy/GCE prepared in Li Cl O₄-water with the potential range of 0?1.7 V was studied.The peak current of the composite electrode for rutin and luteolin increased significantly after incubation in Na OH:Et OH with the volume ratio of1:1.The composite film combined the high electrocatalytic ability of OPEDOT and the high selectivity of OPPy.Both the effective specific surface area and the electron rate constant of the prepared composite electrode after incubation increased significantly when E_up was greater than 1.5 V.By structural analysis,the proposed electrocatalytic mechanism was as follows:before incubation,the-C=O and a small number of-SO₂/-SO groups were formed in OPEDOT-OPPy composite film prepared at the high potential,resulting in shorter conjugated chains and decreased conductivity.The content of-SO₂/-SO groups in the composite film after incubation increased significantly,promoting the thiophene ring from benzene-type to quinone-type structure transformation.The increase of-C=N-groups content in composite film promoted the formation of quinone-type structure and improved conjugation effect of benzene-type pyrrole ring.This may promoted the delocalization of?electrons between interchain and intra-ring in composite films,thus improving the conductivity.Therefore,it can be inferred that there was synergistic effect between OPEDOT and OPPy in the composite film,which jointly improved the electrocatalytic ability of the composite film for rutin and luteolin.Through density functional theory(DFT)calculations,OPEDOT[6]and OPPy[6]in the composite film after incubation had weakened electron-giving ability and enhanced electron acceptance ability,and the smaller energy gap(E_g),which was conducive to the electronic transition from HOMO to LUMO,thus improving conductivity.The optimum geometric structure of both OPEDOT[6]and OPPy[6]after incubation were highly parallel to that of luteolin molecule,which was beneficial to the overlap of?electron clouds and enhancement of hydrogen bonding interaction.The adsorption energy(?E_ad)of the complex of OPEDOT[6]and OPPy[6]with luteolin was smaller than that of the complex before incubation,signifying the enhanced adsorption ability to luteolin.The improvement of these performance was mainly related to the formation of quinone-type structure,-C=N-and-SO₂/-SO groups in OPEDOT-OPPy composite films after incubation,thus illustrating the rationality of the proposed electrocatalytic mechanism.