Study On Charge Transfer And Immunosensing Properties Of Poly-o-phenylenediamine-dopamine Films

Carcinogenesis caused by various factors has become one of the major causes of death worldwide.When the cancer has spread at an advanced stage,neither chemotherapy nor radiation is effective and the chances of survival are expected to be very low.If the cancer is diagnosed before the tumor becomes bigger or the cancer cell has spread,the patient survival rate would be greatly improved.Therefore,it is of significance to develop fast,simple,low-cost,and accurate diagnostic equipments.Electrochemical immunosensor can meet the demand of cancer diagnosis in terms of detection speed and cost.Electrochemical immunosensors can detect cancer by the accurate detection of tumor markers in serum.Therefore,in this article,antibody(Ab)was immobilized on co-poly-phenylenediamine-dopamine to produce electrochemical immunosensor.Electrochemical immunosensor could determine whether there is liver cancer by detecting alphafetoprotein(AFP)in serum.The study of this immunosensor is divided into the following two parts:1.Sutdy on the mechanism of electron and proton transfer in the redox process of poly(o-phenylenediamine)(POPD)films.Poly-o-phenylenediamine(PoPD)as conducting polymer has been used in numerous fileds,but their electrochemical properties are not well justified due to the complexity of the redox process.There are several opinions on the redox reactants and conductivity of various states of PoPD.Therefore,the in-depth research on the electrochemical mechanism and structure of PoPD is required.Firstly,cyclic voltammetry(CV)experiments were carried out on Poly-o-phenylenediamine modified gold electrode(Ge/POPD)in electrolyte solutions with different p H,and the effects of proton concentration on the redox peak potential and current of Ge/POPD were analyzed.The relationship between redox peak potential and proton concentration is obtained by linear fitting of the CV data.It is deduced that the ratio of proton to electron is 1:1 in the redox reaction of semi oxidation state/reduction state,while the ratio of proton and electrons is 2:1 in the redox reaction of semi oxidation state / oxidation state.A redox reaction model of PoPD is proposed based on the effects of proton concentration on the redox peak potential and its rationality is deduced by using Nernst equation.Electrochemical impedance spectroscopy(EIS)further confirmed the characteristics of electron and proton transfer in different redox processes,and confirmed that the reduced POPD had lower charge transfer resistance than the oxided POPD.The linear relationship between the capacitive reactance and the reciprocal of frequency for reduced POPD also shows a rapid charge transfer in reduced POPD accompanied by a1:1 ratio of proton to electron;The nonlinear relationship between the capacitive reactance and the reciprocal frequency of oxidized POPD shows a charge transfer with different charge numbers of protons and electrons,which may be carried out by protons,sulfate ions and electrons.2.Preparation and charge transfer mechanism of poly(o-phenylenediamine dopamine)(POPD-DA)thin films.Based on the preparation method and charge transfer mechanism of poly(o-phenylenediamine)thin films,POPD-DA composite films with rich functional groups were electrodeposited by cyclic voltammetry.The relationship between redox peak potential and electrolyte p H as well as the charge transfer mechanism of POPD-DA were studied.POPD-DA with rich functional groups is helpful to improve the performance of immunosensor.CV analysis showed that there was a linear relationship between the cathodic peak potential and p H with a linear correlation slope of-0.109 V / p H;In the conversion reaction of reduced state and semi oxidized state,the linear change between anode peak and p H is about-0.078V/p H.The results show that the ratio of proton to electron is 2:1 in the reaction of semi oxidation state and oxidation state,and the ratio of proton to electron is 1:1 in the reaction of reduction state and semi oxidation state.POPD-DA has the same electron and proton transfer mechanism as POPD and exhibits similar redox electrochemical properties to POPD.EIS impedance spectra and CV test show that the electrochemical properties of POPD-DA change slightly comparing to POPD with the charge transfer resistance increased slightly and the redox standard electrode potential decreased,but GE/ POPD-DA still retains the redox performance of the main frame of POPD.3.Based on the GE/PoPD-DA electrode,GE/POPD-DA/Ab/BSA electrochemical immunosensor is further designed and prepared by applying and fixing protein for reaction on the electrode.The structure and morphology of PoPD,PDA and PoPD-DA films have been studied by FTIR,UV-Vis,SEM and surface contact angle.The results show that PoPD-DA was not a simple mixture of POPD and PDA,but a composite film of a new compound.The contents of amino and hydroxyl in popd-da were higher than those in POPD and PDA.These amino groups and hydroxyl groups on PoPD-DA can react with alpha fetoprotein antibody(AB)and bovine serum albumin(BSA)under mild conditions by Schiff base reaction and Michael addition reaction,and directly immobilize AB and BSA on the GE/PoPD-DA to obtain the GE/POPD-DA/Ab/BSA electrode.The antigen alpha fetoprotein(AFP)protein in the solution can be directly detected by the specific binding of AB antibody.AB,BSA and AFP on the surface of GE/PoPD-DA electrode will hinder the mass exchange between the membrane and the solution.AFP solutions with different concentrations can be detected by measuring the decrease of the oxidation peak current.The detection of AFP on Ge/POPD-DA/AB/BSA sensor is not interfered by other proteins such as BSA,Ig G and CEA,showing good selectivity,sensitivity,and stability.When applying to the detection of actual human serum,it is found that the detection results are consistent with the expected values.After refrigerated for 21 days,the detection value can still be maintained.Based on the redox properties of POPD,the as-prepared sensor can directly fix and detect antibodies,with the advantages of no marker,no additional redox medium,one-step measurement,low detection limit and simple operation.