Research And Application Of Electrochemical Sensors In The Determination Of Hydroxyl Radicals In Atmosphere

Hydroxyl radicals(·OH)is the main active substance in the troposphere which plays an important role in the oxidation reaction and atmospheric chemistry.Though it’s concentration in the troposphere is very low,it determines the life span of almost all pollutants and the emergence of secondary pollution.It is very important to make full sense of the concentration change of ·OH in the troposphere and its related chemical properties which would help us accurately predict the atmospheric self-purification process and the formation of secondary atmospheric pollutants.Accurately measuring the concentration of·OH in the atmosphere has become one of the important research contents inthe world.In this thesis,salicylic acid was used as the probe molecule for the indirect detection of ·OH.Salicylic acid can react with ·OH quantitatively and controllably to generated electroactive 2,5-dihydroxy benzoic acid(2,5-DHBA).According to the electrochemical activity of 2,5-DHBA,a series of electrochemical methods were constructed for the determination of·OH.This thesis studied the detection principle and performance of these methods systematically.These methods were successfully applied to the detection of ·OH in the atmosphere.The main research contents and results are as follows:(1)A method for the detection of ·OH was reported by using a 2,5-DHBA molecularly imprinted polymer(MIPs)modified glassy carbon electrode(GCE).2,5-DHBA MIPs was prepared by a simple electrochemical polymerization method.Field emission scanning electron microscope(SEM)and electrochemical impedance were used to characterize MIPs/GCE.The performance of the MIPs/GCE electrochemical sensor to 2,5-DHBA was determined by differential pulse voltammetry(DPV).The linear range for the concentration of 2,5-DHBA was from 3.0×10-7 to 4.0×10-5 mol L-1 and the detection limit was down to 1.5×10-8 mol L-1.A linearity for ·OH concentration ranging from 1.8×1014 to 2.4×1016 molecules cm-3 was obtained with the detection limit for ·OH down to 6.1×1012 molecules cm-3 by using the MIPs/GCE.This work has been applied for the detection of ·OH concentration in the atmosphere.(2)2.5-DHBA was imprinted on the reduced graphene oxide(rGO)via a simple electrochemical method by using pyrrole as the functional monomer.rGO not only provides abundant active sites for 2,5-DHBA imprinting but also improves the conductivity and mass transportation,increases the accessibility of active sites for 2,5-DHBA recognition.SEM and electrochemical impedance were used to characterize MIPs/rGO/GCE.The differential pulse voltammetry(DPV)signals of 2,5-DHBA were used to evaluate the performance of the molecularly imprinted polymers(MIPs)based sensor.The linear range for the concentration of 2.5-DHBA was from 5.0×10-8 to 4.5×10-5 M and the detection limit was down to 2.74×10-9 M.A linearity for ·OH concentration ranging from 3.0×1013 to 2.7×1016 molecules cm-3 was obtained.The low detection limit for ·OH was down to 1.6×1012 molecules cm-3 by using this sensor.The atmospheric ·OH concentration in Yangzhou has been successfully detected by this method when the sampling time was 6 h.This work extended the molecularly imprinted polymers to the indirect detection of ·OH in the atmosphere with a simplified sample collection and detection process and good sensitivity.(3)Carbon fiber paper(CFP)was used for the building of an electrochemical sensor because of its good adsorption performance and electrical conductivity.By functionalizing the activated CFP(sc-hrCFP)with salicylic acid,an electrochemical sensor for determination of atmospheric ·OH was constructed.The sensor combined the collection and determination device of ·OH,which simplified the determination process of atmospheric ·OH.The activated CFP(sc-hrCFP)was prepared by pretreatment of CFP with Na3C6H5O7·2H2O combined with hydrothermal treatment which improved the conductivity and hydrophilicity of CFP.X photoelectron spectroscopy(XPS),contact angle(CA)and electrochemical impedance were used to characterize sc-hrCFP.Using the sc-hrCFP as the electrochemical sensor,the amount of the electroactive substance 2,5-DHBA which generated by the reaction of salicylic acid on sc-hrCFP and ·OH was detected by DPV,and the influence of the activation time of sc-hrCFP on the detection of 2,5-DHBA was optimized.This method provided a linearity for 2,5-DHBA from 1.0×10-13 to 3.0×10-10 mol L-1 with a detection limit of 2.1×10-14 mol L-1,which is corresponding to the amount of ·OH from 6.0×107 to 1.8×1011 molecules cm-3 with the detection limit of 1.3 × 107 molecules cm-3.This method has been applied to measure the atmospheric ·OH levels in Yangzhou to demonstrate its feasibility when the sampling time was 1h.(4)Gold nanoparticles(AuNPs)functionalized CFP(AuNPs-CFP)was prepared by using ethylenediamine as the coupling agent because of the good conductivity of AuNPs.The well immobilization and distribution of AuNPs on CFP surface was characterized by element mapping and energy spectrum method.The influence of concentration of electrochemical modified C2H8N2,cycles of electrochemical modified C2H8N2 and self-assembly time of AuNPs on detection of 2,5-DHBA was optimized.This method provided a linearity for 2.5-DHBA from 6.0×10-14 to 6.0×10-11 mol L-1 with a detection limit of 8.7×10-15 mol L-1,which is corresponding to the amount of ·OH from 3.6×107 to 3.6×1010 molecules cm-3 with the detection limit of 5.2×106 molecules cm-3.The sampling time was set at 30 min,and this method has been successfully applied to the determination of ·OH concentration under sunny in Yanghou in spring.Based on the ·OH concentration obtained by the method,the variation of ·OH concentration in one day was analyzed.(5)Based on the good adsorption performance of Covalent Organic Frameworks(COFs)to gas,COF(TpBD)functionalized CFP was prepared as a new electrochemical sensor for determination of ·OH after impregnating SA.The modification of CFP by COF(TpBD)increased the conductivity,the surface area and hydrophilicity of CFP.SEM,CA and fourier transform infrared spectrometer(FR-IR)was used to characterize COF(TpBD)-CFP.The amount of the electroactive substance 2,5-DHBA which generated by the reaction of salicylic acid and ·OH was detected by DPV.This method provided a linearity for 2,5-DHBA from 5.0×10-14 to 1.0×10-9 mol L-1 with a detection limit of 6.9×10-15 mol L-1,which is corresponding to the amount of ·OH from 3.0×107 to 6.0×1011 molecules cm-3 with the detection limit of 4.1×106 molecules cm-3.This method has been successfully used to detect ·OH concentration in different weather conditions in Yangzhou systematically when the sampling time was 30 min,and the conclusion was consistent with the reports in the literature.Compared with the previous methods,it can be found that this method has the advantages of high sensitivity,simple operation,strong stability,and can be reused many times,which provides an important application value for routine air monitoring.