Research And Application Of Chemically Modified Electrodes In The Detection Of Atmospheric Hydroxyl Radicals

Hydroxyl radical(·OH)is considered to be the most important oxidant in the atmosphere,which plays an important role in the photo-oxidation cycle and controls the auto-oxidation capacity of the atmosphere.The main existing detection techniques include laser induced fluorescence(LIF),differential optical absorption spectroscopy(DOAS),chemical ionization mass spectrometry(CIMS)and electron spin-resonance(ESR)spectroscopy.Most of these technologies do not meet the practical requirements of simple operation,low cost,portable or high sensitivity,and are difficult to be applied to large-scale,long-term detection of ·OH in the atmosphere.Therefore,it is of great significance to develop a simple and sensitive method to detect the ·OH in the atmosphere for the atmospheric environment chemistry,and the electrochemical method has the advantages of simple operation and high sensitivity to satisfies the detection demand.This paper mainly introduces several new techniques and applications of the detection of·OH in the atmosphere based on electrochemical methods.They are based on 4-hydroxybenzoic acid(4-HBA)as a molecular probe,which can efficiently capture ·OH and produce a single product-3,4-dihydroxybenzoic acid(3,4-DHBA)with electrochemical activity.3,4-DHBA has been determined by the electrochemical method to detecting atmospheric ·OH indirectly.Part 1.Simple and sensitive determination of hydroxyl radical in atmosphere based on an electrochemically activated glassy carbon electrodeThe glassy carbon electrodes were activated in H2SO4 solution and NaOH solution respectively by polarization or cyclic voltammetry.The surface structure changes of electroactive electrodes under different preparation conditions were analyzed by X-ray photoelectron spectroscopy(XPS).The results showed that GCE activated with anodic polarization in sulfuric acid solution(AGCE-P/H2SO4)performed the highest electrochemical activity.The electrochemical behavior of 3,4-dihydroxybenzoic acid on AGCE-P/H2SO4 was studied.There is a good linear relationship between the oxidation peak currents of DPV response and the standard concentrations in the range of 2.0×10-7～1.0×10-4mol L-1.The detection limit was 6.2×10-8 mol L-1(S/N=3).The feasibility of 3,4-dihydroxybenzoic acid as a target molecule for indirect detecting hydroxyl radicals was studied by the Fenton reaction.This method has been successfully applied for the detection of hydroxyl radical concentration in atmosphere without separation and purification process.Part 2.Determination of hydroxyl radicals in atmosphere based on electropolymerized-molecularly imprinted polymer sensorThe molecularly imprinted polymer film-modified electrode(3,4-DHBA-PPY/TiO2/GCE)was prepared in-situ on the nano-TiO2 modified glassy carbon electrode by electrochemical polymerization.In the sensor,pyrrole played as the functional monomer and 3,4-dihydroxybenzoic acid played as the template molecule.The sensitivity and selectivity of detecting 3,4-dihydroxybenzoic acid were studied and the determination of ·OH indirectly was achieved.The sensor was characterized by scanning electron microscope(SEM)and the electrochemical methods.A few preparation or determination conditions were optimized,such as the number of electropolymerization cycles,pH in the electropolymerization process,elution time.Under the optimum conditions,3,4-DHBA was detected with a linear response in the range of 1.0×10-8～1.0×10-6 mol L-1 and the detection limit was down to 4.2×10-9 mol L-1(S/N=3).The sensor was applied to measure the concentration of hydroxyl radical in the atmosphere,which is a new approach with low cost and convenience.Part 3.Determination of hydroxyl radicals in atmosphere based on molecularly imprinted polymer modified electrodeThe MIP/TiO2 composite was prepared by chemical oxidation on the surface of nano-TiO2.In the synthesis process,pyrrole,3,4-dihydroxybenzoic acid(3,4-DHBA)and FeCl3 took the responsibility of monomer,template molecule and oxidant,respectively.The 3,4-DHBA molecularly imprinted polymer modified electrode(MIP/TiO2/GCE)was prepared by drop-coating method on the surface of glassy carbon electrode.The sensitivity and selectivity of detecting 3,4-dihydroxybenzoic acid was studied and the determination of ·OH indirectly was achieved.The sensor was characterized by scanning electron microscope(SEM),transmission electron microscope(TEM),fourier transform infrared spectroscopy(FT-IR)and the electrochemical methods.A few preparation or determination conditions were optimized,such as mass ratio of pyrrole to 3,4-DHBA,mass ratio of pyrrole to nano-TiO2,polymerization time,incubation time.Under the optimum conditions,3,4-DHBA was detected with a linear response in the range of 1.0×10-10～1.0×10-7 mol L-1 and the detection limit was down to 2.3×10-11 mol L-1(S/N=3).The sensor has been applied to measure the concentration of hydroxyl radical in the atmosphere,and the result is satisfactory.