| The detection and of nitrate content in the environment is of great significance for social development and human health.Traditional detection methods have some disadvantages such as complex equipment and difficult on-site monitoring.Therefore,it is urgent to develop new methods for detection of nitrate.Ion-selective electrodes are an important type of electrochemical sensors,which have the advantages of simple equipment,convenient operation and free from the interference of complex matrix.They have been widely used in environmental monitoring,medical diagnosis,soil determination and so on.Although the traditional ion-selective electrodes with inner filling solution show high potential stability,they require frequent maintenance,due to the leakage of the inner filling solution during use.In recent years,the solid-contact ion-selective electrodes can effectively avoid the influence of the steady-state primary ion fluxes from the membrane phase to the sample solution phase,due to the absence of the inner filling solution.With the advantages of fast potential response,good portability,low cost and easy miniaturization,it has become a new hotspot in the field of the potentiometric sensors.In addition,the response principle of solid-contact polymeric membrane nitrate-selective electrode is based on the relationship between the potential response of the ion-selective membrane and the activity of nitrate ions in accordance with the Nernst equation under the condition of zero current.The Nernst response slope is 59.2m V/decade at 25℃,which means that 1 m V potential change will cause 4%ion activity change.This phenomenon indicates that the potential response obtained under the zero current condition is not favorable for detection of nitrate with small concentration changes.Based on this,two new readout strategies have been proposed to replace the potential signal of solid-contact polymeric membrane nitrate-selective electrode,and the applications of the proposed readout strategies have been demonstrated for determination of nitrate in drinking water.The main contents included the following aspects:1.This chapter mainly was introduced the significance for the detection of nitrate;reviewed the concept,structure,response principle and evaluation parameters of ion selective electrodes;summarized the relevant contents of all-solid-state ion selective electrode,and discussed the response principle of ion selective electrodes with different types of solid-contact conductive layer;summarized the new readout strategy in recent years;proposed the main research contents of this paper.2.The coulometric signal readout of solid-contact polymeric membrane nitrate-selective electrodes were constructed.The principle of the readout method is to apply a constant potential between the solid-contact polymeric membrane nitrate-selective electrodes and the reference electrode.The change of the concentration of nitrate ion in the solution will cause the potential change of the selective sensitive film of nitrate.Meanwhile,electrical double layer capacitance of ordered mesoporous carbon（OMC）compensating the potential change at the ion-selective membrane/solution interface during the measurements.This process will produce a transient current until a new equilibrium state is obtained between the solid-contact polymeric membrane nitrate-selective electrodes and the counter electrode.The electric value is obtained by integrating the current-time real-time.The coulometric readout signals of the solid-contact polymeric membrane nitrate-selective electrode shows two linear responses in the concentration range of 1.0×10-6-8.0×10-6 M and 8.0×10-6-8.0×10-4 M,and the detection limit is 4.0×10-7 M（3σ/s）.In order to demonstrate the analytical applications for the coulometric signal,the OMC-based solid-contact ion-selective electrode was tested by measuring NO3-in mineral water samples.The results detected by the coulometric signal are comparable to those obtained by the traditional potentiometric response and the ion chromatography,which indicates the coulometric signal for the solid-contact polymeric membrane nitrate-selective electrodes is reliable and acceptable.3.The hydrogen peroxide（H2O2）oxidation current readout signal of solid-contact polymeric membrane nitrate-selective electrode was constructed.The proposed readout method is combined with the high sensitivity of current sensor and the high selectivity of potential sensor.The working electrode and the ion-selective electrodes based reference electrode are separately placed in the two electrochemical cells in order to avoid the mutual interfering from the different background solutions,and a salt bridge is used to connect these cells.A constant voltage was applied between the working electrode and the solid-contact polymeric membrane nitrate-selective electrode to promote the oxidation of H2O2 on the working electrode,and obtain the oxidation current.By changing the concentration of nitrate ions,the potential of the solid-contact polymeric membrane nitrate-selective electrode is adjusted to change the potential of the working electrode,and then the oxidation current of H2O2 will change.According to the change of oxidation current of H2O2,the concentration of nitrate ion can be determined.The H2O2oxidation current readout signals of the solid-contact polymeric membrane nitrate-selective electrode shows two linear responses in the concentration range of 5.0×10-6-1.0×10-4 M and 1.0×10-4-1.0×10-3 M,and the detection limit is 3.2×10-6 M（3σ/s）.The concentrations of NO3-in mineral water samples were measured by the proposed amperometric signal of H2O2,and the results obtained by the amperometric signal readout based on H2O2 agree well with those detected by the potentiometric responses,indicating that the transformation from the potential response of ISEs to the amperometric readout is feasible and reliable. |
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