Development Of Solid-state Ion-selective Electrode Detection Systems And Their Applications In Seawater Analysis For Potassium And Calcium

Determination of chemical components in seawater is of great importance.Potassium and calcium are the main elements of alkali and alkaline earth metals in seawater,which play important roles in marine organism systems and marine ecological environment.Nowadays,methods for the determination of potassium and calcium in seawater include atomic spectra,molecular spectrum,mass spectrum and so on,but these methods suffer from problems of complicated equipment,cumbersome operation and expensive apparatus.Ion-selective electrodes?ISEs?are electrochemical sensors that can convert the activity of a target ion into an electrical potential as the measurable signal.As routine analytical tools,ISEs have been widely applied in clinical diagnostics,process control,and environmental monitoring.In a traditional ISE measurement system,a liquid-contact ISE with a polymeric ion selective membrane,an inner solution,and an inner reference electrode is connected with the reference electrode coupled with a salt bridge through a sample solution.However,the presence of the inner solution and inner reference electrode in the liquid-contact ISE,and the usage of the salt bridge containing the aqueous solution in the outer reference electrode can cause problems of difficult maintenance,large volume and inconvenient handling of the ISE.Thus,it is highly desirable to develop all-solid-state ISE measuring systems.Untile now,very few papers for seawater analysis using ISEs have been reported.Fabrication of solid-state ISEs detection systems to analyze seawater components would provide instructive guidelines for marine chemistry.Herein,solid-state ion-selective electrode detection systems based on novel solid-state ISEs and solid-state reference electrodes have been developed and applied for the potentiometric analysis of potassium and calcium ions in seawater.Five ISE systems have been developed and investigated in detail:1.Solid-state ion-selective electrodes with MoS₂ nanoflowers as ion-to-electron transducerCoated-wire electrodes are the earliest solid-state ISEs.However,such electrodes suffer from problems of potential instability,which is due to the lack of defined phase boundary potentials at the interface of the ISE membrane and the underlying solid substrate.Ion-to-electron transducers based on the double-layer capacitance transduction mechanism have been developed to improve the potential stability.However,solid-contact layers based on this transduction mechanism are mainly limited to carbon and noble metal materials.Until now,there are no solid-contact layers based on non-carbon and non-noble metal materials.Herein,MoO₃ nanobelts are used as precusors to synthesize MoS₂ with certain morphologies through a hydrothermal process.Under optimized conditions,MoS₂ nanoflowers and MoS₂ nanoparticles can be obtained,and the corresponding reaction mechanisms have been proposed.Additionally,MoS₂ nanoflowers working as carbon analogues have been developed as novel ion-to-electron transducers to fabricate novel solid-contact K+-ISEs?SC-K+-ISE?.The proposed SC-K+-ISE has been characterized and applied for the determination of K+ in seawater.2.Solid-state ISEs based on MoO₂ microspheres as ion-to-electron transducerEnlightened by the MoS₂ nanoflower working as carbon analogue for the ion-to-electron transducer,we have developed the metal analogue?MoO₂ microspheres?as novel ion-to-electron transducer for use in SC-ISEs based on the double-layer capacitance transduction mechanism.In this work,MoO₃ nanobelts and MoO₃ microrods have been applied as precursors to synthesize MoO₂ in isopropanol via a solvothermal method.Under optimized conditions,mono-dispersed MoO₂ microspheres and conjunctive MoO₂ microspheres have been produced and characterized,respectively.The corresponding reaction mechanisms have been proposed.Due to the poor uniformity in diameter for the conjunctive MoO₂ microspheres,monodispersed MoO₂ microspheres have been used as solid-contact layer for the novel SC-K+-ISE.The ISE shows a Nernstian response of 55.0 mV/decade for potassium activity in the range from 10-5 to 10-3 M,and the limit of detection is 10-5.4 M.The electrode shows a fast and good potential response with excellent resistance to disturbances of light,O₂ and CO₂.The introduction of MoO₂ microspheres has been found to improve the potential stability and capacitance,decrease the impedance,and reduce the water layer.As a result,MoO₂ microspheres working as metal analogue can be used as a new ion-to-electron transducer for developing solid-state ISEs.3.Solid-state ISEs based on Ag@AgCl/KCl transducerIon-to-electron transducers based on the redox capacitance transduction mechanism are the earliest solid-contact layers investigated.Those materials are mainly based on organic materials.Exploring inorganic materials as solid-contact layers,with the redox capacitance transduction mechanism,would pave a new avenue for the development of SC-ISEs.Enlightened by the Ag/AgCl reference electrode,we have developed a novel ion-to-electron transducer based on the inorganic redox material Ag@AgCl/KCl.Herein,Ag@AgCl with core-shell morphologies has been synthesized and applied as the inorganic redox active material.With the use of KCl as Cl-source,a novel redox system can be obtained.By using this inorganic redox transducer,a novel solid-state Ca²⁺ selective electrod has been developed.The potentiometric response,selectivity,stability and the electrochemical properties for the SC-Ca²⁺-ISE with the inorganic redox transducer have been extensively investigated.Experiments show that Ag@AgCl/KCl can be employed as a good ion-to-electron transducer.The proposed SC-Ca²⁺-ISE has been successfully applied for the determination of Ca²⁺ in seawater.4.Solid-state Ca²⁺-ISE based on Ag@AgCl/ionic liquid transducerIt has been realized that the SC-ISE using Ag@AgCl/KCl as the transducer may suffer from problems of instability.With the dissolution of KCl,the hydrophilic ions such as K+ and Cl-would continuously diffuse from the polymeric membrane to the sample solution.This would affect the potential stability and life time of the ISE.To solve this problem,the ionic liquid in the redox system has been used to replace KCl,and a novel inorganic redox transducer Ag@AgCl/TMMCl has been proposed.With the Ca²⁺-ISE as a model,we have fabricated a SC-Ca²⁺-ISE and characterized the electrochemical properties.The developed SC-Ca²⁺-ISE shows a linear response to Ca²⁺ in the activity range of 10-6 to10-2 M with a detection limit of 10-6.5 M.The fabricated SC-Ca²⁺-ISE shows a good potential response and electrochemical properties.Additionally,we have evaluated the selectivity coefficients of this SC-Ca²⁺-ISE and applied it for the determination of Ca²⁺ in sea water.5.Solid-state reference electrodes based on 2,2,6,6-tetramethylpiperidinyloxyl?TEMPO?Conventional reference electrodes are usually based on Ag/AgCl and Hg/Hg2Cl₂ half cells,which are connected to the sample through a salt bridge.An aqueous solution of an equitransferent salt is usually used as the salt bridge to minimize the liquid junction potential at the interface between the salt bridge and the sample solution.Although such reference electrodes are stable and reliable,the salt bridges may suffer from problems of difficult maintenance,large volume and mutual contamination between the bridge electrolyte and sample solution.In this work,the reference membrane has been fabricated by using TEMPO as redox active material,PVC as substrate,o-NPOE as plasterizer and ionic liquid as salt bridge.In order to reduce the presence of the water layer and to improve the potential stability,graphene is used as transducer to develop a novel solid-state reference electrode.The ionic potential responses to solutions,and the pH effect and resistances to disturbances have been tested for the solid state reference electrode.The reliability and accuracy of the novel solid-state reference electrode have been verified by the amperometric and potentiometric analyse,which are comparable with those of the commercial reference electrode.Additionally,with the screen printed electrode configuration,a miniturized potentiometric ISE measurement system for Ca²⁺-ISE has been fabricated and applied in the determination of Ca²⁺ in seawater.