Construction And Applications Of All-solid-state Ion-selective Electrodes With An NiCo₂S₄-based Ion-to-electron Transducer Layer

All-solid-state ion-selective electrodes（ASS-ISEs）have attracted much attention due to their superior properties including simple operation,easy miniaturization,low cost,and scalable capability for mass production.For these electrodes,a solid contact is usually used as an ion-to-electron transducer between the ion-selective membrane and the underlying electron conductor,which can provide the well-defined interfacial potential to improve potential drifts and standard potential（E^o）reproducibility.Although good performances have been reported by using various materials as solid contacts,it is still challenging for developing new high-performance transducer layer materials to improve the potential stability and E^o reproducibility for reliable data.Moreover,the growing demands for micro-area detection of ASS-ISEs are also important motivations for the miniaturization of ASS-ISEs.The conventional liquid membrane ion-selective microelectrodes have the disadvantages of short life time and high detection limit.It is very important to design all-solid-state ion-selective microelectrodes to improve the sensors’performance for micro-area detection.In this work,we introduce nickel cobalt sulfide（NiCo₂S₄）with a large theoretical specific capacitance as a solid contact,and have constructed ASS-ISEs and ASS-ISμEs,respectively.The developed electrodes have been used to detect calcium concentrations in water samples and heavy metal ion fluxes at the surfaces of plant roots.The detailed contents of this thesis are as follows:1.All-solid-state ion selective electrodes with an NiCo₂S₄ microstructure-based ion-to-electron transducer layerIn this work,we introduce nickel cobalt sulfide（NiCo₂S₄）as a solid contact for ion-to-electron transduction based on multiple redox couples.Three different morphologies of NiCo₂S₄ materials can be prepared by using a simple hydrothermal/solvothermal method.The morphologies of NiCo₂S₄ materials can be controlled by changing the polarities of the reaction solvents.The experimental results indicate that NiCo₂S₄ microspheres synthesized with ethanol as the solvent show excellent electrochemical properties.By using the calcium ion as a model and NiCo₂S₄ microspheres as the solid contact material,the developed all-solid-state calcium ion-selective electrode(ASS-Ca²⁺-ISE)exhibits a Nernstian slope of 27.5±0.2 m V/dec in the activity range from 1.0×10^-6 to 2.9×10^-2 M with a detection limit of 5.0×10^-7 M.A variation of the standard potential E°for eight individual solid-contact electrodes and four different batches can be obtained as low as 0.35 and 0.78m V,respectively.Due to the synergistic effect of cobalt and nickel ions in the ternary sulfide,an excellent redox capacitance（565μF）of the buried solid contact coated with the ion-selective membrane can be achieved.In addition,the introduction of NiCo₂S₄ reduces the water layer formed at the sensing membrane/NiCo₂S₄ interface and provides an excellent resistance to the interferences from light,O₂ and CO₂.The prepared ASS-Ca²⁺-ISEs have been used to measure the calcium concentration in seawater.Meanwhile,the all-solid-state potassium ion and nitrate ion-selective electrodes based on NiCo₂S₄ have also been constructed and characterized.The developed all-solid-state potassium ion-selective electrode shows a Nernstian slope of55.6±0.1 m V/dec in the activity range from 1.0×10^-5 to 7.6×10^-2 M with a detection limit of 1.6×10^-6 M,and the proposed all-solid-state nitrate ion selective electrode presents a Nernstian slope of 55.6±0.1 m V/dec in the activity range from1.0×10^-5 to 7.6×10^-2 M with a detection limit of 1.6×10^-6 M.Therefore,NiCo₂S₄could be used as a general ion-to-electron transducer for construction of ASS-ISEs with excellent performance and is a promising alternative for fabrication of calibration-free ASS-ISEs.2.All-solid-state calcium ion-selective electrode based on an in-situ synthesized highly hydrophobic NiCo₂S₄ nanosheet arrayCurrently,the dip-coating method is usually applied to prepare the SC layers.However,this method has a problem of an inefficient surface contact between the SC layer and the conductive substrate,which may deteriorate the performance of the solid contact layer.In this work,the NiCo₂S₄ nanosheets have been in-situ prepared at the surface of the glassy carbon electrode by electro-deposition.The morphology of the material can be controlled by electro-deposition cycle number.The NiCo₂S₄nanosheet array has been used as a solid contact to construct an all-solid-state Ca²⁺-ISE.The proposed Ca²⁺-ISE based on high hydrophobicity NiCo₂S₄ nanosheets exhibits a good Nernstian slope of 29.8±0.4 m V/dec in the activity range from 1.0×10^-6 to 2.9×10^-2 M with a detection limit of 2.0×10^-7 M.The proposed Ca²⁺-ISE shows a high potential stability of 1.9±0.5μV/h over 90 h due to the large redox capacitance of 1.8 m F.The outstanding reproducibilities for the NiCo₂S₄-based Ca²⁺-ISEs can be obtained with the single batch and batch-to-batch E°standard deviations of 0.28（n=6）and 0.67 m V（n=5）,respectively.In addition,the as-prepared NiCo₂S₄ nanosheet array presents a quasi-superhydrophobic with a contact angle of148.3±1.4°,which can effectively suppress the formation of the water layer at the sensing membrane/NiCo₂S₄ interface.The developed NiCo₂S₄ nanosheet arrays based Ca²⁺-ISE has been successfully applied for detection of calcium in water samples.3.All-solid-state lead ion-selective microelectrode with an NiCo₂S₄ nanowire-based ion-to-electron transducer layerFor micro-area detection,it is necessary to develop all-solid-state ion-selective microelectrodes（ASS-ISμEs）.However,it should be noted that the preparation processes of ASS-ISμEs developed so far are complex.Moreover,the lifetimes and stabilities of ASS-ISμEs still need to be improved.In this work,a strategy for water bath method and large-scale synthesis of NiCo₂S₄ nanowires which can be in-situ grown on carbon fibers is proposed,thus simplifying the preparation procedures for microelectrodes.The all-solid-state lead ion selective microelectrodes(ASS-Pb²⁺-ISμEs)can be obtained by filling the lead ion-selective membrane cocktail solution into the tip of a glass capillary by capillarity.The outer tip diameter of the developed ASS-Pb²⁺-ISμE is about 15μm,and the membrane thickness is about 60μm.Moreover,the lifetime of the ASS-Pb²⁺-ISμE extends to 6 days,which may be attributed to the increase in the membrane thickness.The proposed ASS-Pb²⁺-ISμE shows a high potential stability of 42.6±3.7μV/h over 28 h,and a good Nernstian response（a slope of 31.1±0.3 m V/dec）in the Pb²⁺activity range of 3.0×10^-8-8.1×10^-5 M.The prepared ASS-Pb²⁺-ISμE in this work provides an ideal analysis tool for subsequent micro-area ion detection.4.All-solid-state lead ion-selective microelectrode for in-situ monitoring lead ion fluxes near rice root surfacesThe scanning ion-selective electrode technique is promising for noninvasive ion flux detection.The system is usually composed of a liquid membrane ion-selective microelectrode（LM-ISμE）and a traditional Ag/Ag Cl reference electrode.The problems of the liquid membrane ion-selective microelectrode include short life-time and high detection limit.To solve these problems,the ASS-Pb²⁺-ISμE developed in the previous work has been used as the indicator electrode.For comparison,a liquid membrane lead ion-selective microelectrode has also been constructed.Experiments indicate that the ASS-Pb²⁺-ISμE shows a lower detection limit(2.5×10^-8 M)than the LM-Pb²⁺-ISμE(3.2×10^-7 M).In addition,the electrode lifetime has been dramatically improved by using the ASS-ISμE（i.e.,from a few hours to 6 days）.Using the rice roots as a model,both the ASS-Pb²⁺-ISμE and LM-Pb²⁺-ISμE have been applied as the indicator electrodes to monitor the lead ion fluxes near the rice root surfaces via the scanning ion-selective electrode technique.The results indicate that the ASS-Pb²⁺-ISμE can detect the change in ion flux at lower concentrations compared with the LM-Pb²⁺-ISμE.5.All-solid-state micro detection system based on an all-solid-state reference microelectrode and a copper ion-selective microelectrodePotentiometric measuring systems consist of an indicator electrode and a reference electrode.During the measurement process,the reference electrode that exhibits a stable and reproducible potential to obtain reliable data is required.In addition,it is difficult to miniaturize the conventional reference electrode due to the use of the inner filling solution.To meet the demands of micro-area detection,an all-solid-state micro detection system has been constructed,which is based on an all-solid-state reference microelectrode（ASS-μRE）and all solid-state ion-selective microelectrode.The ASS-μRE can be prepared by using NiCo₂S₄ nanowires as SC and an ionic liquid-doped poly（vinyl chloride）membrane as reference membrane.The potential responses of the ASS-μRE have been measured in different electrolyte solutions,which indicates a stable response of the ASS-μRE with a potential change of less 1.0 m V/dec.The developed all-solid-state copper ion selective microelectrode(ASS-Cu²⁺-ISμE)shows a good Nernstian response（a slope of 29.8±0.4 m V/dec）in the activity range of 1.0×10^-7-9.2×10^-5 M with a detection limit of 3.2×10^-8 M.The ASS-μRE and ASS-Cu²⁺-ISμE have been integrated into an all-solid-state micro potentiometric analysis system.Compared with the traditional system,the proposed system has a similar Nernstian response slope and good potential stability.The all-solid-state micro potentiometric analysis system can offer a new detection technology for micro-area detection,and has been successfully applied for detection of copper ion fluxed at the surfaces of Suaeda salsa.