Investigation Of Solid Contacts And Sensing Analytical Application For All-Solid-State Calcium And Magnesium Ion-Selective Electrodes

The quantitative analysis of various ions in environment is an important research subject in the field of environmental chemistry.Among them,the measurement of calcium and magnesium ions is of great significance to promote environmental monitoring and the development of intelligent agriculture.Solid-contact ion selective electrodes（SC-ISEs）have attracted wide attention for its advantages of high sensitivity,good selectivity,simple structure and easy operation.However,water layer,biotoxicity and component leakage are long-term and difficult challenges for traditional SC-ISEs.Therefore,it is of great significance to develop the next generation of stable and reliable ion sensors.Based on this,in order to overcome the challenge of traditional SC-ISEs,this paper chooses modified carbon-based material as the high-efficiency ion-electron transduction layer,and further uses manganese oxide as the bifunctional solid-contact to construct a new SC-ISEs with ISM-free,and uses it for the detection of calcium and magnesium ions in the environment.The main research contents are as follows:1.Three representative carbon-based materials,including nitrogen-doped mesoporous carbon（NMC）,reduced graphene oxide（r GO）and carbon nanotubes（CNT）,were selected as solid-contact（SC）for the preparation of Ca²⁺-SC-ISEs.The results show that the mesoporous structure of NMC and the synergistic effect of N doping make NMC have the highest EDL capacitance and hydrophobicity,which can avoid the formation of water layer between solid-contact and ion selective electrodes（SC/ISM）,and greatly improve the potential stability of the electrode.The NMC-based Ca²⁺-SC-ISEs exhibited a Nernstian slope of 26.3±3.1 m V dec^-1ranging from 10μM to 0.1 M with a detection limit of 3.2μM.Finally,the Ca²⁺concentrations in mineral water and soil leaching solutions were analyzed by the NMC-based solid-contact Ca²⁺-SC-ISEs.The results are consistent with test by ion chromatography,which further proves the practicability of NMC as an efficient ion-electron conversion material in SC-ISEs.2.However,while the solid-contact with strong hydrophobicity and high capacitance can effectively reduce the water effect of SC-ISEs,but as a result of SC|ISM interface on both sides of the ionic strength is different,the osmotic pressure will cause water molecules still inevitably from the solution to SC interface diffusion.In addition,ISM component leakage and biotoxicity are also significant challenges.We proposed to use Ca-Mn oxides as a bifunctional sensing layer to prepare Ca²⁺-SC-ISEs without ion selectivity membrane（ISM）.Firstly,four different Ca-Mn oxides were synthesized by thermal decomposition,and it was found that Ca Mn₂O₄has the best analytical performance,including sensitivity comparable to Ca²⁺ionophore and better selectivity and linear range.Then,a layer of reduced graphene oxide（r GO）containing a small amount of hydroxyl and carboxyl groups was creatively introduced on Ca Mn₂O₄surface to solve the problem that the material is sensitive to p H changes.The p H buffer created effectively reduces the p H interference for the sensor.Finally,the prepared Ca Mn₂O₄-r GO electrode was applied to the detection of Ca²⁺in mineral water,and the online monitoring of Ca²⁺concentration during the growth of garlic was realized.3.On the basis of the above experiments,we further synthesized three different Mg-Mn oxides for the actual detection of Mg²⁺.The ion recognition through ion transport in lattice and ion-to-electron transduction through redox of Mn.Due to the removal of ISM,the traditional"sandwich"structure of three-layer is simplified into"two-layer"structure,which effectively overcomes the water layer effect,component leakage and biological toxicity of SC-ISEs,and the application range of ion-selective electrode is effectively expanded.The results show that Mg₆Mn O₈has sensitivity comparable to Mg²⁺ionophores.The introduction of reductive r GO greatly improves the selectivity of the material for interfering ions,especially for Ca²⁺.The selectivity coefficient is much higher than the traditional SC-ISEs based on Mg²⁺ionophores.In addition,compared with ISM-based SC-ISEs,Mg₆Mn O₈-r GO has a high commercial prospect as Mg²⁺-SC-ISEs at a significantly lower cost.