Metal Oxides-based Membrane-free Solid Contact Potentiometric Ions Sensor For Water Environmental Analysis

Solid-contact ion-selective electrodes（SC-ISEs）have attracted extensive attention in the field of environmental water quality analysis due to their high sensitivity,good selectivity,easy miniaturization and simple structure.However,environmental water is large variation and complex in composition,leading to unpredictable risks for traditional ion-selective membrane（ISM）-based SC-ISEs.One of the biggest challenges is the failure of the ISM.Encouragingly,the recently proposed concept of ISM-free SC-ISEs proved to be an effective solution.Ion-sensitive metal oxides（MO_x）with nanostructures have become one of the best candidates for the construction of membrane-free SC-ISEs due to their high specific surface area,long lifetime,low cost,good chemical and thermal stability.However,MO_x-based sensors generally exhibit low selectivity and sub-Nernstian response.Therefore,it is of great scientific and social value to develop an effective and general strategy to improve the performance of MO_x-based sensors.Based on this,this paper started a series of work to construct and improve the performance of MO_x-based potentiometric sensors:1.Lattice proton intercalation method is proposed for the first time to improve the sensitivity and selectivity of WO₃-based solid-state p H sensors.Proton intercalation can promote the transition of WO₃ from the monoclinic phase to cubic phase,thereby enhancing the ion exchange capacity between WO₃ and H⁺.Meanwhile,after intercalation,the resistance is reduced by more than two orders of magnitude,effectively improving the charge transport on the solid contact interface.In addition,the occupation of the WO₃ lattice by H⁺leads to its ion exchange only with H⁺,which improves the recognition ability of H⁺.The prepared H_xWO₃-based p H electrode has the advantages of high sensitivity,excellent selectivity,excellent resistance to light,gas and organic species,fast response and good reproducibility and repeatability.However,H_xWO₃ has a long-term potential drift due to the slow precipitation of H⁺.2.To solve the problem of long-term potential drift of H_xWO₃,α-Mo O₃ was proposed as a material for lattice proton intercalation to fabricate a solid-state p H sensor.The intercalated H_xMo O₃ electrode exhibits excellent reversible p H response.The great improvement in performance indicates that the strategy of lattice proton intercalation is universal for improving the performance of MO_x-based p H sensors.Meanwhile,proton intercalation reduces the resistance of H_xMo O₃ by more than two orders of magnitude and increases the proton migration rate in its lattice.Furthermore,the intercalated H_xMo O₃ exhibits excellent resistance to current polarization,which facilitates the potential stabilization of the electrode during long-term testing.In addition,H_xMo O₃ exhibits excellent resistance to light,gases and organic species,and good reproducibility.However,H_xMo O₃ has a good response only in the range of p H 1-8,and the detection range is narrow,which may be caused by the decreased exchange ability of H_xMo O₃ with protons in alkaline and weakly alkaline environments.3.Based on the long-term potential drift and narrow detection range of H_xWO₃ and H_xMo O₃,a solid-state p H sensor is prepared by using Na_xWO₃（x=0.1,0.3,0.5,0.8,1.0）.There are significant differences in performance between Na_xWO₃ with different structures.Through crystal structure analysis,it is found that the difference is caused by the structural adjustment caused by proton substitution.After screening,it is determined that Na WO₃ has the best properties,including high sensitivity,excellent selectivity,excellent stability,and good resistance to light,gases,and organic species.In addition,the response of Na_xWO₃ to protons is explained by constructing a response model,which is caused by proton surface substitution to form H_xWO₃,and it is proved by density functional theory（DFT）calculation that this process can proceed spontaneously.The final results show that Na WO₃ effectively solves the problems of H_xWO₃ and H_xMo O₃-based p H sensors,and greatly promotes the further application of MO_x-based p H sensors.4.Using K_xWO₃（x=0.05,0.1,0.2,0.3,0.4,0.5）as the ion-sensitive material,WO₃-based ISM-free SC-ISEs are successfully extended to NH₄⁺detection.With the increase of K content,K_xWO₃ derived different crystal structures and formed ion transport channels of different sizes.Meanwhile,the ion intercalation also effectively improves the conductivity and ion mobility of K_xWO₃.In addition,the ionic selectivity mechanism of K_xWO₃ is verified by DFT.The membrane-free NH₄⁺potentiometric sensor prepared in this way has comparable sensitivity and selectivity to organic ionophore-based SC-ISEs as well as better comprehensive performance（ultrafast response,stability,reproducibility and anti-interference）.Among them,the chronopotentiometry results and response times of the two electrodes are the best results known so far.At the same time,K_xWO₃ also has the advantages of biocompatibility and low cost,in which the cost of making a single electrode is reduced by more than two orders of magnitude.Besides,this work successfully overcomes the long-standing challenge of water-layer effect through a membrane-free structure,providing a new approach to realize low-cost,high-performance and long-life SC-ISEs.5.The p H and NH₄⁺chip electrodes are prepared by integrating the above-mentioned ion-sensitive materials(Na WO₃ and K_0.3WO₃)with the best performance into a miniaturized chip.The sensor chip not only retains the original analytical performance,but also has excellent resistance to current polarization so that it can be used with any potentiometer.At the same time,the sensor chip also has good flexibility and keeps working under the condition of bending60°and 120°.In addition,the wireless sensor system composed of integrated chips and miniature potentiometers has successfully simulated the wireless in situ real-time detection process of water quality in environments such as printing and dyeing wastewater,urban sewage,coking wastewater,soil extract,and pearl river water.The test results are in good agreement with the colorimetric test and commercial p H meter.Valuable environmental water quality information is obtained through the collection and analysis of test data,indicating the great advantages of membrane-free SC-ISEs in integration and the necessity of constructing an efficient in situ real-time detection system.