Construction And Applications Of Miniaturized All-Solid-State Ion-Selective Electrodes

With the rapid development of global industrialization,the contamination of marine environment is increasingly becoming a serious problem throughout the world,especially in estuaries and coastal zones.Hence,it is of great importance to timely and accurately monitor the concentrations of the environmental elements and various pollutants in coastal zones for evaluation of environmental quality and efficient prevention of coastal contamination.Total alkalinity（TA）is an important parameter for characterization of the buffering capacity of seawater.Such parameter is related to not only the adsorption ability of seawater but also the degree of ocean acidification.The measurement of TA in seawater is essential for understanding of the cycling of ocean carbon and the global environmental change.Heavy metals can be transported into the coastal zone through sewage discharge and then entered into the sediment through adsorption,flocculation and precipitation interactions.Parts of heavy metals might be re-suspended and re-dissolved in pore water,thus causing the secondary pollution.Recent researches show that,these heavy metal pollutants could be adsorbed by some of the coastal wetland plants through their roots.Therefore,the determination of heavy metal ions in the pore waters of coastal sediments and at the surfaces of plant roots is very important for us to know the pollution situation of coastal environment and carry out environmental protection and restoration.Solid-state ion-selective electrodes（SC-ISEs）have been recognized as the next generation of ISEs due to their intrinsic advantages of ease of miniaturization,low maintenance,and high robustness.In particular,the miniaturized SC-ISEs with merits of fast response,low detection limit and small sample consumption is a promising technique for analyzing various ionic species in micro-environment and micro-samples.Precious metals such as gold and platium are usually used as the electrode substrates to fabricate miniaturized all-solid-state electrodes.The ion-electron transducors and the polymeric membranes are commonly dip-coated at the tips of the miniatuarized ISEs.It is clear that these substrates are very expensive,the fabrication process is complicated and unsuitable for mass production and the physical strength of the sensing membrane is poor.These pose serious limits to their long-term applications.Herein,based on paper chip and self-supported electrode techiniques,miniaturized H⁺-ISE Pb²⁺-ISE have been developed using carbon materials as solid contacts.The miniaturized H⁺and Pb²⁺ISEs have been developed and applied in detection of TA in seawater,lead ions in porewater and lead ion fluxes at surfaces of plant roots.The specific research work can be divided into three parts:1.All-solid-state portable paper-based electrode system for detection of TA in seawaterA portable paper-based potentiometric sensor using the conductive paste as electrode substrate for detection of TA in seawater is described.The carbon paste was coated with the hydrogen ion-selective membrane to fabricate the H⁺-selective electrode,while the Ag/AgCl paste was coated with the polymeric membrane containing KCl to fabricate the reference electrode.The two parts were integrated on the same paper chip which functions as the sensing system.The seawater samples were firstly mixed with two acidic solutions at different concentrations,and then dropped into the sensor detection areas.The potential difference between two measurements was used to quantify the TA values.The influence of H⁺ionophores on the potential response was tested.The results show that the proposed sensor exhibites a stable Nernstian slope with a response time of 10 s in the pH range of 1～4.5 with a slope of 58.4±0.3 mV/dec（R²=0.992）under the artificial seawater background.The practical application of the proposed paper-based device was successfully carried out for detection of the TA values in real seawater samples,and the results agree well with those obtained by the conventional potentiometric titration method.2.All-solid-state Pb²⁺-selective miniaturized electrode based on disordered mesoporous carbon（DMC）for detection of Pb²⁺in pore waterAn all-solid-state self-supported Pb²⁺-selective miniaturized electrode is proposed.Such electrode was constructed by filling DMC into a capillary glass tube with a tip diameter of～150μm.Subsequently,the cocktail solution of the ion-selective polymeric membrane was absorbed into the porous structure of DMC.The electrochemical behaviors of the proposed miniaturized electrode were characterized by using cyclic voltammetry and electrochemical impedance spectroscopy and the sensing performance was also investigated.Experimental results indicate that the DMC-based miniaturized electrode showes a large double-layer capacitance because of the porous structure,and the proposed all-solid-state Pb²⁺-selective miniaturized electrode exhibites a Nernstian response of 28.9±0.5mV/dec（R²=0.996）in a simulated soil solution over the concentration range of 1.0×10^-7to 1.0×10^-4M with a detection limit of 4.0×10^-8 M.The practical application of the proposed all-solid-state self-supported Pb²⁺-selective miniaturized electrode was carried out for detection of Pb²⁺in pore water,and the results agree well with those obtained by anodic stripping voltammetry.3.All-solid-state Pb²⁺-selective microelectrode based on DMC for detection of Pb²⁺fluxes at the surfaces of Phragmites rootsOn the basis of the successful fabrication of Pb²⁺-selective miniaturized electrodes,a capillary glass tube with a tip diameter of ca.20μm was further used to construct a Pb²⁺-selective microelectrode.In this work,the ion-selective membrane compositions were optimized and the resulting microelectrodes were characterized by using the electrochemical methods.Experimental results show that the DMC-based microelectrode exhibites a typical"S"cyclic voltammetry curve and the proposed Pb²⁺-selective microelectrode showes a linear response in a range of 1.0×10^-8～1.0×10^-4M with a Nernstian slope of 26.5±0.2 mV/dec（R²=0.999）and a detection limit of 8.7×10^-9 M under the simulated soil solution background.The electrode was further used to measure the concentration of Pb²⁺at the surface of Phragmites australis roots from the wetland.After being equilibriumed in the Pb²⁺solution,the miniaturized electrode was moved along the root axis（approximately 100μm increments）.Results show that the lead ion fluxes from at root tip region could be observed.These results are consistent with those obtained by the classical liquid contact ISμEs.