Constructions Of Polymeric Membrane Potentiometric Sensors Based On Dynamic Responses And Their Applications In Environmental Analysis

Polymeric membrane ion-selective electrodes?ISEs?have been widely used in the fields of bioanalysis and environmental monitoring,due to their attractive features including simple instrumentation,rapid response and low cost.The potential response of a conventional ISE based on thermodynamic equilibrium is related to the activity of the ion of interest,which can be expressed by Nernst equation.However,the traditional polymeric membrane ISEs suffer from problems of the low sensitivity during the response slope due to the Nernst equation and of the presence of various interfering ions for environmental monitoring.Additionally,one traditional polymeric membrane ISE only responses to one specific ion,but cannot be used for detection of multiple ananlytes.ISEs based on dynamic responses show the advantages of high sensitivity,good selectivity,excellent reversibility and capability for detection of multiple ananlytes.In this thesis,a series of polymeric membrane ISEs based on dynamic potentiometry are described.By precisely controlling the ion fluxes of polymeric membrane electrodes,the speciation informations and improved sensitivity can be obtained.Transition time,a new readout is proposed to reduce the interference of high background electrolyte.The dual-or multianalyte detection using a single membrane electrode can be achieved by the integration of logic gate operations with potentiometric measurements.A highly sensitive potentiometric biosensor is also proposed based on biological amplification.The contents of the thesis are as follows:1. Detection of free and total ionic concentrations by using calcium-selective polymeric membrane electrodesCalcium is a widespread and important element,and many physiological effects are related to its speciation.The speciation analysis of calcium is of key importance for understanding bioavailability and reactivity in natural and biomedical environments.In this work,we have developed two kinds of calcium-selective polymeric membrane electrodes(Ca²⁺-ISEs)with different inner filling solutions for the detection of free and total ionic concentrations in the presence of EDTA and humic acid.The results indicate that the free Ca²⁺activity,which could be as low as 10^-2 M Na₂EDTA adjusted to p H 9.0,exhibits an apparently super-Nernstian response super-Nernstian potential response range,the potential is found to be linearly related properities,both the total and free ionic concentrations can be obtained by the proposed Ca²⁺-ISEs with different inner filling solutions.The proposed ion-selective polymeric membrane provides a simple and effective method for speciation analysis.2. Current pulse based ion-selective electrodes for chronopotentiometric determination of calcium in seawaterThe theoretical slope of the traditional Ca²⁺-ISE based on thermodynamic equilibrium is about 30 m V/dec,which means that a potential difference of 1 m V causes a 8%change in the activity of Ca²⁺.This may be problemetic for detection of changes in the activity of Ca²⁺in seawater.Therefore,it's required to develop a highly sensitive and accurate method for determination Ca²⁺in seawater.In this work,we have developed a current pulse based ion-selective electrode with enhanced sensitivity for chronopotentiometric measurements of calcium in seawater.The Ca²⁺-selective membrane containing lipophilic salt ETH 500 instead of traditional ion-exchanger is galvanostatically controlled.Under the background solution containing 0.5 M Na Cl,an applied constant cathodic current pulse can leads to the extraction of the calcium ions into the membrane to produce a chronopotential response,which shows a stable and reproducible super-Nernstian response in a narrow calcium activity range.The super-Nernstian region of the electrode depends on not only the magnitude and duration of the applied current pulse but also the interfering ions.Under optimal conditions,the proposed Ca²⁺-ISE exhibits a super-Nernstian response between the calcium concentrations of 10^-2.5–10^-1.5 M with a slope of ca 80 m V/dec.Additionally,satisfactory results.Compared with those of the conventional Ca²⁺-ISE,the standard deviations obtained by proposed Ca²⁺-ISE proposed are significantly reduced.3. Detection of calcium in seawater based on transition time resolved chronopotentiometryIn dynamic potentiometry,the potentiometric responses of polymeric membrane ISEs are reported to be significantly influenced by the presence of the background electrolyte.Therefore,it's necessary to develop a promising approach which is less disturbed by the background electrolyte for determination Ca²⁺concentration in seawater.In this work,an ion-selective electrode with transition time as a readout signal for chronopotentiometric measurement of calcium concentration in seawater is proposed.The proposed Ca²⁺-selective membrane doesn't contain a traditional ion exchanger.When a constant cathodic current pulse with a long duration is applied on the Ca²⁺-selective membrane,the localized depletion of calcium ions at the transition time occurs,which subsequently results in the co-extraction of calcium and sodium ions from the sample solution into the membrane phase.The square root of transition time,i.e.a potential change?inflection point?of the chronopotentiometry is proportional to the calcium concentration,which is can be expressed by the Sand equation.Additionally,it's found that the transition time is related to the magnitude of the applied current and the membrane thickness,rather than the presence of the background electrolyte.The proposed chronopotentiometry with transition time as a readout signal is an attractive methodology to detect the calcium concentration in seawater.4. Chronopotentiometric aptasensing platform based on a G-quadruplex/hemin DNAzyme and logic-gate operations for detection of KANA and OTCConventional potentiometric ion sensors that rely on a specific ion carrier in a polymeric membrane can hardly achieve multianalyte detection.Inspired by the remarkable ability of built-in logic gates sensors for multianalyte detection,herein we report a potentiometric aptasensing platform based on a G-quadruplex/hemin DNAzyme and logic gate operations for determination of two analytes using a single membrane electrode for the first time.A bifunctional aptamer and a signal reporter nucleic acid are assembled on the magnetic beads to form a DNA hybrid structure.The?OR?and?INHIBIT?logic functions can be performed by using the two aptamers and their targets as inputs,and using the chronopotentiometric response based on the G-quadruplex/hemin DNAzyme-H₂O₂-mediated oxidation of TMB as output.Kanamycin?KANA?and oxytetracycline?OTC?have been employed as the models.Under optimal conditions,the dual targets can be sensed in the range from 10to 100 n M,with the detection limits of 7.5 and 9.8 n M?3??for KANA and OTC,respectively.The potentiometric aptasensing protocol has been evaluated with spiked seawater samples with satisfactory results.5. Chronopotentiometric aptasensing with signal amplification based on enzyme-catalyzed surface polymerization for detection of OTCIn recent years,potentiometric biosensing based on the blocking effect of ion fluxes has aroused considerable interest.However,the sensing based on the surface blocking of the passive ion fluxes across the polymeric membrane may suffer from problems of relatively small potentiometric signals and poor reproducibilities.Chronopotentiometry can not only rapidly and precisely control the ion fluxes across the polymeric membrane but also improve the reproducibility of the electrode.In this work,chronopotentiometric biosensor based on in situ enzyme-catalyzed dopamine polymerization is proposed for blocking of the active surface area for a high sensitivity.A signal amplification strategy based on HRP induced biocatalyzed polymerization of dopamine and surface coating has been designed and applied for the chronopotentiometric detection of aptamer-target binding events.Using the pulsed galvanostatic measurement protocol,the polydopamine layer adhering to the ion selective membrane?ISM?would effectively block the current-induced indicator ions and subsequently results in a decrease of the potential response.The target binding detection of target.Oxytetracycline?OTC?,as an extensively used antibiotic,has been employed as a model.Under the optimized condition,the chronopotentiometric aptasensor exhibits high sensitivity for the quantitative detection of OTC with a detection limit of 28 p M?3?/S?.The potentiometric aptasensing protocol has been evaluated with spiked seawater samples with satisfactory results.