Voltammetric and amperometric transduction for solvent polymeric membrane ion sensors

Determination of Single-Ion Partition Coefficients between Water and Plasticized PVC Membrane Using Equilibrium-Based Techniques

An experimentally simple method for the direct determination of single-ion partition coefficients between water and a PVC membrane plasticized with o-NPOE is suggested. The method uses the traditional assumption of equal single-ion partition coefficients for some reference cation and anion, in this case tetraphenylphosphonium (TPP⁺) and tetraphenylborate (TPB^-). The method is based on an integrated approach, including direct study of some salts' distribution between water and membrane phases, estimation of ion association constants, and measurements of unbiased selectivity coefficients for ions of interest, including the reference ones. The knowledge of distribution coefficients together with ion association constants allows for direct calculation of the multiple of the single-ion partition coefficients for the corresponding cation and anion, while the knowledge of unbiased selectivity coefficients together with ion association constants allows for immediate estimation of the single-ion partition coefficients for any ion under study, if the corresponding value for the reference ion is known. Both potentiometric and extraction studies are inherently equilibrium-based techniques, while traditionally accepted methods such as voltammetry and diffusion are kinetical. The inner coherent scale of single-ion partition coefficients between water and membrane phases was constructed.

A review on recent developments in optical and electrochemical aptamer-based assays for mycotoxins using advanced nanomaterials

This review (with 163 refs) covers the recent developments of nanomaterial-based optical and electrochemical sensors for mycotoxins. The review starts with a brief discussion on occurrence, distribution, toxicity of mycotoxins and the legislations in monitoring their levels. It further outlines the research methods, various recognition matrices and the strategies involved in the development of highly sensitive and selective sensor systems. It also points out the salient features and importance of aptasensors in the detection of mycotoxins along with the different immobilization methods of aptamers. The review meticulously discusses the performance of different optical and electrochemical sensors fabricated using aptamers coupled with nanomaterials (CNT, graphene, metal nanoparticles and metal oxide nanoparticles). The review addresses the limitations in the current developments as well as the future challenges involved in the successful construction of aptasensors with the functionalized nanomaterials. Graphical abstract Recent developments in nanomaterial based aptasensors for mycotoxins are summarized. Specifically, the efficiency of the nanomaterial coupled aptasensors (such as CNT, graphene, metal nanoparticles and metal oxide nanoparticles) in optical and electrochemical methods are discussed.

Ion amperometry at the interface between two immiscible electrolyte solutions in view of realizing the amperometric ion-selective electrode

This article reviews the development in ion amperometry at the interface between two immiscible electrolyte solutions (ITIES) in view of realizing the amperometric ion-selective electrode (ISE). The concept of polarizability of ITIES in a multi-ion system is outlined. Principle aspects of ion amperometry at ITIES are discussed including the use of amperometry as a tool for the clarification of the ion sensing mechanism, and for determining the concentrations of ions in the solution. The reference is made to recent amperometric measurements at the supported liquid membrane (SLM) and polymer composite liquid membranes (PCLM), which, together with the micro-hole supported ITIES, appear to be particularly suitable for realization of the amperometric ISE.

Pulsed chronopotentiometric membrane electrodes based on plasticized poly(vinyl chloride) with covalently bound ferrocene functionalities as solid contact transducer

Ion-selective membrane materials based on poly(vinyl chloride) (PVC)-containing covalently attached redox-active ferrocene (Fc) groups are characterized here as all-solid-state pulsed voltammetric ion sensors. The redox capacity of the membrane increases 7-fold with a doubling of the Fc content and 3-fold with the addition of 10 wt % of the lipophilic electrolyte ETH 500, tetradodecylammonium tetrakis(4-chlorophenyl)borate. This salt improves the ionic conductivity of the membrane and appears to make the Fc groups electrochemically more accessible. A too high content of the two, on the other hand, was found to cause undesired sensitivity to redox-active species present in the sample solution. Dilution of the membrane with a plasticizer eliminated this redox sensitivity while preserving its high redox capacity. A practical application of the designed electrodes in electrochemical analysis was demonstrated with a multi-pulse protocol that includes a current-controlled ion uptake pulse, followed by an open-circuit potential (OCP) measurement and a regeneration pulse. Potentiometric calibration curves obtained with this protocol exhibited a linear response with near-Nernstian slopes for acetate, nitrate, chloride, and perchlorate ions with the selectivity expected for an ion-exchanging membrane.

Potentiometric determination of coextraction constants of potassium salts in ion-selective electrodes utilizing a nitrobenzene liquid membrane phase

A theoretical treatment of potentiometric data is applied to calculate coextraction constants (K(IA)) for three potassium salts from water into a liquid nitrobenzene phase. The experiment involves treating nitrobenzene as a membrane and contacting it with two aqueous solutions of different ion activities. In the presence of either a cation or anion exchanger, the ratio of activities of ions in the two aqueous phases gives rise to a potential difference across the membrane that depends upon the nature and charge of the counter ion of the ion-exchanger in excess. Here, the cation exchanger was chosen to be potassium tetrakis(4-chlorophenyl)borate (KTpClPB) and the anion exchanger was tetradodecylammonium chloride (TDDACl). TDDACl was incrementally added to the nitrobenzene phase containing a fixed concentration of KTpClPB, and the corresponding emf was recorded as a function of concentration of TDDACl. The membrane changes from one with cation exchanger properties (excess KTpClPB) to one with anion exchanger properties (excess TDDACl). The potential difference and shape of the titration curve can be predicted by theory based on the phase boundary potential model. Log(K(IA)) values calculated for KCl, KNO(3) and KClO(4) in nitrobenzene were found as: -10.53 (± 0.09), -8.16 (± 0.05) and -5.63 (± 0.03) respectively, in accordance with the Hofmeister series of lipophilicity, and similar to those observed in PVC membranes containing other plasticizers. The method presented here offers the advantage over other methods to calculate K(IA), in that it is relatively experimentally simple without compromising the accuracy of the calculated coextraction constants. The ability to titrate directly into the liquid membrane phase affords a higher precision compared to the preparation of a series of PVC/plasticizer membranes with different compositions.

Background Current Elimination in Thin Layer Ion-Selective Membrane Coulometry

A promising method for the elimination of undesired capacitive currents in view of realizing a potentially calibration free coulometric ion detection system is presented. The coulometric cell is composed of a porous polypropylene tube doped with a liquid calcium-selective membrane and a silver/silver chloride wire as an inner electrode, forming a thin layer sample between wire and tubing. The total charge passed through the system during potential controlled electrolysis of the thin layer sample is indeed found to be proportional to the amount of calcium present, but non-Faradaic processes do contribute to the obtained signal. We introduce here a multi-pulse procedure that allows one to perform a second excitation pulse at the same excitation potential after exhaustive ion transfer voltammetry of calcium has taken place. The intercept of the calibration curve after background subtraction is found as 20.6 +/- 0.6 muC, significantly lower than the value of 54.1 +/- 0.8 muC for the uncorrected curve. Changes in sample temperature (from 23 degrees C to 38 degrees C) did equally not affect the background corrected coulometric readings, supporting that the procedure renders the readout principle more robust.

Calcium pulstrodes with 10-fold enhanced sensitivity for measurements in the physiological concentration range

Ion-selective electrodes ideally operate on the basis of the Nernst equation, which predicts less than 60- and 30-mV potential change for a 10-fold activity change of monovalent and divalent ions measured at room temperature, respectively. Typical concentration ranges in extracellular fluids are quite narrow for the electrolytes of key importance. A range of 2.2-2.6 mM for calcium ions, for instance, translates into just a 2.2-mV potential change. The direct potentiometric measurement of physiological electrolytes is certainly possible with direct potentiometry and is done routinely in clinical analyzers and handheld measuring devices. It places, however, strong demands on the precision of the reference electrode and requires careful temperature control and frequent calibration runs. In this paper, a robust 10-20-fold sensitivity enhancement for calcium measurements is attained by departing from the classical response mechanism and operating in a non-Nernstian response mode. Stable and reproducible super-Nernstian responses of these so-called pulstrodes in a narrow calcium activity range can be controlled by instrumental means in good agreement with theory. The potentials may be measured during a galvanostatic excitation pulse (mode I) or immediately after it (mode II), under open-circuit conditions. Subtraction of the potentials, sampled at different times during a single pulse, allows one to obtain a sensitive differential peak-shaped signal at a critical and fully adjustable analyte activity range. Calcium pulstrodes based on the diamide ionophore AU-1 were characterized and applied to the measurement in model physiological liquids. Super-Nernstian responses exceeding 700 mV/decade were observed in a physiological range of calcium concentration. Such remarkable sensitivity of the pulstrodes, complemented with the well-documented high selectivity of these potentiometric sensors, may provide a significant increase in the accuracy and precision of electrolyte measurements in clinical analysis.

Differential pulse voltammetry and additive differential pulse voltammetry with solvent polymeric membrane ion sensors

The ion transfer across the water-solvent polymeric membrane interface is investigated by using a new device based on a modification of a commercial ion-selective electrode body that permits the accommodation of a platinum counter electrode inside the inner filling solution compartment and, therefore, use of a four-electrode potentiostat with ohmic drop compensation. This device is used here to apply two different double potential pulse techniques--differential pulse voltammetry and additive differential pulse voltammetry--which are more advantageous than other voltammetric techniques, such as normal pulse voltammetry or cyclic voltammetry, for the determination of the characteristic electrochemical parameters of the system. This is due to the concurrence of two factors in these double potential pulse techniques, the peak-shaped response together with a considerable reduction of undesirable current contributions.

Pulsed Galvanostatic Control of Ionophore-Based Polymeric Ion Sensors

This paper describes a pulsed galvanostatic technique to interrogate ion-selective electrodes (ISEs) with no intrinsic ion-exchange properties. Each applied current pulse is followed by a longer baseline potential pulse to regenerate the phase boundary region of the ion-selective membrane. The applied current fully controls the magnitude and sign of the ion flux into the membrane, thus offering instrumental control over an effect that has become very important in ion-selective electrode research in recent years. The resulting chronopotentiometric response curves essentially mimic traditional ISE behavior, with apparently Nernstian response slopes and selectivities that can be described with the Nicolsky equation. Additionally, the magnitude and sign of the current pulse may be used to tune sensor selectivity. Perhaps most important, however, appears to be the finding that the extent of concentration polarization near the membrane surface can be accurately controlled by this technique. A growing number of potentiometric techniques are starting to make use of nonequilibrium principles, and the method introduced here may prove to be very useful to advance these areas of research. The basic characteristics of this pulsed galvanostatic technique are here evaluated with plasticized poly(vinyl chloride) membranes containing the sodium-selective ionophore tert-butyl calix[4]arene tetramethyl ester and a lipophilic inert salt.

Evaluation of the separate equilibrium processes that dictate the upper detection limit of neutral ionophore-based potentiometric sensors

The upper detection limit of polar ionophore-based ion-selective electrode membranes is predicted by utilizing the coextraction constant of dissociated electrolyte, the stability constant of the ionophore, and the membrane composition. The coextraction constant of dissociated electrolytes into the polar poly(vinyl chloride) membrane plasticized with o-nitrophenyl octyl ether (PVC-NPOE) is here measured by a novel approach. The sandwich membrane technique is utilized, with one membrane segment containing a lipophilic cation exchanger and the other containing an anion exchanger. This yields information about the coextraction constant and the free ion concentrations of the electrolyte in the two segments. Predictions correlate quantitatively with the upper detection limit observed for ion-selective electrodes based on the ionophores valinomycin, tert-butylcalix[4]arene tetraethyl ester, and calcimycin. The difficulties of the prediction of the upper detection limit for nonpolar poly(vinyl chloride) membranes plasticized with bis(2-ethylhexyl sebacate) (PVC-DOS) due to ion association are discussed in detail. A thermodynamic cycle experiment with a series of sandwich membranes shows that the principal processes governing the upper detection limit of PVC-DOS membranes are identical to those for the PVC-NPOE membranes. However, the stability of the ion pairs between the ionophore-metal ion complexes and the extracted anion are different from that of ion pairs formed between the same anion and the lipophilic anion exchanger. This makes it difficult to quantitatively predict the upper detection limit on the basis of simple apparent coextraction and complexation data alone. The approach reported herein is useful not only for mechanistic purposes but also to shed light onto the many cases where coextraction effects need to be understood but are not directly experimentally accessible.

Selectivity behavior and multianalyte detection capability of voltammetric ionophore-based plasticized polymeric membrane sensors

The current response features ofvoltammetric ion-selective polymeric membranes doped with neutral ionophores in view of practical sensor development are elucidated. The membranes are designed to extract ions only under applied external potentials and interrogated by normal-pulse voltammetry and pulsed amperometry. They contain two polarizable interfaces to avoid loss of lipophilic ions at the sample side and to maximize the available potential window. A simple theoretical model is developed that describes the observed current at the end of an uptake pulse to the applied membrane potential, which is the sum of both boundary potentials (at the sample and inner electrolyte side) and the membrane internal iR drop. The results describe how the selectivity of the resulting sensor must be dependent on the applied potential. Evidently, the role of the applied potential is akin to incorporating lipophilic cationic and anionic sites with potentiometric ionophore-based membranes, which are well known to considerably affect membrane selectivity and to define the charge type of the assessed ions. This has important implications for sensor design, as the applied cell potential can be used to tune sensor selectivity. Theory also explains the role of the inner electrolyte on sensor behavior. A maximum measuring range is expected with ions in the inner electrolyte that are difficult to extract into the membrane. This corresponds to Kihara's experimental results and contrasts to common ion-selective electrode practice, where a salt of the analyte ion is normally present in the inner electrolyte. Separate and mixed solution experiments with membranes containing the sodium-selective ionophore tert-butyl calix[4]arene tetramethyl ester and the lithium ionophore ETH 1810 agree very well with theoretical expectations. Multianalyte detection capability with a single sensing membrane is demonstrated in a selectivity-modifying pulsed amperometric detection mode, where each applied voltage yields a different practical selectivity of the sensor. The sensor is altered from being sodium to potassium selective as the magnitude of the applied potential is repetitively varied within the pulse sequence. The sensors show high long-term stability under continuous measuring conditions over 15 h.

Scanning electrochemical microscopy. 40. Voltammetric ion-selective micropipet electrodes for probing ion transfer at bilayer lipid membranes

Voltammetric ion-selective micropipet electrodes for use in scanning electrochemical microscopy (SECM) for detection of potassium ion were fabricated. These used pulled borosilicate capillaries with tip orifice radii of 0.7-20 microm with silanized inner walls filled with a solution of 10 mM valinomycin and 10 mM ETH 500 in dichloroethane. The electrodes were characterized by determining the steady-state tip current for K+ concentrations of 0.05-0.3 mM. The tips were used in the SECM feedback and generation-collection modes to study K+ transfer through gramicidin channels in a horizontal bilayer lipid membrane (glycerol monooleate).

Direct potentiometric information on total ionic concentrations

Polymeric membrane ion-selective electrodes exhibit an apparently super-Nernstian response at low sample activities if inner solutions are used that induce strong zero-current fluxes of primary ions toward the inner compartment. This is due to the limited ion fluxes in the aqueous boundary layer near the membrane. In the presence of labile complexes, the effective flux rate is increased and the emf depends on the total concentration of the ions. The concept is illustrated experimentally with calcium-selective electrodes based on the ionophore N,N-dicyclohexyl-N',N'-dioctadecyl-3-oxapentanediamide (ETH 5234) that either respond to total or free ion concentrations. Samples can be distinguished that contain varying levels of total calcium but are all buffered with EDTA to the same free calcium concentration of 5 x 10(-8) M.