Selectivity enhancement of anion-responsive electrodes by pulsed chronopotentiometry

Nonequilibrium Anion Detection in Solid-Contact Ion-Selective Electrodes

Low-cost and portable nitrate and phosphate sensors are needed to improve farming efficiency and reduce environmental and economic impact arising from the release of these nutrients into waterways. Ion selective electrodes (ISEs) could provide a convenient platform for detecting nitrate and phosphate, but existing ionophore-based nitrate and phosphate selective membrane layers used in ISEs are high cost, and ISEs using these membrane layers suffer from long equilibration time, reference potential drift, and poor selectivity. In this work, we demonstrate that constant current operation overcomes these shortcomings for ionophore-based anion-selective ISEs through a qualitatively different response mechanism arising from differences in ion mobility rather than differences in ion binding thermodynamics. We develop a theoretical treatment of phase boundary potential and ion diffusion that allows for quantitative prediction of electrode response under applied current. We also demonstrate that under pulsed current operation, we can create functional solid-contact ISEs using lower-cost molecularly imprinted polymers (MIPs). MIP-based nitrate sensors provide comparable selectivity against chloride to costlier ionophore-based sensors and exhibit >100,000 times higher selectivity against perchlorate. Likewise, MIP-based solid contact ion-selective electrode phosphate sensors operated under pulsed current provide competitive selectivity against chloride, nitrate, perchlorate, and carbonate anions. The theoretical treatment and conceptual demonstration of pulsed-current ISE operation we report will inform the development of new materials for membrane layers in ISEs based on differences in ion mobility and will allow for improved ISE sensor designs.

All-solid-state chip utilizing molecular imprinted polymer for erythromycin detection in milk samples: Printed circuit board-based potentiometric system

Detection of erythromycin (ERY) residues in commercial milk samples is crucial for the safety assessment. Herein, a printed circuit board was patterned as a feasible miniaturized potentiometric sensor for ERY determination in dairy samples. The proposed chip design fits to a 3.5-mm female audio plug to facilitate the potential measurements of working electrode versus reference one in this all-solid-state system. The sensor utilizes molecular imprinted polymer (MIP) for the selective recognition of the studied drug in such challenging matrix. The electrode stability is achieved through the addition of poly (3,4-ethylenedioxythiophene) nano-dispersion on its surface. The proposed device detects down to 6.6 × 10-8 M ERY with a slope of 51 mV/decade in the 1 × 10-7-1 × 10-3 M range. The results display high accuracy (99.9% ± 2.6) with satisfactory relative standard deviation for repeatability (1.6%) and reproducibility (5.0%). The effect of common antibiotic classes, namely, amphenicols, beta-lactams, fluoroquinolones, sulfonamides, and tetracyclines, can be neglected as evidenced by their calculated binding capacities towards the proposed MIP. The calculated selectivity coefficients also show a good electrode performance in the presence of naturally present inorganic ions allowing its application to different milk samples.

Ionophore-Based Ion-Selective Electrodes in Non-Zero Current Modes: Mechanistic Studies and the Possibilities of the Analytical Application

Abstract

This mini review briefly describes (i) literature data on the non-zero current measurements with ionophore-based ion-selective electrodes (ISEs) aimed at fundamental studies of the mechanism of their potentiometric response, and (ii) the data on the possibilities of analytical applications of ISEs in voltametric and constant potential chronoamperometric/coulometric modes, in particular the K+ ion assay in blood serum with the sensitivity of 0.1%. A special attention is paid to the basics of voltammetry and chronoamperometry/coulometry with the ionophore-based ISEs, and to how and why these methods differ from the classical voltammetry and coulometry.

Integration of an Aerosol-Assisted Deposition Technique for the Deposition of Functional Biomaterials Applied to the Fabrication of Miniaturised Ion Sensors

Ion-selective electrodes are at the forefront of research nowadays, with applications in healthcare, agriculture and water quality analysis among others. Despite multiple attempts of miniaturization of these polyvinyl chloride (PVC) gel-based ion sensors, no ion-sensing devices with a thickness below the micrometer range, and operating using open circuit potential, have been developed so far. This work reports the causes of this thickness limitation in potassium-selective sensors. Highly homogeneous ion-sensing films were fabricated by a method based on aerosol assisted chemical vapour deposition, leading to smooth surfaces with 27 ± 11 nm of roughness. Such homogeneity allowed the systematic study of the performance and ionic diffusion properties of the sensing films at sub-micrometer scales. Sensitivities below the Nernst response were found at low thicknesses. The nature of this reduction in sensitivity was studied, and a difference in the superficial and bulk compositions of the films was measured. An optimal configuration was found at 15 µm, with a good selectivity against Na+ (KK+, Na+ = -1.8) a limit of detection in the range of 10-4 M and esponse time below 40 s. The stability of sensors was improved by the deposition of protective layers, which expanded the lifespan of the ion sensors up to 5 weeks while preserving the Nernst sensitivity.

Pulsed galvanostatic control of a polymeric membrane ion-selective electrode for potentiometric immunoassays

Pulsed galvanostatic control of ion fluxes across polymeric membrane ion-selective electrodes (ISEs) is an emerging field for potentiometric sensing. Herein we report a novel potentiometric enzyme immunoassay based on current-controlled release of an enzyme substrate, which eliminates the addition of marker ions in the sample solution. In this method, the carboxylated poly(vinyl chloride) matrix at the outer layer of the ISE membrane is employed to attach a primary antibody. A sandwich immunoassay with an alkaline phosphatase labeled antibody (ALP-Ab) as the reporter is used for the determination of human IgG (as a model protein). The large difference between the lipophilicity of the substrate ion and that of the product ion allows p-nitrophenyl phosphate to be used as the enzyme substrate for potentiometric immunosensors. After the immunoreactions, the captured ALP-Ab catalyzes the hydrolysis of the substrate ions released at the sample-membrane interface by using the pulsed galvanostatic technique. This process can be potentiometrically determined by measuring the open circuit potential of the ISE. Under optimal conditions, the potential response of the proposed immunosensor is proportional to the concentration of human IgG in the range of 50-1000 ng/mL with a detection limit of 30 ng/mL (3σ). Owing to simplicity and independence of sample volume and sample turbidity, the proposed potentiometric immunoassay offers a viable alternative to those based on optical absorbance.

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.

Selectivity Enhancement for Chloride Ion by In(III)-Porphyrin-Based Polymeric Membrane Electrode Operated in Pulsed Chronopotentiometric Mode

A robust selectivity enhancement of an In(III)-porphyrin ionophore-based chloride-selective electrode under pulsed chronopotentiometric measurement mode that enables the detection of chloride ions in the presence of a normally interfering concentration of salicylate ions is described. This enhancement is achieved by the rapid depletion of the surface concentration of the more dilute lipophilic anion during an initial anodic current pulse period due to extraction of this preferred anion into the membrane phase. Measurement of chloride with a detection limit of 8 mM and near Nernstian response slope in the presence of 1 mM salicylate is possible using the pulstrode method.

Reverse current pulse method to restore uniform concentration profiles in ion-selective membranes. 1. Galvanostatic pulse methods with decreased cycle time

The applications of ion-selective electrodes (ISEs) have been broadened through the introduction of galvanostatic current pulse methods in potentiometric analysis. An important requirement in these applications is the restoration of the uniform equilibrium concentration profiles in the ISE membrane between each measurement. The simplest restoration method is zero-current relaxation, in which the membrane relaxes under open-circuit conditions in a diffusion-controlled process. This paper presents a novel restoration method using a reverse current pulse. An analytic model for this restoration method is derived to predict the concentration profiles inside ISE membranes following galvanostatic current pulses. This model allows the calculation of the voltage transients as the membrane voltage relaxes back toward its zero-current equilibrium value. The predicted concentration profiles and voltage transients are confirmed using spectroelectrochemical microscopy (SpECM). The reverse current restoration method described in this paper reduces the voltage drift and voltage error by 10-100 times compared to the zero-current restoration method. Therefore, this new method provides faster and more reproducible voltage measurements in most chronopotentiometric ISE applications, such as improving the detection limit and determining concentrations and diffusion coefficients of membrane species. One limitation of the reverse current restoration method is that it cannot be used in a few applications that require background electrolyte loaded membranes without excess of lipophilic cation exchanger.

Direct sensing of total acidity by chronopotentiometric flash titrations at polymer membrane ion-selective electrodes

Polymer membrane ion-selective electrodes containing lipophilic ionophores are traditionally interrogated by zero current potentiometry, which, ideally, gives information on the sample activity of ionic species. It is shown here that a discrete cathodic current pulse across an H (+)-selective polymeric membrane doped with the ionophore ETH 5294 may be used for the chronopotentiometric detection of pH in well-buffered samples. However, a reduction in the buffer capacity leads to large deviations from the expected Nernstian response slope. This is explained by the local depletion of hydrogen ions at the sample-membrane interface as a result of the galvanostatically imposed ion flux in direction of the membrane. This depletion is found to be a function of the total acidity of the sample and can be directly monitored chronopotentiometrically in a flash titration experiment. The subsequent application of a baseline potential pulse reverses the extraction process of the current pulse, allowing one to interrogate the sample with minimal perturbation. In one protocol, total acidity is found to be proportional to the magnitude of applied current at the flash titration end point. More conveniently, the square root of the flash titration end point time observed at a fixed applied current is a linear function of the total acid concentration. This suggests that it is possible to perform rapid localized pH titrations at ion-selective electrodes without the need for volumetric titrimetry. The technique is explored here for acetic acid, MES and citric acid with promising results. Polymeric membrane electrodes based on poly(vinyl chloride) plasticized with o-nitrophenyl octyl ether in a 1:2 mass ratio may be used for the detection of acids of up to ca. 1 mM concentration, with flash titration times on the order of a few seconds. Possible limitations of the technique are discussed, including variations of the acid diffusion coefficients and influence of electrical migration.

Modern Directions for Potentiometric Sensors

This paper gives an overview of the newest developments of polymeric membrane ion-selective electrodes. A short essence of the underlying theory is given, emphasizing how the electromotive force may be used to assess binding constants of the ionophore, and how the selectivity and detection limit are related to the underlying membrane processes. The recent developments in lowering the detection limits of ISEs are described, including recent approaches of developing all solid state ISEs, and breakthroughs in detecting ultra-small quantities of ions at low concentrations. These developments have paved the way to use potentiometric sensors as in ultra-sensitive affinity bioanalysis in conjunction with nanoparticle labels. Recent results establish that potentiometry compares favorably to electrochemical stripping analysis. Other new developments with ion-selective electrodes are also described, including the concept of backside calibration potentiometry, controlled current coulometry, pulsed chronopotentiometry, and localized flash titration with ion-selective membranes to design sensors for the direct detection of total acidity without net sample perturbation. These developments have further opened the field for exciting new possibilities and applications.

Unbiased selectivity coefficients obtained for the pulsed chronopotentiometric polymeric membrane ion sensors

We report here on the successful observation of the unbiased thermodynamic selectivity of ion-selective sensors working in normal pulse chronopotentiometric mode (pulstrodes). In contrast to ion-selective electrodes, the pulstrodes do not require careful counterbalancing of the transmembrane ionic fluxes to achieve unbiased thermodynamic selectivity. The pulstrodes can work under asymmetric conditions, which are often encountered in practice. The composition of the inner filling solution did not affect the sensor response, indicating that the transmembrane flux of primary ions was indeed effectively suppressed in the absence of ion exchanger. For the K-selective sensor considered here, an improvement of Mg discrimination by a factor of 1000 was demonstrated.

Kinetic modulation of pulsed chronopotentiometric polymeric membrane ion sensors by polyelectrolyte multilayers

Polymeric membrane ion-selective electrodes are normally interrogated by zero current potentiometry, and their selectivity is understood to be primarily dependent on an extraction/ion-exchange equilibrium between the aqueous sample and polymeric membrane. If concentration gradients in the contacting diffusion layers are insubstantial, the membrane response is thought to be rather independent of kinetic processes such as surface blocking effects. In this work, the surface of calcium-selective polymeric ion-selective electrodes is coated with polyelectrolyte multilayers as evidenced by zeta potential measurements, atomic force microscopy, and electrochemical impedance spectroscopy. Indeed, such multilayers have no effect on their potentiometric response if the membranes are formulated in a traditional manner, containing a lipophilic ion exchanger and a calcium-selective ionophore. However, drastic changes in the potential response are observed if the membranes are operated in a recently introduced kinetic mode using pulsed chronopotentiometry. The results suggest that the assembled nanostructured multilayers drastically alter the kinetics of ion transport to the sensing membrane, making use of the effect that polyelectrolyte multilayers have different permeabilities toward ions with different valences. The results have implications to the design of chemically selective ion sensors since surface-localized kinetic limitations can now be used as an additional dimension to tune the operational ion selectivity.