Pulstrodes: triple pulse control of potentiometric sensors

Chronopotentiometric sensors for antimicrobial peptide-based biosensing of Staphylococcus aureus

Charged antimicrobial peptides can be used for direct potentiometric biosensing, but have never been explored. We report here a galvanostatically-controlled potentiometric sensor for antimicrobial peptide-based biosensing. Solid-state pulsed galvanostatic sensors that showed excellent stability under continuous galvanostatic polarization were prepared by utilizing reduced graphene oxide/poly (3,4-ethylenedioxythiophene): poly (4-styrenesulfonate) (rGO/PEDOT: PSS) as a solid contact. More importantly, the chronopotentiometric sensor can be made sensitive to antimicrobial peptides with intrinsic charge on demand via a current pulse. In this study, a positively charged antimicrobial peptide that can bind to Staphylococcus aureus with high affinity and good selectivity was designed as a model. Two arginine residues with positive charges were linked to the C-terminal of the peptide sequence to increase its potentiometric responses on the electrode. The bacteria binding-induced charge or charge density change of the antimicrobial peptide enables the direct chronopotentiometric detection of the target. Under the optimized conditions, the concentration of Staphylococcus aureus can be determined in the linear range 10-1.0 × 105 CFU mL-1 with a detection limit of 10 CFU mL-1. It is anticipated that such a chronopotentiometric sensing platform is readily adaptable to detect other bacteria by choosing the peptides.

Trace-level chronopotentiometric detection in the presence of a high electrolyte background using thin-layer ion-selective polymeric membranes

We propose here a pulsed galvanostatic control of a solid-contact ion-selective electrode coupled with a thin-layer ion-exchanger free membrane, which allows chronopotentiometric trace-level ion detection with a high-interfering background in a rapid and reversible way.

Catalytic Biosensors Operating under Quasi-Equilibrium Conditions for Mitigating the Changes in Substrate Diffusion

Despite the success of continuous glucose measuring systems operating through the skin for about 14 days, long-term implantable biosensors are facing challenges caused by the foreign-body reaction. We present a conceptually new strategy using catalytic enzyme-based biosensors based on a measuring sequence leading to minimum disturbance of the substrate equilibrium concentration by controlling the sensor between "on" and "off" state combined with short potentiometric data acquisition. It is required that the enzyme activity can be completely switched off and no parasitic side reactions allow substrate turnover. This is achieved by using an O₂ -independent FAD-dependent glucose dehydrogenase embedded within a crosslinked redox polymer. A short measuring interval allows the glucose concentration equilibrium to be restored quickly which enables the biosensor to operate under quasi-equilibrium conditions.

Unlocking All-Solid Ion Selective Electrodes: Prospects in Crop Detection

This paper reviews the development of all-solid-state ion-selective electrodes (ASSISEs) for agricultural crop detection. Both nutrient ions and heavy metal ions inside and outside the plant have a significant influence on crop growth. This review begins with the detection principle of ASSISEs. The second section introduces the key characteristics of ASSISE and demonstrates its feasibility in crop detection based on previous research. The third section considers the development of ASSISEs in the detection of corps internally and externally (e.g., crop nutrition, heavy metal pollution, soil salinization, N enrichment, and sensor miniaturization, etc.) and discusses the interference of the test environment. The suggestions and conclusions discussed in this paper may provide the foundation for additional research into ion detection for crops.

Anodic Stripping Voltammetry on a Carbon-based Ion-Selective Electrode

In this study, we demonstrated the unique capability of carbon-based ion-selective electrode (ISE) to perform highly sensitive square wave anodic stripping voltammetry, while maintaining all the properties of an ISE, in terms of sensitivity, detection limit, response time and selectivity. Square wave anodic stripping voltammetry involves deposition and dissolution steps of metal ions, which means adsorption and desorption of metal ions on the conductive ion-selective membrane without losing its ion-sensing property. To demonstrate this capability, we chose a Ca2+ ion-selective microelectrode (μISE) as a potentiometric method and Cu2+-stripping voltammetry as an amperometric method. The carbon-based ISE surface is capable of quantifying nanomolar to micromolar Cu2+ in both a standard acetate buffer and a complex water sample. The Ca2+-μISE also showed a Nernstian slope of 29 mV / log [Ca2+] and a detection limit of 1 μM within the linear range of 1 μM to 10 mM. It thus opens an opportunity to use the low detection limit of anodic stripping voltammetry and the high selectivity of ISE-based potentiometry.

Integrated multi-ISE arrays with improved sensitivity, accuracy and precision

Increasing use of ion-selective electrodes (ISEs) in the biological and environmental fields has generated demand for high-sensitivity ISEs. However, improving the sensitivities of ISEs remains a challenge because of the limit of the Nernstian slope (59.2/n mV). Here, we present a universal ion detection method using an electronic integrated multi-electrode system (EIMES) that bypasses the Nernstian slope limit of 59.2/n mV, thereby enabling substantial enhancement of the sensitivity of ISEs. The results reveal that the response slope is greatly increased from 57.2 to 1711.3 mV, 57.3 to 564.7 mV and 57.7 to 576.2 mV by electronic integrated 30 Cl⁻ electrodes, 10 F⁻ electrodes and 10 glass pH electrodes, respectively. Thus, a tiny change in the ion concentration can be monitored, and correspondingly, the accuracy and precision are substantially improved. The EIMES is suited for all types of potentiometric sensors and may pave the way for monitoring of various ions with high accuracy and precision because of its high sensitivity.

A paper based, all organic, reference-electrode-free ion sensing platform

We present a reference-electrode free, all organic K⁺ sensitive ion sensing platform fabricated by simplest means on a plain sheet of paper.

No more conventional reference electrode: transition time for determining chloride ion concentration

Ion selective electrodes (ISE) are used extensively for the potentiometric determination of ion concentrations in electrolytes. However, the inherent drift in these measurements and the requirement of a stable reference electrode restrict the feasibility of this method for long-term in-situ applications. This work presents a chronopotentiometric approach to minimize drift and avoid the use of a conventional reference electrode for measuring chloride ion concentration. An anodic current pulse is applied to a Ag/AgCl working electrode which initiates a faradaic reaction that depletes the chloride ions near the electrode surface. The rate of change in potential at the Ag/AgCl electrode, due to chloride ion depletion, reaches an inflection point once the chloride ions deplete completely near the electrode surface. The moment of the inflection point, also known as the transition time, is a function of the chloride ion concentration and is described by the Sand equation. It is shown that the square root of the transition time is linearly proportional to the chloride ion concentration. Drift in the response over two weeks is negligible: 59 μM/day when measuring 1 mM of Cl(-) ions using a 10 A m(-2) current pulse. The transition time at a specific ion concentration can be tuned by the applied current pulse, e.g., in a solution containing 5 mM chloride ions, the transition times with current pulses of 10 and 20 A m(-2) are 1.56 and 0.25s, respectively. The moment of inflection determines the response, and thus is independent of the absolute potential of reference electrode. Therefore, any metal wire can act as a pseudo-reference electrode, enabling this approach for long-term and integrated-sensor applications such as measurement inside concrete structures.

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.

Detection of protease activities by flash chronopotentiometry using a reversible polycation-sensitive polymeric membrane electrode

A novel electrochemical method, termed flash chronopotentiometry (FCP), is used to develop a rapid and sensitive method for detecting protease activities. In this method, an appropriate current pulse is applied across a polycation-selective polymer membrane to induce a strong flux of the polycationic peptides from the sample phase into the organic membrane of the electrode. During this current pulse, the cell potential (EMF) is monitored continuously, and is a function of the polypeptide concentration. The imposed current causes a local depletion of the polypeptide at the sample/membrane interface, which yields a drastic potential change in the observed chronopotentiogram at a characteristic time, called the transition time (τ). For a given magnitude of current, the square root of τ is directly proportional to the concentration of the polypeptide. Proteases cleave polypeptides into smaller fragments that are not favorably extracted into the membrane of the sensor. Therefore, a decrease in the transition time is observed during the proteolysis process. The degree of change in the transition time can be correlated to protease activity. To demonstrate this approach, the activities of trypsin and α-chymotrypsin are detected using protamine and synthetic polycationic oligopeptides that possess specific cleavage sites that are recognized by these proteases.

Ion transfer through solvent polymeric membranes driven by an exponential current flux

General analytical equations which govern ion transfer through liquid membranes with one and two polarized interfaces driven by an exponential current flux are derived. Expressions for the transient and stationary E-t, dt/dE-E and dI/dE-E curves are obtained, and the evolution from transient to steady behaviour has been analyzed in depth. We have also shown mathematically that the voltammetric and stationary chronopotentiometric I(N)-E curves are identical (with E being the applied potential for voltammetric techniques and the measured potential for chronopotentiometric techniques), and hence, their derivatives provide identical information.

Reversible detection of heparin and other polyanions by pulsed chronopotentiometric polymer membrane electrode

The first fully reversible polymeric membrane-based sensor for the anticoagulant heparin and other polyanions using a pulsed chronopotentiometry (pulstrode) measurement mode is reported. Polymeric membranes containing a lipophilic inert salt of the form R(+)R(-) (where R(+) and R(-) are tridodecylmethylammonium (TDMA(+)) and dinonylnaphthalene sulfonate (DNNS(-)), respectively) are used to suppress unwanted spontaneous ion extractions under zero-current equilibrium conditions. An anodic galvanostatic current pulse applied across the membrane perturbs the equilibrium lipophilic ion distribution within the membrane phase in such a way that anions/polyanions are extracted into the membrane from the sample. The membrane is then subjected to an open-circuit zero current state for a short period, and finally a 0 V vs reference electrode potentiostatic pulse is applied to restore the membrane to its initial full equilibrium condition. Potentials are sampled as average values during the last 10% of the 0.5 s open circuit phase of the measurement cycle. Fully reversible and reproducible electromotive force (emf) responses are observed for heparin, pentosan polysulfate (PPS), chondroitin sulfate (CS), and oversulfated chondroitin sulfate (OSCS), with the magnitude of the potentiometric response proportional to charge density of the polyanions. The sensor provides an emf response related to heparin concentrations in the range of 1-20 U/mL. The responses to variations in heparin levels and toward other polyanions of the pulstrode configuration are analogous to the already established single-use, nonreversible potentiometric polyion sensors based on membranes doped only with the lipophilic anion exchanger TDMA(+).

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.

Current-polarized ion-selective membranes: The influence of plasticizer and lipophilic background electrolyte on concentration profiles, resistance, and voltage transients

Lipophilic background electrolytes consisting of a lipophilic cation and a lipophilic anion, such as tetradodecylammonium tetrakis(4-chlorophenyl) borate (ETH 500), or bis(triphenylphosphoranylidene) ammonium tetrakis[3,5bis(trifluoromethyl) phenyl] borate (BTPPATFPB) are incorporated into the membranes of ion-selective electrodes (ISEs) to improve the detection limit and selectivity of the electrodes and decrease the resistance of the sensing membrane. In this work, spectroelectrochemical microscopy (SpECM) is used in conjunction with chronopotentiometry to quantify the effects of a lipophilic background electrolyte on the concentration profiles induced inside current-polarized membranes and on the measured voltage transients in chronopotentiometric experiments. In agreement with the theoretical model, the lipophilic background electrolyte incorporated into o-NPOE or DOS plasticized membranes decreases the membrane resistance and thus the contribution of migration in the overall transport across ion-selective membranes. Consequently, it has a significant influence on the changing concentration profiles of the ion-ionophore complex during chronopotentiometric experiments.

Ion channel mimetic chronopotentiometric polymeric membrane ion sensor for surface-confined protein detection

The operation of ion channel sensors is mimicked with functionalized polymeric membrane electrodes, using a surface confined affinity reaction to impede the electrochemically imposed ion transfer kinetics of a marker ion. A membrane surface biotinylated by covalent attachment to the polymeric backbone is used here to bind to the protein avidin as a model system. The results indicate that the protein accumulates on the ion-selective membrane surface, partially blocking the current-induced ion transfer across the membrane/aqueous sample interface, and subsequently decreases the potential jump in the so-called super-Nernstian step that is characteristic of a surface depletion of the marker ion. The findings suggest that such a potential drop could be utilized to measure the concentration of protein in the sample. Because the sensitivity of protein sensing is dependent on the effective blocking of the active surface area, it can be improved with a hydrophilic nanopore membrane applied on top of the biotinylated ion-selective membrane surface. On the basis of cyclic voltammetry characterization, the nanoporous membrane electrodes can indeed be understood as a recessed nanoelectrode array. The results show that the measuring range for protein sensing on nanopore electrodes is shifted to lower concentrations by more than 1 order of magnitude, which is explained with the reduction of surface area by the nanopore membrane and the related more effective hemispherical diffusion pattern.

Determination of unbiased selectivity coefficients using pulsed chronopotentiometric polymeric membrane ion sensors

A new procedure for the determination of selectivity coefficients of neutral carriers using pulsed chronopotentiometric ion selective sensors (pulstrodes) is established. Pulstrode membrane which lacks an ion-exchanger suppresses the zero current ion flux, allowing a Nernstian response slope for even highly discriminated ions. Unlike previously developed methods, unbiased selectivity remains unaltered even with the exposure to the primary ion solution for prolonged time. Studies with potassium-, silver-, and calcium-selective electrodes reveal that pulstrodes yield the same or slightly favorable unbiased selectivity coefficients than reported earlier. In contrast to alternative methods for the determination of unbiased selectivity, this technique offers a unique simplicity and reliability. Therefore the new procedure promises to be a valuable additional tool for the characterization of unbiased selectivity coefficients for the ISEs.

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.

Theory and Computer Simulation of the Time-Dependent Selectivity Behavior of Polymeric Membrane Ion-Selective Electrodes

A theoretical treatment of the time-dependent potential response of ion-selective electrodes to sample solutions containing primary and interfering ions is presented. The theory accounts for the influence of ion fluxes in the electrode membrane and the contacting aqueous sample layer and describes the variations in the apparent selectivity behavior as a function of the measuring time. The applicability of the theory is demonstrated by comparing predicted response curves with results of virtual experiments based on computer simulation. A close and convincing agreement was achieved for a large series of different examples, which confirms that the new theory can be successfully applied for general cases.

Mathematical model of current-polarized ionophore-based ion-selective membranes

A mathematical model is presented to describe the effects of constant current on ion-selective membranes using theta functions. The model provides exact analytic solutions for calculating the concentration polarization of the ionophore, the ion-ionophore complex, and the charged mobile sites in space and time within the membrane. It also predicts the time course of the membrane potential and the electric field inside the membrane following the application of constant current. This analytic solution is faster to compute than numerical simulations and provides the solution for any given time or position directly. The simulated concentration profiles compare favorably with concentration profiles recorded experimentally using spectro-electrochemical microscopy.

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.

Reversible detection of proteases and their inhibitors by a pulsed chronopotentiometric polyion-sensitive electrode

Polymer membrane electrodes operated by pulsed chronopotentiometry have recently been introduced to replace traditional ion-selective electrodes for a number of applications. While ion-selective electrodes for the polycation protamine have been reported, for instance, a pulsed chronopotentiometric readout mode (called here pulstrode) provides improved stability and reproducibility while exhibiting sufficient selectivity for the direct detection of protamine in undiluted whole blood samples. Here, such protamine-sensitive pulstrodes are applied for the real-time detection of the activity of the protease trypsin and its soybean inhibitor. This is possible because small fragments produced by the trypsin digestion are not detectable by the protamine-sensing membrane. The real-time response to the proteolytic reaction is shown to exhibit good reproducibility and reversibility, and the initial reaction rate is dependent on the concentration of the protease and its inhibitor.