Modeling Potentiometric Sensitivity of Conducting Polymers

Combining an Integrated Sensor Array with Machine Learning for the Simultaneous Quantification of Multiple Cations in Aqueous Mixtures

The direct quantification of multiple ions in aqueous mixtures is achieved by combining an automated machine learning pipeline with transient potentiometric data obtained from a single miniaturized array of polymeric sensors electrodeposited on a conventional printed circuit board (PCB) substrate. A proof-of-concept system was demonstrated by employing 16 polymeric sensors in combination with features extracted from the transient differential voltages produced by these sensors when transitioning from a reference solution to a test solution, thereby obviating the need for a conventional reference electrode. A tree-based regression model enabled concentrations of various metal cations in pure solutions to be determined in less than 2 min. In a model mixture comprising Al³⁺, Cu²⁺, Na⁺, and Fe³⁺, the mean relative error was found to depend on the type of ion and varied between 1% for Fe³⁺ and 44% for Na⁺ in the concentration range 1-10 mg/L. Overall, a mean relative error of 16% was obtained for quantification of these four ions across a total of 124 tests in different solutions spanning concentrations between 2 and 360 mg/L. These results demonstrate how the analytical capability of a multiselective sensor array can leverage data-driven approaches through training by examples for accelerated testing and can be proposed to complement traditional analytical tools to meet industrial demands, including traceability of chemicals.

Unintended Changes of Ion-Selective Membranes Composition-Origin and Effect on Analytical Performance

Ion-selective membranes, as used in potentiometric sensors, are mixtures of a few important constituents in a carefully balanced proportion. The changes of composition of the ion-selective membrane, both qualitative and quantitative, affect the analytical performance of sensors. Different constructions and materials applied to improve sensors result in specific conditions of membrane formation, in consequence, potentially can result in uncontrolled modification of the membrane composition. Clearly, these effects need to be considered, especially if preparation of miniaturized, potentially disposable internal-solution free sensors is considered. Furthermore, membrane composition changes can occur during the normal operation of sensors—accumulation of species as well as release need to be taken into account, regardless of the construction of sensors used. Issues related to spontaneous changes of membrane composition that can occur during sensor construction, pre-treatment and their operation, seem to be underestimated in the subject literature. The aim of this work is to summarize available data related to potentiometric sensors and highlight the effects that can potentially be important also for other sensors using ion-selective membranes, e.g., optodes or voltammetric sensors.

Electrocoating Polypyrrole on Gold-Wire Electrode as Potential Mediator Membrane Candidate for Anionic Surfactant Electrode Sensor

The development of polypyrrole as a potential mediator membrane candidate for sodium dodecyl sulfate (SDS) sensor electrode has been investigated. The polypyrrole membrane was synthesized electrochemically from the pyrrole and coated at the surface of a 1.0 mm diameter of the gold-wire electrode. Electropolymerization of pyrrole and coating of the polypyrrole produced was performed by cyclic voltammetry technique in the electrochemical cell containing supporting electrolyte of 0.01 M NaClO4 with an optimum potential range of -0.9 V–1.0 V, the scanning rate of 100 mV/s, an electric current of 2 mA, and running of potential scanning of 10 cycles. By using the similar optimal parameters of cyclic voltammetry, electropolymerization of 0.01 M pyrrole solution containing 0.001 M SDS also produces a polypyrrole membrane coated at the gold-wire electrode surface. These coated electrodes have the potential response-ability toward DS- anions in the concentration range of 10-7 M–10-5 M with a limit of detection of 10-7 M and sensitivity of electrode of 9.9 mV/decade. This finding shows that the SDS solution’s role is as supporting electrolyte and also as a source of DS- dopant during the pyrrole electropolymerization processes. Dopants are trapped in the polymer membrane during the electrochemical formation of polypyrrole and role as ionophores for DS- anion in the analyte solution. A potential response to the electrode phenomena is excellent basic scientific information for further synthesis of conducting polymer and development of conducting polymer-coated wire electrode model, especially in the construction of ion-selective electrode (ISE) for the determination of anionic surfactants with those models.

A miniaturized solid-contact potentiometric multisensor platform for determination of ionic profiles in human saliva

This paper describes a miniaturized multisensor platform (MP-ISES) consisting of electrodes: a reference one (RE) and ion-selective electrodes (ISEs) for monitoring Na+, K+, Ca2+, Mg2+, Cl−, and SCN− ions and pH in human saliva. Gold electrode surface was modified by deposition of two layers: electrosynthesized PEDOT:PSS forming an intermediate layer, and ion-selective membrane. The developed ISEs were characterized by a wide linear range and sensitivity consistent with the Nernst model. The entire MP-ISEs are characterized by satisfactory metrological parameters demonstrating their applicability in biomedical research, in particular in measurements concerning determination of ionic profiles of saliva. Saliva samples of 18 volunteers aged from 20 to 26 participating in a month experiment had been daily collected and investigated using the MP-ISEs assigned individually to each person. Personalized profiles of ions (ionograms) in saliva, such as Na+, K+, Ca2+, Mg2+, Cl−, SCN−, and H+, were obtained.

Synthesis of Polypyrrole Induced by [Fe(CN)₆]3- and Redox Cycling of [Fe(CN)₆]4-/[Fe(CN)₆]3

Chemical synthesis of the conducting polymer polypyrrole induced by [Fe(CN)₆]^3- is reported. Reaction kinetics were characterized spectrophotometrically. Reaction rate was evaluated at several different pH levels in the presence of [Fe(CN)₆]^3- and [Fe(CN)₆]^4- ions. The formation of polypyrrole at aerobic and anaerobic conditions was evaluated. We report that at anaerobic conditions [Fe(CN)₆]^4- cannot initiate oxidative polymerization, while its oxidized form [Fe(CN)₆]^3- successfully initiates and maintains the pyrrole polymerization reaction. The formation of polypyrrole was also observed in the solution containing a pyrrole monomer, [Fe(CN)₆]^4- and dissolved oxygen due to re-oxidation (redox cycling) of [Fe(CN)₆]^4- into [Fe(CN)₆]^3- by dissolved oxygen. Experiments to determine the polymerization reaction rate were performed and showed the highest rate in the presence of 0.5 mM of [Fe(CN)₆]^3- at pH 9.0, while the polymerization reaction performed at pH 7.0 was determined as the slowest. This investigation opens new horizons for the application of [Fe(CN)₆]^4-/[Fe(CN)₆]^3--based redox cycling reactions in the synthesis of the conducting polymer polypyrrole and potentially in the formation of other conducting polymers which can be formed by oxidative polymerization.

A new poly(phthalazine ether sulfone ketone)-coated fiber for solid-phase microextraction to determine nitroaromatic explosives in aqueous samples

A novel polar solid-phase microextraction (SPME) fiber coated with poly(phthalazine ether sulfone ketone) (PPESK) was prepared by immersion precipitation technique. The microstructure of the coating exhibits a sponge-like sublayer supporting a dense cracking shaped top layer (about 1 microm in thickness). This coating shows long lifetime (up to 100 times) and is stable at desorption temperature up to 290 degrees C due to the rigid aromatic rings in chemical structure. We evaluated the extraction-desorption properties of the PPESK fiber for nitroaromatic explosives in aqueous samples. The parameters affecting the extraction were optimized, including extraction temperature and time, salt addition, desorption temperature and time. Limits of detection (LOD), precisions and linear dynamic range for the analysis of explosives by SPME-GC/TSD or ECD were evaluated. Limits of detection of the new fiber was three orders of magnitude lower than those with carbowax/divinylbenzene (CW/DVB), and the relative standard deviation (RSD) of single fiber and fiber-to-fiber were less than 9.3 and 10.4%, respectively. The results demonstrated that the PPESK coating exhibited high extraction efficiency for nitroaromatic compounds due to the pi-pi interaction, dipole-dipole interactions and interactions by polar functional groups. The method was applied to the analysis of nitroaromatic explosives in real aqueous samples including seawater and groundwater samples, with relative recoveries better than 90.7%.

Effect of interferents present in the internal solution or in the conducting polymer transducer on the responses of ion-selective electrodes

The effect of interferents present on the opposite side of the Pb2+-selective membrane has been studied for both internal solution and all-solid-state sensors with a conducting polymer (CP) transducer. For interferents with moderate selectivity coefficients (sodium cations) present in the internal solution or in the CP transducer phase, super-Nernstian responses were obtained. For sensors containing strongly discriminated interferents (lithium ions), however, responses typical of conventional electrodes are observed, despite the low activity of primary ions on the opposite side of the membrane. This effect is attributed to hindered incorporation of interfering ions into the membrane, which also impairs the long term stability of the potential. Because of the relatively small absolute amounts of interferents in the transducer of all-solid-state sensors, their exchange for primary ions occurs quickly. Thus, transformation of the sensor to one with a micromolar detection limit and high potential stability is observed.

Optimizing the analytical performance and construction of ion-selective electrodes with conducting polymer-based ion-to-electron transducers

All-solid-state ion-selective electrodes that use a conducting polymer as the ion-to-electron transducer have emerged as one of the most promising classes of all-solid-state potentiometric sensors in recent years. This is largely because it has many analytical advantages, including high response stability, which is unique in the field of internal-solution-free ion-selective electrodes. This paper reviews the considerable progress that has been made in this area of sensing in recent years, in terms of detection limits, selectivity coefficients and novel construction methods.

Junction-less reference electrode for potentiometric measurements obtained by buffering pH in a conducting polymer matrix

A reference electrode for potentiometric measurements based on conducting polymers (CP) doped with pH buffering ligands is described. Both the CPs and doping ligands are selected and adjusted in such a way that possible ionic and redox sensitivity is hampered, while the pH buffering property of the CP film is exposed. In this way, the electric potential drop at the conducting polymer|solution interface is stabilized and close to constant over a certain pH range. The electrode behaves as a pseudo-reference electrode in amphiprotic solvents or their mixtures, e.g. water-alcohol mixtures. For the first time titration of sulfates with lead(ii) in water-methanol solution using two "plastic" electrodes, CP-based Pb(2+)-sensitive indicator and CP-based reference electrode, is shown. Because the electrode is junction-less it may easily be miniaturized and maintained and thus may serve in frontier applications of sensors.

An experimental study of membrane materials and inner contacting layers for ion-selective K+ electrodes with a stable response and good dynamic range

The goal was to identify formulations for use in valinomycin K(+) ion-selective electrodes that could routinely achieve a detection limit of <10(-6) M, even after repeated use and exposure at higher K+ activity (0.1 M) and without the requirement for special pretreatment or conditioning in low K+ activity (10(-3) M). Electrodes that would be characterized by high potential stability were sought in this work. Valinomycin-containing membranes with diffusion coefficient of approximately 10(-11) cm(2) s(-1), formulated from methacrylic/acrylic polymers with or without plasticizer, were compared with plasticized PVC membranes (diffusion coefficient 10(-8) cm(2) s(-1)). The methacrylic/acrylic membranes without plasticizer were shown to give an order of magnitude lower detection limit, when compared with PVC-dioctyl sebacate and o-nitrophenyl octyl ether plasticized methacrylic/acrylic polymers under the same conditions, highlighting the influence of plasticizer on the detection limit. As predicted from current theoretical derivation, the inner contacting layer in the ion-selective electrode construction was shown to be highly influential in maintaining the detection limit below 10(-6) M with use and with poly(pyrrole) providing the inner contact ion-to-electron transduction function, a further order of magnitude improvement in the lower detection limit could be maintained for both chloride and hexacyanoferrate doped poly(pyrrole), when 2% ionophore was employed in the ion-selective membrane. This formulation showed extraordinary stability and reproducibility in terms of measurement range and drift over extended measurement testing, with close to Nernstian slopes. At higher ionophore concentrations (4%), the apparent selectivity of the electrode was improved at the expense of detection limit and the nature of the poly(pyrrole) dopant ion became important in determining the dominant exchange processes at the poly(pyrrole)/ion-selective membrane interface.

An ultrasensitive and stable potentiometric immunosensor

We describe a novel quantitative polypyrrole based potentiometric biosensor that provides broad-spectrum assay capability. The biosensor allows for capture of analytes of interest from complex real samples such as serum and whole blood, and subsequent measurement in a controlled matrix environment. The technology is rapid (<15 min), ultrasensitive (<50 fM) and reproducible (CV<5% at 0.1 ng/ml). In addition the system has shown a wide dynamic range (four to five orders of magnitude), and good stability, 37 degrees C for at least 4 months. This potentiometric biosensor detects enzyme labelled immuno-complexes formed at the surface of a polypyrrole coated, screenprinted gold electrode. Detection is mediated by a secondary reaction that produces charged products (a 'charge-step' procedure). A shift in potential is measured at the sensor surface, caused by local changes in redox state, pH and/or ionic strength. The magnitude of the difference in potential is related to the concentration of the formed receptor-target complex. The potentiometric sensing technology has been demonstrated in assays for hepatitis B surface antigen (HBsAg) (Mw>300 kDa), Troponin I (Mw approximately 23 kDa), Digoxin (Mw 780 Da) and tumour necrosis factor (hTNF-alpha) (Mw approximately 23 kDa). These model targets were chosen to represent analytes of a range of molecular weights, and because of their requirement for assays of high analytical sensitivity and precision. All these assays were performed using complex fluid samples and the presence of any non-specific binding has no significant effect on the final measurement. New assays can be transferred and optimised readily.

Evolution of solid-phase microextraction technology

The main objective of this contribution is to describe the development of the concepts, techniques and devices associated with solid-phase microextraction, as a response to the evolution of understanding of the fundamental principles behind this technique. The discussion begins with an historical perspective on the very early work conduced almost a decade ago. As new fundamental understanding about the functioning of the technology developed, new ways of constructing and using the SPME devices evolved.