Mechanism of ionic and redox sensitivity of p-type conducting polymers

Conducting polymer functionalization in search of advanced materials in ionometry: ion-selective electrodes and optodes

Functionalized conducting polymers (FCPs) have recently garnered attention as ion-selective sensor materials, surpassing their intrinsic counterparts due to synergistic effects that lead to enhanced electrochemical and analytical parameters. Following a brief introduction of the fundamental concepts, this article provides a comprehensive review of the recent developments in the application of FCPs in ion-selective electrodes (ISEs) and ion-selective optodes (ISOs), particularly as ion-to-electron transducers, optical transducers, and ion-selective membranes. Utilizing FCPs in these devices offers a promising avenue for detecting and measuring ions in various applications, regardless of the sample nature and composition. Research has focused on functionalizing different conducting polymers, such as polyaniline and polypyrrole, through strategies such as doping and derivatization to alter their hydrophobicity, conductance, redox capacitance, surface area, pH sensitivity, gas and light sensitivity, etc. These modifications aim to enhance performance outcomes, including potential stability/emission signal stability, reproducibility and low detection limits. The advancements have led to the transition of ISEs from conventional zero-current potentiometric ion sensing to innovative current-triggered sensing approaches, enabling calibration-free applications and emerging concepts such as opto-electro dual sensing systems. The intrinsic pH cross-response and instability of the optical signal of ISOs have been overcome through the novel optical signal transduction mechanisms facilitated by FCPs. In this review, the characteristics of materials, functionalization approaches, particular implementation strategies, specific performance outcomes and challenges faced are discussed. Consolidating dispersed information in the field, the in-depth analysis presented here is poised to drive further innovations by broadening the scope of ion-selective sensors in real-world scenarios.

Recent Advances in Wearable Potentiometric pH Sensors

Wearable sensors reflect the real–time physiological information and health status of individuals by continuously monitoring biochemical markers in biological fluids, including sweat, tears and saliva, and are a key technology to realize portable personalized medicine. Flexible electrochemical pH sensors can play a significant role in health since the pH level affects most biochemical reactions in the human body. pH indicators can be used for the diagnosis and treatment of diseases as well as the monitoring of biological processes. The performances and applications of wearable pH sensors depend significantly on the properties of the pH–sensitive materials used. At present, existing pH–sensitive materials are mainly based on polyaniline (PANI), hydrogen ionophores (HIs) and metal oxides (MO_x). In this review, we will discuss the recent progress in wearable pH sensors based on these sensitive materials. Finally, a viewpoint for state–of–the–art wearable pH sensors and a discussion of their existing challenges are presented.

Pt/Polypyrrole Quasi-References Revisited: Robustness and Application in Electrochemical Energy Storage Research

Choosing reference electrodes for nonaqueous electrochemical measurements, especially in energy storage research, is challenging due to lengthy experiments (>1 day), the lack of alternatives to the commonly used Ag/Ag⁺ reference electrode (RE), the introduction of junction potentials, and the possibility of sample contamination. Often, quasi-reference electrodes (QREs) such as Ag wires and Li metal strips are used. However, small changes in electrolyte composition can cause large potential drifts, and their surfaces may be reactive to the solution. Here, we propose an alternative QRE based on polypyrrole electrodeposited on Pt wire (PPyQRE) encased in a glass tube with the open end sealed with commercial frits. While freestanding PPyQRE wires have been reported in the literature, simple encasing of the PPyQRE overcomes the above-mentioned drawbacks of QREs while providing a reliable reference potential that is closer to the performance of an RE. During cyclic voltammetric and bulk electrolysis testing of a redox mediator in solution, the encased PPyQRE exhibited stable reference potentials over multiple charge/discharge cycles with minimal drift (∼5 mV) after ∼2.25 days of operation. We also tested the reliability of our reference during the testing of multilayer graphene Li-ion anodes, which often involve cycling samples at highly reducing potentials (<-3 V vs Fc/Fc⁺) over long durations (>1 day). In the same testing conditions, the Ag/Ag⁺ electrode led to observable Ag deposits on the graphene and large potential drifts (∼50 mV), while the PPyQRE exhibited no measurable drift and revealed changes in voltammetric features that were obscured by reference drift when using Ag/Ag⁺. Minor reference drifts of ∼30 mV over long usage of the PPyQRE (∼2 months) can be addressed by calibration with a ferrocene couple at the end of experiments. These results highlight the advantages of using an encased PPyQRE as a simple and practical reference electrode for electrochemical measurements in the field of nonaqueous energy storage research.

Selective Nonenzymatic Amperometric Detection of Lactic Acid in Human Sweat Utilizing a Multi-Walled Carbon Nanotube (MWCNT)-Polypyrrole Core-Shell Nanowire

Lactic acid plays an important role as a biochemical indicator for sports medicine and clinical diagnosis. The detection of lactic acid in sweat is a promising technique without any intrusive inconvenience or risk of infection. In this study, we present a selective nonenzymatic amperometric detection method for lactic acid in human sweat utilizing a multi-wall carbon nanotube (MWCNT)-polypyrrole core-shell nanowire. Because polypyrrole is a p-type conducting polymer, onto which anions are exclusively doped, leading to charge transfer, it offers selective detection for lactate anions at a specific potential, while being inert to the neutral and cationic species contained in human sweat. A chronoamperometric study reveals good sensing performance for lactic acid with a high sensitivity of 2.9 μA mM⁻¹ cm⁻² and detection limit of 51 μM. Furthermore, the MWCNT-polypyrrole nanowire exhibits excellent selectivity for lactic acid over interfering species, such as sodium chloride, glucose, urea, and riboflavin, which coexist with lactic acid in sweat. Finally, a nonenzymatic amperometric sensor for the selective detection of lactic acid in human sweat is demonstrated on commercial flexible electrodes. The results demonstrate the potential applications of the MWCNT-polypyrrole core-shell nanowire as a nonenzymatic amperometric lactate sensor.

Coulometric response characteristics of solid contact ion-selective electrodes for divalent cations

The chronoamperometric and coulometric response of solid contact ion-selective electrodes (SCISEs) for the detection of divalent cations was investigated in order to provide a more complete description of the mechanism of the recently introduced coulometric transduction method for SCISEs. The coulometric transduction method has earlier been employed only for SCISEs that were selective to monovalent ions. The SCISEs utilized poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrene sulfonate) (PSS−) as the solid contact (ion-to-electron transducer). PEDOT(PSS) was electrodeposited on glassy carbon and covered with plasticized PVC-based ion-selective membranes (ISMs) that were selective towards divalent cations (Ca2+, Pb2+). In contrast to earlier studies, the results obtained in this work show that the coulometric response for the Pb2+-SCISE was limited mainly by ion transport in the PEDOT(PSS) layer, which was not the case for the Ca2+-SCISE, nor was it observed earlier for the monovalent ions. The exceptional behavior of the Pb2+-SCISE was explored further by electrochemical impedance spectroscopy, and it was shown that the effective redox capacitance of PEDOT(PSS) was significantly higher for the Pb2+-SCISE than for the Ca2+-SCISE although the polymerization charge of PEDOT(PSS) was the same. The slow transport of Pb2+ in PEDOT(PSS) was tentatively related to complexation between Pb2+ and PEDOT(PSS).

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.

Smart material platforms for miniaturized devices: implications in disease models and diagnostics

Smart materials are responsive to multiple stimuli like light, temperature, pH and redox reactions with specific changes in state. Various functionalities in miniaturised devices can be achieved through the application of "smart materials" that respond to changes in their surroundings. The change in state of the materials in the presence of a stimulus may be used for on demand alteration of flow patterns in devices, acting as microvalves, as scaffolds for cellular aggregation or as modalities for signal amplification. In this review, we discuss the concepts of smart trigger responsive materials and their applications in miniaturized devices both for organ-on-a-chip disease models and for point-of-care diagnostics. The emphasis is on leveraging the smartness of these materials for example, to allow on demand sample actuation, ion dependent spheroid models for cancer or light dependent contractility of muscle films for organ-on-a-chip applications. The review throws light on the current status, scope for technological enhancements, challenges for translation and future prospects of increased incorporation of smart materials as integral parts of miniaturized devices.

Galvanic cell without liquid junction for potentiometric determination of copper

This paper describes potentiometric measurements in an integrated galvanic cell with both indicator and reference electrodes. Both electrodes are conducting polymer-based. The copper-sensitive indicator electrode is made by using poly(3,4-ethylenedioxythiophene) (PEDOT) doped with 2-(o-arsenophenylazo)-1,8-dihydroxynaphthalene-3,6-disulphonic sodium salt (Arsenazo-I) as the electroactive substance in the film, while the reference electrode is based on PEDOT doped by 2-morpholineoethanesulfonic acid (MES). It is shown that the galvanic cell can be used for determination of copper both in non-aqueous media (where all PVC-based membranes failed) and in the presence of chloride ions, which disturb the signal of conventional copper ion-selective electrodes with solid-state membranes. It is further shown that the titration of copper ions can be successfully monitored using the described electrochemical cell.

Biosensing and drug delivery by polypyrrole

Conducting polypyrrole is a biological compatible polymer matrix wherein number of drugs and enzymes can be incorporated by way of doping. The polypyrrole, which is obtained as freestanding film by electrochemical polymerization, has gained tremendous recognition as sophisticated electronic measuring device in the field of sensors and drug delivery. In drug delivery the reversing of the potential 100% of the drug can be released and is highly efficient as a biosensor in presence of an enzyme. In this review we discuss the applications of conducting polypyrrole as biosensor for some biomolecules and drug delivery systems.

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.

All-solid-state reference electrodes based on conducting polymers

A novel construction of solution free (pseudo)reference electrodes, compatible with all-solid-state potentiometric indicator electrodes, has been proposed. These electrodes use conducting polymers (CP): polypyrrole (PPy) or poly(3,4-ethylenedioxythiophene) (PEDOT). Two different arrangements have been tested: solely based on CP and those where the CP phase is covered with a poly(vinyl chloride) based outer membrane of tailored composition. The former arrangement was designed to suppress or compensate cation- and anion-exchange, using mobile perchlorate ions and poly(4-styrenesulfonate) or dodecylbenzenesulfonate anions as immobilized dopants. The following systems were used: (i) polypyrrole layers doped simultaneously by two kinds of anions, both mobile and immobilized in the polymer layer; (ii) bilayers of polypyrrole with anion exchanging inner layer and cation-exchanging outer layer; (iii) polypyrrole doped by surfactant dodecylbenzenesulfonate ions, which inhibit ion exchange on the polymer/solution interface. For the above systems, recorded potentials have been found to be practically independent of electrolyte concentration. The best results, profound stability of potentials, have been obtained for poly(3,4-ethylenedioxythiophene) or polypyrrole doped by poly(4-styrenesulfonate) anions covered by a poly(vinyl chloride) based membrane, containing both anion- and cation-exchangers as well as solid potassium chloride and silver chloride with metallic silver. Differently to the cases (i)-(iii) these electrodes are much less sensitive to the influence of redox and pH interferences. This arrangement has been also characterized using electrochemical impedance spectroscopy and chronopotentiometry.

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.