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

High Active Material Loading in Organic Electrodes Enabled by a Multifunctional Binder

Organic electrodes are promising candidates for next-generation lithium-ion batteries due to their low cost and sustainable nature; however, they often suffer from very low conductivity and active material loadings. The conventional binder used in organic-based Li-ion batteries is poly(vinylidene fluoride) (PVDF), yet it is electrochemically inactive and thus occupies volume and mass without storing energy. Here, we report an organic mixed ionic-electronic conducting polymer, poly[norbornene-1,2-bis(C(O)OPEDOT)]₂₅-b-[norbornene-1,2-bis-(C(O)PEG₁₂)]₂₅ denoted PEDOT-b-PEG for simplicity, as a cathode binder to address the aforementioned issues. The polymer contains a poly(3,4-ethylenedioxythiophene) (PEDOT) functionality to provide electronic conductivity, as well as poly(ethylene glycol) (PEG) chains to impart ionic conductivity to the cathode composite. We compare electrodes containing a perylene diimide (PDI) active material, conductive carbon, and a polymeric binder (either PVDF or PEDOT-b-PEG) with different weight ratios to study the impact of active material loading and type of binder on the performance of the cell. The lithium-ion cells prepared with the PEDOT-b-PEG polymer binder result in higher capacities and decreased impedance at all active material loadings compared to cathodes prepared with the PVDF-containing electrodes, demonstrating potential as a new binder to achieve higher active material loadings in organic electrodes. The strategy of preparing these polymers should be broadly applicable to other classes of mixed polymer conductors.

Single-Step Integration of Poly(3-Octylthiophene) and Single-Walled Carbon Nanotubes for Highly Reproducible Paper-Based Ion-Selective Electrodes

Calibration of ion-selective electrodes (ISEs) is cumbersome, time-consuming, and constitutes a significant limitation for the development of single-use and wearable disposable sensors. To address this problem, we have studied the effect of ion-selective membrane solvent on ISE reproducibility by comparing tetrahydrofuran (THF) (a typical solvent for membrane preparation) and cyclohexanone. In addition, a single-step integration of semiconducting/transducer polymer poly(3-octylthiophene) (POT) with single-walled carbon nanotubes (SWCNTs) into the paper-based ISEs (PBISEs) substrate was introduced. PBISEs for potassium and sodium ions were developed, and these ISEs present outstanding sensor performance and high potential reproducibility, as low as ±1.0 mV (n = 3).

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.

Ion-Selective Carbon Nanotube Field-Effect Transistors for Monitoring Drug Effects on Nicotinic Acetylcholine Receptor Activation in Live Cells

We developed ion-selective field-effect transistor (FET) sensors with floating electrodes for the monitoring of the potassium ion release by the stimulation of nicotinic acetylcholine receptors (nAChRs) on PC12 cells. Here, ion-selective valinomycin-polyvinyl chloride (PVC) membranes were coated on the floating electrode-based carbon nanotube (CNT) FETs to build the sensors. The sensors could selectively measure potassium ions with a minimum detection limit of 1 nM. We utilized the sensor for the real-time monitoring of the potassium ion released from a live cell stimulated by nicotine. Notably, this method also allowed us to quantitatively monitor the cell responses by agonists and antagonists of nAChRs. These results suggest that our ion-selective CNT-FET sensor has potential uses in biological and medical researches such as the monitoring of ion-channel activity and the screening of drugs.

Paper Strip and Ceramic Potentiometric Platforms Modified with Nano-Sized Polyaniline (PANi) for Static and Hydrodynamic Monitoring of Chromium in Industrial Samples

Screen-printed membrane sensors based on the use of paper and ceramic substrates are fabricated, characterized, and used for rapid batch and continuous monitoring of CrIII in the form of CrO42- in some industrial products and wastewater samples. Strips of paper and ceramic platforms (15 × 5 mm) were covered with conductive carbon paint and then modified with polyaniline (PANI) film, to act as an ion-to-electron transducer, followed by a drop casting of plasticized poly (vinyl chloride) (PVC) Rhodamine-B chromate membrane as a recognition sensing material. In a 5.0 mmol L-1 Trizma buffer solution of pH ~8, the fabricated paper and ceramic based membrane sensors exhibited a near Nernstian response for CrVI ion with slopes of -29.7 ± 0.5 and -28.6 ± 0.3 mV decade-1, limit of detection 2.5 × 10-5 and 2.4 × 10-6 mol L-1 (1.3-0.12 µg mL-1), and linear concentration range 7.5 × 10-3-5.0 × 10-5 and 7.5 × 10-3-1.0 × 10-5 mol L-1 (390-0.5 µg mL-1), respectively. Both sensors exhibited fast and stable potentiometric response, excellent reproducibility, and good selectivity with respect to a number of common foreign inorganic species. Impedance spectroscopy and chronopotentiometry data revealed a small resistance and a larger double layer capacitance due to the presence of the intermediate polyaniline (PAN) conductive layer. Furthermore, the formation of a water layer between the ion selective membrane (ISM) and the underlying conductor polymer and between the conducting polymer and the carbon conducting surface was greatly reduced. The developed disposable solid-contact potentiometric sensors offer the advantages of simple design, long term potential stability, flexibility, miniaturization ability, short conditioning time, and cost effectiveness that enable mass production. The sensors were successfully used for static and hydrodynamic measurements of total chromium in some leather tanning wastewater and nickel-chrome alloy samples. The results compare favorably with data obtained by atomic absorption spectrometry.

Disposable Multi-Walled Carbon Nanotubes-Based Plasticizer-Free Solid-Contact Pb2+-Selective Electrodes with a Sub-PPB Detection Limit †

Potentiometric plasticizer-free solid-contact Pb²⁺-selective electrodes based on copolymer methyl methacrylate-n-butyl acrylate (MMA-BA) as membrane matrix and multi-walled carbon nanotubes (MWCNTs) as intermediate ion-to-electron transducing layer have been developed. The disposable electrodes were prepared by drop-casting the copolymer membrane onto a layer of MWCNTs, which deposited on golden disk electrodes. The obtained electrodes exhibited a sub-ppb level detection limit of 10⁻¹⁰ mol·L⁻¹. The proposed electrodes demonstrated a Nernstian slope of 29.1 ± 0.5 mV/decade in the linear range from 2.0 × 10⁻¹⁰ to 1.5 × 10⁻³ mol·L⁻¹. No interference from gases (O₂ and CO₂) or water films was observed. The electrochemical impedance spectroscopy of the fabricated electrodes was compared to that of plasticizer-free Pb²⁺-selective electrodes without MWCNTs as intermediated layers. The plasticizer-free MWCNTs-based Pb²⁺-selective electrodes can provide a promising platform for Pb(II) detection in environmental and clinical application.

Biochemical sensing with macrocyclic receptors

Preventive healthcare asks for the development of cheap, precise and non-invasive sensor devices for the early detection of diseases and continuous population screening. The actual techniques used for diagnosis, e.g. MRI and PET, or for biochemical marker sensing, e.g. immunoassays, are not suitable for continuous monitoring since they are expensive and prone to false positive responses. Synthetic supramolecular receptors offer new opportunities for the creation of specific, selective and cheap sensor devices for biological sensing of specific target molecules in complex mixtures of organic substances. The fundamental challenges faced in developing such devices are the precise transfer of the molecular recognition events at the solid-liquid interface and its transduction into a readable signal. In this review we present the progress made so far in turning synthetic macrocyclic hosts, namely cyclodextrins, calixarenes, cucurbiturils and cavitands, into effective biochemical sensors and the strategies utilized to solve the above mentioned issues. The performances of the developed sensing devices based on these receptors in detecting specific biological molecules, drugs and proteins are critically discussed.

Application of the interface equilibria-triggered dynamic diffusion model of the boundary potential for the numerical simulation of neutral carrier-based ion-selective electrodes response

It is shown that a simple dynamic diffusion model of the boundary potential based on a separate, step-by-step, account of ion transfer across the membrane/aqueous solution interface and the diffusion processes within both phases which was proposed earlier for describing the response of ionophore-free membranes, can be successfully used for ionophore-based membranes as well. The model makes it possible to carry out both separate and joint account of the effects of co-extraction, transmembrane transport and ion exchange on the boundary potential and retains robustness in all the variants studied. The model adequately describes the ionophore-based electrode response over the entire range of concentrations and allows one to clearly demonstrate the dependence of lower detection limit on such parameters as the diffusion coefficients and the concentration of electroactive substances in the membrane phase, the thickness of the diffusion layer in the sample solution, the duration of the measurement, and the composition of the internal reference solution. The results of numerical simulation are in good agreement with the experimental data presented in the literature. As all the factors of influence considered above can easily be regulated in more or less wide limits, but at the same time, an estimation of their cumulative effect is not always possible on an intuitive level, the present model can be of practical interest for justifying the ways of optimizing the design of the ISE and the algorithm for performing measurements in solving specific analytical problems.

Rapid Bacterial Detection via an All-Electronic CMOS Biosensor

The timely and accurate diagnosis of infectious diseases is one of the greatest challenges currently facing modern medicine. The development of innovative techniques for the rapid and accurate identification of bacterial pathogens in point-of-care facilities using low-cost, portable instruments is essential. We have developed a novel all-electronic biosensor that is able to identify bacteria in less than ten minutes. This technology exploits bacteriocins, protein toxins naturally produced by bacteria, as the selective biological detection element. The bacteriocins are integrated with an array of potassium-selective sensors in Complementary Metal Oxide Semiconductor technology to provide an inexpensive bacterial biosensor. An electronic platform connects the CMOS sensor to a computer for processing and real-time visualization. We have used this technology to successfully identify both Gram-positive and Gram-negative bacteria commonly found in human infections.

Solid contact potassium selective electrodes for biomedical applications - a review

Ion-selective electrodes (ISE) are used in several biomedical applications, including laboratory sensing of potassium concentration in blood and urine samples. For on-site determination of potassium concentration and usage in other applications such as determination of extracellular potassium concentration, miniaturization of the sensors is required. To that extent, solid contacts have proven to be an adequate substitute of liquid contacts as inner layer for ion-to-electron transduction, allowing industrial production of miniaturized ISEs. This review paper covers relevant developments of solid-state ISEs in the past decade, critically compares current potassium ISEs and discusses future prospects for biomedical applications. Performances of three main types of solid contact materials in potassium sensing are compared, namely polypyrrole, polythiophenes and conducting nanomaterials. With these new materials, numerous improvements in stability, selectivity and time response of solid-state ISEs have been made. Current developments are new operational methods of sensing, flexible miniaturized sensors and multi-electrode designs able to measure electrolyte concentrations in one-drop blood samples or transmembrane ionic flows.

Amperometric Ion Sensing Approaches at Liquid/Liquid Interfaces for Inorganic, Organic and Biological Ions

Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) has become an invaluable tool for the selective and sensitive detection of cationic and anionic species, including charged drug molecules and proteins. In addition, neutral molecules can also be detected at the ITIES via enzymatic reactions. This chapter highlights recent developments towards creating a wide spectrum of sensing platforms involving ion transfer across the ITIES. As well as outlining the basic principles needed for performing these sensing applications, the development of ITIES-based detection strategies for inorganic, organic, and biological ions is discussed.

Selective ion-sensing with membrane-functionalized electrolyte-gated carbon nanotube field-effect transistors

In this work the ion-selective response of an electrolyte-gated carbon-nanotube field-effect transistor (CNT-FET) towards K(+), Ca(2+) and Cl(-) in the biologically relevant concentration range from 10(-1) M to 10(-6) M is demonstrated. The ion-selective response is achieved by modifying the gate-electrode of an electrolyte-gated CNT-FET with ion-selective membranes, which are selective towards the respective target analyte ions. The selectivity, assured by the ion-selective poly(vinyl chloride) based membrane, allows the successful application of the herein proposed K(+)-selective CNT-FET to detect changes in the K(+) activity in the μM range even in solutions containing different ionic backgrounds. The sensing mechanism relies on a superposition of both an ion-sensitive response of the CNT-network as well as a change of the effective gate potential present at the semiconducting channel due to a selective and ion activity-dependent response of the membrane towards different types of ions. Moreover, the combination of a CNT-FET as a transducing element gated with an ion-selective coated-wire electrode offers the possibility to miniaturize the already well-established conventional ion-selective electrode setup. This approach represents a valuable strategy for the realization of portable, multi-purpose and low-cost biosensing devices.

Solid contact ion-selective electrodes with a well-controlled Co(II)/Co(III) redox buffer layer

Solid contact ion-selective electrodes (ISEs) typically have an intermediate layer between the ion-selective membrane and the underlying solid electron conductor that is designed to reduce the irreproducibility and instability of the measured electromotive force (emf). Nevertheless, the electrode-to-electrode reproducibility of the emf of current solid contact ISEs is widely considered to be unsatisfactory. To address this problem, we report here a new method of constructing this intermediate layer based on the lipophilic redox buffer consisting of the Co(III) and Co(II) complexes of 1,10-phenanthroline ([Co(phen)3](3+/2+)) paired with tetrakis(pentafluorophenyl)borate as counterion. The resulting electrodes exhibit emf values with an electrode-to-electrode standard deviation as low as 1.7 mV after conditioning of freshly prepared electrodes for 1 h. While many prior examples of solid contact ISEs also used intermediate layers that contained redox active species, the selection of a balanced ratio of the reduced and oxidized species has typically been difficult and was often ignored, contributing to the emf irreproducibility. The ease of the control of the [Co(phen)3](3+)/[Co(phen)3](2+) ratio explains the high emf reproducibility, as confirmed by the emf decrease of 58 mV per 10-fold increase in the ratio of the reduced and oxidized redox buffer species. Use of a gold electrode modified with a self-assembled 1-hexanethiol monolayer as underlying electron conductor suppresses the formation of a water layer and results in an electrode-to-electrode standard deviation of E° of 1.0 mV after 2 weeks of exposure to KCl solution.

Solid-state reference electrodes based on carbon nanotubes and polyacrylate membranes

A novel potentiometric solid-state reference electrode containing single-walled carbon nanotubes as the transducer layer between a polyacrylate membrane and the conductor is reported here. Single-walled carbon nanotubes act as an efficient transducer of the constant potentiometric signal originating from the reference membrane containing the Ag/AgCl/Cl(-) ions system, and they are needed to obtain a stable reference potentiometric signal. Furthermore, we have taken advantage of the light insensitivity of single-walled carbon nanotubes to improve the analytical performance characteristics of previously reported solid-state reference electrodes. Four different polyacrylate polymers have been selected in order to identify the most efficient reservoir for the Ag/AgCl system. Finally, two different arrangements have been assessed: (1) a solid-state reference electrode using photo-polymerised n-butyl acrylate polymer and (2) a thermo-polymerised methyl methacrylate:n-butyl acrylate (1:10) polymer. The sensitivity to various salts, pH and light, as well as time of response and stability, has been tested: the best results were obtained using single-walled carbon nanotubes and photo-polymerised n-butyl acrylate polymer. Water transport plays an important role in the potentiometric performance of acrylate membranes, so a new screening test method has been developed to qualitatively assess the difference in water percolation between the polyacrylic membranes studied. The results presented here open the way for the true miniaturisation of potentiometric systems using the excellent properties of single-walled carbon nanotubes.

Determination of choline and derivatives with a solid-contact ion-selective electrode based on octaamide cavitand and carbon nanotubes

A new solid-contact ion-selective electrode has been developed for determining choline and derivatives in aqueous solutions. The backbone of this new potentiometric sensor is the conjunction of the cavitand receptor, as the molecular recognition element, and a network of non-carboxylated single-walled carbon nanotubes, acting as a solid transducer material. The octaamide cavitand, a synthetic receptor that is highly selective for biologically important trimethyl alkylammonium cations such as choline, acetylcholine or carnitine, makes the selective determination of these compounds possible for the first time. The guest-host interaction takes place in the acrylate ion-selective membrane of the solid-contact electrode. The sensor was characterized by electrochemical impedance spectroscopy and environmental scanning electron microscopy. The new electrode displays a nearly Nernstian slope (57.3+/-1.0 mV/decade) and very stable behaviour (DeltaE/Deltat=224 muVh(-1)) throughout the dynamic range (10(-5) to 10(-1)M). The limit of detection of 10(-6.4)M and the high selectivities obtained will enable choline and derivatives to be determined in biological samples. Finally, the stability of the electrical potential of the new solid-contact electrode was examined by performing current-reversal chronopotentiometry and the influence of the interfacial water film was evaluated by the potentiometric water layer test.

Subnanomolar Detection Limit Application of Ion-Selective Electrodes with Three-Dimensionally Ordered Macroporous (3DOM) Carbon Solid Contacts

Solid-contact ion-selective electrodes (SC-ISEs) can exhibit very low detection limits and, in contrast to conventional ISEs, do not require an optimization of the inner filling solution. This work shows that subnanomolar detection limits can also be achieved with SC-ISEs with three-dimensionally ordered macroporous (3DOM) carbon contacts, which have been shown recently to exhibit excellent long-term stabilities and good resistance to the interferences from oxygen and light. The detection limit of 3DOM carbon-contacted electrodes with plasticized poly(vinyl chloride) as membrane matrix can be improved with a high polymer content of the sensing membrane, a large ratio of ionophore and ionic sites, and conditioning with a low concentration of analyte ions. This permits detection limits as low as 1.6×10(-7) M for K(+) and 4.0×10(-11) M for Ag(+).

Developments in the Field of Conducting and Non-conducting Polymer Based Potentiometric Membrane Sensors for Ions Over the Past Decade

Many research studies have been conducted on the use of conjugated polymers in the construction of chemical sensors including potentiometric, conductometric and amperometric sensors or biosensors over the last decade. The induction of conductivity on conjugated polymers by treating them with suitable oxidizing agents won Heeger, MacDiarmid and Shirakawa the 2000 Nobel Prize in Chemistry. Common conjugated polymers are poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, poly(terthiophene)s, poly(aniline)s, poly(fluorine)s, poly(3-alkylthiophene)s, polytetrathiafulvalenes, polynapthalenes, poly(p-phenylene sulfide), poly(p-phenylenevinylene)s, poly(3,4-ethylenedioxythiophene), polyparaphenylene, polyazulene, polyparaphenylene sulfide, polycarbazole and polydiaminonaphthalene. More than 60 sensors for inorganic cations and anions with different characteristics based on conducting polymers have been reported. There have also been reports on the application of non-conducting polymers (nCPs), i.e. PVC, in the construction of potentiometric membrane sensors for determination of more than 60 inorganic cations and anions. However, the leakage of ionophores from the membranes based on these polymers leads to relatively lower life times. In this article, we try to give an overview of Solid-Contact ISE (SCISE), Single-Piece ISE (SPISE), Conducting Polymer (CP)-Based, and also non-conducting polymer PVC-based ISEs for various ions which their difference is in the way of the polymer used with selective membrane. In SCISEs and SPISEs, the plasticized PVC containing the ionophore and ionic additives govern the selectivity behavior of the electrode and the conducting polymer is responsible of ion-to-electron transducer. However, in CPISEs, the conducting polymer layer is doped with a suitable ionophore which enhances the ion selectivity of the CP while its redox response has to be suppressed.

Method of achieving desired potentiometric responses of polyacrylate-based ion-selective membranes

We introduce a simple procedure allowing preparation of cation-selective electrodes with poly( n-butyl acrylate)-based membranes containing different proportions of primary and interfering ions introduced already at the membrane preparation step, by using two different liphophilic salts of the same anion. With this approach the time required to achieve saturation of polyacrylate membranes with primary ions can be significantly shortened. Moreover, depending on the ratio of the primary and interfering ions introduced to the membrane cocktail, different potentiometric responses are obtained ranging from typical (with micromolar detection limit), through lower detection limits to super-Nernstian ones.