Solvent-Processible Polymer Membrane-Based Liquid Junction-Free Reference Electrode

Reference Electrodes Based on Ionic Liquid-Doped Reference Membranes with Biocompatible Silicone Matrixes

Many reference electrodes with an ionic liquid-doped reference membrane contain a plasticizer that can gradually leach out into the sample. However, because many common plasticizers are known to be endocrine disruptors and may induce inflammatory reactions, they are preferably avoided for wearable or implantable sensors. Therefore, this work tested polymeric reference electrode membranes prepared by solvent casting from seven commercially available biocompatible silicones that are widely used in implantable devices. Only reference electrodes with membranes consisting of poly(3,3,3-trifluoropropylmethylsiloxane) (Fluorosilicone 1) and one of several 1-methyl-3-alkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids provided a stable and sample-independent potential in electrolyte solutions spanning the range of electrolyte concentrations in human blood, with more hydrophobic ionic liquids performing better. Over 8 days at 37 °C in artificial blood electrolyte solutions, the reference membranes doped with 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide exhibited a potential drift as low as 20 μV/h. In 10% animal serum, a 112 μV/h drift was observed over 5.8 days. The other six silicone materials doped with an ionic liquid either failed to form self-standing membranes or did not provide a sample-independent potential in the ionic concentration range tested. In case of the functional reference electrodes, differential scanning calorimetry confirmed good miscibility between the ionic liquid and the polymer matrix, whereas the poor miscibility of four polymer matrixes and the ionic liquids-as confirmed by differential scanning calorimetry-correlated with an undesirable sample dependence of the reference potential.

Reference Electrodes with Polymer-Based Membranes-Comprehensive Performance Characteristics

Several types of liquid membrane and solid-state reference electrodes based on different plastics were fabricated. In the membranes studied, equitransferent organic (QB) and inorganic salts (KCl) are dispersed in polyvinyl chloride (PVC), polyurethane (PU), urea-formaldehyde resin (UF), polyvinyl acetate (PVA), as well as remelted KCl in order to show the matrix impact on the reference membranes’ behavior. The comparison of potentiometic performance was made using specially designed standardized testing protocols. A problem in the reference electrode research and literature has been a lack of standardized testing, which leads to difficulties in comparing different types, qualities, and properties of reference electrodes. Herein, several protocols were developed to test the electrodes’ performance with respect to stability over time, pH sensitivity, ionic strength, and various ionic species. All of the prepared reference electrodes performed well in at least some respect and would be suitable for certain applications as described in the text. Most of the reference types, however, demonstrated some weakness that had not been previously highlighted in the literature, due in large part to the lack of exhaustive and/or consistent testing protocols.

Reference Electrodes with Ionic Liquid Salt Bridge: When Will These Innovative Novel Reference Electrodes Gain Broad Acceptance?

In this paper, we raise questions that researchers have to ask if they intend to replace a conventional reference electrode with an ionic liquid-based reference electrode and try to answer these questions based on our experiences and literature data. Among these questions, the most important is which ionic liquid should be used. However, beyond the chemical composition of the ionic liquid, to realize all the potential benefits of ionic-liquid based reference electrodes, there are additional, equally important considerations. Through examples we will show the importance of the (i) purity of the ionic liquid and the consequences of deviations from its stoichiometric salt composition, (ii) form of implementation of the ionic liquid-based reference electrode membrane (free-flowing salt bridge, or ionic liquid embedded in a membrane), (iii) membrane/gelling agent material and its composition, and (iv) experimental conditions (steady state or flowing conditions) under which it will be used. Finally, we recommend methods to test the performance criteria of the ionic liquid-based reference electrodes.

in vivo ischemia monitoring array for endoscopic surgery

An array with all-solid-state, potentiometric, miniaturized sensors for pH and potassium was developed to be introduced into the stomach or other sectors of the digestive tract by means of flexible endoscopy. These sensors perform continuous and simultaneous measurement of extracellular pH and potassium. This detection seeks to sense ischemia in the gastric mucosa inside the stomach, an event indicative of local microvascular perfusion and tissue oxygenation status. Our array is proposed as a medical tool to identify the occurrence of the ischemia after gastrointestinal or gastroesophageal anastomosis. The stability and feasibility of the miniaturized working and reference electrodes integrated in the array were studied under in vitro conditions, and the behavior of the potassium and pH ion-selective membranes were optimized to work under acidic gastric conditions with high concentrations of HCl. The array was tested in vivo in pigs to measure the ischemia produced by clamping the blood flow into the stomach. Our results indicate that ischemic and reperfusion states can be sensed in vivo and that information on tissue damage can be collected by this sensor array. The device described here provides a miniaturized, inexpensive, and mass producible sensor array for detecting local ischemia caused by unfavorable anastomotic perfusion and will thus contribute to preventing anastomotic leakage and failure caused by tissue necrosis.

Real-time monitoring of ischemia inside stomach

The low pH in the gastric juice of the stomach makes it difficult to fabricate stable and functional all-solid-state pH ISE sensors to sense ischemia, mainly because of anion interference and adhesion problem between the ISE membrane and the electrode surface. In this work, the adhesion of ISE membrane on solid surface at low pH was improved by modifying the surface with a conductive substrate containing hydrophilic and hydrophobic groups. This creates a stable and robust candidate for low pH applications. Moreover, anion interference problem at low pH was solved by integration of all-solid-state ISE and internal reference electrodes on an array. So, the same tendencies of anion interferences for all-solid-state ISE and all-solid-state reference electrodes cancel each other in differential potentiometric detection. The developed sensor presents a novel all-solid-state potentiometric, miniaturized and mass producible pH ISE sensor for detecting ischemia on the stomach tissue on an array designed for endoscopic applications.

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.

Nanoporous platinum solid-state reference electrode with layer-by-layer polyelectrolyte junction for pH sensing chip

A novel solid-state reference electrode was developed by combining nanoporous Pt with polyelectrolyte junction. The polyelectrolyte junction was formed in the microchannel connecting the nanoporous Pt and the sample solution, and had layer-by-layer structure of oppositely charged polyelectrolytes. The layer-by-layer polyelectrolyte junction effectively blocked the mass transport of ions and maintains constant pH environments on the surface of the nanoporous Pt. The assembly of the polyelectrolyte junction and the nanoporous Pt, which produced reportedly a stable open-circuit potential in response to constant pH, exhibited outstanding performance as a solid-state reference electrode (e.g., excellent reproducibility of ±4 mV (n = 5), good long term stability of ±1 mV (for 50 h), and independence of solution environments like pH and ionic strength). A working principle of the solid-state reference electrode with layer-by-layer polyelectrolyte junction was suggested in terms of the roles of each layer and the effect of the neighboring layer. As a demonstrative application of the solid-state reference electrode, a miniaturized chip-type solid-state pH sensor comprised of two nanoporous Pt electrodes and a micro-patterned layer-by-layer polyelectrolyte junction was developed. The solid-state pH sensing chip showed reliable pH responses without liquid junction and successfully worked in a variety of buffers, beverages, and biological samples, showing its potential utility for practical applications. In addition, the solid-state pH sensing chip was integrated in a microfluidic system to be utilized for pH monitoring in microfluidic flow.

Plastic reference electrodes and plastic potentiometric cells with dispersion cast poly(3,4-ethylenedioxythiophene) and poly(vinyl chloride) based membranes

A simple procedure of preparing low cost, planar and disposable reference electrodes for potentiometric applications is presented. This method is essentially the same as used for obtaining all-plastic ion-selective electrodes and thus promising for simple fabrication of complete cells. Commercially available aqueous dispersion of poly(3,4-ethylenedioxythiophene) doped by poly(4-styrenesulfonate) ions (PEDOT-PSS, Baytron P) is simply cast on a non-conducting plastic support (transparent foil for laser printers). This layer is covered by a non-selective poly(vinyl chloride) based membrane containing solid AgCl and KCl, added to obtain a stable potential. The conducting polymer layer plays a double role, of electrical contact and ion-to-electron transducer, enhancing the potential stability. The reference electrodes obtained exhibit independence of the kind and concentration of electrolyte applied as well as very low sensitivity to interferences: redox reactants and H+ ions; they are also characterized by both potential stability and low polarisability, sufficient for potentiometric applications. Cells of plastic electrodes (indicator and reference ones) are tested using an arrangement with Pb2+ or Ca2+ selective sensors. Potentiometric characteristic of such cells is satisfactory, well comparable with that using a classical electrode arrangement.

An all-solid-state reference electrode based on the layer-by-layer polymer coating

A solid-state reference electrode (SSRE) was fabricated by layering a silicone rubber (SR) film containing KCl on an AgCl surface, then a perfluorinated ionomer film, and finally a polyurethane-based membrane containing an ionophore, a lipophilic ionic additive, and a plasticizer, respectively. The addition of SiCl4 to the polyurethane-based membrane layer enhanced the strength of the membrane in an aqueous solution. The morphologies of the membranes were studied separately by SEM. The fabrication of the Ag/AgCl electrode through this layer-by-layer polymer coating improved the electrode stability enormously. In addition, the potential drift of the SSRE according to the pH of the medium was minimized by introducing a H+-ion-selective ionophore (tridodecylamine; TDDA) into the outmost polymer membrane. The cyclic voltammetric and potentiometric responses using the SSRE and a conventional reference electrode, respectively, were consistent. The SSRE exhibited little potential variation even in the case of the addition of very high concentrations of various salts, such as Na salicylate, LiCl, KCl, CaCl2, MgCl2, KNO3, NaCl, and NaHCO3. The practicability of the proposed SSRE was tested for the determination of blood pH and pCO2 in a flow cell system. The SSRE fabricated in the present study was stable over two years.

Extremely Base-Resistant Organic Phosphazenium Cations

A series of peralkylated polyaminophosphazenium cations exhibiting extraordinary base resistance under phase-transfer conditions were efficiently synthesized from readily available starting materials. Their half lives under these conditions exceed those of the most stable conventional organic cations by factors of up to 3000.

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.

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.

The study of some possible measurement errors in clinical blood electrolyte potentiometric (ISE) analysers

The understanding of the most important sources of error in potentiometric blood analyser which might contribute to better instruments measurement repeatability is very often marginalized in fabrications and daily operation of some commercial blood analysers. In this paper ISEs-potentiometric measurements were performed and validated in Clinical Institute of Laboratory Diagnosis of the Zagreb University School of Medicine and Clinical Hospital Centre, using a carefully designed and constructed fully automated (computerised) homemade ISE-based blood electrolyte analyser constructed with an in-line five-channel flow-through measuring cell. The influence of electrolyte concentration of the salt bridge is reported. Special attention has been paid to the reference electrode design, and constructions which can operate in open liquid junction and membrane restricted liquid junction modes are described.

Ion chromatography detector based on solid-state ion-selective electrode array

A variety of neutral carrier type ionophores for monovalent cations were employed to prepare solid-state cation-selective electrodes (SSEs) for use as a detector in single-column ion chromatography (IC). The polyurethane-based pseudoreference electrode made it possible to assemble an array type SSE detector for IC. An SSE-based detector provides not only the overall chromatogram for the separated ion species (monensin methyl ester-nonactin-based membrane), but also the enhanced chromatogram for specified ions of interest (valinomycin as K+ and nonactin for NH4+). This feature makes it possible to perform highly quantitative analysis with low detection limits even if the separation efficiency of the ion-exchange is not sufficient. Since SSE-based IC detectors are easily miniaturized and replaceable at low cost, they are an ideal component of a portable IC system.