Solid-state Reference Electrode Research Articles

Solid State Reference Electrodes with Conducting Polymers and Their Application in Potentiometric Cells

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Solid-state reference electrodes for potentiometric sensors

The availability and application of solid-state reference electrodes for potentiometric electrochemical sensors are briefly reviewed. For a long time, considerable efforts have been made to combine solid-state indicator electrodes with equivalent reference electrodes to take advantage of the absence of liquid system components to full capacity. In spite of various suggestions to solve the problem, no type of solid-state reference electrode is so far available with properties completely identical to conventional ones.

New Class of Ag/AgCl Electrodes Based on Hydrophobic Ionic Liquid Saturated with AgCl

A new type of Ag/AgCl electrodes based on a hydrophobic ionic liquid has been proposed. The electrode consists of a Ag/AgCl electrode immersed in or coated with a AgCl-saturated ionic liquid, 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide ([C(8)mim+][C(1)C(1)N-]), instead of the internal aqueous solution. The [C(8)mim+][C(1)C(1)N-] phase plays dual roles, that is, as a medium dissolving AgCl and an ionic-liquid-type salt bridge upon contact with an aqueous solution. The gelation of the [C(8)mim+][C(1)C(1)N-] phase allows us to prepare coated-wire-type solid-state reference electrodes with a well-defined thermodynamic basis for the electrode potential. Both gelled and nongelled types show stable electrode potentials against the change in the concentration of KCl between 0.05 mmol dm(-3) and 2 mol dm(-3). This new class of reference electrodes opens the way for a variety of miniaturized and solid-state reference electrodes.

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.

Solid-state Cd/Cd 2+ reference electrode based on PMMA gel electrolytes

All solid-state reference electrode based on Cd/Cd 2+ system was developed for the electrochemical measurements in aprotic organic carbonates (propylene PC, ethylene EC and dimethyl carbonate DMC). The electrode was designed by using poly(methyl methacrylate) – propylene carbonate polymer electrolyte with immobilised Cd(ClO 4) 2 solution and a cadmium wire. The electrode performance in various solvents was studied including potential determination and its dependence on the solution ionic strength. Cyclic voltammetry of ferrocene in particular solvents showed good reproducibility of the electrode potential within the set of 18 electrodes: E(PMMA-Cd-Cd 2+) = − 0.439 ± 0.013 vs. SCE in propylene carbonate. Also long-term chronopotentiometric measurements showed good potential stability and no potential drift within 3-week continuous experiment. Three versions of electrode material filling were studied (glass frit, ceramic separator and graphite rod) with the aim to reduce unwelcome penetration of chemicals into the electrode material and leakage of cadmium ions into solution. Reliable electrochemical data were obtained during the 5-year employment of the PMMA-Cd-Cd 2+ electrode.

A polymeric junction membrane for solid-state reference electrodes

A polymer membrane system composed of 30 wt.% cellulose acetate (CA) and polyurethane (PU) has been developed for the fabrication of mass producible solid-state reference electrodes. The CA/PU membrane exhibits appropriate adhesion to common substrates (e.g., ceramic and silicon chips) of miniaturized electrochemical sensors, allows quick hydration of internal hydrogel layer (3 M KCl with a 6 wt.% water soluble polymer) deposited on micro-patterned electrodes resulting in fast preconditioning time (∼100 s), and provides a stable reference potential for an extended period (use-lifetime: 25–90 min) by limiting the diffusion of internal electrolytes. The potentiometric responses of the CA/PU membrane-based solid-state reference electrodes drifted rapidly (40–90 mV/h) after their use-lifetime; the results suggest that the internal electrolyte slowly fills the micro-channels of the outer membrane while maintaining stable potential, and begins to diffuse away at an increased rate from the membrane/aqueous solution interface. The potentiometric responses of the polymer membrane-based ion-selective electrodes (for Na +, K +, Ca 2+, Cl − and H +) formed on the same chip with the CA/PU membrane-coated Ag/AgCl reference electrode were examined both in aqueous and physiological samples; their analytical performance closely matched that of sensors measured against a conventional reference electrode.

Composites: A novel alternative to construct solid state Ag/AgCl reference electrodes

An Ag/AgCl solid-state reference electrode is developed by means of a graphite–AgCl–Silver dag-epoxy resin composite. The response of the composite reference electrode (CRE) to chloride ions is evaluated; a linear non-nernstian response is observed associated to the following equation E = −15.15 (±1.10) − 44.05 (±0.38) log[Cl −]. Comparing the CRE's response potential versus a saturated commercial Ag/AgCl reference in KCl 0.1 M, a mean of 40.7 ± 0.4 mV of the distribution of potential versus time data is observed over a period of 1 h. The performance of the CRE as reference for a glass membrane electrode by means of direct pH measurements and quantitative determination of acids by acid–base titrations is evaluated obtaining statistically stable, precise and exact results compared with those obtained using a combined glass electrode. The typical cylindrical configuration of the CRE is changed to adapt it to a FIA system for the determination of ammonium ion, obtaining a sensitivity 50.30 ± 0.26 mV/log[NH 4 +] and a linear range 8.5 × 10 −5 to 0.1 M, which are analytical parameters statistically equivalent to those presented by the classical determination system.

A miniaturized low-power wireless remote environmental monitoring system based on electrochemical analysis

In this paper, we report a miniaturized low-power wireless remote environmental monitoring system. This system has been developed for on-site monitoring of water pollution by heavy-metal ions. The system is composed of three parts: an electrochemical sensor module using microfabricated electrodes for detecting heavy-metal contamination in sample water; a custom potentiostat module including readout circuitry, analog-to-digital converter and microcontroller; and a radio frequency (RF) module for sending detected signals to a base station through wireless communication. The electrochemical sensor module is implemented using microfabricated mercury working electrodes (WEs), solid-state reference electrode (SSRE), and platinum counter electrode (CE). For the low-power operation, direct frequency-shift keying (FSK) modulation and simple binary FSK demodulation methods are used for RF module which is realized using 0.18 μm CMOS technology. All the modules are hybrid integrated in a printed circuit board (PCB) and low-power consumption below 1 mW has been achieved.

Solid-state internal reference electrode based on quinhydrone for hydrogen sensor with acid-doped polybenzimidazole

An electrode consisting of a mixture of quinhydrone (QH) and polybenzimidazole (PBI) with carbon was investigated as a solid reference electrode for hydrogen sensors. The vibrational behaviour of the polybenzimidazole/quinhydrone blend was investigated by infrared spectroscopy. The results suggest the existence of hydroquinone in the blend. The quinone/hydroquinone redox couple was evidenced by cyclic voltammetry. The potential of the solid-state reference electrode was found to be very stable over a period of 700 h. The drift was less than 0.1 mV per day. This electrode was successfully implemented in a potentiometric sensor. The standard potential was of the order of −550 mV at room temperature and increased as a function of the temperature with a slope of 1 mV/°C. The standard potential was also insensitive to changes in relative humidity (rh) in the sample gas over a period of 1 day.

Improvement of integrated Ag/AgCl thin-film electrodes by KCl-gel coating for ISFET applications

To eliminate the need of a separate reference electrode and facilitate the use of ion-sensitive field-effect transistor (ISFET), an all-solid-state reference electrode integrated with ISFET in one chip has been developed. A novel agarose-stabilized KCl-gel membrane was introduced to serve both as a polymer-supported solid reference electrolyte and an ionic bridge for Ti/Pd/Ag/AgCl electrode. Calibrated against the commercial macro Ag/AgCl reference electrode the new miniaturized reference electrode shows its cell potential variation was ±0.9–1.4 mV (equivalent to about ±0.015–0.023 pH) in 1 h, less than 2 mV variation over pH 4–10, and almost insensitive to changes in Cl − ion concentration (about 0.02–0.25 mV/pKCl). The planar solid-state reference electrode also shows a very small offset voltage of 0.45 mV and reproducible to within ±0.5 mV among the batch fabricated electrodes. ISFET pH-sensor with integrated all-solid-state reference electrode characterized in standard pH solutions show a linear sensitivity of 56 mV/pH, these results agree very well with the theoretical value of 59 mV/pH. Dynamic response and hysteresis characteristics of the pH-sensor are also studied and discussed.

Fabrication and characterization of a new planar solid-state reference electrode for ISFET sensors

A new technique to fabricate planar solid-state reference electrodes for ISFET sensors is reported. Agarose powder was introduced to gel the KCl saturated electrolyte to serve both as a polymer-stabilized internal electrolyte and an ion-diffusion membrane for Ti/Pd/Ag/AgCl planar electrode. It enables the integration of a reference electrode on the sensor chip such as ISFET for ion sensing measurement. This new planar integrated reference electrode has not only eliminated the fabrication problems associated with the filling of the inner reference liquid electrolyte into a miniature micromachined cavity, but also has many excellent performance characteristics with respect to the reference potential stability and its insensitivity to the changes of pH values and Cl − ion concentrations in the sample solutions under test. Calibrated against the standard Ag/AgCl reference electrode the new planar reference electrode shows its cell potential variation less than 1 mV over 30 min; less than 2 mV variation over pH values ranging from 4 to 10; and almost insensitive (approx. 0.02∼0.25 mV/pKCl) to changes in Cl − ion concentration. The reference electrode also shows a very small offset voltage of 0.45 mV and reproducible to within 0.5 mV among the batch fabricated electrodes.

Potentiometric CO 2 gas sensor with lithium phosphorous oxynitride electrolyte

Potentiometric cell, Au/LiCoO 2 5 m/o Co 3O 4/Li 2.88PO 3.73N 0.14/Li 2CO 3/Au, has been fabricated and investigated for monitoring CO 2 gas. A LiCoO 2–Co 3O 4 mixture was used as the solid-state reference electrode instead of a reference gas. The idea is to keep the lithium activity constant on the reference side using thermodynamic equilibrium at a given temperature. The thermodynamic stability of the reference electrode was studied from the phase stability diagram of Li–Co–C–O system. The Gibb’s free energy of formation of LiCoO 2 was estimated at 500°C from the measured value of the cell emf. The sensors showed good reversibility and fast response toward changing CO 2 concentrations from 200 to 3000 ppm. The emf values were found to follow a logarithmic Nernstian behavior in the 400–500°C temperature range. CH 4 gas did not show any interference effect. Humidity and CO gas decreased the emf values of the sensor slightly. NO and NO 2 gases affect this sensor significantly at low temperatures. However, increased operating temperature seems to reduce the interference.

Solid state potentiometric sensor at medium temperatures (150–300°C) for detecting oxidable gaseous species in air

The development of a solid state potentiometric sensor able to work up to 300°C is described. In this sensor, a very thin membrane of α-zirconium acid phosphate, obtained by colloidal dispersion of exfoliated α-Zr(HPO 4) 2 and acting as a proton conductor, is interposed between a catalytic electrode (platinum) and a solid state reference electrode (a silver disk anodically coated with Ag 2O). Both the proton conductor and the reference electrode were thermally stable at the above temperature. The sensing electrode became more and more active with the increasing temperature, allowing the detection even of some species towards which the sensor is inert at lower temperature. Preliminary investigations showed that its emf depends on the partial pressure of oxygen. When used in air at 200–300°C, the sensor can detect oxidable gaseous species such as hydrocarbons and alcohols. The sensitivity obtained is of the order of a few ppm. Some considerations on the detection mechanism, as well as on the potentiality of this sensor for its use in the detection of oxidable polluting agents in air are also reported.

Analysis of Heavy-Metal Ions Using Mercury Microelectrodes and a Solid-State Reference Electrode on a Si Wafer

Micro working electrodes and a solid-state reference electrode have been fabricated using semiconductor processes for electrochemical detection of heavy-metal ions in aqueous media. The working electrodes are mercury-coated platinum microelectrodes 5 µm, 10 µm and 25 µm in diameter. The smaller electrode shows the higher sensitivity and the detection limits of the 5-µm-diameter electrodes are about 0.2 ppb and 0.4 ppb for cadmium and lead ions, respectively. The reference electrode is a Ag/AgCl electrode coated by polyvinyl chloride (PVC)/Nafion layer containing Cl- ions. The potential variation of the fabricated solid-state reference electrodes is about 55 mV under a 0.1 M Cl- concentration.

Effects of mercury electrodeposition on the surface degradation of microlithographically fabricated iridium ultramicroelectrodes

The surface degradation of a microlithographically fabricated array of 10 mm diameter iridium ultramicroelectrodes (UME) was investigated using atomic force microscopy (AFM), Auger scanning electron microscopy (SEM), and stripping voltammetry. Micrometer-size accumulations could be observed on the iridium surface after mercury was electrochemically deposited and stripped approximately five times in a perchloric acid media with a Ag AgCl 3M NaCl reference electrode. AFM images of the accumulations revealed that they were pyramidal in shape and had a tendency to form in clusters. Elemental analysis of the clusters with Auger SEM identified them as mercury and a small amount of chloride. The accumulations were determined to be mercurous chloride (Hg2Cl2) with the chloride contamination resulting both from leakage of the reference electrode and the decay of the perchloric acid. During subsequent depositions of mercury, chloride ions were eliminated by using a nitric acid media and a polyurethane solid state reference electrode. With chloride eliminated, mercury could be electrodeposited and stripped at least ten times on the iridium UME array without any visible surface degradation. © 1998 Elsevier Science S.A. All rights reserved.

Evaluation of miniaturised solid state reference electrodes on a silicon based component

A study of two different types of micro reference electrodes with respect to preparation and performance is described. The first, Type 1 Ag/AgCl, consists of a silver/silver chloride ink screen printed onto platinum on silicon. The second, Type 2 Ag/AgCl, uses the screen printed reference electrode just described, Type 1 Ag/AgCl, covered with a chloride doped vinyl ester resin. The latter eliminates some of the difficulties and drawbacks associated with Type 1 Ag/AgCl because the requirement for a constant chloride concentration in the analyte solution is removed when using Type 2 Ag/AgCl. Type 1 Ag/AgCl acted as a chloride selective electrode with a slope of 55.83 mV per decade for chloride activity and Type 2 Ag/AgCl potential versus SCE varied from −40.1 to −20.3 mV for KCl concentrations of 10 −7 and 1 M, respectively. The performance of a commercially available macro solid state reference electrode (called MacroRefex in this work), fabricated from the same materials as Type 2 Ag/AgCl, was also investigated.

Towards a solid state reference electrode

The construction of a solid state reference electrode is presented. The solid state reference electrode has two membranes arranged in parallel. The membranes are anion and cation conducting, and both of them are in contact with the sample solution. Resistors with considerably larger resistance than that of the ion-exchange membranes are soldered onto each of the two electrode parts, to give the same amount of charge transferred in the parallel paths. The anion and cation conducting paths of the electrode then give contributions to the e.m.f. that cancel, and we show that the cell e.m.f. varies with the concentration of the ion reversible to the ion selective electrode (ISE) in a Nernstian way. As an example of the ISE we have studied a fluoride selective electrode. We have also tested materials for the anion and cation conducting parts of the reference electrode, and found that they behave close to what is predicted. The performance of the total reference electrode can be compared to a commercially available electrode. Procedures for further progress in the realization of the ideal reference electrode are discussed on the background of our theoretical development.

Fabrication and Characterization of a Solid State Reference Electrode for Electroanalysis of Natural Waters with Ultramicroelectrodes

As an alternative to standard commercial reference electrodes, a solid state reference electrode is fabricated for in situ voltammetric analysis in solutions containing little or no added supporting electrolyte. In the fabrication process, a Ag/AgCl wire is coated with an electrolyte immobilized and protected with Nafion or polyurethane. The electrode potential is measured as a function of time, ionic strength, and pH. A stable potential is obtained for Nafion and polyurethane within 30−35 min in water. Both the polyurethane and Nafion solid state reference electrodes are stable for at least 90 days, but the Nafion electrode fluctuates more than the polyurethane electrode. The electrode is demonstrated using square wave anodic stripping voltammetry at a large electrode (3 mm), an ultramicroelectrode (10 μm), and an array of ultramicroelectrodes.

Effect of pressure on the potentiometric response of ion-selective electrodes and reference electrodes

A study is presented on the influence of pressure of up to 30 bar on potentiometric cells. Combinations of a poly(vinyl chloride) (PVC) matrix nitrate ion-selective electrode with inner filling solution, a solid contact PVC nitrate electrode, a capillary flow type reference electrode, and a solid state reference electrode were tested. A linear increase of the potentials as a function of pressure was observed in all cases after an initially non-linear phase. The overall potential changes in the investigated pressure range were between 0.2 mV and 1.5 mV, slopes being typically 10 μV bar −1 and 40 μV bar −1 for nitrate electrodes with liquid filling solutions and for solid contact nitrate electrodes, respectively.

Thick-film reference electrodes for solid state pH measurement

The fabrication and performance details of thick-film solid state reference electrodes are reported and limitations of range and longevity are given. These planar structures are designed as references for solid state pH sensors. Practical and theoretical aspects of their operational mechanisms are discussed.