Solid-state Reference Research Articles

Disposable Solid-State pH Sensor Using Nanoporous Platinum and Copolyelectrolytic Junction

A disposable solid-state pH sensor was realized by utilizing two nanoporous Pt (npPt) electrodes and a copolyelectrolytic junction. One nanoporous Pt electrode was to measure the pH as an indicating electrode (pH-IE) and the other assembled with copolyelectrolytic junction was to maintain constant open circuit potential (<TEX>$E_{oc}$</TEX>) as a solid-state reference electrode (SSRE). The copolyelectrolytic junction was composed of cationic and anionic polymers immobilized by photo-polymerization of N,N'-methylenebisacrylamide, making buffered electrolytic environment on the SSRE. It was expected to make. The nanoporous Pt surrounded by a constant pH excellently worked as a solid state reference electrode so as to stabilize the system within 30 s and retain the electrochemical environment regardless of unknown sample solutions. Combination between the SSRE and the pH-IE commonly based on nanoporous Pt yielded a complete solid-state pH sensor that requires no internal filling solution. The solid state pH sensing chip is simple and easy to fabricate so that it could be practically used for disposable purposes. Moreover, the solid-state pH sensor successfully functions in calibration-free mode in a variety of buffers and surfactant samples.

Development of an Ion Sensitive Field Effect Transistor Based Urea Biosensor with Solid State Reference Systems

Ion sensitive field-effect transistor (ISFET) based urease biosensors with solid state reference systems for single-ended and two-ended differential readout electronics were investigated. The sensing membranes of the biosensors were fabricated with urease immobilized in a conducting polymer-based matrix. The responses of 12.9∼198.1 mV for the urea concentrations of 8∼240 mg/dL reveal that the activity of the enzyme was not significantly decreased. Biosensors combined with solid state reference systems were fabricated, and the evaluation results demonstrated the feasibility of miniaturization. For the differential system, the optimal transconductance match for biosensor and reference field-effect transistors (REFET) pair was determined through the modification of the membranes of the REFETs and enzyme field-effect transistors (EnFETs). The results show that the transconductance curve of polymer based REFET can match with that of the EnFET by adjusting the photoresist/Nafion™ ratio. The match of the transconductance curves for the differential pairs provides a wide dynamic operating measurement range. Accordingly, the miniaturized quasi-reference electrode (QRE)/REFET/EnFET combination with differential arrangement achieved similar urea response curves as those measured by a conventional large sized discrete sensor.

AC Impedance Characterization of Highly Resistive Media Using Four-Electrode Electrochemical Cells

Electrochemical impedance spectroscopy (EIS) was employed for characterization studies of highly resistive non-aqueous media using four-electrode cells with solid state reference electrodes. The influence of internal resistance of the impedance analyzer, the media's temperature and conductivity, shielding of the cables, and the electrochemical cell geometry and configuration on the impedance results were investigated. The most accurate EIS measurements can be made in the 2-electrode cells with active shields where a single arc at high frequencies and a complicated low frequency interfacial impedance feature were observed. When 4-electrode cells were employed, the relative impedances of the sample and the analyzer, geometry and positioning of voltage electrodes, and capacitive coupling between the signal lines introduced two types of impedance measurement artifacts. A capacitive-resistive low frequency load was interpreted as a measurement artifact originating from the voltage electrodes' geometry and positioning. The medium frequency artifact combining resistive, capacitive and inductive features is intrinsic to the experimental setup resulting in a voltage divider effect.

All-Solid-State pH Sensor Based on Conducting Polymer

A solid-state (free of internal electrolyte solution) reference electrode based on metallic silver is simply fabricated via a simple electrodeposition from AgNO3. Potentiometry is carried out to study the performance of the solid-state reference electrode in comparison with the commercial Ag/AgCl reference electrode. In addition, the proposed solid-state reference electrode has been integrated with the PPy-modified pH electrode to form an all-solid-state pH sensor, showing the capability of pH measurement. Absence of the internal solution leads to the pH sensor that is convenient to use and maintenance. In addition, the proposed pH sensor is possibly applied to biochemical and medical processes as well as flow infection analysis.

A Hydrogen Sensor Using SrCe 0.95Yb 0.05O 3-α as Proton Conductor and YH x+YH 2− z as Reference Electrode for Determining Hydrogen Pressure in Solid Steel

The steel contains a small amount of hydrogen which will escape during the heat treatment. The hydrogen pressure in 16MnRE steel was investigated with a hydrogen sensor, which used SrCe 0.95 Yb 0.05 O 3− a proton conductor as a solid electrolyte, YH x +YH 2− z as a solid state reference electrode and Ni wire as electrode constructing a hydrogen concentration cell, shown as Ni | YH x +YH 2− z |SrCe 0.95 Yb 0.05 O 3− a |[H] steel | Ni. The response time of sensor is less than 10 s. The relational expression of hydrogen partial pressure with temperatures was determined using two shape proton conductors. The results showed the regularity in experimental temperature range, and the hydrogen partial pressure increased as its temperature was raised.

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.

The structures of the precursor Hf(OnBu)4 and its modification in solution: EXAFS-investigation in combination with XANES- and IR-spectroscopy

The solution structure of hafnium-n-butoxide Hf(OnBu)4, an important precursor for the sol–gel preparation of hafnium oxide containing materials, was studied in solution in pure toluene, and in presence of acetylacetone, iso-propanol and tetrahydrofuran by EXAFS-, XANES- and IR-spectroscopy. The combination of these three methods and the additional measurements and discussions of well known solid state references allow us to identify structural motives and to gain a detailed picture of the structures formed in solution without any a priori knowledge about these structures. The dimeric structure of crystalline Hf(OnBu)4 is not preserved in solution and a trimeric cyclic structure is formed. While the addition of HOiPr and THF does not induce any change of the formed complexes, one equivalent of Hacac reduces the degree of aggregation by the partial formation of dimers. In the presence of two and three equivalents of Hacac, solely monomeric species are formed. The obtained structural parameters are used to establish for the first time an empirical correlation of a spectral feature in the XANES region with the average oxygen coordination number.

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.

The titanium/hydrogen system as the solid-state reference in high-temperature proton conductor-based hydrogen sensors

The titanium/hydrogen system as the solid-state reference in high-temperature proton conductor-based hydrogen sensors

Infrared Balloon Experiment: improved instrumental configuration and assessment of instrument performance

During the 2002 environmental satellite Envisat mid-latitude validation campaign, a new upgraded configuration of the Infrared Balloon Experiment (IBEX) Fourier transform spectrometer, which had its first flight in 1978, performed a stratospheric balloon flight across the Mediterranean Sea. Among the substantial upgrades made to the instrument, the use of photon-noise-limited detectors permitted us to reach the theoretical limits in terms of signal-to-noise ratio. Also, important modifications were made to the interferometric system and electronics, such as the installation of a solid-state laser reference source and an onboard data recording system. During the flight, measurement of volume-mixing-ratio vertical profiles of O3, HNO3, N2O, and ClO from an altitude of approximately 38 km were performed with a vertical resolution of approximately 1.5 km.

Filling in the Hexagonal Close-Packed Unit Cell

The illustrations of the hcp unit cell that are used in textbooks at all levels and also in crystallography and solid-state reference works are incomplete, in that they fail to include fractions of middle layer atomic spheres with centers lying outside of the unit cell.

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.

Oxygen sensor with solid-state CeO 2–ZrO 2–TiO 2 reference

Development of an oxygen sensor for automotive applications with a solid-state reference is reported. Unlike conventional oxygen sensors used in monitoring exhaust gas emissions, in which one side of the sensor must be directly exposed to the air reference, the new sensor employs a ternary mixed oxide of CeO 2–ZrO 2–TiO 2 (hereafter referred to as CZT) as the reference. The sensor consists of YSZ electrolyte and CZT reference layers with platinum coating as electrodes. The response of the sensor was investigated in a gas mixture of 6 mol% CO (in He) and air at 400, 500 and 600 °C. At 400 °C, the sensor generates a wide hysteresis in the lean region, while at higher temperatures a narrower hysteresis occurs at λ = 1. The sensor also exhibits variations in the voltage, from about +300 mV in the rich region to about −250 mV in the lean region, an interesting behavior not observed in air-referenced structures.

An yttria-doped ceria-based oxygen sensor with solid-state reference

A self-contained, miniaturized, electrochemical-type solid-reference oxygen sensor is described which uses yttria-doped ceria (YDC) as an oxygen ion conducting solid electrolyte. Unlike conventional oxygen sensors, in which one side of the sensor needs to be directly exposed to the air reference, the oxygen sensor presented in this paper exploits a solid solution of 25mol% zirconia in ceria as the reference. Designing a combination structure, the sensing capability of this sensor is compared with a YSZ electrolyte oxygen sensor under the same conditions. This novel oxygen sensor exhibits an anomalous behavior in transition from rich-to-lean regimes where the polarity of the created voltage changes from a positive value of +500mV to a negative value of −150mV at λ = 1.01.

A novel miniaturized oxygen sensor with solid-state ceria–zirconia reference

Development of a novel oxygen sensor with a solid-state reference is reported. Unlike conventional oxygen sensors used in monitoring exhaust gas emissions, in which one side of the sensor must be directly exposed to the air reference, the oxygen sensor presented here employs a solid solution of 25 mol% zirconia in ceria as the reference. The sensor consists of yttria-stabilized-zirconia (YSZ) electrolyte and ceria–zirconia reference layers with platinum coating as electrodes. Tests have been performed with two simulated gases mixture of (91.6% He, 8.4% CO) and (93.8% He, 6% CO, 0.2% C 2H 6) at operating temperatures of 300, 325, 350, 400 and 450 °C. For the He/CO gas mixture, a sharp transition occurs at the stoichiometric value. For the He/CO/C 2H 6 gas mixture, hysteresis was observed at temperatures 300 and 325 °C, while at higher temperatures a sharp step on the rich side ( λ=0.92) was seen. Unlike conventional sensors, where the developed voltage varies between 1000 and 100 mV, the present sensor exhibits a sharp variation in the voltage, from 350 to −80 mV, an anomalous and interesting behavior not observed in air-referenced structures. The oxygen sensor can be miniaturized, yielding a low-cost sensor with a fast response, especially during cold start.

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.