Solid-state Reference Electrode Research Articles

Synthesis, properties, and applications of all-solid-state reference electrode based on AgTi2(PO4)3

High-density AgTi2(PO4)3 as a solid electrolyte was synthesised by a high-temperature solid-state process. The optimal sintering temperature was determined to obtain the best structural and electrical performance. Based on the as-prepared AgTi2(PO4)3 fast-ion conductor, a new type of all-solid-state reference electrode was prepared, and its electrochemical properties, such as cyclic voltammetry curves and potentials at varying pH, were investigated. A pH sensor was designed by combining an AgTi2(PO4)3 reference electrode with an Ir/IrO2 pH electrode. This pH sensor exhibited a good linear response to pH values in aqueous solutions and simulated seawater samples. In addition, a metal ion sensor, comprising the as-fabricated solid-state reference electrode, a Pt counter electrode, and a glassy carbon working electrode, was employed to quantify Cu2+, Hg2+, and Pb2+ concentrations using differential-pulse anodic stripping voltammetry. The resulting peak currents correlated with the metal ion concentrations and exhibited a low detection limit, good linearity, and wide detection range, which is suitable for performing chemical analysis in harsh marine environments.

Development of Solid State Reference Electrode Based on Polypyrrole (PPy) and Screen Printed Electrode (SPE)

In this study, a Polypyrrole doped KCl/KCl saturated (PPy-Cl/KCl) reference electrode had been successfully developed. In this development process, the PPy-Cl/KCl reference electrode that had been provided consisted of two stages of refinement, specifically the PPy-Cl/KCl 1 reference electrode which became the PPy-Cl/KCl 2 reference electrode. Furthermore, the PPy-Cl/KCl 2 SRE was made into 3 types with cycle variations of 10, 15, and 20 cycles and the mV response value test and Scanning Electron Microscopy (SEM) test were carried out. Regarding the variation results, PPy-Cl/KCl 2 SRE was obtained with 15 cycles as the best PPy-Cl/KCl 2 reference electrode with ∆mV value of 3.43 mV. Subsequently, the PPy-Cl/KCl 2 SRE with 15 cycles was involved in the characterization test using cyclic voltammetry (CV) testing, response testing vs Cl sensors and drift testing. In the CV testing, the PPy-Cl/KCl 2 SRE with 15 cycles indicated a voltammogram graphic pattern similar to the Ag/AgCl reference electrode as a commercial reference electrode. Moreover, in the response testing vs the Cl sensor, the PPy-Cl/KCl 2 SRE with 15 cycles provided a Nernstian number value of -54.5 mV/s with a testing range of 0.1-104 M, and in the PPy-Cl/SRE drift testing, KCl 2 with 15 cycles indicated fairly good stability, particularly with a drift value of 1 mV/minute which was achieved after the conditioning process after 3.5 minutes.

Performance of Disposable Cu/CuSO4 Reference Electrode in Reinforced Concrete Corrosion Analysis

The investigation of reference electrode performance for corrosion analysis has been widely conducted over the last decade. The need for a disposable and reliable reference electrode is a specific need for corrosion investigation. This research investigated the effectiveness of liquid-based and solid-state reference electrodes, which are disposable and provide reliable performance. The electrode was prepared with low-cost material preparation (disposable usage) within a Copper-Copper Sulphate (CCS) liquid-based and solid-state electrodes. The potential stability was already tested for dependability by measuring the Open Circuit Potential (OCP) on the rebar concrete specimen. The behaviour of solid-state reference and liquid-based reference electrodes in terms of polarization was studied in a corroded environment. Real-time potential data was also obtained for the corrosion rate investigation. The result shows the consistency performance of (OCP) value during corrosion measurement. The deviation of the maximum to a minimum potential value less than 0.02 V, verifies that the liquid-based copper (Cu) electrode and solid-state copper (Cu) electrodes are reliable, adaptable, and disposable for reinforced concrete corrosion measurement. In terms of corroded rebar potential measurement, the real-time polarization measurement result shows that the high-risk corroding occurred on the embedded rebar and was validated by directly visualizing the rebar concrete specimen. Therefore, the results obtained allow us to conclude that the disposable of both reference electrodes shows their stability and significantly reduces the reinforced concrete corrosion research cost.

Electrochemical Investigation of the Solid-State Reference Electrode Based on Activate Carbon for Corrosion Monitoring of Reinforced Concrete Structures

Electrochemical Investigation of the Solid-State Reference Electrode Based on Activate Carbon for Corrosion Monitoring of Reinforced Concrete Structures

Studies on corrosion monitoring of reinforced concrete in a chloride environment using an embedded solid-state reference electrode

In the present study, the corrosion status of steel rebars embedded in concrete with various cover thicknesses and exposed to conditions simulating a marine environment (3.5 % NaCl) was monitored by an embedded solid-state reference electrode (ESSRE). Electrochemical impedance spectroscopy (EIS) results confirm that corrosion initiation of the steel rebars occurred at 702, 1144, 1469 and 2454 h for C10, C15, C20 and C30 concrete specimens, respectively. Further, the relationship between the free and bound chlorides for C15, C20 and C30 specimens were defined by Temkin isotherm. In addition, the time to crack initiation was predicted by the El Maaddawy model. This study has successfully demonstrated that the solid-state MnO2 electrode is reliable for monitoring the onset of steel rebar corrosion in concrete by a non-destructive method.

Facile fabrication of tailorable Ag/AgCl reference electrodes for planar devices

• Ag/AgCl planar reference electrodes were fabricated using a facile process. • Planar reference electrodes show an open circuit potential of ∼15 mV vs a commercial. • The reference electrodes showed 70 h of stability in a chloride solution. • Planar reference electrodes were used for electrodeposition and glucose sensing. We report the fabrication of tailorable and planar silver-silver chloride (Ag/AgCl) solid-state reference electrodes (SSREs) deposited onto a flexible acetate. A silver layer was thermally evaporated and then was chlorinated in an iron (III) chloride solution. The solid salt reservoir composite consisted of salt crystals embedded in a pre-cured poly(dimethylsiloxane) matrix solution. We performed cyclic voltammetry to study the suitability of the fabricated SSREs in a three-electrode system and showed a negligible difference in the voltammograms obtained by the redox reaction of Fe 3+ /Fe 2+ compared to a commercial Ag/AgCl reference electrode, showing the reduction and oxidation peaks very slight potential shifts. Also, we used the planar SSRE in a glucose reactive strip sensor and for electroplating deposition of four different metals. These planar SSREs have a lifetime of at least 70 h of continuous monitoring at open-circuit potential, a result that makes possible the use of these electrodes in disposable devices. The principal advantage of this system is that the SSREs can be tailored to any desired form to fabricate planar devices that require reference electrodes, like most of chemical and biological sensors, and could be used where access to a laboratory is difficult.

Screen-printed Electrodes with Stable Solid-state Reference Electrode for Oxidation-reduction Potential (ORP) Measurement Applications

ORP is defined as the electromotive force between a noble metal electrode and a reference electrode when immersed in solution [1]. The electrochemical setup typically involves a platinum wire as a working electrode (sometimes gold, rarely silver) and a well-controlled reference electrode. When these electrodes are immersed together into a solution, an electron migration is produced between the working electrode and the solution. Depending on the nature of the species diluted, the working electrode loses electrons if the media is an oxidant, so the potential measured becomes positive. If the media is reductive, the working electrode gains electrons, and the resulting potential will be negative. Voltage is measured between both electrodes (working and reference electrode), so the ORP value is usually given in millivolts.ORP provides valuable information because it is a measure of sanitizer activity and water quality, due to the changes in ORP being associated with the existence of contaminants or biological activity. ORP measurements are also useful in several major industrial applications where maintaining potential in a production stage under control is a critical parameter or when it is necessary to assure the quality of water. Examples include cyanide oxidation or chrome reduction in wastewater treatment of metal-plating industry residues, bleach production and bleaching of paper, biological growth control in cooling towers and water quality monitoring in swimming pools, hot tubs, spas, or aquafarming.Screen-printed electrodes (SPEs) offer several interesting advantages in industrial applications such as point-of-care testing, usability, and a wide range of possible customizable setups. However, most SPEs include a pseudo-reference electrode fabricated with silver or silver/silver chloride. Since the KCl electrolyte is absent, this pseudo-reference electrode is exposed to media implying that the reference electrode is not under control, especially in aggressive media. In this publication, an SPE with a platinum paste working electrode and a controlled solid-state reference electrode is presented. Several experiments are included to show that these platforms offer ORP monitoring in household, industrial or laboratory applications.

Self-repairing nanoporous MCNT/Ag/AgCl composite electrode as a low-cost and long-term stable marine potentiometric sensor

Maintenance-free, cost-effective, and long-lasting potentiometric sensors are highly anticipated for marine detection. Solid-state silver/silver chloride (Ag/AgCl) is considered the most suitable non-polarizable electrode for that purpose. However, the conventional fabrication of thin-film or porous Ag/AgCl electrodes has limitations in the case of simultaneous cost-effectiveness and stability endurance. In the present study, the multi-walled carbon nanotube (MCNT)/Ag/AgCl composite electrode with a 70 mg/cm2 Ag load was prepared, having nanoporous MCNT/Ag conductive skeleton, and a three-dimensionally spreading Ag/AgCl thin layer. The prepared MCNT/Ag/AgCl composite electrode showed an excellent electrode performance, including low resistance, high exchange current density, and stable potential with long-term stability and low cost. The process kinetics and self-repairing mechanism between Ag and AgCl with the help of MCNT were elucidated. The usage of MCNT/Ag/AgCl composite electrode as a solid-state reference electrode and ocean electric field sensor was successfully evaluated, which provides a promising marine potentiometric sensor application for this nanoporous metal composite electrode via a novel preparation method.

Electropolymerised pH Insensitive Salicylic Acid Reference Systems: Utilization in a Novel pH Sensor for Food and Environmental Monitoring.

This work summarizes the electrochemical response of a salicylic acid-based carbon electrode for use as a novel solid-state reference electrode in a redox-based pH sensor. This novel reference produces a pH insensitive response over a range of pH 3–10 in solutions with low buffer concentrations, different compositions, conductivities, and ionic strengths is produced. The pH of the local environment is shown to be determined by the chemistry and the electrochemical response of the redox active species on the surface of the electrode; the local pH can be controlled by the electropolymerized salicylic acid moieties due to the acid concentration on the surface, avoiding any perturbation in environmental pH and leading to a stable novel reference system. Sensitivities of −7.1 mV/pH unit, −2.4 mV/pH unit, −0.2 mV/pH unit, and 2.5 mV/pH units were obtained for different food medias, hydroponic solution, seawater, and cell-culture media, respectively, confirming its ability to control the local pH of the electrode. This reference system is paired with a new pH sensing element based on electropolymerized flavanone to provide a calibration free, pH sensitive sensor to effectively and accurately measure the pH of various media with high viscosity, low conductivity, low/high buffer concentration or cell-culture environment, presenting a maximum error of +/−0.03 pH units.

Gold-modified paper as microfluidic substrates with reduced biofouling in potentiometric ion sensing

Microfluidic sampling media based on paper and its modifications with either gold nanoparticles or sputtered gold were evaluated for potentiometric determination of Na+, K+, and Cl– ions in clinically relevant samples. The measurements were conducted in comparison to other commonly considered microfluidic substrates, i.e. sponge, polyester textile, and polyamide textile. Ion determination was done by using solid-contact ion-selective electrodes based on plasticized PVC membranes for Na+, K+, and Cl– ions and utilizing PEDOT(PSS) or PEDOT(Cl) as the ion-to-electron transducer. The solid-contact ion-selective electrodes and a solid-state reference electrode were placed directly on the substrate into which the sample solution was wicked. Transport of bovine serum albumin (BSA) through the paper substrate was studied by ellipsometry. Modification of the paper substrates by gold nanoparticles (AuNPs) was found to slow down the transport of BSA through the paper, when compared with unmodified paper substrates and when compared with all the other alternative sampling matrices studied. The retention of BSA obtained with AuNP-modified paper substrates significantly improved the accuracy of the potentiometric ion determinations in sweat, saliva, artificial tears, and artificial serum. The potentiometric results were validated by inductively coupled plasma optical emission spectrometry (ICP-OES) and ion chromatography (IC). The study indicates that modification of paper by AuNPs is a feasible approach to reduce biofouling of sensors that are used in the paper-based analysis of clinically relevant samples.

A Paper-Based Potentiometric Platform for Determination of Water Hardness

A novel paper-based potentiometric platform for the simple and fast monitoring of water hardness is presented. First, potentiometric ion-selective electrodes for calcium and magnesium printed on a paper substrate were built and optimized. These sensors, which display near-Nernstian sensitivity, were used for the determination of the concentration of these cations and the calculation of the water hardness. Second, the incorporation of a solid-state reference electrode allowed building an integrated paper-based potentiometric cell for the determination of the hardness of artificial and real samples (mineral waters). The validation of the results shows good ability to predict hardness in the conventional scale. Truly decentralized measurements were demonstrated by integration of a miniaturized instrument and dedicated software in a portable device. The measurements were able to be performed in just under two minutes, including a two-point calibration. Since the method is simple to use and cost-effective, it can be implemented in domestic and industrial settings.

An unusually stable solid state Ag|AgCl reference electrode for long term continuous measurements based on a crosslinked poly(vinyl acetate)/KCl composite

Electrochemical measurements are attractive for continuous or long-term monitoring of chemical species in a range of analytical applications including health, environmental and agricultural monitoring, as well as food analysis and industrial process control. A significant barrier to long-term electrochemical measurements in continuous online monitoring applications is the requirement for stable and durable reference electrodes. Most reference electrodes require regular maintenance in such situations to maintain the concentration of the internal electrolyte reservoir and prevent fouling of the electrode. Solid state reference electrodes (SSREs) are well suited to these long-term monitoring applications, but most only have their short-term performance reported or are designed for disposable applications. Additionally, polymer crosslinking has been demonstrated as an effective way to slow leakage of suspended solutes, improve chemical resistance, and control a polymer's physical properties; but application of this technique in SSRE materials is yet to be reported. This work describes the construction and performance of a novel SSRE with exceptional long-term stability, using an ethylene glycol dimethacrylate (EGDMA) crosslinked PVAc/KCl composite material. Open circuit potentiometry and electrochemical impedance spectroscopy were used to study the electrochemical performance. Conductivity measurements and micro X-ray fluorescence mapping were used to examine the effect of EGDMA crosslinking on electrolyte leakage and electrode lifetime. A SSRE with a 1% molar ratio of EGDMA/vinyl acetate proved to be exceptionally stable and provided a reliable reference potential for at least 175 days in a 1 M Na2SO4 solution. PVAc/KCl composites without crosslinker were found to leak electrolyte ca. 30% faster than those with 1% EGDMA. EGDMA crosslinking also was shown to improve electrochemical performance and SSRE lifetime in both highly acidic and strongly alkaline solutions, suggesting a broad range of applications. Additionally, a 1% crosslinked SSRE demonstrated comparable performance to a commercial reference electrode in potentiometric and voltammetric measurements. The results demonstrate that crosslinked polymer-electrolyte composites provide a promising new approach for improving reference electrode performance in electrochemical measurements, particularly in continuous long-term online monitoring applications.

Conformational and intermolecular structure of stereoregular isomers of poly(acrylic acid) (PAA) and Na+-poly(acrylate) polyelectrolyte (Na+-PAA) in dilute aqueous solution: a molecular dynamics simulation study

ABSTRACT Polyelectrolytes are useful in a variety of applications such as detergents, consumer care, electrochemical devices, solid-state reference electrode, water treatment, paper production, contact lenses, drug delivery systems, tissue engineering and biosensors. The behaviour of polyelectrolytes in solution is influenced by stereochemistry. The influence of tacticity on conformations of carboxylate anionic polyelectrolytes isotactic poly(acrylic acid) (i-PAA) and syndiotactic poly(acrylic acid) (s-PAA) in water was investigated by explicit solvent atomistic molecular dynamics (MD) simulations using different generic force-fields (CHARMM27, GROMOS 53a6) and water models (SPC-E and TIP4P). Simulations with uniformly distributed charges were performed using 20, 30, and 60 repeat unit chains of i-PAA and s-PAA. CHARMM27 FF with SPC/E water model correctly depict the helical conformational structure of the i-PAA (tg + = 76%) in comparison with experimental data (tg + = 72%). Simulation results show i-PAA to be helical as compared to s-PAA which is extended (trans), in qualitative agreement with experimental data on i-PAA and s-PMA. Counter-ion condensation is stronger for i-PAA which may be partly responsible for its helical conformation. The novelty of the present work is the origin of the conformational behaviour of the anionic polyelectrolyte with such information having potential biological applications such as gene therapy. Abbreviations: i-PAA: isotactic-poly(acrylic acid); s-PAA: syndiotactic-poly(acrylic acid); s-PMA: syndiotactic-poly(methacrylic acid); PEA: Poly(ethacrylic acid); SPC: Simple Point Charge; SPC/E: Extended Simple Point Charge; ST2: Based on the Ben-Naim and Stillinger model; TIPS: A Set of Transferable Intermolecular Potential function; TIP3P: Three-point Transferable Intermolecular Potential; TIP4P, TIP4P-Ew, TIP4P/Ice, TIP4P/2005: Four-point Transferable Intermolecular Potential, for Ewald technique, for Ice, reparametrised original TIP4P model, respectively; TIP5P: Five-point Transferable Intermolecular Potential; R g: Radius-of-gyration; R: End-to-end distance; RDF: Radial Distribution Function; RB: Ryckaert-Bellemans; COM: Centre-of-Mass; CHARMM: Chemistry at HARward Macromolecular Mechanics; GROMOS: GROningen MOlecular Simulation; GROMACS: GROningen MAchine for Chemical Simulations; MD: Molecular Dynamics; PME: Particle Mesh Ewald; LINCS: LINear Constraint Solver; VR: Velocity Rescaling; NPT: Constant Number of particle, Pressure, and Temperature; NVT: Constant Number of particle, Volume, and Temperature; L-J: Lennard-Jones; vdW: van der Waals; FF, FF1, FF2: Force-Field, Force-Field 1, Force-Field 2 respectively; f Charge fraction or charge density or degree-of-ionisation; ACF: Autocorrelation Function.

Novel MgHf4P6O24 as a Solid Electrolyte in Mg-Sensors

In this research paper, the results of the ac-electrochemical impedance spectroscopy (EIS) of MgHf4P6O24 solid electrolyte employing a two-probe technique between 182-764oC, and from 100 mHz to 32 MHz have been reported for the first time. The results indicate that MgHf4P6O24 is an excellent Mg2+-cation conducting solid electrolyte. The ionic conductivity of MgHf4P6O24 was found to be 4.52 x 10-4 Scm-1 at 747oC and the material exhibited no signature of phase change unlike the MgZr4P6O24 solid electrolyte during the sintering and impedance measurements suggesting that both materials and operational stability has been successfully maintained at 1000 ≤ T/oC ≤ 1300. In addition to this, the design, fabrication and testing of solid-state electrochemical Mg-sensors incorporating MgHf4P6O24 as a solid electrolyte has been successfully demonstrated in molten Al-Mg dilute alloys for the first time in this study. The high-temperature Mg-sensors were designed using the novel Mg2+-cation conducting solid electrolyte by incorporating a biphasic powder mixture of MgCr2O4 + Cr2O3 + O2 as a solid-state reference electrode, which has shown promising trend after successfully sensing the Mg dissolved in molten Al at 700±5oC.

Solid-State pH Sensors for Mine Water Monitoring

Introduction The monitoring of water quality parameters such as pH, redox potential, and conductivity is essential in order to investigate the variety of hydrochemical processes occurring in aqueous environments. For a better understanding of these processes, measurements with high temporal resolution are needed, which cannot be achieved with manual sampling procedures. Hence, alternative technical analysis methods, such as sensors, are used to monitor relevant parameters in-situ. Unfortunately conventional probes often experience harsh or detrimental conditions during on-site deployment. All-solid-state potentiometric sensors have been proposed as robust alternatives for demanding environments that deliver results with sufficient accuracy, at relatively low-cost, with experimental and instrumental simplicity [1].This work focuses on the development of a solid-state pH sensor for the monitoring of mine water and acid mine drainage (AMD). AMD is the consequence of uncontrolled sulfidic mineral oxidation, typically as a result of ore or coal mining practices. The resulting outflow of acidic water, which can contain high concentrations of metal ions, is potentially disastrous to aquatic life and also poses a great threat to water supplies [2]. Our aim is to develop a robust and cost-effective sensor that enables in-situ AMD monitoring in mines and surrounding waterways, as well as pit lakes formed as the result of flooded open-pit mines. With increased spatial and temporal resolution in comparison to manual sampling, these sensors will help to predict hazardous conditions and increase the understanding of the complex underlying geochemical mechanisms. Methods The proposed sensor consists of a pH sensitive iridium oxide (IrOx) layer in combination with a solid-state reference electrode. During sensor fabrication structured titanium and gold electrodes were deposited on oxidised silicon substrates using standard cleanroom techniques. IrOx was subsequently electrochemically coated on these electrodes via cyclic voltammetry. The deposition solution was prepared according to the protocol published by Marzouk [3]. In a three electrode configuration (reference: 3 M KCl Ag/AgCl, counter: platinum gauze) the prepared metal electrodes functioned as the working electrodes. Homogenous IrOx layers with a thickness around 80 nm were obtained after 100 cycles at 0.1 V/s between -0.5 V and 0.85 V or -0.7 V and 0.9 V for the gold and titanium electrodes respectively (see Figures 1 and 2).Several approaches for the construction of solid state reference electrodes (SSREs) have been presented in literature [4]. In this work, salt-saturated polymers were deposited on Ag/AgCl layers, similar to [5]. The selected polymers were either unsaturated polyester resins or UV-curing epoxy-based resins. These resins were mixed with up to 70 ma% of finely ground KCl. The resulting slurry was dropped on Ag/AgCl electrodes and cured to form the SSREs.Data collection was performed with EZO-circuits from Atlas Scientific in connection with a RaspberryPi or an ESP32-based microcontroller using I2C or serial protocols.The fabricated IrOx electrodes were characterised regarding their pH sensing performance. Their sensitivity was evaluated by recording the open circuit potential (OCP) vs. a commercial reference electrode (3 M KCl, Ag/AgCl) during a titration of a Briton-Robinson buffer with 0.5 M KOH at 25°C. Drift rates were measured in standard buffer solutions. In order to determine the SSRE’s electrochemical potential stability, OCP was recorded versus a conventional reference electrode in KCl solutions of varying concentration at 25°C. Results and Conclusions As shown in Figure 3, the IrOx electrodes exhibit a super-Nernstian pH sensitivity of around -73 mV/pH between pH 2 to 11.5. These values are comparable to published results [3]. Electrodes from several batches fabricated on different days show high reproducibility with respect to sensitivity and intercept, as indicated by the closely overlaying data points in Figure 3. These results suggest a high robustness of the coating process. The drift rate of the IrOx electrodes is less than 1 mV/h which is sufficient for the intended application. The functionality of the SSRE can be deducted from the data given in Figure 4: An uncoated Ag/AgCl electrode exhibits Nernstian behavior to changes in chloride ion concentration whereas the fabricated SSRE with a polymer coating shows a stable potential. The combination of SSRE and IrOx electrode results in a potentiometric pH sensor whose output changes with pH as shown in Figure 5. Outlook Future work will focus on improving the sensing characteristics as well as measurements in AMD samples. Eventually on-site deployment of the sensors is planned. Additionally, first experiments to transfer the electrodes to flexible and chemically inert substrates for ultimate robustness and potential applications in smart robotic skins have been conducted (see Figure 6). Acknowledgments This research has been funded by the European Social Fund (ESF) and the Free State of Saxony. (ARIDuA , project number 100310491).

CSIRO SENSEI™ in Situ System for Groundwater Monitoring

Currently, standard electroanalytical multi-sensor systems used in-situ to monitor the chemistry of aqueous systems are based on conventional glass ion-selective electrodes. Issues with these electrodes are common and range from their fragile construction, to fouling of the junction and often reference electrode poisoning. Remote operations, such as mining sites, require new technologies to overcome these challenges, encouraging the development of low-maintenance options for monitoring mineral processes and environmental pollution in-situ.CSIRO is developing a robust and cost-effective solid-state sensor and analytics platform, termed SENSEI™, which includes solid-state electrodes and system electronics embedded directly into the measurement unit. The monitoring system includes CSIRO’s patented pH and reference electrodes together with ORP and conductivity sensors. Third party sensors can be included as part of the unit via MODBUS connection. The all-solid construction of the unit enables operation at elevated pressures and in the presence of external forces, and direct contact of the electrodes with the electronics removes the need for analogue cables, reducing electronic interference.A key feature of the SENSEI™ system is the patented solid-state reference electrode. Due to the issues mentioned earlier, conventional reference electrodes were not suitable in an all-solid system and this led to CSIRO developing a new solid salt-polymer composite reference electrode. The SENSEI™ reference electrode includes an Ag/AgCl reference element embedded in a composite material of crosslinked polyvinyl acetate and KCl. After the initial stabilisation time the potential of the reference electrodes stabilises at -45±5 mV vs. SCE. According to laboratory and pilot-scale experiments, the electrode functions maintenance-free during continuous online monitoring for 9-12 months in aqueous systems, including groundwater and acidic mineral leaching systems.The first large-scale application for the system is in environmental groundwater monitoring of an in-situ recovery operation at Heathgate Resources Pty Ltd. In in-situ recovery mining systems the leaching solution is pumped underground, where it dissolves the metals of the ore deposit, and afterwards pumped back to the surface for post-processing, so proper process control is crucial for a safe and environmentally friendly operation to ensure mining fluids are contained within the mining region. Continuous monitoring of the lateral, overlying and underlying aquifers which ring the mining operation enables quick corrective actions to be taken in the event of possible contamination from a leaching solution. The field trial began in 2019 and will continue until the end of 2020.Figure. A schematic drawing of the SENSEI™ sensor unit. The sensor pack includes solid-state (1) ORP electrode, (2) reference electrode, (3) pH electrode and (4) conductivity/temperature sensor. Figure 1

Ultrathin, long-term stable, solid-state reference electrode enabled by enhanced interfacial adhesion and conformal coating of AgCl

Continuous biochemical monitoring with a flexible electrochemical sensor offers a new wearable electronic system that can measure real-time voltage and current signals. The signal quality is determined by long-term stability of a silver/silver chloride reference electrode (Ag/AgCl RE). However, it is very challenging for any solid-state electrode to have a long-term stable operation. Even though new membrane technologies have improved the voltage stability, the existing thin film Ag/AgCl REs have limitations of insufficient film adhesion and structural instability. Here, this paper introduces an ultrathin, all-solid-state RE that demonstrates a long-term functional stability for more than two weeks via enhanced interfacial adhesion and conformal coating of AgCl. An optimization of chlorination factors allows a highly uniform, 800 nm-thick RE surface. The voltage response of the sensor in a saline solution shows a change of 0.09 mV/h for 18 days with a quasi-stable behavior, capturing the potential as an amperometric biosensor. Moreover, the characterization with an enzymatic working electrode verifies that two-electrode system using the thin-film RE has a sensitivity (S = 606 nA·mmol−1. cm−2), compatible to that with a commercial RE (S = 532 nA·mmol−1. cm−2). Collectively, this work provides a comprehensive study of materials and surface functionalization of all-solid-state REs for thin-film biosensors, which will pave the way for long-term usable wearable biosystems.

Chloride and pH Determination on a Wireless, Flexible Electrochemical Sensor Platform

A stamp-sized, flexible, wireless, generic electrochemical sensor platform was demonstrated for use as a potentiometric pH and amperometric Cl− sensor. The platform combines flexible printed circuit technology with screen-printed electrodes, which enables cheap and easy quantification of biorelevant analytes in many different environments. pH sensors with graphene oxide as the recognition element was constructed on a ferrocyanide modified carbon electrode, to maintain a well-defined redox potential even at high alkalinity. A solid-state reference electrode based on a NaCl-doped polyvinyl butyral membrane ensured applicability towards sample matrices of unknown composition by remining unsensitive (±7.1 mV) to external Cl− in the range pCl 0–3. The platform was able to linearly (−26.3 mV pH−1, r2 = 0.99) quantify pH in the range of 2–9 with a maximum inter-sensor standard deviation of 0.43 pH units. Secondly, a Cl− sensor combining three unmodified Ag/AgCl electrodes with squarewave amperometry was designed, simulated and tested. A sensitivity of (−147.7 $\mu \text{A}$ pCl−1, r2 = 0.97) was obtained, with a maximum inter-sensor standard deviation of 0.27 pCl units in the range pCl 0–3.

A Solid-State Reference Electrode Based on a Self-Referencing Pulstrode.

The design of solid-state reference electrodes without a liquid junction is important to allow miniature and cost-effective electrochemical sensors. To address this, a pulse control is proposed using an Ag/AgI element as reliable solid-state reference electrode. It involves the local release of iodide by a cathodic current that is immediately followed by an electromotive force (EMF) measurement that serves as the reference potential. The recapture of iodide ions is achieved by potentiostatic control. This results in intermittent potential values that are reproducible to less than one millivolt (SD=0.27 mV, n=50). The ionic strength is shown to influence the activity coefficient of released iodide in accordance with the extended Debye-Hückel equation, resulting in a predictable change of the potential reading. The principle is applied to potentiometric potassium detection with a valinomycin-based ion-selective electrode (ISE), demonstrating a completely solid-state sensor configuration.

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.