Micro-reference Electrode Research Articles

Functionalised ZnO-nanorod-based selective electrochemical sensor for intracellular glucose

In this article, we report a functionalised ZnO-nanorod-based selective electrochemical sensor for intracellular glucose. To adjust the sensor for intracellular glucose measurements, we grew hexagonal ZnO nanorods on the tip of a silver-covered borosilicate glass capillary (0.7 μm diameter) and coated them with the enzyme glucose oxidase. The enzyme-coated ZnO nanorods exhibited a glucose-dependent electrochemical potential difference versus an Ag/AgCl reference microelectrode. The potential difference was linear over the concentration range of interest (0.5–1000 μM). The measured glucose concentration in human adipocytes or frog oocytes using our ZnO-nanorod sensor was consistent with values of glucose concentration reported in the literature; furthermore, the sensor was able to show that insulin increased the intracellular glucose concentration. This nanoelectrode device demonstrates a simple technique to measure intracellular glucose concentration.

Performance of high-power lithium-ion cells under pulse discharge and charge conditions

Lithium-ion cells being designed for transportation applications must sustain high current pulses under rapid discharge and rapid charge conditions without significant degradation of cell performance. In this article we examine the pulse discharge and charge performance of cells, containing a LiNi 0.8 Co 0.15 Al 0.05 O 2 -based cathode, a graphite-based anode, and a LiPF 6 -bearing EC:EMC electrolyte, at current rates ranging from 3 to 25 C. Impedance data indicate that 18650-type cells containing this chemistry can withstand 18s discharge pulses at rates up to 17 C in the 3.7-4.0V voltage window. Data from cells containing a LiSn micro-reference electrode show that the positive electrode impedance increases, whereas the negative electrode impedance decreases, with increasing magnitude of the discharge current pulse. The discharge pulse-current that can be sustained by the cell is limited by lithium diffusion into oxide particles of the positive electrode.

Localized electrochemical deposition of micrometer copper columns by pulse plating

Micrometer copper column fabrication by localized electrochemical deposition (LECD) was investigated in this study. To obtain columns with uniform diameter, compact structure and a smooth surface, LECD conducted in pulse current (PC) mode was better than that conducted in direct current (DC) mode. A micro-reference electrode was used to monitor the potential at the LECD site. Measurement of this potential permitted estimation of the local copper ion concentration resulting from their dynamic consumption by electrochemical reduction and their supply by mass transport. The electroplating current was measured in order to evaluate the rate of electrochemical reduction. The mass-transfer rate of copper ions was estimated using a theoretical calculation based on diffusion. The surface morphology and internal structure were significantly affected by this local concentration, which was in turn governed by the electrical voltage and the duty cycle employed. The mechanism for LECD conducted in PC mode is discussed.

Improved solid-state planar <inline-formula><math display="inline" overflow="scroll"><mrow><mi mathvariant="normal">Ti</mi><mo>∕</mo><mi mathvariant="normal">Pd</mi><mo>∕</mo><mi mathvariant="normal">Ag</mi><mo>∕</mo><mi mathvariant="normal">Ag</mi><mi mathvariant="normal">Cl</mi><mo>∕</mo><mi mathvariant="normal">K</mi><mi mathvariant="normal">Cl</mi></mrow></math></inline-formula>-gel microreference electrode by silicon cap sealing package

Although solid-state microreference electrodes (µREs) have been developed for electrochemical and biomedical sensing applications for more than one decade, there are very few studies devoted to improving their performance through packaging. Using planar technology and a silicon cap sealing method, we implement a packaged Ti/Pd/Ag/AgCl/KCl-gel µRE with a total dimension of only 9 mm (L)×6 mm (W)×1 mm (H), a hundredfold less than the commercial Ag/AgCl reference electrode (RE). Compared with the unpackaged µRE, the presented chip-level packaged µRE demonstrates many improved characteristics, including a very stable cell potential (5-mV drift voltage in 30,000 s), an approximately zero offset voltage (−7 mV), a very low impedance (1.50 k) and phase shift (8.98 deg) at 1-kHz operation frequency, a very small double-layer capacitance (0.04 µF), and a very high reproducibility (±3.1-mV).

Determination of ζ-Potential and Tortuosity in Rat Organotypic Hippocampal Cultures from Electroosmotic Velocity Measurements under Feedback Control

Extracellular translational motion in the brain is generally considered to be governed by diffusion and tortuosity. However, the brain as a whole has a significant zeta-potential, thus translational motion is also governed by electrokinetic effects under a naturally occurring or applied electric field. We have previously measured zeta-potential and tortuosity in intact brain tissue; however, the method was tedious. In this work, we use a four-electrode potentiostat to control the potential difference between two microreference electrodes in the tissue, creating a constant electric field. Additionally, some alterations have been made to simplify our previous procedure. The method entails simultaneously injecting two 70 kDa dextran conjugated fluorophores into rat organotypic hippocampal cultures and observing their mobility using fluorescence microscopy. We further present two methods of data analysis: regression and two-probe analysis. Statistical comparisons are made between the previous and current methods as well as between the two data analysis methods. In comparison to the previous method, the current, simpler method with data analysis by regression gives statistically indistinguishable mean values of zeta-potential and tortuosity, with a similar variability for zeta-potential, -21.3 +/- 2.8 mV, and a larger variability for the tortuosity, 1.98 +/- 0.12. On the other hand, we find that the current method combined with the two-probe analysis produces accurate and more precise results, with a zeta-potential of -22.8 +/- 0.8 mV and a tortuosity of 2.24 +/- 0.10.

Corrosion behavior of reinforcing steel in simulated concrete pore solutions: A scanning micro-reference electrode study

The scanning micro-reference electrode (SMRE) technique was used to study the corrosion behavior of reinforcing steel in simulated concrete pore (SCP) solutions with different pH values. The early stage as well as the propagation of the localized corrosion of reinforcing steel in different solutions was explored. The results indicated that the potential distribution on the reinforcing steel surface changed in homeostasis and the steel remained passive in the pure simulated concrete pore solution. The solution pH had a significant effect on the localized corrosion of reinforcing steel, and the critical pH value for localized corrosion of reinforcing steel in SCP solutions was between 11.46 and 11.31.

Development of a miniaturised electrochemical cell integrated on epoxy-glass laminate

The development of the miniaturised electrochemical cell integrated on epoxy-glass laminate support is reported. Potassium-selective microelectrodes based on valinomycin and two polymeric matrices [plasticized poly(vinyl chloride) and polyurethane] have been fabricated on planar Au or Ag/AgCl transducers. Polymeric layers containing ionic liquid were cast on the surface of Ag/AgCl microelectrodes to form the reference half-cells. Performances of the ion-sensitive and reference microelectrodes (i.e., ion-selectivity, slopes of calibration curves, signal stability and repeatability, long-term stability) were studied. The influence of the method of membrane deposition on sensor characteristics was not significant, however the microelectrodes with polyurethane membranes exhibited better durability. Theoretical calibration curves were obtained when K+-selective electrodes and reference electrodes were integrated on a single substrate. The designed electrochemical cell may be helpful in further studies devoted to the development of miniaturised electrode arrays for multi-component analysis or electronic tongues purposes.

Detection of DNA and proteins using amorphous silicon ion-sensitive thin-film field effect transistors

Amorphous silicon-based ion-sensitive field-effect transistors (a-Si:H ISFETs) are used for the label-free detection of biological molecules. The covalent immobilization of DNA, followed by DNA hybridization, and of the surface adsorption of oligonucleotides and proteins were detected electronically by the a-Si:H ISFET. The ISFET measurements are performed with an external Ag/AgCl microreference electrode immersed in 100 mM phosphate buffer electrolyte with pH 7.0. Threshold voltage shifts in the transfer curve of the ISFETs are observed resulting from successive steps of surface chemical functionalization, covalent DNA attachment to the functionalized surface, surface blocking, and hybridization with a complementary target. The surface sensitivity achieved for DNA oligonucleotides is of the order of 1 pmol/cm 2. Point-of-zero charge estimations were made for the functionalized surfaces and for the device surface after DNA immobilization and hybridization. The results show a correlation between the changes in the point-of-zero charge and the shift observed in the threshold voltage of the devices. Electronic detection of adsorbed proteins and DNA is also achieved by monitoring the shifts of the threshold voltage of the ISFETs, with a sensitivity of approximately 50 nM.

Spatially-Resolved Interfacial Electrochemistry: Ohmic Microscopy

A method is herein described that allows spatially resolved capacitive currents at electrode|electrolyte interfaces to be recorded in situ by monitoring the ohmic drop in the electrolyte, Δφsol, using two microreference electrodes. Measurements were performed in 0.1 M H2SO4 aqueous solutions using Au and Pt either as single or dual working electrodes, that is, connected to one another, in an otherwise conventional three-electrode electrochemical cell. Plots of Δφsol versus E, the potential of the dual Au|Pt electrode with respect to the main reference electrode, recorded during voltammetric cycles, yielded curves bearing features characteristic of the Au|0.1 M H2SO4 or Pt|0.1 M H2SO4 interface depending on whether the microreference electrodes were placed close to the Au or Pt electrode surface, respectively. Extensions of this methodology to measurements in the micrometer domain are discussed briefly.

Miniaturized electronic tongue with an integrated reference microelectrode for the recognition of milk samples

This work presents the application of a new construction of an electronic tongue for the classification of milk originating from various producers. Integrated array of microelectrodes was fabricated from epoxy-glass laminate. PVC membranes with various additives were used as chemosensitive layers. The developed device is capable of recognition of milk samples with high correctness. Moreover, the application of miniaturized reference electrode, integrated on the same substrate, also provided satisfactory results, which can be helpful in future constructions of hand-held electronic tongue systems.

Existence of an electrically insulating layer in amalgam-containing galvanic couples

Objective Avoiding the placement of amalgam and noble metal restorations in interproximal contact is recommended due to anticipated galvanic corrosion of the amalgam. There is a similar concern for amalgam/amalgam galvanic couples. It was the objective of this study to determine if an electrically insulating layer forms in the contact area of these galvanic couples. The existence of an electrically insulating layer, which could reduce the galvanic corrosion rate, would be indicated by different corrosion potentials for the two restorations of the couple. Methods Using a convenience sample of 158 human subjects, corrosion potentials were measured on each restoration of three types of galvanic couples: amalgam/noble metal ( n = 8), amalgam/amalgam ( n = 93) and noble metal/noble metal ( n = 7). Measurements were made with a Ag/AgCl micro-reference electrode and a high impedance voltmeter. All restorations were at least 6 months old. Results Statistical analysis showed that the mean absolute corrosion potential differences and the simultaneous confidence intervals of the couples were, respectively, amalgam/noble metal: 62 (31) mV and (27, 99) mV, amalgam/amalgam: 11 (14) mV and (7, 15) mV and noble metal/noble metal: 7 (10) mV and (0, 19) mV. Significance The amalgam/noble metal couples had consistent and mostly large corrosion potential differences between their restorations, which indicated the presence of an electrically insulating layer. An electrically insulating layer was also indicated for the amalgam/amalgam and noble metal/noble metal couples. The layer is probably composed of non-metallic corrosion products, biofilms, and possibly, dental calculus, which could reduce galvanic corrosion rates to small or negligible values.

Studies on the degradation of Li-ion batteries by the use of microreference electrodes

Li-ion batteries made by the Lithylene technology were investigated after extensive cycling for a mechanistic understanding of the capacity fade phenomena. The batteries cycled 500 times at 0.5 C were found to lose 13% of their original capacity, which was solely due to the loss of active materials. The negative electrode maintained its capacity to contain Li + ions from the positive electrode. The loss of positive electrode materials was attributed to formation and thickening of the surface layer and structure disorder evidenced by XRD measurements. In situ impedance measurements revealed that the positive electrode was also responsible for the impedance rise upon cycling. The charge transfer resistance was found to be the most influential factor in the battery impedance, which increased exponentially during cycling. This increase was proved not due to the decrease of positive electrode surface area but resulted from growth of the surface layer.

Galvanic Coupling Between Pure Copper and Pure Aluminum: Experimental Approach and Mathematical Model

The corrosion behavior of a pure aluminum/pure copper couple in a weakly conductive sulfate solution was investigated. Potential and current distributions on the surface of the model couple at the beginning of immersion were obtained by solving the Laplace equation using a finite element method (FEM) algorithm. The potential distribution predicted by the calculations was checked using a microreference electrode. A good agreement was found between experimental and theoretical results. It was shown that the reaction occurring at the copper electrode was oxygen reduction, while aluminum remote from the interface remained in the passive state. Moreover, calculations predicted a large cathodic current, related to an increase in oxygen reduction, restricted to copper at the interface. This led to a local pH increase reaching values higher than 9, allowing the dissolution of aluminum to occur close to the interface. Combining these data with optical and scanning electron microscope observations after of immersion in the sodium sulfate solution allowed a three-step mechanism to be proposed to explain the corrosion damage, and particularly the presence of a copper deposit on the aluminum surface, some distance from the interface, a phenomenon currently observed in commercial copper-rich aluminum alloys.

Diffusion of Chlorine Dioxide Through Aqueous and Oil Films

Since chlorine dioxide generation methods have become simpler and safer, this antimicrobial agent is used in an increasing number of applications in the food industry. Contaminated food processing equipment is now commonly sanitized by immersion or spray application of chlorine dioxide solutions. The contact time needed to kill or deactivate bacteria is in part related to the solution concentration. Accordingly, a working ClO 2 microelectrode was coupled with a reference microelectrode to investigate the depletion of ClO 2 concentration in thin aqueous films. The depletion of chlorine dioxide concentration in thick and thin films was recorded. Thin films were examined both with and without the addition of a surfactant. Profiles showed that the ClO 2 residual in thick films lasted up to 6 h. In thin films, the addition of a surfactant decreased the film thickness by almost half, and the ClO 2 profile depletion time was reduced from about an hour to about 25 min. A correlation relating the film depth to the recorded decay coefficient is reported. The behaviour of chlorine dioxide was investigated to determine whether chlorine dioxide could partition with and break through fatty acid layers that would be encountered in soiled food processing equipment.Experiments with caprylic acid showed the ability of chlorine to break through lipid layers and thus disinfect the biofilm that might exist below it.

A Novel Method for the In Situ Determination of Concentration Gradients in the Electrolyte of Li‐Ion Batteries

An electrochemical method has been developed for the in situ determination of concentration gradients in the electrolyte of sealed Li-ion batteries by measuring the potential difference between microreference electrodes. Formulas relating the concentration gradient and the potential difference between the microreference electrodes were derived from the Nernst-Planck equation. The concentration gradients in Li-ion batteries are theoretically and experimentally proven to be linear at steady state under galvanostatic charging and discharging conditions. Based on this finding, both the diffusion coefficient of the lithium ions in the electrolyte and the diffusion overpotential related to the concentration gradient have been calculated. It was found that the concentration gradient is proportional to the applied current over a wide current range and that the obtained diffusion coefficient of lithium ions is almost constant. For the batteries studied in this work, the diffusion overpotential is already noticeable at 0.30 A and the limiting current is estimated to be 1.1 A, corresponding to a C-rate of 3.7.

Corrosion potential recovery of dental amalgam restorations following prophylaxis

Objective Dental amalgam restorations are subjected to abrasion during selective prophylaxis that can damage or remove the protective oxide and result in increased rates of corrosion and chemical dissolution of mercury. It was the objective of this research to study the corrosion potential change of dental amalgam restorations to obtain an indication of the time required for in vivo repassivation following prophylaxis. Methods The corrosion potentials of 27 Class I and Class II amalgam restorations were measured pre- and post-prophylaxis using a high impedance voltmeter and a Ag/AgCl micro-reference electrode. Prophylaxis was performed for approximately 2 s on each amalgam surface using a slow-speed handpiece with a rubber-cup and commercial abrasive paste. Subjects thoroughly rinsed before the post-prophylaxis corrosion potentials were measured. The data were analyzed using a confidence interval, a t-test and correlation analysis. Results The pre- and post-prophylaxis mean corrosion potentials were, respectively, −132 (27) mV and −126 (27) mV. The mean of the differences between the pre- and post-prophylaxis corrosion potentials was 6.1 (28) mV, with an associated 95% confidence interval of (−4.8, 17) mV. A t-test showed the mean absolute difference in corrosion potential was less than 50 mV ( p < 0.0001). Significance The results of this study show that the post-prophylaxis recovery of the corrosion potential of amalgam restorations occurred by at most 10–44 min, indicating that the period of elevated corrosion rate and elevated chemical dissolution rate of mercury, due to oxide damage or removal, may be short-lived.

Solid‐State Microelectrodes for Flow‐Cell Analysis Based on Planar Back‐Side Contact Transducers

AbstractThis paper presents an overview of the studies carried out in the recent years in our laboratory, on the construction of different solid‐state microelectrodes for flow‐cell analysis. The microelectrodes were based on planar transducers made from a printed circuit board and manufactured as back‐side contact structures, which facilitate their mounting in a flow‐cell. Performances of the polymeric membrane ion‐selective: coated‐wire microelectrodes and microsensors with a polyHEMA layer containing internal electrolyte are compared. Conducting polymer – polypyrrole doped with ester of sulfosuccinic acid – was added to the PVC membranes to develop all‐solid‐state microelectrodes without internal electrolyte solution, exhibiting good potential stability in time. Finally, all‐solid‐state reference microelectrodes based on back‐side contact transducers are prepared and examined. These structures were applied as reference electrodes in flow‐cell measurements with potassium‐selective planar microelectrodes.

All-solid-state miniaturised planar reference electrodes based on ionic liquids

The paper presents the design of all-solid-state miniaturised reference electrodes suitable for flow-through analysis. The sensors are based on Ag/AgCl planar microelectrodes covered with PVC membranes containing an ionic liquid (IL, 1-dodecyl-3-methylimidazolium chloride). The membrane with IL additive provides an internal solid electrolyte, maintaining constant chloride concentration in the layer and thus constant potential of the Ag/AgCl electrode in solution of different concentration of chloride and other interfering anions. The performance of the reference microelectrodes with different ionic liquid content in the membrane was determined. The electrodes exhibited good potential stability, reproducibility and long-term stability. The structures were applied as reference electrodes in flow-cell measurements with potassium-selective planar microelectrodes. Comparable calibration curves of K+-selective electrodes determined using the designed reference microelectrodes based on ionic liquid and a commercial Ag/AgCl reference electrode were obtained.

Corrosion potential variation of aged dental amalgam restorations over time

The corrosion potential of a dental amalgam restoration is generally determined using a single measurement, even though environmental factors and abrasion can continuously alter the surface state and reactivity of this alloy. It was, therefore, the purpose of this study to determine the maximum variability of the corrosion potential of aged dental amalgam restorations, for 28 days. The corrosion potentials of 148 aged dental amalgam restorations in 12 human subjects were measured at t = 0 and 4 h, and 4, 7, 14, 21 and 28 days. Measurements were made with a high impedance voltmeter and a Ag/AgCl micro-reference electrode. The subjects were instructed not to alter their usual eating and oral hygiene routines. Corrosion potential changes occurred throughout the 28 days. They were both positive and negative for the same restoration, and were sometimes very large. Only 4 h after the initial measurement, the absolute value of the corrosion potential changes ranged between 18 and 287 mV for 50% of the restorations. The largest maximum absolute corrosion potential change for each subject's restorations ranged between 85 and 329 mV. Statistical analysis showed that the overall mean maximum absolute corrosion potential change for the subjects' restorations was 74 mV. It was shown that the corrosion potential of aged dental amalgam restorations varies substantially over time, and that a single measurement is not representative of short- or long-term electrochemical behavior. This finding has implications regarding the corrosion rate of dental amalgam restorations, particularly those that are part of a galvanic couple.

Ag/AgCl microelectrodes with improved stability for microfluidics

A method for fabricating Ag/AgCl planar microelectrodes for microfluidic applications is presented. Micro-reference electrodes enable accurate potentiometric measurements with miniaturized chemical sensors, but such electrodes often exhibit very limited lifetimes. Our goal is to construct Ag/AgCl microelectrodes reliably with improved potential stability that are compatible with surface mounted microfluidic channels. Electrodes with geometric surface areas greater than or equal to 100 μm 2 were fabricated individually and in an array format by electroplating silver, greater than 1 μm thickness, onto photolithographically patterned thin-film metal electrodes. The surface of the electroplated silver was chemically oxidized to silver chloride to form Ag/AgCl micro-reference electrodes. Characterization results showed that Ag/AgCl microelectrodes produced by this fabrication method exhibit increased stability compared with many devices previously reported. Electrochemical impedance spectroscopy allowed device specific parameters to be extracted from an equivalent circuit model, and these parameters were used to describe the performance of the microelectrodes in a microfluidic channel. Thus, stable Ag/AgCl microelectrodes, fabricated with a combination of photolithographic techniques and electroplating, were demonstrated to have utility for electrochemical analysis within microfluidic systems.