Solid-Contact Ion-Selective Electrodes with Copper Hexacyanoferrate in the Transducer Layer

The possibility of using mixed Fe²⁺/Fe³⁺ copper hexacyanoferrate (CuHCF) as the material for the transducer layer of solid-contact ion-selective electrodes (SC-ISEs) with plasticized polyvinylchloride membranes is studied. The study is performed for K⁺-SC-ISEs and water-hardness SC-ISEs. It is shown that CuHCF combines the ion-exchange and redox properties and, hence, in principle, should be suitable for SC-ISEs. However, the reproducibility of SC-ISE potentials from one electrode to another and their stability in time are far below those of conventional ISEs with internal aqueous solution. The potentials of individual SC-ISEs can be brought closer to one another by their polarization using a potentiostat or by their short-circuiting to a saturated silver-chloride reference electrode.     

A portable wireless-sensor system from all-in-one sensor array based hybrid solid contact layer for point-of-care ion monitoring in river basin

The sensor arrays are considered as a promising way to aim at point-of-care (POC) testing for the detection of different ions in environment through the use of solid-contact ion-selective electrodes (SC-ISEs). However, the exorbitant equipment and intricate process for the production of SC-ISEs arrays has limit its application. Herein, the Au with nano-branched structure (Au NBS) and poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT : PSS) were decorated on the surface of bare pad of traditional printed circuit boards coated with Au (Au-PCB) to construct a hybrid solid-contact layer including double-layer capacitance and pseudocapacitance of SC-ISEs synergistically, thus the multichannel SC-ISEs exhibited high stability and specificity. Overall, based on portable multichannel sensor and homemade APP (Ion Analysis), a wireless all-in-one sensor system was constructed to detect ions with the limit of NO₃⁻, Cl⁻, Na⁺, K⁺, Ca²⁺ and Mg²⁺ as 0.047 mM, 0.039 mM, 0.022 mM, 0.066 mM, 0.045 mM and 0.024 mM, respectively. Besides, it can detect such ions in river basin successfully. With the cost of each electrode less than 0.5 $, this sensor arrays enable a wide range of hazardous ion monitoring in water environment.     

A ladder conjugated polymer transducer for solid-contact Cu2+-selective electrodes

In recent years, there has been a pronounced interest in solid-contact ion-selective electrodes （SC-ISEs）, with emphasis on the use of conducting polymers as ion-to-electron transducer. In this work, a ladder conjugated polymer, thieno[3,2-b]thiophene （LCPT）, was investigated in fabricating Cu2＋-selective electrodes for the first time. The resulting electrodes were characterized by electrochemical impedance spectroscopy （EIS）, chronopotentiometry, and the water layer test. Results proved that the active LCPT facilitates the ion-to-electron transduction, and avoids the detrimental aqueous layer formed at the interface of SC-ISEs.     

Solid Contact Ion-selective Electrodes on Printed Circuit Board with Membrane Displacement

Multiplexed solid-contact ion-selective electrodes (SCISEs) are fabricated using printed circuit board (PCB) and mesoporous carbon black (MCB) as ion-to-electron transducer (solid contact). Four sensor configurations were examined and showed that in addition to MCB, the sensor configuration plays crucial role in the stability of the potential response. The enhanced sensor stability was also linked with suppression of transmembrane flux of water. The sensors exhibited near-Nernstian sensitivity (58.1 mV/dec for K⁺ ISEs and −55.1 mV/dec for NO₃^- ISEs), low detection limits (1.5–2.2 μM), and good short-term stability (∼0.1 mV/min). Sensors can be stored dry and used without preconditioning. This work demonstrates a promising approach to combining PCB technology and carbon black for large-scale production of low cost ISEs for point-of-care testing, wearables, or in situ field measurements.     

Stabilizing Potential of Solid-Contact Sensors Selective towards Surface-Active Substances

Electrochemical and operation parameters of solid-contact sensors with silver and graphite current leads selective towards surface-active substances (SAS) are studied. Graphite has advantage as an electronic conductor. Parameters of solid-contact SAS electrodes with graphite (response time, potential drift, lifetime) are more stable, and the electrodes have a lower SAS detection limit.     

Direct Electrochemical Oxidation of DNA on Polycrystalline Gold Electrodes

Electrochemical CV and SWV studies were performed with double stranded DNA from salmon testes (dsDNA) and single stranded DNAs, containing 25 nucleotides (ssDNA) directly adsorbed at polycrystalline Au electrodes. A distinct oxidation peak at +730 mV (SWV, scan rate 0.248 V s^?1) or at +730 – +780 mV (CV, scan rate from 0.3 to 1 V s^?1) was obtained with DNA‐modified Au electrodes after a time‐dependent prepolarization step at a positive potential value, i.e., at +500 mV (vs. Ag|AgCl), performed with the DNA‐modified Au electrodes dipped in a blank buffer solution. No electrochemical activity was detected when ssDNA, containing no guanines, was used for adsorptive modification of the Au electrodes. Electrochemical impedance measurements registered a possible reorganization of the adsorbed DNA layer in the course of the prepolarization, accompanied by decreasing in‐phase impedance. The results enable us to relate the oxidation process observed at the DNA‐modified Au electrodes with the oxidation of guanine residues in DNA.     

Sensing the Lactic Acid in Probiotic Yogurts Using an L-Lactate Biosensor Coupled with a Microdialysis Fiber Inserted in a Flow Analysis System

An amperometric biosensor for the determination of L-lactic acid in probiotic yogurts has been assembled using L-lactate dehydrogenase (EC 1.1.1.27, LDH) entrapped in 1% (v/v) neutralized Nafion® solution deposited on Variamine blue redox mediator modified screen-printed electrodes. The Variamine blue was previously covalently linked to oxidized single-walled carbon nanotubes and used for modifying screen-printed electrodes. The electrochemical cell, containing the L-lactate biosensor operating at an applied working potential of +200 mV vs. Ag|AgCl, was coupled with a microdialysis fiber and connected with a flow system, thus obtaining a microdialysis based sampling experimental set-up. Various analytical parameters, such as the cofactor concentration (2 mM, NAD⁺), the flow rate (10.5 μL/min), the applied working potential (+200 mV vs. Ag|AgCl), the working buffer (50 mM phosphate buffer +0.1 M KCl), and pH (7.5), were optimized in batch amperometric experiments. The dynamic linear working range was comprised between 2·10^?4 and 1·10^?3 M. The proposed biosensor was challenged with real samples of yogurt, properly diluted in working buffer, and the performances of the L-lactate biosensor were compared with a commercially available kit for the determination of L-lactic acid in foodstuffs from R-Biopharm GmbH, Germany, showing a good agreement.     

Preparation and Characterization of a Lignin Modified Electrode

Alkali lignin undergoes strong adsorption on polycrystalline gold electrodes. Subsequent oxidation in a sulfuric acid solution leads to a restructured redox‐active polymer that shows features characteristic for surface confined species. Surface coverage of up to 4.40×10^?10 mol cm^?2 may be obtained depending on the adsorption time or lignin concentration in the adsorption solution. Using Laviron's approach the electron‐transfer rate constant and the transfer coefficient were found to be 8.9 s^?1 and 0.35, respectively. The formal potential of the redox couple shifted negatively with pH at a rate of ca. 60 mV/pH unit, suggesting a 2 e/2 H⁺ reaction. The redox couple was also found to be a good mediator for electrochemical ascorbic acid oxidation in neutral phosphate buffer with ca. 250 mV reduction of the oxidation overpotential.     

Dispersion of multi-wall carbon nanotubes in polyhistidine: Characterization and analytical applications

We report for the first time the use of polyhistidine (Polyhis) to efficiently disperse multiwall carbon nanotubes (MWCNTs). The optimum dispersion MWCNT–Polyhis was obtained by sonicating for 30 min 1.0 mg mL⁻¹ MWCNTs in 0.25 mg mL⁻¹ Polyhis solution prepared in 75:25 (v/v) ethanol/0.200 M acetate buffer solution pH 5.00. The dispersion was characterized by scanning electron microscopy, and by cyclic voltammetry and amperometry using ascorbic acid as redox marker. The modification of glassy carbon electrodes with MWCNT–Polyhis produces a drastic decrease in the overvoltage for the oxidation of ascorbic acid (580 mV) at variance with the response observed at glassy carbon electrodes modified just with Polyhis, where the charge transfer is more difficult due to the blocking effect of the polymer. The reproducibility for the sensitivities obtained after 10 successive calibration plots using the same surface was 6.3%. The MWCNT-modified glassy carbon electrode demonstrated to be highly stable since after 45 days storage at room temperature the response was 94.0% of the original. The glassy carbon electrode modified with MWCNT–Polyhis dispersion was successfully used to quantify dopamine or uric acid at nanomolar levels, even in the presence of large excess of ascorbic acid. Determinations of uric acid in human blood serum samples demonstrated a very good correlation with the value reported by Wienner laboratory.     

Miniature Ion-selective Electrodes with Mesoporous Carbon Black as Solid Contact

Herein, we demonstrated miniature solid-contact ion-selective electrodes (ISEs) using a commercial mesoporous carbon black (MCB) as ion-to-electron transducer. MCB is attractive in its high surface area, good conductivity, relative low cost and availability. ISEs for potassium (K⁺) and nitrate (NO₃⁻) ions were prepared by subsequently coating the sealed glass capillaries (1.5 mm) with MCB and ion-selective membranes. Addition of MCB substantially stabilized electrode response by providing adequate double-layer capacitance and lowering resistance by more than 100× compared to the coated-wire electrodes. The electrodes exhibited near-Nernstian slopes of 59.6 (K⁺ ionophore), 57.8 (K⁺ ion-exchanger) and −54.8 (NO₃⁻ ion-exchanger) with standard solutions in the range of 10⁻⁵ to 10⁻¹ M. Fast response (∼10 s) and reproducible sensitivities were also obtained in a mixed electrolyte containing interfering ions, although with a baseline drift of 2–10 mV/day in the long term. Importantly, the electrodes can be simply stored in air between measurements and used directly without conditioning in solutions. With simple fabrication and free maintenance, these sensors offer a low cost and convenient alternative to bulk ISEs, especially when sample volumes or space are limited.     

Flow Injection Analysis of Maleic Hydrazide Using an Electrochemical Sensor Based on Palladium‐Dispersed Carbon Paste Electrode

A new analytical methodology for the electrochemical detection of the herbicide maleic hydrazide (3,6‐dihydroxypyridazine) by flow injection analysis is presented. This method is supported by the novel application of a palladium‐dispersed carbon paste electrode as an amperometric sensor for this herbicide. Maleic hydrazide shows anodic electrochemical activity on carbon‐based electrodes (glassy carbon or carbon paste electrodes) in all the pH range. This electrochemical activity is enhanced using metal‐dispersed carbon paste electrodes, especially at Pd‐dispersed CPE which displays good oxidation signals at 690 mV (0.050 M phosphate buffer pH 7.0), 140 mV lower than at unmodified electrodes. Under the optimized conditions, the electroanalytical performance of Pd‐dispersed CPE in flow injection analysis was excellent, with good reproducibility (RSD 3.3%) and a wide linear range (1.9×10^?7 to 1.0×10^?4 mol L^?1). A detection limit of 1.4×10^?8 mol L^?1 (0.14 ng maleic hydrazide) was obtained for a sample loop of 100 μL at a fixed potential of 700 mV in 0.050 M phosphate buffer solution at pH 7.0 and a flow rate of 2.0 mL min^?1. The proposed method was applied for the maleic hydrazide detection in natural drinking water samples.     

Solid-contact Cu(II) ion-selective electrode based on 1,2-di-(<Emphasis Type="Italic">o</Emphasis>-salicylaldiminophenylthio)ethane

The development of Cu(II) solid-contact ion-selective electrodes, based on 1,2-di-(o-salicylaldiminophenylthio)ethane as a neutral carrier, is presented. For the electrodes construction, unmodified carbon ink (type 1 electrode) and polymer membrane-modified carbon ink (type 2 electrode) were used as solid support and transducer layer. Also, carbon ink composite polymer membrane electrode (type 3 electrode) was prepared. The analytical performance of the electrodes was evaluated with potentiometry, while bulk and interfacial electrode features were provided with electrochemical impedance spectroscopy. It is shown that modification of carbon ink with polymer membrane cocktail decreases the bulk contact resistance of the transducer layer and polymer membrane, thus enhancing the analytical performance of the electrode in terms of sensitivity, linear range, and stability of potential. The optimized electrodes of types 2 and 3 exhibit a wide linear range with detection limits of 1.8 × 10⁻⁶ and 1.6 × 10⁻⁶ M, respectively. They are suitable for determination of Cu²⁺ in analytical measurements by direct potentiometry and in potentiometric titrations, within pH between 2.3 and 6.5. The electrodes are selective for Cu²⁺ over a large number of tested transition and heavy metal ions.     

Mediated electron transfer of cellobiose dehydrogenase and glucose oxidase at osmium polymer-modified nanoporous gold electrodes

Nanoporous and planar gold electrodes were utilised as supports for the redox enzymes Aspergillus niger glucose oxidase (GOx) and Corynascus thermophilus cellobiose dehydrogenase (CtCDH). Electrodes modified with hydrogels containing enzyme, Os-redox polymers and the cross-linking agent poly(ethylene glycol)diglycidyl ether were used as biosensors for the determination of glucose and lactose. Limits of detection of 6.0 (±0.4), 16.0 (±0.1) and 2.0 (±0.1) μM were obtained for CtCDH-modified lactose and glucose biosensors and GOx-modified glucose biosensors, respectively, at nanoporous gold electrodes. Biofuel cells composed of GOx- and CtCDH-modified gold electrodes were utilised as anodes, together with Myrothecium verrucaria bilirubin oxidase (MvBOD) or Melanocarpus albomyces laccase as cathodes, in biofuel cells. A maximum power density of 41 μW/cm² was obtained for a CtCDH/MvBOD biofuel cell in 5 mM lactose and O₂-saturated buffer (pH 7.4, 0.1 M phosphate, 150 mM NaCl).     

Subnanomolar detection limit application of ion-selective electrodes with three-dimensionally ordered macroporous (3DOM) carbon solid contacts

Solid-contact ion-selective electrodes (SC-ISEs) can exhibit very low detection limits and, in contrast to conventional ISEs, do not require an optimization of the inner filling solution. This work shows that subnanomolar detection limits can also be achieved with SC-ISEs with three-dimensionally ordered macroporous (3DOM) carbon contacts, which have been shown recently to exhibit excellent long-term stabilities and good resistance to the interferences from oxygen and light. The detection limit of 3DOM carbon-contacted electrodes with plasticized poly-(vinyl chloride) as membrane matrix can be improved with a high polymer content of the sensing membrane, a large ratio of ionophore and ionic sites, and conditioning with a low concentration of analyte ions. This permits detection limits as low as 1.6 × 10⁻⁷ M for K⁺ and 4.0 × 10⁻¹¹ M for Ag⁺.     

Ion-Selective Electrodes With Sensing Membranes Covalently Attached to Both the Inert Polymer Substrate and Conductive Carbon Contact

The use of solid-contact ion-selective electrodes (ISEs) is of interest to many clinical, environmental, and industrial applications. However, upon extended exposure to samples and under thermal and mechanical stress, adhesion between these membranes and underlying substrates often weakens gradually. Eventually, this results in the formation of a water layer at the interface to the underlying electron conductor and in delamination of the membrane from the electrode body, both major limitations to long-term monitoring. To prevent these problems without increasing the complexity of design with a mechanical attachment, we use photo-induced graft polymerization to simultaneously attach ionophore-doped crosslinked poly(decyl methacrylate) sensing membranes covalently both to a high surface area carbon as ion-to-electron transducer and to inert polymeric electrode body materials (i.e., polypropylene and poly(ethylene-co-tetrafluoroethylene)). The sensors provide high reproducibility (standard deviation of E⁰ of 0.2 mV), long-term stability (potential drift 7 μV h⁻¹ over 260 h), and resistance to sterilization in an autoclave (121 °C, 2.0 atm for 30 min). For this work, a covalently attached H⁺ selective ionophore was used to prepare pH sensors with advantages over conventional pH glass electrodes, but similar use of other ionophores makes this approach suitable to the fabrication of ISEs for a variety of analytes.     

Recovery of nanomolar detection limit of solid-contact lead (II)-selective electrodes by electrode conditioning

Solid-contact Pb²⁺-selective electrodes (Pb²⁺-ISEs) were prepared by using polybenzopyrene doped with eriochrome black T as solid contact material and a conventional polyvinyl chloride membrane with lead ionophore IV as selective compound. Nernstian response down to 10^?9?mol?dm^?3 Pb²⁺ was obtained by careful control of the electrode conditioning process. Furthermore, the response at lowest concentrations was retained by exposing the solid-contact Pb²⁺-ISEs to a solution containing Na₂EDTA. Finally, the solid-contact Pb²⁺-ISEs were used in the determination of lead in a synthetic sample (pPb²⁺?=?7.40). The analysis of the sample was done with direct potentiometry (pPb²⁺?=?7.64?±?0.11) and single standard addition method (pPb²⁺?=?7.27?±?0.07). These results were in good agreement with those obtained by inductively coupled plasma–mass spectrometry (pPb?=?7.34). The renewable response of the Pb²⁺-ISEs at low concentrations opens interesting possibilities when dealing with trace-level measurements of Pb²⁺.     

Characterization of graphite electrodes modified with laccase from Trametes versicolor and their use for bioelectrochemical monitoring of phenolic compounds in flow injection analysis

Spectrographic graphite electrodes were modified through adsorption with laccase from Trametes versicolor. The laccase-modified graphite electrode was used as the working electrode in an amperometric flow-through cell for monitoring phenolic compounds in a single line flow injection system. The experimental conditions for bioelectrochemical determination of catechol were studied and optimized. The relative standard deviation of the biosensor for catechol (10 μM, n=12) was 1.0% and the reproducibility for six laccase-modified graphite electrodes, prepared and used different days was about 11%. The optimal conditions for the biosensor operation were: 0.1 M citrate buffer solution ( at pH 5.0), flow rate of 0.51 ml min⁻¹ and a working potential of −50 mV versus Ag|AgCl. At these conditions the responses of the biosensor for various phenolic compounds were recorded and the sensor characteristics were calculated and compared with those known for biosensors based on laccase from Coriolus hirsutus, cellobiose dehydrogenase (CDH) from Phanerochaete chrysosporium and horseradish peroxidase (HRP).     

Progress of Organic Electrodes in Aqueous Electrolyte for Energy Storage and Conversion

Aqueous batteries using inorganic compounds as electrode materials are considered a promising solution for grid-scale energy storage, while wide application is limited by the short life and/or high cost of electrodes. Organics with carbonyl groups are being investigated as the alternative to inorganic electrode materials because they offer the advantages of tunable structures, renewability, and they are environmentally benign. Furthermore, the wide internal space of such organic materials enables flexible storage of various charged ions (for example, H⁺, Li⁺, Na⁺, K⁺, Zn²⁺, Mg²⁺, and Ca²⁺, and so on). We offer a comprehensive overview of the progress of organics containing carbonyls for energy storage and conversion in aqueous electrolytes, including applications in aqueous batteries as solid-state electrodes, in flow batteries as soluble redox species, and in water electrolysis as redox buffer electrodes. The advantages of organic electrodes are summarized, with a discussion of the challenges remaining for their practical application.     

Preparation and comparison of Proteus vulgaris and Proteus mirabilis bacterial electrodes for the determination of DL-phenylalanine

Bacterial electrodes for DL-phenylalanine were prepared by immobilizing the bacteria Proteus vulgaris and Proteus mirabilis on an ammonia gas-sensor. The response of the Proteus vulgaris bacterial electrode, when 10 mg of the bacteria was used, had a linear range between 3.0 × 10⁻⁴ and 1.0 × 10⁻² M DL-phenylalanine with a response slope of 43 mV/decade in pH 7.0, 0.1 M phosphate buffer solution at 30°C, while the response of the Proteus mirabilis bacterial electrode, when 3 mg of the bacteria was used, had a linear range between 3.0 × 10⁻⁴ and 3.0 × 10⁻² M DL-phenylalanine with a response slope of 49 mV/decade in pH 7.2, 0.1 M phosphate buffer solution at 30°C. The most important interferents were urea and l-asparagine, and inorganic salts reacted as an inhibitor. The Proteus vulgaris bacterial electrode could be used directly for the determination of DL-phenylalanine in nearly the same linear range during 3 days. On the other hand, the Proteus mirabilis bacterial electrode could be used continuously during 7 days in the above linear range.     

All-solid-state potassium-selective electrode using graphene as the solid contact

Graphene sheets are used for the first time to fabricate a new type of solid-contact ion-selective electrode (SC-ISE) as the intermediate layer between an ionophore-doped solvent polymeric membrane and a glassy carbon electrode. The new transducing layer was characterized by transmission electron microscopy, scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The performance of the new K(+-)selective electrodes was examined by a potentiometric water layer test, potentiometric measurements, and current reversal chronopotentiometry. The obtained potentiometric sensors were characterized with a calibration line of slope close to Nernstian (59.2 mV/decade) within the activity from 10(-4.5) to 0.1 M. The high capacitance of the graphene solid contacts results in a signal that is stable over one week. The short response time is less than 10 s for activities higher than 10(-5) M. The potential drift of the electrodes was calculated from the slope of the curves at longer times (ΔE/Δt = 1.2 × 10(-5) V s(-1) (I = 1 nA) and ΔE/Δt = 5.5 × 10(-5) V s(-1) (I = 5 nA)). All the results indicate that graphene is a promising material for use as a transducer layer for SC-ISEs.