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 solid-state redox buffer as interface of solid-contact ISEs

Graphite with a surface-confined redox buffer system was used as solid contact in solid-contact ion-selective electrodes (SC-ISE). Potentiostatically preconditioning of the redox buffer ensures that the ratio of oxidized and reduced groups is unity, i.e., a maximum buffer capacity. These SC-ISEs exhibited a very high reproducibility and stability of potentials. Graphite modification was achieved by generating carboxylic surface groups via oxidation with nitrosulfuric acid and subsequent modification of the acidic groups with n-(2,5-dimethoxyphenyl)ethyl-1-amine (DMPEA). Composite electrodes of modified graphite and polymethylmetharcrylate (PMMA) as binder were used as solid-contact of polymer membrane K⁺-and F^?-sensitive SC-ISEs. The solid-state redox buffer decreased potential variation from several hundred mV to just a few mV.     

A simple approach for fabricating solid-contact ion-selective electrodes using nanomaterials as transducers

A simple and robust approach for the development of solid-state ion-selective electrodes (ISEs) using nanomaterials as solid contacts is described. The electrodes are fabricated by using the mixture of an ionic liquid (IL) and a nanomaterial as intermediate layer, formed by melting the IL. Tetradodecylammonium tetrakis(4-chlorophenyl)borate (ETH 500) is chosen as an model of IL to provide strong adhesion between the inner glassy carbon electrode and the intermediate layer. Nanomaterials including single-walled carbon nanotubes (SWCNTs) and graphene were used as active ion-to-electron transducers between the glassy carbon electrode and the ionophore-doped ISE membrane. By using the proposed approach, the solid-contact Cu²⁺- and Pb²⁺-selective electrodes based on ETH 500/SWCNTs and ETH 500/graphene as transducers, respectively, have been fabricated. The proposed electrodes show detection limits in the nanomolar range and exhibit a good response time and excellent stability.     

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⁺.     

Plastic reference electrodes and plastic potentiometric cells with dispersion cast poly(3,4-ethylenedioxythiophene) and poly(vinyl chloride) based membranes

A simple procedure of preparing low cost, planar and disposable reference electrodes for potentiometric applications is presented. This method is essentially the same as used for obtaining all-plastic ion-selective electrodes and thus promising for simple fabrication of complete cells. Commercially available aqueous dispersion of poly(3,4-ethylenedioxythiophene) doped by poly(4-styrenesulfonate) ions (PEDOT-PSS, Baytron P) is simply cast on a non-conducting plastic support (transparent foil for laser printers). This layer is covered by a non-selective poly(vinyl chloride) based membrane containing solid AgCl and KCl, added to obtain a stable potential. The conducting polymer layer plays a double role, of electrical contact and ion-to-electron transducer, enhancing the potential stability. The reference electrodes obtained exhibit independence of the kind and concentration of electrolyte applied as well as very low sensitivity to interferences: redox reactants and H+ ions; they are also characterized by both potential stability and low polarisability, sufficient for potentiometric applications. Cells of plastic electrodes (indicator and reference ones) are tested using an arrangement with Pb2+ or Ca2+ selective sensors. Potentiometric characteristic of such cells is satisfactory, well comparable with that using a classical electrode arrangement.     

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.     

AgI-Ag2O-V2O5 glasses as ion-to-electron transducers for the construction of all-solid-state microelectrodes

A method for the fabrication of ion-selective all-solid-state microelectrodes is presented. The ion-to-electron transduction process takes place into the transducer material. In this approach, AgI-Ag₂O-V₂O₅ glasses, which exhibit ionic and electrical conductivity are applied as ion-to-electron transducers of polymeric membrane microelectrodes. All-solid-state electrodes based on potassium-sensitive poly(vinyl chloride) membranes, deposited directly on the surface of glass composites, exhibited theoretical responses. Their selectivity and durability were comparable to planar microelectrodes containing an internal electrolyte immobilized in the intermediate hydrogel layer. The only disadvantage of the proposed structures was their limited reproducibility. Moreover, it was found that the unmodified AgI-Ag₂O-V₂O₅ glasses can be applied as ion-sensitive membrane of solid-state microelectrodes for the determination of Ag⁺ and I⁻ ions.     

Potassium Ion-selective Electrode with a Sensitive Ion-to-Electron Transducer Composed of Porous Laser-induced Graphene and MoS2 Fabricated by One-step Direct Laser Writing

In this study, an ion-selective electrode with a sensitive ion-to-electron transducer composed of porous laser-induced graphene (LIG) and MoS₂ (LIG-MoS₂/ISE) was fabricated to measure the potassium ion concentration in a greenhouse nutrient solution for soilless culture. Additionally, a more effective and low-cost method was proposed for the large-batch production and manufacture of potassium ion-selective electrodes (K⁺-ISEs) using the direct laser writing technique, which differs considerably from existing methods. Moreover, the sensing mechanism of the proposed LIG-MoS₂/ISE for potassium ion detection was investigated. The morphology and physical properties of the LIG-MoS₂/SC-K⁺-ISEs were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and energy-dispersive spectroscopy. Potentiometric measurements, chronopotentiometry, potentiometric water layer tests, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to evaluate the analytical performance of the newly developed K⁺-ISEs. A Nernstian slope of 30.1 mV/decade for the activity of potassium ions in a concentration range from 10⁻⁷ to 10⁻² M was determined. The EIS and chronopotentiometry results revealed that the LIG-MoS₂/SC-K⁺-ISE had a larger resistance and double-layer capacitance than the LIG/SC-K⁺-ISE. The ion-selective membrane (ISM) and solid-contact layer did not have any water film between them, according to the potentiometric water layer test. The results proved that the LIG-MoS₂ nanocomposite could possibly be used as a sensitive ion-to-electron transducer to fabricate K⁺-ISEs. The K⁺-ISE fabrication method using the direct laser writing technique had a higher efficiency, enabling its broad application prospects in agriculture.     

Fullerene-based electrochemical buffer layer for ion-selective electrodes

In this work, C(60) fullerene is used as an electrochemical mediator for the development of an all-solid-state ISE. The unique electrochemical characteristics of the fullerenes allow for the facile ion-to-electron transduction across the ionically active polymeric ion-selective membrane and the electrochemically active glassy carbon transducer. The interfacial ion-to-electron charge transfer was investigated by Electrochemical Impedance Spectroscopy. The study of the analytical characteristics of a model potassium-selective electrode, together with the EIS studies, reveals that, indeed, the interfacial C(60) electrochemically active layer facilitates the ion-to-electron transduction, providing a stable and reversible solid-state ISE system. This finding is a significant contribution to the efforts aiming at overcoming one of the most significant drawbacks of the solid-state ISEs, that is the potential drift observed during continuous measurements, and could lead to the development of both cation- and anion-sensitive systems.     

Influence of inner transducer properties on EMF response and stability of solid-contact anion-selective membrane electrodes based on metalloporphyrin ionophores

The performance of solid-contact coated-wire-type electrodes with plasticized poly(vinyl chloride) membranes containing metalloporphyrins as anion-selective ionophores is reported. The membranes are deposited on transducers based on graphite pastes and graphite rods. The hydrophobicity of the underlying conductive transducer surface is found to be a key factor that influences the formation of an aqueous layer beneath the polymer film. Elimination of this ill-defined water layer greatly improves the electrochemical properties of the ion-sensors, such as electromotive force stability and lifetime. Only highly lipophilic electrode substrates, namely graphite paste with mineral oil, were shown to prevent the formation of aqueous layer underneath the ion-sensing membrane. The possibility of employing Co(III)-tetraphenylporphyrin both as NO₂ ⁻ selective ionophore and as electron- and ion-conducting species to ensure ion-to-electron translation was also discussed based on the results of preliminary experiments.     

An effective nanostructured assembly for ion-selective electrodes. An ionophore covalently linked to carbon nanotubes for Pb2+ determination

We report on the synthesis of a new hybrid material, i.e. benzo-18-crown-6 covalently linked to multi-wall carbon nanotubes, and its use in solid-state ion-selective electrodes both as a receptor and an ion-to-electron transducer. This new concept leads to potentiometric sensors with extremely high selectivity.     

Optimizing the analytical performance and construction of ion-selective electrodes with conducting polymer-based ion-to-electron transducers

All-solid-state ion-selective electrodes that use a conducting polymer as the ion-to-electron transducer have emerged as one of the most promising classes of all-solid-state potentiometric sensors in recent years. This is largely because it has many analytical advantages, including high response stability, which is unique in the field of internal-solution-free ion-selective electrodes. This paper reviews the considerable progress that has been made in this area of sensing in recent years, in terms of detection limits, selectivity coefficients and novel construction methods.     

Ligand-modified multi-walled carbon nanotubes for potentiometric detection of silver

Three novel hybrid materials have been synthesized by ligands: N-(2-vinylsulfanyl-ethylidene)-benzene-1,2-dimine (SBD), N-pyridin-2-ylmethylene-benzene-1,2-dimine (NBD) and N-furan-2-ylmethylene-benzene-1,2-dimine (OBD), covalently linking to multi-walled carbon nanotubes (MWCNTs). These MWCNT hybrid materials were used both as ionophores and as ion-to-electron transducers to construct Ag(+) carbon paste electrodes. The resulting electrodes show higher selectivity to Ag(+) than other cations tested. Among the three electrodes, the electrode based on SBD-g-MWCNTs with optimum composition shows the best performance to Ag(+). It exhibits an excellent Nernstian response to Ag(+) in the concentration range from 8.8 × 10(-8) to 1.0 × 10(-1) M with a detection limit of 6.3 × 10(-8) M, and it can also be used over a wide pH range of 3.0-8.0 with a quick response time of 5 s. The response mechanism of the proposed electrode was also investigated by using AC impedance and UV-vis spectroscopy techniques.     

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.     

All-plastic, disposable, low detection limit ion-selective electrodes

A novel concept for all-plastic and all-solid-state ion-selective electrodes (ISEs) is introduced. Planar, flexible ion-selective electrodes, comprising only polymeric materials, with no internal solution, were obtained. The cast conducting polymer layer (obtained from aqueous suspension) was covered with a solvent polymeric based membrane to obtain a planar all-plastic sensor. The conducting polymer layer served both as electrical contact and as ion-to-electron transducer. To illustrate this concept, the conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) ions (PEDOT-PSS, Baytron P) was chosen. Due to interaction, analyte cations-poly(4-styrenesulfonate) anions, an extended linear range of potentiometric responses was obtained, with lowered detection limit.As example, Ca²⁺-selective and K⁺-selective all-plastic electrodes were fabricated and yielded with high selectivity, near Nernstian slopes and fast responses. The detection limits obtained for Ca²⁺- and K⁺-selective sensors were 5 × 10⁻⁹ M CaCl₂ and 4.4 × 10⁻⁷ M KCl, respectively.The possibilities of modifying the conducting polymer-phase composition is highlighted. This method is extremely useful to tune the desired type of responses, and cannot be directly applied for electrochemically deposited conducting polymers.     

A solid-contact Pb(2+)-selective electrode using poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene) as ion-to-electron transducer

In this work, a novel all-solid-state polymeric membrane Pb(2+)-selective electrode was developed by using for the first time poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) as solid contact. To demonstrate the ion-to-electron transducing ability of MEH-PPV, chronopotentiometry and electrochemical impedance spectroscopy measurements were carried out. The proposed electrodes showed a Nernstian response of 29.1 mV decade(-1) and a lower detection limit of subnanomolar level. No water film was observed with the conventional plasticized PVC membrane. This work demonstrates a new strategy for the fabrication of robust potentiometric ion sensors.     

A solid-contact Pb-selective electrode using poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) as ion-to-electron transducer

In this work, a novel all-solid-state polymeric membrane Pb²⁺-selective electrode was developed by using for the first time poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) as solid contact. To demonstrate the ion-to-electron transducing ability of MEH-PPV, chronopotentiometry and electrochemical impedance spectroscopy measurements were carried out. The proposed electrodes showed a Nernstian response of 29.1 mV decade⁻¹ and a lower detection limit of subnanomolar level. No water film was observed with the conventional plasticized PVC membrane. This work demonstrates a new strategy for the fabrication of robust potentiometric ion sensors.     

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.     

导电聚合物构建的高性能固态离子选择电极

基于最新研究文献和自身研究工作,系统总结了以导电聚合物构建的各种高性能固态离子选择电极.导电聚合物所特有的共轭结构以及电子导电和离子导电的双重导电功能使其可以作为离子-电子转换器,从而实现对离子的传感响应与探测.由聚苯胺、聚吡咯和聚噻吩等导电聚合物为转换中间层而构建的离子选择电极可以实现纳摩尔浓度水平的离子传感探测,有望在环境监测、药物医疗和食品安全等诸多方面发挥重要作用.