Micro Solid‐Contact Ion‐Selective Electrode Using a Carbon Nanotube Tower as Ion‐to‐Electron Transducer and Conductive Substrate

Solid contact (SC) ion‐selective electrodes (ISEs) have been recognized as the next generation of ISEs. In this work, the electrical conductivity and mechanical strength of a carbon nanotube (CNT) tower enable it to play the dual roles of transducer and substrate for micro SC‐ISEs. The electrode had a close to Nernstian slope of 35 mV/decade a_Ca2+, a linear range of four orders of magnitude of calcium ion activity (10^?5.6 to 10^?1.8 M), and a detection limit of 1.6×10^?6 M. The simplified fabrication by a one‐step drop casting makes miniaturizing SC‐ISEs and fabricating sensor arrays easier to achieve.     

Ion‐selective Electrodes with 3D Nanostructured Conducting Polymer Solid Contact

Until now both ion‐to‐electron transducers as well as large surface area nanostructured conducting materials were successfully used as solid contacts for polymer‐based ion‐selective electrodes. We were interested to explore the combination of these two approaches by fabricating ordered electrically conducting polymer (ECP) nanostructures using 3D nanosphere lithography and electrosynthesis to provide a high surface area and capacitive interface for solid contact ion‐selective electrodes (SC‐ISEs). For these studies we used poly(3,4‐ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT(PSS)) films with 750 nm diameter interconnected pores as the intermediate layer between a glassy carbon electrode and a Ag⁺ ‐selective polymeric membrane. We also investigated the feasibility of loading the voids created in the polymer film with a lipophilic redox mediator (1,1’‐dimethylferrocene) to provide the respective ISEs with well‐defined/controllable E⁰ values. These expectations were fulfilled as the standard deviation of E⁰ values were reduced with almost an order of magnitude for 3D nanostructured SC‐ISEs filled with the redox mediator as compared to their redox mediator‐free analogs. The detrimental effect of the redox mediator extraction into the plasticized PVC‐based ion‐selective membrane (ISM) was efficiently suppressed by replacing the PVC‐based ISMs with a low diffusivity silicone rubber matrix.     

Synthesis of a Tweezer—like Bis（Phenylthiapropoxy）calix[4]—arene as a Cation／π Enhanced Sensor for Ion—Selective Electrodes

Two novel 25,27-dihydroxy-26,28-bis(3-phenylthiapropxy)-calix[4]arene(3) and 25,27-dihydroxy-26,28-bis(3-phenylthiapropoxy)-5,11,17,23-tetra-tert-butylcalix[4] arene (4) were synthesized for the evaluation of their ion-selectivity in ion-selective electrodes(ISEs).ISEs based on 3 and 4 as neutral ionophores were prepared,and their selectivity coefficients for Ag^ (lg KAg,M^pot)were investigated against other alkali metal,alkaline-earth metal,aluminum,thallium(Ⅰ）,Lead and some transition metal ions using the separate solution method (SSM).These ISEs showed excellent Ag^ seletivity over most of the interfering cations examined,except for Hg^2 and Fe^2 having relative smaller interference(lg KAg,M^pot≤-2.1).     

Ion‐Selective Electrodes Based on Hydrophilic Ionophore‐Modified Nanopores

We report the synthesis and analytical application of the first Cu²⁺‐selective synthetic ion channel based on peptide‐modified gold nanopores. A Cu²⁺‐binding peptide motif (Gly‐Gly‐His) along with two additional functional thiol derivatives inferring cation‐permselectivity and hydrophobicity was self‐assembled on the surface of gold nanoporous membranes comprising of about 5 nm diameter pores. These membranes were used to construct ion‐selective electrodes (ISEs) with extraordinary Cu²⁺ selectivities, approaching six orders of magnitude over certain ions. Since all constituents are immobilized to a supporting nanoporous membrane, their leaching, that is a ubiquitous problem of conventional ionophore‐based ISEs was effectively suppressed.     

Investigation of Protein Binding With All Solid‐State Ion‐Selective Electrodes

A potentiometric sensor for studying charge based adsorption of proteins was created using a single‐piece polyaniline‐PVC ion‐selective electrode (ISE). Three different ISEs, two for Na⁺ and one for Cl^? ion determination, were studied. The Na⁺‐ISEs consisted of a neutral calixarene‐based ionophore and one with a charged carrier dinonylnapthalenesulfonic acid (DNNSA) whereas for the Cl^? ISE, an anion exchanger tridodecylmethylammonium chloride (TDDMA⁺Cl^?), was used. The Na⁺ ISE with DNNSA as the charged carrier was successfully able to discriminate the binding of two different proteins (bovine serum albumin and lysozyme) based on their intrinsic charge.     

Development and Optimization of an Ion‐selective Electrode for Serotonin Detection

Our studies are focused on the development of novel potentiometric sensors for the quantification of the neurotransmitter serotonin. Therefore, ion‐selective electrodes based on plasticized PVC membranes are applied. The electroactive part of the membrane consists of an ion pair complex formed between the protonated analyte and a carborane anion [Co(1,2‐C₂B₉H₁₁)₂]⁻. The analytical performance of the electrode was studied regarding sensitivity, concentration range, limit of detection and potential stability. The ion‐selective electrodes were optimized with respect to the material of the transducing element, as well as the membrane thickness and its composition. Stable, all solid state ISEs could be developed, using the non‐polar plasticizer NPOE and a graphite rod with high surface area as transducing element. We thus achieved a near Nernstian response over three decades of concentration (2.25⋅10^‐5‐1.00⋅10^‐2 M) and a limit of detection in the μ‐molar range for the optimized electrodes. The electrodes could successfully be miniaturized using carbon based screen printed electrodes.     

Lipophilic Multi‐walled Carbon Nanotube‐based Solid Contact Potassium Ion‐selective Electrodes with Reproducible Standard Potentials. A Comparative Study

The two most promising approaches for preparing solid contacts (SCs) for polymeric membrane based ion‐selective electrodes (ISEs) are based on the use of large surface areas conducting materials with high capacitance (e. g., various carbon nanotubes) and redox active materials (e. g. conducting polymers). While many of the essential requirements for the potential stability of SCISEs were addressed, the E⁰ reproducibility and its predictability, that would enable single use of such electrodes without calibration is still a challenge, i. e., the fabrication of electrodes with sufficiently close E⁰ and slope values to enable the characterization of large fabrication batches through the calibration of only a small number of electrodes. The most generic solution seems to be the adjustment of the E⁰ potential by polarization prior to the application of the ion‐selective membrane. This approach proved to be successful in case of conducting polymer‐based solid contacts, but has to be still explored for capacitive solid contact based ISEs, which is the purpose of this paper. We have chosen a well‐established highly lipophilic multi‐walled carbon nanotube (MWCNT), i. e. octadecane modified MWCNT (OD‐MWCNT), that is investigated in the comparative context of a similarly lipophilic conducting polymer solid contact (a perfluorinated alkanoate side chain functionalized poly(3,4‐ethylenedioxythiophene)). While, the OD‐MWCNT based SCISEs had inherently small standard deviation of their E⁰ values (less than 5 mV) this could be further improved by external polarization and short circuiting the SCISEs.     

Conducting Polymer‐Based Solid‐State Ion‐Selective Electrodes

Conducting polymers, i.e., electroactive conjugated polymers, are useful both as ion‐to‐electron transducers and as sensing membranes in solid‐state ion‐selective electrodes. Recent achievements over the last few years have resulted in significant improvements of the analytical performance of solid‐contact ion‐selective electrodes (solid‐contact ISEs) based on conducting polymers as ion‐to‐electron transducer combined with polymeric ion‐selective membranes. A significant amount of research has also been devoted to solid‐state ISEs based on conducting polymers as the sensing membrane. This review gives a brief summary of the progress in the area in recent years.     

Characteristics of Lariat Crown Ether‐Copper (II) Ion‐Selective Electrodes

An ion‐selective electrode using ionophore 2′‐picolyl sym‐dibenzo‐16‐crown‐5 ether as membrane carrier, sodium tetraphenylborate (NaTPB) as an anion excluder, and 2‐nitrophenyl‐octyl ether (NOPE) as the plasticing solvent mediator has been successfully developed. This electrode exhibits al in ear response with a slope of 42 mV/decade in concentration ranging from 10^?5 molL^?1 to 10^?1 molL^?1, slightly larger than the 30 mV expected from the one‐to‐one complex. The reason for the super‐Nernstain slope is the partial dimmer formation in side the membrane of the electrode, because this dimmer [Cu(C₂₅H₂₇NO₆)₂(H₂O)₂] 2ClO₄, has been isolated and confirmed by single crystal X‐ray crystallography. The detection limit for the cop per (II) ion was estimated to be 1 × 10^?6 molL^?1. Electrades composed of other plasticing solvent mediators such as tris(2‐ethylhexyl) phosphate (TOP), bis (2‐ethylhexyl) sebacate (DOS) and dibutyl phthalate (DBP) were also investigated. Stability constants (logKs) of the two to one and the one to one 2‐picolylsym‐dibenzo‐16‐crown‐5 ether‐Cu (II) complexes have been determined by potentiometric titration in methanol.     

Responses of Metalloporphyrin‐Based Ion‐Selective Electrodes to pH

The ISEs based on [M(tpp)Cl] (M: Al, Ga, In, Mn, Fe; H₂tpp: tetraphenylporphin) had pH responses across their respective pH ranges, which had some correlation with the pH ranges of the two‐phase hydrolysis. Such pH responses are ascribed to the phase boundary potentials relating to the acid‐base pairs of [M(tpp)(H₂O)]⁺ and [M(tpp)(OH)] and/or [M₂(tpp)₂O]. The potential responses of the In and Fe complexes had the upper limitation to pH of 90 % hydrolysis, whereas those of the Al and Ga complexes had the extension to at least pH 12, indicating stable existence of [M(tpp)(H₂O)]⁺ even in contact with strongly alkaline solutions.     

Ion-Selective Electrodes Based on Bilayer Film Coating

The electrode characteristics of ion-selective electrodes (ISEs) for K⁺, Na⁺, NH₄ ⁺, and Ca²⁺ based on bilayer film coatings, where the inner layer films are electroactive electropolymerized ones and the outer layer films are composed of conventional ion-sensitive materials, have been examined. These ISEs of the coated-wire electrode type have no conventional internal reference solution and reference electrode, but the inner films may be considered to function as the “internal standard solution.” The ion selectivity coefficients and the activity range showing Nernstian response were almost comparable to those of conventional liquid-membrane electrodes. The bilayer-coated ISEs showed insensitivity to O₂ and CO₂, long-term stability, and little drift. It was also found that the electrode performance is practically unchanged after sterilization in an autoclave. The results demonstrate that the bilayer-coated ISEs examined are promising for the determination of K⁺, Na⁺, NH₄ ⁺, or Ca²⁺ activity in biological and environmental systems.     

Stability and Lifetime of Potassium Solid‐Contact Ion Selective Electrodes for Continuous and Autonomous Measurements

The current generation of solid‐contact ion‐selective electrodes (SC‐ISE) suffer from lack of stability and lifetime. When using such sensors for remote, continuous, or autonomous measurements, these analytical characteristics are especially critical. In this work we compare several different configurations of ISEs to be deployed for monitoring in extreme environments, and present a novel configuration to improve performance. In particular we compare a polymeric hydrogel‐based ISE, used previously in the Wet Chemistry Lab on the Phoenix Mars Lander, with three variations of solid supported nanoporous carbon‐based ISEs. The symmetric membrane (SM) solid contact ISE (SM‐SC‐ISE) shows promise in overcoming many of the analytical problems encountered with hydrogel and solid‐state devices. The results indicate that sensors based on the SM configuration provide improvements in both stability, and most importantly reproducibility, over other existing SC‐ISEs. Future work will continue testing the SM configuration for use in a variety of extreme environments, including continuous monitoring and in‐situ analyses in extraterrestrial environments.     

Novel Cesium‐Selective Electrodes Based on Lipophilic 1,3‐Bisbridged Cofacial‐calix[6]crowns

Five bisbridged calix[6]crowns have been investigated as Cs⁺ ionophore in PVC membrane electrodes. As ionophores, three 1,3‐bisbridged calix[6]crown‐4‐ethers( I–III ), 1,3‐bisbridged calix[6]crown‐5‐ether( IV ), and 1,3‐bisbridged calix[6]crown‐6‐ether( V ) have been evaluated. The membranes all give good Nernstian response in the concentration range from 1×10^?7 to 1×10^?1 M of cesium ion. The best detection limits (?log aequation/tex2gif-inf-1.gif=7.08–7.36) are obtained for electrode membranes containing 1,3‐bisbridged cofacial‐calix[6]crown‐4‐ethers( I‐III ), and the values are the lowest compared with those reported previously. The highest selectivity coefficients [ 3.74(Cs/K), 2.63(Cs/Rb)] are obtained for the membrane of 1,3‐bisbridged calix[6]crown‐4‐ether( II ), and these values are also the highest compared with previous reports for Cs⁺‐ISEs. The highest selectivity towards cesium ion is attributed to the geometrically cofacial positions of two crown‐ethers in calix[6]crowns in order to provide the complex of cesium ion and eight oxygens of cofacial crowns.     

Potentiometric Determination of Non‐Ionic Surfactants by Liquid Membrane Electrodes

It is well known that non‐ionic surfactants (NIS) influence remarkably the potentiometric measurements with liquid membrane ion selective electrodes (ISEs), interfering particularly on performance of ISEs for earth‐alkali metals, for which the loss of selectivity with regard to alkali metals has been documented. These studies indicate that such interferences are due to the extraction of surfactants within the membrane, where a competition takes place between the originally present ionophore and the surfactant which also acts as a ligand for alkali metals. The interpretation of such phenomena enabled one to exploit this interference for analytical purposes by membrane/solution extraction experiment monitored by UV measurements and by impedance FRA analysis on coated wire electrodes. Using Ca/Mg ISEs based on the neutral ionophore ETH 4030, it has been established that the logarithm of the Ca/Mg over Na potentiometric selectivity constant is linearly correlated with the concentration of NIS like Tegopren 5863 and Triton X‐100. The proposed method has been applied for the development of a new potentiometric analytical procedure for the determination of Tegopren 5863 in synthetic seawater (SSW), ranging from 0.25 to 5 ppm. Our procedure consists in the exposure of the electrode to stirred SSW containing the surfactant; the progressive extraction of Tegopren 5863 causes a growth in electrode's sensitivity to Na⁺ and K⁺, losing selectivity for Ca²⁺ and Mg²⁺. In turn this induces an increase of EMF, as all these ions are present in the studied matrix. The potential drift was monitored for 15 hours, showing that the process reaches thermodynamic equilibrium after about 12 hours of exposure. This method presents a value of 210 ppb of Tegopren 5863 as detection limit.     

Semifluorinated Polymers as Ion‐selective Electrode Membrane Matrixes

Most commercially available fluorous polymers are ill suited for the fabrication of ion‐selective electrode (ISE) membranes. Therefore, we synthesized semifluorinated polymers for this purpose. Ionophore‐free ion‐exchanger electrodes made with these polymers show a selectivity range (≈14 orders of magnitude) that is nearly as wide as found previously for liquid fluorous ion‐exchanger electrodes. These polymers were also used to construct ISE membranes doped with fluorophilic silver ionophores. While the resulting ISEs were somewhat less selective than their fluorous counterparts, the semifluorinated polymers offer the advantage that they can be doped both with fluorophilic ionophores and traditional lipophilic ionophores, such as the silver ionophore Cu(II)‐I (o ‐xylylenebis[N,N ‐diisobutyldithiocarbamate]). We also cross‐linked these polymers, producing very durable membranes that retained broad selectivity ranges. K⁺ ISEs made with the cross‐linked semifluorinated polymer and the ionophore valinomycin showed selectivities similar to those of PVC membrane ISEs but with a superior thermal stability, the majority of the electrodes still giving a theoretical (Nernstian) response after exposure to a boiling aqueous solution for 10 h.     

Electrochemical Response Characteristics and Analytical Application of Papaverine Ion‐Selective Membrane Electrodes

Abstract

The construction and electrochemical response characteristics of poly(vinyl chloride) matrix ion‐selective electrodes (ISEs) for papaverine hydrochloride are described. The membranes incorporate ion association complexes of papaverine with tetraphenylborate {[B(C₆H₅)₄]^?}, picrate {[C₆H₂(NO₂)₃O]^?}, tetraiodomercurate [(HgI₄)]^2?, tetraiodobismuthate [(BiI₄)^?], Reinecke salt {[Cr(NH₃)₂(SCN)₄]^?} and heteropolycompounds of Keggin structure–molybdophosphoric acid [H₃(PMo₁₂O₄₀)], tungstophosphoric acid [H₃(PW₁₂O₄₀)], molybdosiliconic acid [H₄(SiMo₁₂O₄₀)], and tungstosiliconic acid [H₄(SiW₁₂O₄₀)] as electroactive materials for ionometric sensor controls. These ISEs show linear response for papaverine hydrochloride over the range from 1×10^?5 up to 5×10^?2 mol/l with cationic slopes from 42 up to 58 mV per concentration decade. These ISEs exhibit fast response time (up to 1 min), low determination limit (up to 1×10^?5 M), good stability (3–5 weeks) and reasonable selectivity. The ISEs were used for direct potentiometry and potentiometric titration {Na[B(C₆H₅)₄]} of papaverine hydrochloride in pharmaceutical preparations. Results with mean accuracy of 98.6±0.9% of nominal were obtained, which correspond well to data obtained with the European Pharmacopoeial method.     

Triiodide PVC Membrane Electrode Based on a Charge‐Transfer Complex of Iodine with Ditertbutyl‐Dicyclohexyl‐18‐Crown‐6

A new triiodide ion‐selective electrode based on a charge‐transfer complex of iodine with ditertbutyl‐dicyclohexyl‐18‐crown‐6 (t‐Bu)₂DC18C6 as membrane carrier was prepared. The electrode has a linear dynamic range from 6.3 × 10^?3‐5 × 10^?6 with a Nernstian response of 58.6 ± 1 mV decade^?1 and a detection limit of 1.3 × 10^?6 M. The response time of the sensor was 25 s. The membrane could be used for two months without any divergence in potentials. The electrode exhibits an anti‐Hofmeistetr selectivity sequence with a preference for triiodide at pH 2.0‐10.0. The response mechanism of the electrode was investigated by Uv‐Vis spectroscopic technique. The electrode can be used for the determination of ascorbic acid in orange juice.     

An Arrayed Micro‐glutamate Sensor Probe Integrated with On‐probe Ag/AgCl Reference and Counter Electrodes

Complete all‐in‐one multi‐arrayed glutamate (Glut) sensors have been constructed on a silicon‐based micromachined probe composed of micro‐platinum (Pt) working electrodes, a micro‐silver/silver chloride (Ag/AgCl) reference electrode (RE), and a micro‐Pt counter electrode (CE). The OCP shift of the electrodeposited Ag/AgCl on‐probe micro‐reference electrode compared with a Ag/AgCl wire is <0.1 mV/h. The composition ratio of Ag, Cl, and Pt on the electrodeposited on‐probe micro‐reference electrode is observed to be 1.00 : 0.48 : 0.02 analyzed by EDS. The miniaturized amperometric Glut biosensors were constructed on working electrode sites (electrode area: ∼8.5×10⁻⁵ cm²) of the microprobe modified with glutamate oxidase (GlutOx) enzyme layers for the selective, fast, and continuous detection of L‐glutamate. The sensor selectivity towards common electroactive interferents has been improved significantly by coating the electrode surface with perm‐selective polymer layers, overoxidized polypyrrole (PPY) and Nafion®. The sensitivity, detection range, and response time of the proposed all‐in‐one Glut biosensors are 204.7±5.8 nA μM⁻¹ cm⁻² (N=5), 4.99–109 μM, and 2.7±0.3 sec, respectively and no interferent signals of AA and DA were observed. The sensor can be reused over 19 times of continuous repetitive operation (total measurement time: ∼4 hours) and the sensor sensitivity can retain up to four weeks of storage.     

Novel Plastic Chromium(III)‐Ion Selective Electrodes Based on Different Ionophoric Species and Plasticizing Solvent Mediators

Different ionophoric species, viz.: 18‐crown‐6 (18C6), dibenzo‐18‐crown‐6 (DB18C6) and calix[6]arene (CAX), as electroactive materials, with 2‐nitrophenyloctylether (2‐NPOE), bis(ethylhexyl)sebacate (DOS), dioctyl phthalate (DOP), and didecyl phthalate (DDP) as plasticizing solvent mediators were used to construct Cr³⁺ selective electrodes in a PVC matrix in the ratio (w/w) PVC: ionophore: plasticizer (60 : 2 : 120). Seven electrodes out of the fabricated 12 electrodes, gave best results in terms of working concentration range (1.0×10^?5?1.0×10^?1 M) with a close to Nernstian slope of 18.5 and a Nernstian slope of 20.0 mV/decade of activity. The usable pH range of the sensors is 4.0–7.0. The detection limit of the selected electrodes is ≤1.0×10^?7 M. The response time of the sensors is 8–35 s, depending on the concentration of Cr³⁺ used. The selectivity coefficient values indicate that the electrodes are highly selective for Cr³⁺ over a number of other cations except Pb²⁺ and Na⁺ (for some electrodes). The electrodes have successfully been used to determine Cr³⁺ in certified and real alloys and in effluents of electroplating shops with a precision as relative standard deviation (RSD)<3%, for each of the proposed Cr³⁺‐ion selective electrodes. The results obtained by the proposed ISEs are in good agreement with the results obtained by direct flame AAS method.     

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.