Double matrix membranes for potentiometric cation selective disposable sensors

The construction of double matrix membranes (DMM) for disposable NH ₄ ⁺ , Na⁺ and pH sensors is described. The usual polymer matrix membrane was incorporated in an inert micro fibre matrix (MFM). Capillary forces distribute the cocktail homogeneously in the MFM; this also determines the thickness. Sensors using the new DMM have shown selectivity, detection limits and slope comparable with polymer matrix membranes. The advantages of DMMs are their mechanical stability, the feasibility of low cost mass production with identical parameters and improved handling.     

Ion-selective potentiometric sensors with silicone sensing membranes: A review

Ion-selective electrodes (ISEs) are used widely in mainframe analyzers for clinical chemistry, but there is also an increasing interest in the development of paper-based devices, wearable and implantable sensors, and other miniaturized ISEs. This trend is spurring much research in developing solid contact materials that enable miniaturization. The development of suitable polymeric matrixes for such sensors has only received less attention. In particular, in spite of lifetime limitations and toxicity concerns, polymeric matrixes comprising plasticizers are still commonly used. To that end, we note the benefits of silicone materials as alternative polymeric matrixes and, in particular, their promise for enhanced biocompatibility. While there has been steady progress in the development of ISEs with silicone membranes, this topic has not been reviewed for many years. This review critically discusses key fundamental characteristics of ISEs with silicone sensing and reference membranes, including their biocompatibility, adhesion to device substrates, water uptake, polarity, common impurities, and commercial availabilities. This is followed by a discussion of specific types of silicones and their use in ISEs, with the goal to inform and stimulate future research efforts into such devices.     

Determination of fluoxetine in pharmaceutical preparations and biological samples using potentiometric sensors based on polymeric membranes

The construction and general performance of four novel potentiometric membrane sensors for the determination of fluoxetine have been described. The sensors are based on the formation of the ion association complex of fluoxetine with sodium tetraphenylborate and phosphotungstic acid as electroactive materialles, dispersed in a PVC matrix with dibuthyl sebacate (or diethyl sebacate) as a plasticizer. These sensors exhibit fast, stable and near-Nernstian response for the monocharged fluoxetine cation over wide concentration range from 3.0 × 10^?6 to 1.2 × 10^?2 M and pH 4.0–7.5. No interferences are caused by many inorganic and organic species. Direct potentiometric determinations of 5–100 μg/mL of fluoxetine in drug and urine samples show good recovery of fluoxetine. The developed membrane electrodes have been used as an end point indicator electrode; the potentiometric titration of fluoxetine with sodium tetraphenylborate as a titrant has been monitored.     

Response of DNA fragments to potentiometric sensors studied using HPLC

Potentiometric sensors are studied as viable candidates for the construction of high throughput DNA arrays. For preliminary investigations, such sensors were used in an HPLC setup in the present work. This avoided errors due to ionic contaminants or additives in the commercial samples. The oligonucleotides dT(10), dT(20) and dT(30) were used as test substances. The potentiometric sensors were of the coated wire type, containing PVC, DOP, MTDDACl and a synthetic podand urea receptor. The HPLC system consisted of a reversed phase column eluted with a phosphate buffer, triethylammoniumacetate (TEAA), and an acetonitrile gradient. Molar responses and sensitivities increased with increasing chain length of oligonucleotides, yielding detection limits as low as 10(-6)M (dT(30), injected concentration). The slopes of the calibration graphs were at least 23 mV/decade (dT(10)), which was much higher than expected. The results are discussed in view of the potential use of this sensor type in high throughput microarrays.     

Determination of some pharmaceuticals using simple potentiometric sensors of coated-wire type

Summary The preparation of simple potentiometric sensors of the coated-wire type is described and working instructions for their use are given. The sensors prepared from an ordinary aluminium conductor, being simple and cheap, can well serve to monitore the titrations based on ion-pair formation. The application possibilities are discussed and shown as practical examples on analyses of nasal and eye drops containing cationic surfactants or salts of alkaloids.

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Bestimmung einiger Arzneimittel mit Hilfe einfacher potentiometrischer Sensoren

Zusammenfassung Die Herstellung einfacher potentiometrischer Sensoren mit isoliertem Draht und deren Verwendung wurde beschrieben. Aus einfachem Aluminiumdraht sind sie billig und können für Titrationen auf der Grundlage von Ionenpaar-Bildung dienen. Die Möglichkeiten ihrer Anwendung wurden erörtert und praktiziert für die Untersuchung von Nasen- und Augentropfen mit kationischen Schaumbildnern oder von Alkaloidsalzen.

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According to the Poster XII-23 presented at Euroanalysis V, Cracow, August 1984; the comments inserted into the text are a consequence of the discussion.     

Microsized graphite sensors for potentiometric determination of cyclobenzaprine hydrochloride in pure powder, tablets, and plasma

Two cyclobenzaprine hydrochloride (CZ) microsized graphite selective sensors were investigated with dibutylsebacate as a plasticizer in a polymeric matrix of carboxylated polyvinyl chloride (PVC-COOH) in the case of sensor 1, based on the interaction between the drug and the dissociated COOH groups in the PVC-COOH. Sensor 2 was based on the interaction between the drug and ammonium reineckate, which acted as anionic electroactive material in the presence of polyvinyl chloride matrix. The two sensors were constructed by using 2-hydroxy propyl beta-cyclodextrin as an ionophore, which has a significant influence on increasing the membrane sensitivity and selectivity of both sensors. Fast and stable Nernstian responses of 1 x 10(-5) - 1 x 10(-2) and 1 x 10(-4) - 1 x 10(-2) M for the two sensors, respectively, with slopes of 58.6 and 55.5 mV/decade, respectively, over the pH range 2-4 were obtained. The proposed method displayed useful analytical characteristics for determination of CZ in its pure powder form with average recoveries 99.95 +/- 0.23 and 99.61 +/- 0.34% for sensors 1 and 2, respectively, and in plasma with good recoveries. The sensors were also used to determine the intact drug in the presence of its degradate and, thus, could be used as stability-indicating methods. The obtained results by the proposed methods were statistically analyzed and compared with those obtained by the U.S. Pharmacopeia method; no significant difference for either accuracy or precision was observed. Results obtained with the two electrodes revealed their performance characteristics, which were evaluated according to International Union of Pure and Applied Chemistry recommendations.     

Improved potentiometric response of solid-contact lanthanum (III) selective electrode

For the first time, the analytical application of integrate ionophore-transducer material based on magnetic graphene hybrids and 2,2-dithiodipyridine (DTDP) in solid-contact lanthanum (III) selective electrode is reported. The attachment of Fe₃O₄ nanoparticles (NPs) to graphene oxide (GO) for magnetic graphene hybrid is achieved by covalent bonding, and the universal problem, Fe₃O₄ NPs may easily leach out from the graphene during application, is successfully solved by the method above. The proposed electrode exhibits an excellent near-Nernstian response to lanthanum (III) ranging from 1.0 × 10⁻⁹ to 1.0 × 10⁻³ M with a slope of 17.81 mV/dec. Moreover, the excellent performance on fairly good selectivity, wide applicable pH range (3.0^_8.0), fast response time (10 s) and long life time (2 months) reveal the superiority of the electrode. Most importantly, we have made a great improvement in the detection limit (2.75 × 10⁻¹⁰ M), which brings new dawn to the real-time detection of lanthanum (III) using ion selective electrode.     

Chemically modified screen-printed electrodes as efficient potentiometric sensors for cyclobenzaprine hydrochloride determination in pure and pharmaceutical preparations

Chemically modified screen-printed electrodes (SPEs) with 3,4-dihydroxy-9,10-dioxo-2-anthracenesulfonic acid sodium salt [Alizarin Red (ALZ.R)] and tricresylphosphate (TCP) were fabricated and examined for potentiometric determination of cyclobenzaprine hydrochloride (CZP.HCl) in pure and pharmaceutical samples. The content of ALZ.R in the prepared SPEs varied, which significantly affected their potentiometric response during CZP.HCl estimation. The measured potential value was dependent on the pH of the studied solution. A stable modified SPE sensor was prepared with reproducible readings within 22 days. A wide linear concentration range could be measured with Nernestian slope value of 60.12 ± 0.18 mV decade⁻¹. Based on the recorded analytical parameters of these modified SPEs, a sensitive and reliable detection of CZP.HCl in pharmaceutical samples could be attained.     

Optical humidity and ammonia gas sensors using Reichardt's dye-polymer composites

Optical intensity reflected at 750 nm by Reichardt's betain dye-polymer (PMMA or PEO) composite increased with an increase in humidity and was not affected by ammonia vapor. For the composites treated with hydrochloric acid, the intensity at around 590 nm decreased with an increase in ammonia concentration, and the sensitivity was enhanced by the presence of water vapor. The treatment with hydrochloric acid induced the formation of phenolic group, O⁻---H⁺ and N⁺---Cl⁻ in the dye. The sorption of NH₃ decreases the ionic interactions.     

All-solid-state potentiometric sensors for ascorbic acid by using a screen-printed compatible solid contact

The development of all-solid-state potentiometric ion selective electrodes for monitoring of ascorbic acid, by using a screen-printed compatible solid contact is described. The applied methodology is based on the use of PVC membrane modified with some firstly-tested ionophores (triphenyltin(IV)chloride, triphenyltin(IV)hydroxide and palmitoyl-l-ascorbic acid) and a novel one synthesized in our laboratory (dibutyltin(IV) diascorbate). Synthesis protocol and some preliminary identification studies are given. A conductive graphite-based polymer thick film ink was used as an internal solid contact between the graphite electrode and the PVC membrane. The presence and the nature of the solid contact (plain or doped with lanthanum 2,6-dichlorophenolindophenol (DCPI)) seem to enhance the analytical performance of the electrodes in terms of sensitivity, dynamic range, and response time. The analytical performance of the constructed electrodes was evaluated with potentiometry, constant-current chronopotentiometry and electrochemical impedance spectroscopy (EIS). The interference effect of various compounds was also tested. The potential response of the optimized Ph₃SnCl-based electrode was linear against ascorbic acid concentration range 0.005-5.0 mM. The applicability of the proposed sensors in real samples was also tested. The detection limit was 0.002 mM ascorbic acid (50 mM phosphate, pH 5 in 50 mM KCl). The slope of the electrodes was super-Nernstian and pH dependent, indicating a mechanism involving a combination of charge transfer and ion exchange processes. Fabrication of screen-printed ascorbate ISEs has also been demonstrated.     

Pulstrodes: triple pulse control of potentiometric sensors

Ion-selective electrode membranes based on hydrophobic materials doped with chemically selective host molecules are an attractive sensing technology but normally suffer from a limited sensitivity, given by the Nernst equation, and a direct reliance on the reference electrode potential, which makes miniaturization difficult. These fundamental problems are addressed here by imposing a multipulse electrochemical excitation signal onto ion-selective membranes that lack ion-exchange properties. Current pulses are responsible for the generation of ion fluxes in the direction of the membrane, which give reproducible super-Nernstian response slopes that originate from depletion processes at the membrane surface. Membranes may also be measured at zero current after this pulse, giving super-Nernstian response regions at lower concentrations. Difference potentials obtained from subsequent pulses give about 10-fold higher sensitivities than predicted on the basis of the Nernst equation.     

Fluorous bulk membranes for potentiometric sensors with wide selectivity ranges: observation of exceptionally strong ion pair formation

Potentiometric sensors based on fluorous membranes doped with a fluorophilic tetraphenylborate derivative are shown to have a remarkably wide range of selectivities that exceeds the selectivity range of conventional polymeric membranes by 8 orders of magnitude. The fluorous character of these sensing membranes explains the formation of ion pairs of unprecedented strength. Ion pair formation constants in perfluoroperhydrophenanthrene, as measured in this work, are on the order of 1020 and exceed previously reported values for ion pair formation in nonpolar solvents by 6 orders of magnitude. The low solubility of lipids in such fluorous phases makes them very promising for the reduction of biofouling.     

A sequential injection electronic tongue employing the transient response from potentiometric sensors for anion multidetermination

Intelligent and automatic systems based on arrays of non-specific-response chemical sensors were recently developed in our laboratory. For multidetermination applications, the normal choice is an array of potentiometric sensors to generate the signal, and an artificial neural network (ANN) correctly trained to obtain the calibration model. As a great amount of information is required for the proper modelling, we proposed its automated generation by using the sequential injection analysis (SIA) technique. First signals used were steady-state: the equilibrium signal after a step-change in concentration. We have now adapted our procedures to record the transient response corresponding to a sample step. The novelty in this approach is therefore the use of the dynamic components of the signal in order to better discriminate or differentiate a sample. In the developed electronic tongue systems, detection is carried out by using a sensor array formed by five potentiometric sensors based on PVC membranes. For the developed application we employed two different chloride-selective sensors, two nitrate-selective sensors and one generic response sensor. As the amount of raw data (fivefold recordings corresponding to the five sensors) is excessive for an ANN, some feature extraction step prior to the modelling was needed. In order to attain substantial data reduction and noise filtering, the data obtained were fitted with orthonormal Legendre polynomials. In this case, a third-degree Legendre polynomial was shown to be sufficient to fit the data. The coefficients of these polynomials were the input information fed into the ANN used to model the concentrations of the determined species (Cl⁻, and ). Best results were obtained by using a backpropagation neural network trained with the Bayesian regularisation algorithm; the net had a single hidden layer containing three neurons with the tansig transfer function. The results obtained from the time-dependent response were compared with those obtained from steady-state conditions, showing the former superior performance. Finally, the method was applied for determining anions in synthetic samples and real water samples, where a satisfactory comparison was also achieved.      

Stabilized potentiometric solid-state polyion sensors using silver-calixarene complexes as additives within ion-exchanger-based polymeric films

Novel solid-state sensors for biomedically important polyions (i.e., heparin and protamine) that exhibit significantly enhanced initial EMF stability are prepared by incorporation of lipophilic silver-calixarene complexes along with the required ion-exchangers within polymeric films. A dithioether of tertiary butyl calix[4]arene together with its silver complex are added to the polyion sensing membranes which are deposited on a silver-epoxy conductor of a solid-state transducer. The silver-complex serves as a reversible electron transfer agent between the organic polymer film and the underlying solid-state conductor, resulting in highly reproducible starting EMF values and improved initial signal stability. This approach is further employed to devise a heparin sensing cartridge for blood measurements based on a novel differential measurement mode. One sensor responds to heparin, while the second identical solid-state sensor serves as a pseudo reference electrode with all heparin present in the sample within this half-cell complexed by the addition of excess protamine. The cartridges are evaluated by monitoring heparin response in spiked dog blood using poly(vinyl chloride) (PVC) blends and fluorinated silicone rubber (Dow Corning RTV 730) films containing appropriate levels of ion-exchanger and Ag⁺-calixarene/free calixarene additives.     

Theoretical predictions on the response properties of potentiometric gas sensors based on internal polymer membrane electrodes

The appropriate equilibrium expressions and known thermodynamic equilibrium constants are used in calculations on the expected response properties of polymer membrane electrode-based ammonia and carbon dioxide gas sensors. Slopes, detection limits, Nernstian response ranges and selectivities of such devices are shown to be a function of the initial pH, ionic strength and equilibrium constant of the internal electrolyte buffer used within these probes. Previously reported data for an ammonia sensor of this type correlate well with the theory. The poor response characteristics of carbon dioxide sensors based on internal carbonate-responsive membranes is also explained via the model presented. Future prospects and considerations for the development of other gas sensors of this type are discussed.     

Measurements of the capacitance and the response time of solid-state potentiometric sensors by an electrochemical time-of-flight method

A micro-electrochemical time-of-flight technique relying on galvanostatic ion generation and potentiometric sensing (P-ETOF) has been developed in order to characterize dynamic properties of solid-state potentiometric sensors and to measure their capacitance. Lithographically-fabricated generator-sensor devices featuring 10 m-wide micro-electrodes spaced by 20–50 m are used in either an open face mode, where hemi-cylindrical diffusion governs mass transport of generated ions, or in a narrow channel mode, where linear diffusion describes transport of ions between the generator and sensor micro-electrodes in a thin (~3 m) layer of electrolyte. P-ETOF-open face mode is capable of generating silver ions or protons at a rate equivalent to a current density of 10 A/cm². This induces a maximum, mass transport controlled rate of a sensor potential rise of ~80 V/s. P-ETOF was used to characterize anodically electrodeposited iridium oxide film pH micro-sensors. Their maximum rate of potential change was determined to depend inversely on the oxide films thickness and varied from 34 V/s for 40 nm-thick films to 9 V/s for the 650 nm-thick iridium oxide films. The capacitances of these pH micro-sensors were determined to depend linearly on their thicknesses, with a slope of 3 F cm^–2 nm^–1 and a specific capacitance of 30 F/cm³. These results point to slow proton transport in the bulk of the oxide film as a mechanistic element of their pH response.Dedicated to Zbigniew Galus on the occasion of his 70th birthday.