Potentiometric Electronic Tongue for Quantitative Ion Analysis in Natural Mineral Waters.

The present paper addresses the development and use of a new potentiometric electronic tongue for both qualitative and quantitative characterization of natural mineral waters. The electronic tongue is particularly related to the conductivity and ion content of/in the water sample. The analytical system is based on six ion-selective electrodes whose membranes are formulated to provide either cationic or anionic response and considering plasticizers with different dielectric constants (bis(2-ethylhexyl) sebacate, 2-nitrophenyl octyl ether or tricresylphosphate), while keeping the polymeric matrix, i.e., poly(vinyl chloride). Notably, the absence of any ionophore in the membrane provides a general response profile, i.e., no selectivity toward any special ion, which is convenient for the realization of an effective electronic tongue. The dynamic response of the tongue toward water samples of different chemical compositions and geographical locations has been obtained. At the optimized experimental conditions, the tongue presents acceptable repeatability and reproducibility (absence of hysteresis). The principal component analysis of the final potential values observed with the six electrodes allows for the differentiation and classification of the samples according to their conductivity, which is somehow related to the mineralization. Moreover, quantitative determination of the six main ions in the water samples (i.e., chloride, nitrate, hydrogen carbonate, sulfate, sodium, calcium, and magnesium) is possible by means of a simple linear calibration (and cross-validation) model.

A sustainable amperometric biosensor for the analysis of ascorbic, benzoic, gallic and kojic acids through catechol detection. Innovation and signal processing.

In this work, we present a new catechol amperometric biosensor fabricated on the basis of naturally available enzymes in common mushrooms. The biosensor response mechanism comprises the reduction of the quinone exclusively produced in the oxidation of the catechol present in the sample, which is catalyzed by tyrosinase enzyme. The new catechol biosensor has demonstrated excellent analytical performance at increasing catechol concentrations in the sample solution, which includes superior reproducibility for several electrodes and long-term stability. On top of that, the biosensing element used in the fabrication is a sustainable material, of low-cost and presents an excellent lifetime of years. Whether the catechol biosensor is operating in the presence of a compound influencing the reactions underlying the amperometric response (such as ascorbic, benzoic, gallic and kojic acids), this serves as an analytical platform to detect these compounds in real samples. Particularly, we introduce herein for the first time different treatments to process the current signal of the biosensor pursuing the linearity needed for the analytical application in real samples. In this sense, the catechol biosensor has been successfully applied to the detection of benzoic, gallic and kojic acids in juices, teas and cosmetic products, respectively.

Binding studies and anion-selective electrodes with neutral isophthalamide-based receptors.

Two acyclic isophthalamide-based hosts have been synthesised and their anion binding properties have been evaluated by (1)H-NMR titrations. Different binding modes have been detected for the series of tested anions. The attachment of aminomethylpyrrole groups resulted in an improved binding selectivity. Additionally, the receptors have been incorporated as ionophores in plasticised polymeric membrane-based anion-selective electrodes. The potentiometric studies were in agreement with the NMR experiments and revealed a good sensing ability, considering the structural simplicity of the receptors and their interactions purely based on hydrogen bonding. These preliminary experiments have revealed an interesting selectivity towards highly hydrophilic anions such as fluoride and sulfate. Moreover, a particularly low detection limit (9 × 10(-7) M) has been determined for the fluoride anion.

Kinetic effects of the complexation reaction in the facilitated ion transfer at liquid membrane systems of one and two polarized interfaces. Theoretical insights.

An in-depth study of the ion transfer facilitated by complexation in the organic phase (TOC mechanism) in liquid membrane systems of one and two polarized interfaces is carried out by taking into account the kinetic effects associated with the complexation reaction. Explicit analytical equations for the normal pulse voltammetric (I/E) and chronoamperometric (I/t) responses with an explicit dependence on the kinetic parameters of the chemical complexation are presented for both kinds of membrane system, which could be useful for modeling artificial and biological membranes. The equations are compared with those obtained by using the widely used approximation of total equilibrium conditions that leads to the transfer by interfacial complexation mechanism (TIC), which only depends on thermodynamic parameters. Simple methods are proposed that allow quantitative determination of the equilibrium and kinetic constants of the complexation reaction in the organic phase for both kinds of membrane system.

Use of a new ziprasidone-selective electrode in mixed solvents and its application in the analysis of pharmaceuticals and biological fluids.

The construction and characterization of a new ion-selective electrode for the determination of the antipsychotic ziprasidone in mixed solvents is presented. The electrode contains a plasticized polymeric membrane based on a ziprasidone-tetraphenylborate ion-exchanger. The influence of membrane composition on the electrode response towards ziprasidone in hydroalcoholic solutions was studied. The electrode displayed a stable response in a 2:3 (v/v) methanol/water medium from a ziprasidone concentration of 3 × 10(-6) M with a fast response time of less than 20 s. The electrode also showed good selectivity towards ziprasidone over common inorganic and organic compounds and several species with pharmacological activity. The electrode was successfully applied to the determination of ziprasidone in pharmaceuticals and human urine and serum.

Considerations about the Continuous Assay Methods, Spectrophotometric and Spectrofluorometric, of the Monophenolase Activity of Tyrosinase.

With the purpose to obtain the more useful tyrosinase assay for the monophenolase activity of tyrosinase between the spectrofluorometric and spectrophotometric continuous assays, simulated assays were made by means of numerical integration of the equations that characterize the mechanism of monophenolase activity. These assays showed that the rate of disappearance of monophenol (VssM,M) is equal to the rate of accumulation of dopachrome (VssM,DC) or to the rate of accumulation of its oxidized adduct, originated by the nucleophilic attack on o-quinone by a nucleophile such as 3-methyl-2-benzothiazolinone (MBTH), (VssM, A-ox), despite the existence of coupled reactions. It is shown that the spectrophotometric methods that use MBTH are more useful, as they do not have the restrictions of the L-tyrosine disappearance measurement method, of working at pH = 8 and not having a linear response from 100 µM of L-tyrosine. It is possible to obtain low LODM (limit of detection of the monophenolase activity) values with spectrophotometric methods. The spectrofluorimetric methods had a lower LODM than spectrophotometric methods. In the case of 4-hydroxyphenil-propionic acid, the LODM obtained by us was 0.25 U/mL. Considering the relative sensitivities of 4-hydroxyanisole, compared with 4-hydroxyphenil-propionic acid, LODM values like those obtained by fluorescent methods would be expected.

Physical insights of salt transfer through solvent polymeric membranes by means of electrochemical methods.

A combined voltammetric study of the joint transfer of the two constituting ions of a water-soluble salt has been carried out using normal-pulse voltammetry, linear-sweep voltammetry and square-wave voltammetry in a system with two liquid-liquid polarized interfaces. As a result, we have explained the voltammetric features that allow us to distinguish this uptake from that corresponding to two equally charged ions, in spite of the appearance in both situations of two current peaks with the same sign in both square-wave and linear-sweep voltammograms, and we have found that linear-sweep voltammetry and square-wave voltammetry complement each other excellently.A theoretical comparison with a system of a single polarized interface has also been made, showing that these systems are much less appropriate for characterizing these salt-ion transfers.

Development of membrane selective electrode for determination of the antipsychotic sulpiride in pharmaceuticals and urine.

The construction and electrochemical response characteristics of a poly(vinyl chloride) (PVC) membrane selective electrode for the determination of sulpiride (SPD) are described. The sensing membrane comprised an ion-exchanger formed between the protonated drug and tetraphenylborate (TPB(-)) in a plasticized PVC matrix. The influence of membrane composition on the electrode response was studied. The electrode showed a fast, stable and Nernstian response over a sulpiride concentration range (1 × 10(-4) - 1 × 10(-2) M) with a mean slope of 58.4 ± 0.9 mV dec(-1) of concentration, a mean detection limit of 4.2 × 10(-5) ± 1.2 × 10(-5) M, a wide working pH range (2 - 8) and a fast response time (< 15 s). The electrode showed good selectivity towards sulpiride with respect to some inorganic and organic compounds. When the electrode was applied to the determination of sulpiride in pharmaceuticals and human urine, a high percentage of recovery was attained with no need for sample pretreatment procedures because of the lack of interfering matrix effects.

Flow-Injection Coulometric Detection Based on Ion Transfer and Its Application to the Determination of Chlorpromazine.

A flow-injection coulometric method for the determination of chlorpromazine based on ion transfer into a plasticized poly(vinyl chloride) (PVC) membrane, was developed. The detector used consists of a flow-through cell that incorporates a plasticized poly(vinyl chloride) (PVC) membrane which contains tetrabutylammonium tetraphenylborate as electrolyte. The membrane is located between the flowing solution and an inner aqueous electrolyte solution. Two pairs of electrodes, each pair formed by a reference electrode and a working electrode, are used, one pair in each solution. The potential between the reference electrodes was controlled by a four-electrode potentiostat with ohmic drop compensation. A potential step capable of producing the transfer of the chlorpromazine ion into the membrane was applied during the passage of a wide portion of sample plug through the cell and the corresponding quantity of the electricity was measured. In the selected conditions, a linear relationship was observed between the quantity of electricity and chlorpromazine concentrations over a range of 1x10-6 -1x10-4 M. The detection limit was 2 x 10-7 M. Good repeatability and between-day reproducibility was obtained. No interference was observed on the part of some common ions and pharmaceutical excipients. The method proposed was applied satisfactorily to the determination of chlorpromazine in pharmaceuticals and human urine.

Comparative enzymatic studies using ion-selective electrodes. The case of cholinesterases.

The application of traditional ion-selective electrodes for comparative enzymatic analysis was demonstrated for the first time in this study. A kinetic-potentiometric method based on the monitoring of the concentration of the ionic substrate involved in the enzymatic reaction catalyzed by different cholinesterases is used for this purpose. A comparative study was performed comprising both enzymatic assays using different ionic substrates and the corresponding inhibited reactions in presence of neostigmine (a synthetic anticholinesterase). The developed approach is used to obtain valuable comparative results through calculation of kinetic parameters, such as Michaelis and inhibition constants. Interesting results were obtained for acetylcholinesterase and butyrylcholinesterase enzymes, which were selected as proof-of-concept: (i) the binding affinity that these enzymes have for their natural substrates showed to be higher (acetylcholine and butyrylcholine respectively) than for their corresponding thiol derivatives (acetylthiocholine and butyrylthiocholine), which are traditionally used in spectrophotometric enzymatic assays; (ii) as expected, the maximum hydrolysis rate found in the assays of each enzyme was independent of the substrate used; (iii) acetylcholinesterase enzyme inhibition due to neostigmine was found to be higher (higher inhibition constant). Advantageously, the use of ion-selective electrodes permits to perform cholinesterases' enzymatic assays using their natural substrates and under physiological conditions, unlike the traditional spectrophotometric methods used in routine enzymatic assays. Importantly, while well-known enzymes are use throughout this work, this approach can be extended to other types of enzymatic assays as a tangible alternative to traditional spectrophotometric methods.

New carbazolo[1,2-a]carbazole derivative as ionophore for anion-selective electrodes: remarkable recognition towards dicarboxylate anions.

A new carbazolo[1,2-a]carbazole derivative was synthesized by expanding the binding cavity to explore the possibility of hosting larger anions such as dicarboxylate anions. The compound was incorporated as an ionophore into a membrane for an anion-selective electrode. The response of the electrode was evaluated for oxalate, malonate, succinate, glutarate and adipate in terms of calibration characteristics (slope, limit of detection and linear range of the response), response time, repeatability, reproducibility and selectivity. Nernstian reproducible responses, with very good detection limits, fast responses and selectivity not previously observed, were found for all the dicarboxylates anions, and the results were especially good in the case of glutarate. In order to obtain additional structural information about the complex formed between the ionophore and the dicarboxylate anions, (1)H NMR and fluorescence studies were carried out. The observed potentiometric selectivity depends on the good correspondence between the size of the carbazolocarbazole cavity and the length of the dicarboxylate anion, as supported by the NMR and fluorescence studies.

Rubber-based substrates modified with carbon nanotubes inks to build flexible electrochemical sensors.

The development of a solid-contact potentiometric sensor based on conducting rubbers using a carbon nanotubes ink is described here. Commercial rubbers are turned into conductive ones by a simple and versatile method, i.e. painting an aqueous dispersion of single-walled carbon nanotubes on the polymer surface. On this substrate, both the working ion-selective electrode and the reference electrode are built in order to form an integrated potentiometric cell. As a proof-of-principle, selective potassium electrodes are fully characterized giving comparable performances to conventional electrodes (sensitivity, selectivity, stability, linear range, limit of detection and reproducibility). As an application of the rubber-based electrodes, a bracelet was constructed to measure potassium levels in artificial sweat. Since rubbers are ubiquitous in our quotidian life, this approach offers great promise for the generation of chemical information through daily objects.

A SO2-selective electrode based on a Zn-porphyrin for wine analysis.

This work describes the assessment of a SO2-selective electrode based on the use of the neutral carrier 5,10,15,20-tetraphenyl(porphyrinate)zinc(II) in a PVC membrane plasticized with 2-nitrophenyl phenyl ether. After being conditioned in 2 mol L(-1) diethylamine solution for 24 h, the electrode exhibited selective anionic response toward the analyte in a concentration interval of more than four decades, with an slope of -59.5 mV dec(-1), a practical detection limit of 3.7×10(-6) mol L(-1) and a low limit of linear range of 7.2×10(-6) mol L(-1). The response mechanism is based on the displacement of the diethylamine:metalloporphyrin complex equilibrium within membrane bulk, inducing a variation in the cationic-sites to ionophore ratio. In turn, free hydroxyl ions are complexed by the displaced ionophore in a ratio 1:1 and translated as single negative charge nernstian response. Finally, the selectivity of the electrode is evaluated in view of its application to wine analysis. Results had high accuracy and precision when compared with a reference method.

Flow-injection potentiometric determination of triiodide by plasticized poly(vinyl chloride) membrane electrodes and its application to the determination of chlorine-containing disinfectants.

Plasticized poly(vinyl chloride) (PVC) membranes of different compositions were tested for use in the construction of potentiometric flow detectors for triiodide. A membrane with a 2:1 (w/w) 2-nitrophenyl octyl ether to PVC ratio was selected. The influence of thiosulphate in the carrier solution composition and of the flow-injection variables on the determination of triiodide was studied. In the selected conditions, a linear relationship between peak height and log[I(3)(-)] was obtained between 5 x 10(-6) and 1 x 10(-4)moll(-1) triiodide. Peak height relative standard deviations for 2 x 10(-5) and 1 x 10(-4)moll(-1) triiodide were +/-0.4 and +/-1.8%, respectively, and sampling frequency was 80 samples per hour. The method proposed was applied satisfactorily to the iodometric determination of different chlorine-containing disinfectants, among them trichloroisocyanuric acid and dichloroisocyanurate in several types of commercial sample.