Novel potentiometric copper (II) selective membrane sensors based on cyclic tetrapeptide derivatives as neutral ionophores

Two novel membrane sensors sensitive and reasonably selective for Cu²⁺ ions are described. These are based on the use of newly synthesized cyclic tetrapeptide derivatives as neutral ionophores and sodium tetraphenylborate (NaTPB) as an anionic excluder in plasticized PVC membranes. The sensors exhibit fast and stable near-Nernstian response over the concentration range 1.0 × 10⁻⁶ mol l⁻¹ to 1.0 × 10⁻² mol l⁻¹ Cu²⁺ with a cationic slope of 30.2-25.9 mV per decade at pH 4.5-7 with a lower detection limit of 0.05-0.13 μg ml⁻¹. Effects of plasticizers, lipophilic salts and various foreign common ions are tested. The sensors display long life-span, long term stability, high reproducibility, and short response time. Selectivity of both sensors is significantly high for Cu²⁺ over Fe³⁺, Al³⁺, Zn²⁺, Cd²⁺, Hg²⁺, Ni²⁺, Co²⁺, Mn²⁺, alkaline earth and alkali metal ions. The sensors are used for direct measurement of copper content in different rocks and industrial wastewater samples from electroplating factories. The results agree fairly well with data obtained using atomic absorption spectrometry.     

All-solid-state potentiometric sensors: A new wave for in situ aquatic research

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The HLD–NAC equation of state for microemulsions formulated with nonionic alcohol ethoxylate and alkylphenol ethoxylate surfactants

The net–average curvature (NAC) model (Langmuir, 19, 186, 2003) is a simple, yet powerful, equation of state to fit and predict the phase behavior of microemulsions (μEs) formulated with ionic surfactants. However, its use for nonionic surfactants systems has not been evaluated until now. The objective of this work is to use the NAC equation of state to model the phase behavior of μEs formulated with pure alcohol ethoxylate (CiEj) and commercial nonylphenol ethoxylate (NPEj) surfactants with a range of alkanes (Cn). The NAC model requires three basic parameters: the characteristic curvature of the nonionic surfactant (Ccn), the characteristic length (ξ), and the scaling length parameter (L). The first part of this contribution presents a brief review of the NAC model and other thermodynamic models of μEs. Later, five example applications of the NAC model are presented and discussed: (I) fit/prediction of phase inversion temperature (PIT) values, (II) prediction of phase transition temperatures and characteristic lengths, (III) fit/prediction of interfacial tensions, (IV) prediction of “fish” phase diagrams, and (V) prediction of the composition of bicontinuous systems. The NAC model was able to fit the phase behavior observed in all the experimental scenarios, and predict within 30% the value of phase volumes and transition temperatures for CiEj systems. The largest deviations can be attributed to a poor estimation of ξ obtained through empirical correlations. It was also noted that for commercial NPEj surfactants, the NAC model can also fit and predict the phase behavior of these systems, only that more complex empirical correlations are needed for ξ and Ccn due to the partition of different surfactant n-mers into the excess oil and aqueous phases. In all cases, a good agreement was found when the length parameter (L) was estimated as 1.4 times the extended length of the surfactant tail.     

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.     

Solid-contact potentiometric sensors based on ion pairs of cetylpyridinium with dodecylsulfate and tetraphenylboratc for various surfactants

Compositions of active membrane components for solid-contact potentiometric sensors selective to various surfactants were proposed. The stoichiometric ratios of components and the solubility and thermal stability of organic ion-exchangers were estimated. The main electroanalytical parameters of sensors were determined. Procedures for the potentiometric determination of nonionic surfactants in wastewater, cationic surfactants in hair rinses, and separate determination of nonionic and anionic surfactants in shampoos and synthetic detergents were developed. Presented at the V All-Russian Conference with the Participation of CIS Countries on Electrochemical Methods of Analysis (EMA-99), Moscow, December 6–8, 1999.     

Simultaneous potentiometric determination of cationic and ethoxylated nonionic surfactants in liquid cleaners and disinfectants

A sensitive potentiometric surfactant sensor based on a highly lipophilic 1,3-didecyl-2-methyl-imidazolium cation and a tetraphenylborate (TPB) antagonist ion was used as the end-point detector in ion-pair potentiometric surfactant titrations using sodium TPB as a titrant. Several analytical and technical grade cationic and ethoxylated nonionic surfactants (EONS) and mixtures of both were potentiometrically titrated.The sensor showed satisfactory analytical performances within a pH range of 3-10 and exhibited satisfactory selectivity for all CS and EONS investigated. Ionic strength did not influence the titration except at 0.1 M NaCl, in which a slight distortion of the second inflexion corresponded with the nonionic surfactant.Two-component combinations of four CS and three EONS were potentiometrically titrated using the sensor previously mentioned as the end-point detector. The quantities of the surfactants varied between 2 and 6 μmol for CS and 2.50 and 7.50 μmol for EONS. The known addition methodology was used for determination of the surfactant with considerably lower concentration in the mixture.Three commercial products containing cationic surfactants as disinfectants and nonionic surfactants were potentiometrically titrated, and the results for both type of surfactants were compared with those obtained with standard conventional methods.     

Silver(I)-selective membrane potentiometric sensor based on two recently synthesized ionophores containing calix[4]arene

Two new PVC membrane electrodes that are highly selective to Ag(I) ions were prepared using (L₁) calyx[4]arene (L₂) as two suitable neutral carriers. The silver(I) ion selective electrodes exhibit a good response for silver ion over a wide concentration range of 1.0 × 10⁻¹ to 4.2 × 10⁻⁶ M (L₁) and 1.0 × 10⁻¹ to 6.5 × 10⁻⁶ M (L₂) with a Nernstian slope of 60 mV per decade (L₁) and 58 mV per decade (L₂) at 25°C, and was found to be very selective, precise, and usable within the pH range 4.0–8.0. They have a response time of <15 s and can be used for at least 3 months without any measurable divergence in potential. The proposed sensors show a fairly good discriminating ability towards Ag⁺ ion in comparison to some hard and soft metal ions. The electrodes were used as indicator electrodes in the potentiometric titration of silver ion and in the determination of Ag⁺ in photographic emulsion and radiographic and photographic films. Published in Russian in Elektrokhimiya, 2009, Vol. 45, No. 7, pp. 862–868. The article is published in the original.     

Neutral bidentate organophosphorus compounds as novel ionophores for potentiometric membrane sensors for barium(II)

Bis(diarylphosphine oxide) naphthalene compounds are used as novel ionophores in plasticized poly(vinyl chloride) matrix membrane sensors for barium ions. The most favorable sensor was 1,2-bis(diethylphenylphosphine oxide)naphthalene containing potassium tetrakis(4-chlorophenyl)borate as lipophilic salt and o-nitrophenyloctyl ether as plasticizer for ion-selective electrode membrane construction. The electrode showed excellent properties. It gave a linear response with a Nernstian slope of 30 mV per decade within the concentration range 10(-1)-10(-5) mol L(-1) BaCl2. The electrode exhibits a high selectivity towards Ba2+ with respect to Li+, Na+, K+, Rb+, Cs+, NH4+, Ag+, Mg2+, Ca2+, Sr2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Al3+, La3+, and Ce3+ ions. The electrode response was stable over a wide pH range (3-11). The lifetime of the electrode was about 2 months. It was successfully applied to the determination of Ba2+ contents in some rocks.     

PVC membrane based potentiometric sensors for uranium determination

Two novel uranyl PVC matrix membrane sensors responsive to uranyl ion are described. The first sensor incorporates tris(2-ethylhexyl)phosphate (TEHP) as both electroactive material and plasticizer and sodium tetraphenylborate (NaTPB) as an ion discriminator. The sensor displays a rapid and linear response for UO₂²⁺ ions over the concentration range 1×10⁻¹–2×10⁻⁵ mol l⁻¹ UO₂²⁺ with a cationic slope of 25.0±0.2 mV decade⁻¹. The working pH range is 2.8–3.6 and the life span is 4 weeks. The second sensor contains O-(1,2-dihydro-2-oxo-1-pyridyl)-N,N,N′,N′-bis(tetra-methylene)uronium hexafluorophosphate (TPTU) as a sensing material, sodium tetraphenylborate as an ion discriminator and dioctyl phenylphosphonate (DOPP) as a plasticizer. Linear and stable response for 1×10⁻¹–5×10⁻⁵ mol l⁻¹ UO₂²⁺ with near-Nernstian slope of 27.5±0.2 mV decade⁻¹ are obtained. The working pH range is 2.5–3.5 and the life span of the sensor is 6 weeks. Interference from many inorganic cations is negligible for both sensors. However, interference caused by some ions (e.g. Th⁴⁺, Cu²⁺, Fe³⁺) is eliminated by a prior ion exchange or solvent extraction step. Direct potentiometric determination of as little as 5 μg ml⁻¹ uranium in aqueous solutions shows an average recovery of 97.2±1.3%. Application for the determination of uranium at levels of 0.01–1 wt.% in naturally occurring and certified ores gives results with good correlation with data obtained by X-ray fluorescence.     

Comparison of microbial sensors based on amperometric and potentiometric electrodes

Microbial sensors based on oxygenn and carbon dioxide electrodes coupled with immobilized Saccharomyces are compared for measurements of glucose and other carbohydrates. With the oxygen sensor, the yeast works under aerobic conditions but anaerobically with the carbon dioxide sensor. The two metabolisms of the same strain make little difference to the lifetimes (> 15 days), selectivities and response rates (5–10 min) of the sensors. The effects of pH are very different owing to the pH sensitivity of the carbon dioxide sensor. The viable concentration ranges overlap; the oxygen-based sensor is more useful for low concentrations of glucose (0.01–1 mmol l^?1) while the carbon dioxide-based sensor is better suited for 1–10 mmol l^?1. With the oxygen-based sensor, the response time is governed by the rate of metabolism; with the carbon dioxide-based sensor, the response time of the potentiometric carbon dioxide electrode is the rate determining step.     

Electronic tongue based on an array of metallic potentiometric sensors

An electronic tongue system based on the array of six metallic potentiometric sensors (metallic wires) was developed and utilized for discrimination of foodstuffs: several types of vinegar and fruit juices. Copper, tin, iron, aluminum, brass and stainless steel wires were included in the array and supplemented by pH glass electrode. The response of potentiometric metallic sensors towards various organic acids has been studied and possible sensitivity mechanisms were discussed. Overall potential changes of metallic sensors were exanimate as complex mixed signals influenced by several components presenting in analyte employing chemometric approach. The multisensor array of such a type can be useful for several applications since of simplicity in handling, low cost of sensors and easy measure procedure.     

Novel potentiometric sensors based on polysulfone immobilized metallothioneins as metal-ionophores

We show here the use of immobilized metal-binding biomolecules for metal analysis by using novel potentiometric sensors. To this end and as a model, Ag⁺-ISEs were developed using polysulfone matrix embedding metallothioneins as ionophores (mouse MT1 (P1) or sea urchin SpMTA (P2)). Polysulfone, a porous polymer that was not used until the present in potentiometric biosensors, has the advantage of being compatible with biological materials. Also, the phase inversion procedure allows protein incorporation into the membrane with minima alterations, since it always remains in the aqueous phase. Construction of these biosensors required small amounts of protein; they can be dry-stored and have long lifetimes. They exhibited linear responses with slopes of ca. 61 mV per decade within the 10⁻⁵ to 10⁻² M Ag⁺ concentration range, detection limits of about 10⁻⁵ M, and worked in the 2-to-8 pH range. Except for Hg²⁺, the Pb²⁺, Zn²⁺, Cd²⁺, Cu²⁺ cations do not interfere with Ag⁺ determination. Significantly, different affinities of Pb²⁺ and Zn²⁺ towards P1- and P2-ISE were found, in good correlation with the higher affinity of these cations towards SpMTA than to MT1. Consequently, the distinct metal-binding features of each MT are conserved and determine the differential properties of their biosensors. These results open a broad range of possibilities for the use of proteins as ionophores in what could be considered a new type of potentiometric biosensor if their response mechanism is taken into account.     

Determination of cationic surfactants in pharmaceutical disinfectants using a new sensitive potentiometric sensor

A new sensitive potentiometric surfactant sensor was prepared based on a highly lipophilic 1,3-didecyl-2-methyl-imidazolium cation and a tetraphenylborate antagonist ion. This sensor was used as a sensing material and incorporated into the plasticized PVC-membrane. The sensor responded fast and showed a Nernstian response for investigated surfactant cations: cetylpyridinium chloride (CPC), hexadecyltrimethylammonium bromide (CTAB) and Hyamine with slope 59.8, 58.6 and 56.8 mV/decade, respectively. The sensor served as an end-point detector in ion-pair surfactant potentiometric titrations using sodium tetraphenylborate as titrant. Several technical grade cationic surfactants and a few commercial disinfectant products were also titrated, and the results were compared with those obtained from a two-phase standard titration method. The sensor showed satisfactory analytical performances within a pH range of 2-11, and exhibited excellent selectivity performance for CPC compared to all of the organic and inorganic cations investigated. The influence of the nonionic surfactants on the shape of titration curves was negligible if the mass ratio of ethoxylated nonionic surfactants and cationic surfactants (EONS:CS) was not greater than 5.     

Potentiometric sensors based on fluorous membranes doped with highly selective ionophores for carbonate

Manganese(III) complexes of three fluorophilic salen derivatives were used to prepare ion-selective electrodes (ISEs) with ionophore-doped fluorous sensing membranes. Because of their extremely low polarity and polarizability, fluorous media are not only chemically very inert but also solvate potentially interfering ions poorly, resulting in a much improved discrimination of such ions. Indeed, the new ISEs exhibited selectivities for CO(3)(2-) that exceed those of previously reported ISEs based on nonfluorous membranes by several orders of magnitude. In particular, the interference from chloride and salicylate was reduced by 2 and 6 orders of magnitude, respectively. To achieve this, the selectivities of these ISEs were fine-tuned by addition of noncoordinating hydrophobic ions (i.e., ionic sites) into the sensing membranes. Stability constants of the anion-ionophore complexes were determined from the dependence of the potentiometric selectivities on the charge sign of the ionic sites and the molar ratio of ionic sites and the ionophore. For this purpose, a previously introduced fluorophilic tetraphenylborate and a novel fluorophilic cation with a bis(triphenylphosphoranylidene)ammonium group, (R(f6)(CH(2))(3))(3)PN(+)P(R(f6)(CH(2))(3))(3), were utilized (where R(f6) is C(6)F(13)). The optimum CO(3)(2-) selectivities were found for sensing membranes composed of anionic sites and ionophore in a 1:4 molar ratio, which results in the formation of 2:1 complexes with CO(3)(2-) with stability constants up to 4.1 × 10(15). As predicted by established theory, the site-to-ionophore ratios that provide optimum potentiometric selectivity depend on the stoichiometries of the complexes of both the primary and the interfering ions. However, the ionophores used in this study give examples of charges and stoichiometries previously neither explicitly predicted by theory nor shown by experiment. The exceptional selectivity of fluorous membranes doped with these carbonate ionophores suggests their use not only for potentiometric sensing but also for other types of sensors, such as the selective separation of carbonate from other anions and the sequestration of carbon dioxide.     

Novel nonionic surfactants based on sulfoxides. 2. Homo- and copolymers

The synthesis of new nonionic amphiphilic polymers of the polysoap type is described, as well as their general behaviour in aqueous solution. The polymers were prepared by homopolymerisation of acrylate polymerisable surfactants (or "surfmers"), which bear the nonionic sulfoxide moiety. Alternatively, small hydrophilic acrylate and acrylamide monomers bearing the nonionic sulfoxide moiety were copolymerised with dodecylacrylate and N-dodecyl- N-methylacrylamide. Compared to many other nonionic hydrophilic fragments, a single sulfoxide group behaves as a strongly hydrophilic fragment of small volume. However, its relative hydrophilicity depends sensitively on its precise positioning in the polymers. Properly placed, three sulfoxide groups can balance up to two dodecyl chains to obtain still water-soluble polymers that exhibit typical polysoap properties. Some of the new nonionic polymers show lyotropic liquid-crystalline behaviour at ambient temperature.     

Electrochemical sensors based on ionophores: Current state, trends, and prospects

The review considers ionophore-based electrochemical sensors (ion-selective electrodes). In general the progress in the field is subdivided in two distinctly different trends: (1) an extensive development (enhancing of the list of ions whose concentration can be selectively measured with the corresponding new sensors) and (2) an intensive development (improvement of the analytical capabilities of already known sensors). Recent articles (mostly published after the year 2000) describing the ionophore-based ion-selective electrodes are classified and briefly characterized from this viewpoint.     

Novel thiocyanate-selective membrane sensors based on di-, tetra-, and hexa-imidepyridine ionophores

Potentiometric thiocyanate-selective sensors based on the use of three synthesized di-, tetra-, and hexa-imidepyridine derivatives as novel anionic neutral ionophores in plasticized poly(vinyl chloride) (PVC) membranes are described. The sensors exhibit significantly enhanced response towards thiocyanate ions over the concentration range 5×10⁻⁶ to 1.0×10⁻² M with a lower detection limit of 0.3 μg ml⁻¹ and slopes ranging from −55.6 to −58.3 mV per decade. Fast and stable response, good reproducibility, long-term stability, applicability over a wide pH range (2-8) and high selectivity for SCN⁻ ion in the presence of 18 common anions are demonstrated. The sensors are used for direct potentiometric measurements of thiocyanate ions over the concentration range 0.2-580 μg ml⁻¹ and for monitoring sequential titration of some metal ions (e.g. Ag⁺, Tl⁺, Cu²⁺, Pb²⁺) in binary and ternary mixtures. Sequential binding of these metal ions with SCN⁻ ensures share stepwise titration curves with consecutive end point breaks at the equivalent points. Recoveries of 98.5-99.1±0.3% are obtained for metal ion concentrations of 0.06-4 mg ml⁻¹.     

Effects of nonionic surfactants on seeded polymerizations

The rate of polymerization of vinyl acetate in systems containing poly(vinyl acetate) seed particles was investigated. A significant reduction in rate was observed when nonionic surfactants were adsorbed onto the particles prior to polymerization. The molecular weight of the poly(ethylene oxide) moieties in the surfactants was varied over two decades. The diminution in rate was relatively insensitive to this molecular weight variation, at least for higher molecular weights (≥20,000). A minimum in the rate was apparent at lower molecular weights. No new nucleation was detectable in any of these systems. The results have been interpreted in terms of two properties of the dissolved poly(ethylene oxide) layers through which the oligomeric free radicals must diffuse in order to reach the major loci of polymerization inside the particles. These two properties, the effective thickness of the dissolved poly(ethylene oxide) layer and its average microscopic viscosity, are expected theoretically to be approximately self-compensating when the molecular weight is varied.     

Hydrolyzable nonionic surfactants: stability and physicochemical properties of surfactants containing carbonate, ester, and amide bonds

A linear and a branched nonionic cleavable surfactants containing a carbonate bond have been prepared from tetra(ethylene glycol) and an alkylchloroformate. The stability of these carbonate surfactants was determined by investigating their hydrolysis and biodegradability characteristics. The hydrolysis was catalyzed by alkali or enzymes (esterase from porcine liver and lipases from Mucor miehei and Candida antarctica B) and was monitored using 1H NMR. It was found that the stability toward alkali was higher for a carbonate surfactant than for a corresponding surfactant with an ester as weak bond. Biodegradation tests resulted in more than 60% degradation after 28 days for both carbonate surfactants. Physicochemical properties, such as critical micelle concentration (CMC), cloud point, area per molecule, and surface tension at the CMC, were determined and compared to those obtained from similar surfactants containing ester, amide, or ether bonds. It was found that the carbonate linkage is hydrophobic and that the oxycarbonyl part of the carbonate group is equivalent, in a formal sense, to an extra methylene group in the alkyl chain of the surfactant.