Elżbieta Malinowska

Potentiometric and spectroscopic characterization of anion selective electrodes based on metal(III) porphyrin ionophores in polyurethane membranes

Abstract The potentiometric anion responses of membrane electrodes formulated with In(III), Ga(III) and Co(III) metalloporphyrins as ionophores in polyurethane (PU) films are examined and compared to the response behavior found previously using conventional poly(vinyl chloride) (PVC) as the membrane matrix. A dimer–monomer equilibrium determined recently to occur in PVC membranes for In(III) and Ga(III) porphyrins is also observed to occur (via UV–VIS spectrophotometry) for these same porphyrins in the PU matrix. However, the equilibrium constants for dimer–monomer reactions appear to differ in PU membranes compared to PVC films, as determined from the degree of super-Nernstain responses towards target anions (chloride for In(III) and fluoride for Ga(III)), as well as the anion concentration ranges required to break the dimer as determined spectroscopically. Formation of dimeric species, and optimal potentiometric selectivities for membranes formulated with In(III) and Ga(III) porphyrins is dependent on the addition of exogenous lipophilic anion sites (e.g. potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) to the organic PU membrane. In the absence of such an additive, the presence of endogenous cationic sites in the PU prevents dimer formation, and decreases the anion selectivity of membranes prepared with the Ga(III) and In(III) species, which are known to function as charged carrier type anion ionophores. In the case of Co(III) porphyrins, which function as neutral carriers for nitrite anion, the presence of endogenous cationic sites in PU actually helps stabilize the formation of a negatively charged dinitrite complex, thereby enabling electrodes to yield nearly theoretical response to nitrite even in the absence of added cationic sites. This behavior is in sharp contrast to conventional PVC, where endogenous anionic site impurities will prevent anion (nitrite) response from occurring, unless significant amounts of exogenous lipophilic quaternary ammonium species are added to the membrane formulation.

Novel approach of immobilization of calix[4]arene type ionophore in 'self-plasticized' polymeric membrane

Abstract The design of ion-selective membranes based on the immobilization of the calix[4]arene tetraethyl ester containing at their wider rim one polymerizable group in a linear isodecyl acrylate (IDA)/methyl methacrylate (MM) copolymer is reported. Preliminary studies were focused on an optimization of polymeric matrix based on IDA/MM copolymers to make them suitable for preparing ion-selective membranes. The effect of polymer composition (IDA:MM ratio), matrix features (glass transition temperature and molecular weight) and compatibility with electro-active components was examined. It was found that the copolymer of IDA:MM=3:7 ratio exhibited the best mechanical properties. The feasibility of the application of IDA/MM matrix for ion-selective membranes was examined using as a model ionophore, 4- tert -butyl calix[4]arene tetraethyl ester. Electrodes based on these membranes showed near theoretical sensitivity towards sodium concentration and selectivity similar to reported for classical plasticized PVC-based membranes. Methacrylamide derivative of calixarene was immobilized in IDA/MM polymer of optimized composition. Potentiometric measurements revealed that the selectivity and the slope of the response curve obtained for electrodes employing membranes with immobilized calixarene are essentially the same as for the membranes containing a free ionophore, while the lifetime was improved from 2 to 6 months.

Lead-selective membrane electrodes based on neutral carriers. Part I. Acyclic amides and oxamides

A group of amides and oxamides has been used as potential ionophores for lead ions in solvent polymeric membrane electrodes. The most favourable analytical results (slope, log kPb,M and lifetime) were obtained with membranes incorporating ionophore ETH 295 and potassium tetrakis(p-chlorophenyl)borate. With the use of these membranes: (i) a linear response with a slope of 35.3 mV per pPb unit in PbII and an activity range of 10–5.2–10–1.0M were obtained; (ii) the selectivity factors (log kPb,M) were lower than –1.5 with respect to the ions studied (with the exception of Ag+ and H+ ions); (iii) the electrodes can be used with no change in their properties for at least 2 months.

Polymeric membrane electrodes with enhanced fluoride selectivity using Zr(IV)-porphyrins functioning as neutral carriers

Poly(vinyl chloride) polymeric membranes plasticized with o-NPOE (o-nitrophenyl octyl ether) or DOS (dibutyl sebacate) and containing Zr(IV)-octaethyl(OEP)- or Zr(IV)-tetraphenylporphyrins (TPP) along with lipophilic cationic additives (tridodecylmethylammonium chloride; TDMACl) are examined potentiometrically and optically with respect to their response toward fluoride. It is shown that these zirconium porphyrins can function as neutral anion carriers within the organic membranes of the electrodes. Spectrophotometric measurements of thin polymeric films indicate that the presence of lipophilic cationic sites in the form of TDMA(+) and use of lower dielectric constant plasticizer (DOS) prevents formation of metalloporphyrin dimers in the organic polymer phase, which have been observed previously in polymeric membranes formulated with the same Zr(IV) porphyrins but with lipophilic anion site additives. By preventing dimer formation, rapid and Nernstian potentiometric response of the corresponding membrane electrodes toward fluoride ion is observed. Indeed, electrodes prepared with PVC/DOS membranes containing Zr(IV)-OEP and 15mol% of TDMACl (relative to the ionophore) exhibit fast (t(95)<15s) and reversible response toward fluoride. The slope of calibration plots are near-Nernstian (-59.9mV per decade). Such electrodes display the following selectivity pattern: ClO(4)(-)>SCN(-)>F(-)>NO(3)(-)>Br(-)>Cl(-), which differs significantly from the classical Hofmeister series, with greatly enhanced potentiometric selectivity toward fluoride. The data presented herein, coupled with results from a previous study, confirm that Zr(IV) porphyrins can serve as either charged or neutral type anion carriers with respect to their enhanced interactions with fluoride when used as ionophores to prepare liquid-polymeric membrane electrodes, and that the nature of membrane additives and plasticizer dictates the response mechanism at play for given membrane formulations.

CMP(O) tripodands: synthesis, potentiometric studies and extractions

Ligand systems containing three carbamoylmethylphosphonate (CMP) or -phosphine oxide (CMPO) moieties attached to a tripodal platform have been synthesized for metal complexation and subsequent extraction from HNO3 solutions. The incorporation into ion selective electrodes (ISE) and picrate extractions with Na+, K+, Ag+, Ca2+, Cd2+, Hg2+, Pb2+, Cu2+, Eu3+ and Fe3+ shows that CMPO tripodand 3 is very selective for Eu3+ and forms a very stable complex (logML = 28.3). Liquid–liquid extractions performed with Eu3+ and Am3+ show reasonable extraction properties of the CMP(O) tripodands 3, 11 and 13 in 1,1,2,2-tetrachloroethane, while in 1-octanol for all tripodands studied the distribution coefficients are low. Upon addition of the synergistic agent hexabrominated cobalt bis(dicarbollide) anion (bromo-COSAN) the distribution coefficients for Am3+ and Eu3+ extraction increase considerably for CMP(O) tripodands 3 and 4. Covalently linked COSAN only enhances the extraction of Am3+ and Eu3+ at 0.001–0.01 M HNO3. The functionalization of dendrimer coated magnetic silica particles with CMP(O) tripodands led to very effective particles ( 31 and 32) for Am3+ and Eu3+ removal from 0.01 M HNO3 solutions.

Potentiometric response of magnesium-selective membrane electrode in the presence of nonionic surfactants

Abstract Although not normally present in biological samples, nonionic surfactants can be present in calibrating, rinse, and quality control solutions used within modern automated electrolyte analyzers. This paper describes the effect of nonionic surfactants containing poly(ethylene oxide) units (Brij 35 and Triton X-100) within these structures and alkyl- N -methylglucamide-based (MEGA) surfactants on the potentiometic response of neutral carrier-based magnesium-selective electrodes. Results are presented for plasticized PVC membranes doped with magnesium carrier ETH 7025 along with lipophilic anionic additives. Experiments were carried out in model electrolyte solutions with a composition similar to that of the blood serum. It is shown that the ion selectivities of magnesium-selective membrane electrodes can be affected greatly by the presence of Brij 35 or Triton X-100 in the sample. This phenomenon can be explained by the partitioning of nonionic surfactants into the membrane phase and the concomitant enhanced extraction of cations (mainly potassium and sodium) present in the sample phase. Experimental results show that the dramatic loss in selectivity for magnesium membrane makes it essentially impossible to measure magnesium ions by direct potentiometry when Brij 35 or Triton X-100 are present. The poisoning of the magnesium ISE in the presence of Brij 35 or Triton X-100 can be almost completely eliminated by changing the type of nonionic surfactant to one void of poly(ethylene oxide)-groups. It will be shown that the N -methylglucamide-based surfactants can be employed for this purpose.

Fluoride-selective polymeric membrane electrodes based on Zr(IV)- and Al(III)-salen ionophores of various structures.

Al(III)- and Zr(IV)-salophens of novel structures were tested as anion-selective ionophores. It was shown that these compounds are highly selective to fluoride and give selectivity greatly deviating from classical Hofmeister pattern, when doped into the polymeric membrane of ion-selective electrode (ISE). The following selectivity sequence has been recorded for both ionophores: F(-) > ClO(4)(-) > SCN(-) > NO(3)(-) approximately Br(-) approximately Cl(-). The results of potentiometric and spectrophotometric measurements allow to conclude that the nature and structure of salophen ligands influence stability of ISE working parameters. An increase in salophen ligands lipophilicity results in prolongation of the ISE lifetime, most likely due to slower ionophore decomposition caused by the hydrolysis of imine bonds in salophen structure. Ion-selective electrodes (ISEs) with the most successful Al(III)-salophen exhibited a stable, fast and near-Nernstian fluoride response and a functional lifetime near 3 weeks and selectivity coefficients with log K(F(-), Y(-))(pot) as follows: -2.8 (Y(-)=Br(-)), -2.7 (Cl(-)), -2.8 (NO(3)(-)), -1.5 (SCN(-)), -1.3 (ClO(4)(-)), which is better than for other ones based on Zr(IV)- and Al(III)-salophens and salens described to date.

Electrochemical oligonucleotide-based biosensor for the determination of lead ion

The possibility of utilization of gold electrodes modified with short guanine-rich ssDNA probes for determination of Pb(2+) was examined. Interaction between guanine residues and lead ion followed by formation of G-quadruplex structures was confirmed by electrochemical impedance spectroscopy investigations. An external cationic redox label, methylene blue, was employed in voltammetric measurements for analytical signal generation. It was shown that due to the G-quadruplex formation, the oligonucleotides in the recognition layer fold, which enhances the electron transfer between methylene blue and the electrode surface. The MB current signal rises proportionally to the lead ion concentration in the range from 0.05 to 1μmol/L. The developed biosensor demonstrated high selectivity towards Pb(2+) ion, with only minor response towards interfering metal cations. The calculated limit of detection was of 34.7nmol/L. The utilization of the biosensor for Pb(2+) determination in real samples of water was also tested.

Towards advanced chemical microsensors—an overview

The paper presents design and performance of miniaturized chemical sensors based on silicon transducers: ion-sensitive field effect transistor (ISFET) and solid-state electrode (SSE). The sensors were fabricated as back-side contact structures, which facilitate their mounting in a flow-cell. The role of an intermediate layer between the transducer and the ion-selective membrane is discussed. Various polymeric matrices were used to manufacture microsensors: polysiloxanes, polyacrylates (polymethacrylates), polyurethanes.

Design of miniaturized nitrite sensors based on silicon structure with back-side contacts

Already reported “solid-state” miniaturized ion-selective electrodes (ISEs) have been fabricated by casting polymer-based ion-selective membranes on metal electrodes patterned on a silicon wafer [1–3]. Silicon structures employed for this purpose possessed both: the sensing and electrical contact sites on the front side of the chip. Such sensors’ design requires an additional encapsulation layer that covers the electrical contact pads. Insufficient quality of this encapsulation layer might cause a solid problem when sensors are used in an electrolyte solution. In order to eliminate this problem, Potentiometric sensors with a back-side electrical contact, fabricated with silicon technology, have been designed and applied for the fabrication of miniaturized BSC solid-state sensors.