Polymer with ionophore

Erbach R, Hoffmann B, Schaub M and Wegner G (1992) Application of rod-like polymers with ionophores as Langmuir-Blodgett membranes for Si-based ion sensors. Sens Actuators B-Chem 6 211-216. 

Polymers with low glass transition do not require plasticizers. However, these compounds are often unpolar (Table 4) and, consequently, they are unsatisfactory solvents for polar ligands, ionophores, dyes and analytes. 

Similar but even more pronounced anionic effects were observed with liquid membranes based on ionophore 80 and exhaustively purified organic solvents (nitrobenzene or 1,2-dichloroethane) but no polymer matrix. The use of such membranes allows to minimize the concentration of ionic impurities that could take the role of anionic sites. For a membrane with ionophore 80, the SHG response... 

Chemisorption. A key step in the development of a successful membrane modified FET is the adherence and longevity of the membrane and the success of the encapsulation procedure. Sudholter et al (35) have proposed a method for the attachment of the ion selective membrane to a silylated SiC>2 gate oxide. An organofunctional silane, for example methacryloxypropyl trimethoxysilane, forms a bond between the surface and a photocrosslinkable polymer (eg polybutadiene). The ionophore would be directly bound to the polymer backbone, thus eliminating the need for plasticiser. Without any further modification with ionophore the membrane is sensitive to pH and shows a long life time (some months). Similar approaches have also been proposed by other workers, eg (36)... 

Table 23-4 lists some typical commercially available liquid-membrane electrodes. The anion-sensiiive electrodes contain a solution of an anion exchanger in an organic. solvent. As mentioned earlier, many of ihc so-cailcd liquid-membrane electrodes arc in fact solids in which the liquid is contained in a polymer (plastic) matrix. The first and most widely used polymer for membrane electrodes is PVC. but other materials have been used as well for compatibility with ionophores and fabrication materials. Polymer-based electrodes are somewhat more convenient to use and more rugged than the older porous disk electrodes. All electrodes listed in Tabic 23-4 are of the plastic-membrane type. 

A closely related group of LC substances is formed by iono-phoric LC polymers with side aown ether groups, which were first synthesized by Percec." The effect of ionophores is based on their ability to form complexes with cations of alkali and alkali-earth metals transported through biological membranes. This phenomenon opens fascinating possibilities for the creation of new synthetic anisotropic membranes and sensors that are based on ionophoric LC polymers. ... 

Potentiometric methods for the detection of phosphate based on polymer wire coated or membrane ion selective electrodes have until recently suffered from poor selectivity, sensitivity, and lifetime, and have been unsuitable for most water analysis applications (Table 8.2) [111,112]. Recent developments in membrane formulation involving PVC containing vanadyl salophen [113] or polystyrene-polybutadienene block polymers with the phosphate ionophore, 3-allyl-l,5,8-triazacyclodecane-2,4-dione [114], show much improved membrane lifetime, sensitivity, and selectivity for phosphate, and offer considerable promise for future water-monitoring applications. 

Cross, G.G., T.M. Fyles, and V.V. Suresh. 1994. Coated-wire electrodes containing polymer inunobilized ionophores blended with poly(vinyl chloride). Talanta 41 1589-1595. 

A composite polymer membrane has also been used as an effective amperometric detector for ion exchange chromatography [42] and showed detection limits similar to those obtained with a conductivity detector. An advantage of the amperometric detector based on micro-ITIES over the conductometric detector is that selectively can be tailored by proper choice of the ionophore. For instance, the selectivity of the membrane toward ammonium in the presence of an excess of sodium could be substantially increased by introducing an ammonium-selective ionophore (such as valinomycin) in the gel membrane [42]. 

The optical sensors are composed of ion-selective carriers (ionophores), pH indicator dyes (chromoionophores), and lipophilic ionic additives dissolved in thin layers of plasticized PVC. Ionophores extract the analyte from the sample solution into the polymer membrane. The extraction process is combined with co-extraction or exchange of a proton in order to maintain electroneutrality within the unpolar polymer membrane. This is optically transduced by a pH indicator dye (chromoionophore)10. 

The sensor layer consists of a selective ionophore (e.g. valinomycin for potassium), a lipophilic anionic site (borate) and the cationic PSD. Before interaction with potassium, a lipophilic ion pair between the cationic PSD and borate anion is formed in the polymer layer. When valinomycin (also contained in the layer) selectively extracts potassium into the layer, then the positively charged valinomycin-potassium complex forms an ion pair with... 

Solvent polymeric membranes, conventionally prepared from a polymer that is highly plasticized with lipophilic organic esters or ethers, are the scope of the present chapter. Such membranes commonly contain various constituents such as an ionophore (or ion carrier), a highly selective complexing agent, and ionic additives (ion exchangers and lipophilic salts). The variety and chemical versatility of the available membrane components allow one to tune the membrane properties, ensuring the desired analytical characteristics. 

The main classes of plasticizers for polymeric ISEs are defined by now and comprise lipophilic esters and ethers [90], The regular plasticizer content in polymeric membranes is up to 66% and its influence on the membrane properties cannot be neglected. Compatibility with the membrane polymer is an obvious prerequisite, but other plasticizer parameters must be taken into account, with polarity and lipophilicity as the most important ones. The nature of the plasticizer influences sensor selectivity and detection limits, but often the reasons are not straightforward. The specific solvation of ions by the plasticizer may influence the apparent ion-ionophore complex formation constants, as these may vary in different matrices. Ion-pair formation constants also depend on the solvent polarity, but in polymeric membranes such correlations are rather qualitative. Insufficient plasticizer lipophilicity may cause its leaching, which is especially undesired for in-vivo measurements, for microelectrodes and sensors working under flow conditions. Extension of plasticizer alkyl chains in order to enhance lipophilicity is only a partial problem solution, as it may lead to membrane component incompatibility. The concept of plasticizer-free membranes with active compounds, covalently attached to the polymer, has been intensively studied in recent years [91]. 

Size-related problems may become important for all microsensors. Leakage of sensing materials from a small membrane may lead to rapid deterioration of sensor properties [104], While the lipophilicity of membrane components cannot be increased infinitely, immobilization of ionophore and ion exchanger in the polymer by covalent attachment or molecular imprinting along with utilization of plasticizer-free membranes could help solve the leakage problem. 

The fourth type of mediator-based cation optical sensing is using potential sensitive dye and a cation selective ionophore doped in polymer membrane. Strong fluorophores, e.g. Rhodamine-B C-18 ester exhibits differences in fluorescence intensity because of the concentration redistribution in membranes. PVC membranes doped with a potassium ionophore, can selectively extract potassium into the membrane, and therefore produce a potential at the membrane/solu-tion interface. This potential will cause the fluorescent dye to redistribute within the membrane and therefore changes its fluorescence intensity. Here, the ionophore and the fluorescence have no interaction, therefore it can be applied to develop other cation sensors with a selective neutral ionophore. 

Lower detection limits for Ca21. Cd24. Ag. K+, Na, I. and OO4 ion-selective electrodes were demonstrated when concentrations were reduced in the internal filling solutions 26 A future improvement will come when the ionophore is dissolved in a conductive polymer in direct electrical contact with a metal conductor.27 This electrode entirely omits the inner filling solution. 

These devices are fabricated by doping a polymer membrane with sensing ingredients such as an ionophore which selectively binds a targeted ion and an ion exchanger which attracts a fixed concentration of analyte and preserves charge balance into the membrane phase... 

The membrane used to activate this potassium-selective IWAO [134] consists of a potassium bulk optode based on 0.5 wt % chromoionophore ETH 5294, 1.0 wt% ionophore valinomycin, 0.5 wt% ionic additive potassium tetrakis(4-chlorophenyl)borate (KtpClPB), 31.0 wt % polymer PVC, 67.5 wt % organic solvent and plasticizer bis(2-ethylhexyl)sebacate (DOS) [142], This commercially available optode not only acts as an example of the development of an enhanced ion-selective IWAO, but also serves to validate the previously remarked features, because results can be compared with the ones obtained with membranes of the same composition and thickness in a con-... 

A more sophisticated class of optical sensors with high selectivity towards ions are the ion-selective optodes (ISOs) [21], where the matrix (hydrophobic polymer such as PVC) contains a selective lipophilic ionophore (optically silent), a chromoionophore, a plasticizer and an anionic additive. The measurement principle is based on a thermodynamic equilibrium that controls the ion exchange (for sensing cations) or ion coextraction (for sensing anions) with the sample. The source of optode selectivity is a preferential interaction between the target ion and an ionophore. For a dye to act as a chromoionophore, it must... 

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