Valinomycin-based membranes

Valinomycin-based membranes (Table 2.4) have been the predominant choice for potassium sensing in soil and other environmental samples. Considerable research effort has focused on improving the adhesion of the PVC membrane to extend the consistent sensitivity period, and thus, the lifetime of the electrode. Valinomycin ionophores have exhibited strong K selectivity and sensitivity sufficient to quantify variations in the typical range in soil K where additional fertilizer is recommended. ... 

Without question, sodium and potassium have been the analytes receiving the most attention in conjunction with the development of new analyzers. Almost all instruments on the market utilize the potassium-selective membrane system based on the antibiotic valinomycin in a PVC membrane matrix. For blood measurements, such a membrane is quite adequate. However, in undiluted urine samples, a negative error in the measurement of potassium has been reported (KIO). Apparently, this interference comes from a negatively charged lipophilic component of the urine which can partition into the PVC membrane, reducing the membrane potential (i.e., the membrane is not permselective). Fortunately, this problem can be overcome by incorporating the valinomycin in a silicone rubber-based membrane matrix (A4) into which the unknown anionic component apparently has a less favorable partition coefficient. 

The purpose of the reference barrel is to allow for the simultaneous measurement of the cell membrane potential while measuring the intracellular ion activities with the ISE portion of the device. Tims, in practice, a second single-barrel reference micropipet electrode is placed in the bathing solution outside the cell so that the potential between the two KCl-filled electrodes can always be monitored to obtain the instantaneous cell membrane potential (E ). The potential of the liquid membrane ISE barrel can also be monitored versus the external reference electrode. In this manner, potential changes due to variations in the cell membrane potential can be taken into account when calculating the intracellular ion activities. Alternatively, only the potential difference between both barrels of the electrode could be monitored. This potential should only be dependent on the intracellular activity of the analyte ion (not affected by the cell membrane potential). For certain ion measurements, e.g., K using a valinomycin based liquid micropipet electrode, leakage of K+ from the reference barrel could present a problem. In such cases, the reference barrel and the outer reference pipet should be filled with a solution other than 3 M KCl. 

A4. Anker, P., Jenny, H., Wuthier, U., Asper, R., Ammann, D., and Simon, W., Determination of [K + ] in blood serum with a valinomycin-based silicone rubber membrane of universal applicability to body fluids. Clin. Chem. (Winston-Salem, N.C.) 29, 1447-1448... 

Fiedler U, Ruzicka J (1973) Selectrode—the universal ion-selective electrode Part VII. A valinomycin-based potassium electrode with nonporous polymer membrane and solid-state inner reference system. Anal Chim Acta 67 179-193... 

Although rum ammonia levels are not routinely measured, it is a useful indicator of Reye s syndrome and should be monitored in newborns at risk of developing hyperammonemia Ammonia is produced in many analytically useful enzyme reactions and the ammonium ISE has been used as the base sensor in several enzyme electrodes (see next section). In addition to valinomycin, other antibiotics such as the nonactin homalogs and gramicidins also behave as ionophores. The nonactin homolo were originally studied for their ability to selectively bind potassiiun ions It was then discovered that ammonium ions were preferred over potassium ions, and the selectivity coefficient Knh+ = 0.12 was reported. Since ammonia is present at fairly low levels in serum, this selectivity is not sufficient to to accurately measure NH4 in the presence of K. An extra measure of selectivity can be gained by using a gas permeable membrane to separate the ammonia gas from the sample matrix... 

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]. 

However, there seems to be some drawback in the solubility or dispersibility of ion-sensing material in silicone rubber. This is mainly because silicone rubber does not contain a large quantity of plasticizer as the membrane solvent, in which neutral carriers can be dissolved easily, unlike in plasticized-PVC ion-sensing membranes. This issue is serious, especially with silicone-rubber membranes containing neutral carriers that show high crystallinity. Valinomycin, a typical ionophore, seems applicable to silicone-rubber-based K" -selec-tive electrodes [7,8,12-14]. Conventional crown-ether-based neutral carriers are also quite soluble in silicone rubber. 

FIG. 7 Potential response of K -ISFETs based on valinomycin-encapsulated sol-gel-derived membranes fabricated with initial DEDMS/TEOS ratios of 1 (A), 2 (B), and 3 (O)- (From Ref. 27.)... 

Selectivity coefficients values for K - and Na -ISFETs with the optimized ion-sen-sing membranes encapsulating valinomycin and bis(12-crown-4) are summarized in Fig. 9. The selectivity coefficient for with respect to Na in the K -ISFET is 2 x 10 " and that for Na with respect to in the Na -ISFET is 3 x 10. The selectivity coefficient values are similar to those for the ISFETs and ion-selective electrodes with the previous membrane materials containing the same neutral carriers. The high sensitivity and selectivity for the neutral-carrier-type ISFETs based on sol-gel-derived membranes can last for at least 3 weeks. 

FIG. 8 Potential response of K+- and Na+-ISFETs based on optimized sol-gel-derived membranes of valinomycin ( ) and bis(12-crown-4) (O) with interlayer of polythiophene [27]. The initial DEDMS/TEOS ratio for sol-gel processing is 3. 

The sol-gel-derived membrane ISFETs based on bis(12-crown-4) and valinomycin were applied for Na+ and K+ assay in blood sera [27]. For reproducibility checking, the metalion assay was carried out ten times for each sample, using two different control sera. The ISFETs gave reliable data both in Na+ and K+ assay. The coefficient of variation ranges between 0.45 and 0.78%, thus indicating excellent reproducibility in the serum cation assay with the ISFETs of neutral-carrier-type sol-gel-derived membranes. The results for real blood sera for normal human for the cation assay showed relative errors within 2% for the Na assay and within 4% for the assay. 

Fig. 7.3. Correlation between the selectivity coefficient and the ratio of stability constants in water for a liquid membrane ISE based on valinomycin dissolved in nitrobenzene. (After Morf [151].)...

Of the K ISEs, the best properties are those of the electrode based on the ionophore valinomycin XXII [51,196], marked by high selectivity for potassium with respect to sodium. Esters of phthalic acid (9,10) are used as membrane solvents and it is preferable for the reasons given in section 3.3 if the membrane contains the potassium salt of hydrophobic anion XV or XVI [119, 166]. The ISE containing cyclic polyether XXV is useful for only some applications [183] because its selectivity for potassium with respect to sodium is much smaller than with the valinomycin ISE. 

Figure 5.25 — Flow-through ion-selective optrode based on a multilayer lipidic membrane prepared by the Langmuir-Blodgett method. (A) Cross-sectional view of the composite six-layer membrane (four layers of arachidic acid/ valinomycin covered by an arachidic acid and rhodamine dye bilayer). (B) Optical arrangement integrated with the sensor, which is connected to a flow system. LS light source Ml and M2 excitation and emission monochromator, respectively FI and F2 primary filters M mirror LB lipid-sensitive membrane in a glass platelet FC flow-cell A amplifier D display P peristaltic pump. (Reproduced from [107] with permission of the Royal Society of Chemistry).

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 major breakthrough in the development and routine application of PVC type ISEs was the discovery by Simon and co-workers that the neutral antibiotic valinomycin could be incorporated into organic liquid membranes (and later plasticized PVC membranes), resulting in a sensor with high selectivity for K over Na (Kk/ns = 2.5 X 10 ), The ISE based on valinomycin was the first example of a neutral carrier ISE and is extensively used today for the routine measurement of in blood. Figure 4-2 shows the response of the... 

Membrane Potential produced b Diffusion Potential. In 1973,Uribe and reported that addition of valinomycin and to chloroplasts enhanced ATP synthesis elicited by acid-base transition and attributed... 

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