Potentiometric response

Fig. 9. Representation of the use of the modification of an ionophore s potentiometric response in order to detect antibody binding. A constant ion activity (in this case K ) must be maintained in the sample solution...

Figure 5 also shows the effect of the ionophore concentration of the Langmuir type binding isotherm. The slope of the isotherm fora membrane with 10 mM of ionophore 1 was roughly three times larger than that with 30 mM of the same ionophore. The binding constant, K, which is inversely proportional to the slope [Eq. (3)], was estimated to be 4.2 and 11.5M for the membranes with 10 mM and 30 mM ionophore 1, respectively. This result supports the validity of the present Langmuir analysis because the binding constant, K, should reflect the availability of the surface sites, the number of which should be proportional to the ionophore concentration, if the ionophore is not surface active itself In addition, the intercept of the isotherm for a membrane with 10 mM of ionophore 1 was nearly equal to that of a membrane with 30 mM ionophore 1 (see Fig. 5). This suggests the formation of a closest-packed surface molecular layer of the SHG active Li -ionophore 1 cation complex, whose surface concentration is nearly equal at both ionophore concentrations. On the other hand, a totally different intercept and very small slope of the isotherm was obtained for a membrane containing only 3 mM of ionophore 1. This indicates an incomplete formation of the closest-packed surface layer of the cation complexes due to a lack of free ionophores at the membrane surface, leading to a kinetic limitation. In this case, the potentiometric response of the membrane toward Li+ was also found to be very weak vide infra).

As shown in Fig. 6(a), the SHG response of membrane 2 to aqueous KSCN was found to be different from that to KCl. Upon increasing the KSCN concentration, the SHG signal initially increased but reached a maximum at 0.2 M, and then decreased. The potentio-metric response of the same membrane also exhibited a maximum at a K" " ion activity of ca. 0.1 M [see inset in Fig. 6(a)]. Thus, the decrease in the SHG and potentiometric responses were found to start roughly at the same KSCN concentration. This may be attributed to a decrease in the number of oriented K -ionophore 2 complexes at the interface with the appearance of SCN ions in the membrane. 

Use of ionophore-incorporated membranes leads thus to the same conclusions as described above for the ionophore-free membranes. Here too, the SHG measurements suggest that a permanent, primary ion-dependent charge separation at the liquid-liquid interface, and therefore a potentiometric response, is only possible when the membrane contains ionic sites. 

Fluhler E, Burnham VG, Loew LM (1985) Spectra, membrane-binding, and potentiometric responses of new charge shift probes. Biochemistry 24(21) 5749-5755... 

Wuskell JP, Boudreau D, Wei MD, Jin L, Engl R, Chebolu R, Bullen A, Hoffacker KD, Kerimo J, Cohen LB, Zochowski MR, Loew LM (2006) Synthesis, spectra, delivery and potentiometric responses of new styryl dyes with extended spectral ranges. J Neurosci Meth 151(2) 200-215... 

FIGURE 4.9 Potentiometric response of heparin (a) and protamine (b) selective electrodes after different 5min and 24h equilibration times, respectively. Sample solutions contained 0.1 M NaCl and 50mM TRIS (pH 7.40) [39]. 

E. Bakker, R.K. Meruva, E. Pretsch, and M.E. Meyerhoff, Selectivity of polymer membrane-based ion-selective electrodes — self-consistent model describing the potentiometric response in mixed ion solutions of different charge. Anal. Chem. 66, 3021—3030 (1994). 

R.A. Llenado, Potentiometric response of the calcium selective membrane electrode in the presence of surfactants ,... 

In some applications, silver/silver chloride or calomel electrodes are considered cumbersome to use and maintain. More importantly, they are extremely difficult to miniaturize particularly with regard to their combined use with potentiometric membrane electrodes (see Section 18a.4.5.4) that have been fabricated into highly miniaturized and compact screen-printed sensor arrays for clinical use. Thus, several reference electrodes are manufactured with the same polymeric materials that are needed to design the responsive ion-selective membranes [7]. Incorporation of suitable active agents into such membranes leads to potentiometric responses that are ideally independent of the sample... 

Redox-based biosensors. Noble metals (platinum and gold) and carbon electrodes may be functionalized by oxidation procedures leaving oxidized surfaces. In fact, the potentiometric response of solid electrodes is strongly determined by the surface state [147]. Various enzymes have been attached (whether physically or chemically) to these pretreated electrodes and the biocatalytic reaction that takes place at the sensor tip may create potential shifts proportional to the amount of reactant present. Some products of the enzyme reaction that may alter the redox state of the surface e.g. hydrogen peroxide and protons) are suspected to play a major role in the observed potential shifts [147]. 

Ever since an ISFET that was chemically modified by a valinomycin-containing PVC membrane was reported [141], there has been general consensus on the advantages of this type of microsensor over conventional ISEs. Some serious problems have also been acknowledged, though e.g. the low mechanical stability of the membranes, the interference of COj in the potentiometric response, the lack of a stable micro-reference electrode and the relatively high drift rate of ISFETs). Attachment of the membrane can... 

The existence of the nitryl ion in the presence of acidic substances suggested that nitrate ion in fused alkali nitrates might dissociate into N02+ and 0 2 ions (5). To determine the extent of the dissociation, it was necessary to develop an electrode potentiometrically responsive to either N02+ or 0 2. The only possibility for a reversible N02+ electrode that came to mind was nitrogen dioxide gas bubbling over platinum. This electrode did respond to N02+ in acidic solutions, but as ex-... 

Figure 6. Potentiometric responses to anions by o-NPOE/PVC (2 1 wt/wt) membranes containing (a) 1 wt% receptor 15 and 55 mol% of cationic site 9 (relative to the receptor), and (b) 6 wt% of cationic site 9 (without receptor). Measured at pH 6.8 (0.1 M HEPES-NaOH buffer).

Electrodes based on 9 but no nucleobase derivative [3.0 wt% 9 bis(2-ethylhexyl) phthalate ( dioctyl phthalate , DOP) as the membrane solvent] showed similar potentiometric responses to 5 -GMP and 5 -AMP (Figure 8a), which is not surprising because cation 9 cannot interact specifically with the base pairing site of nucleotides. The EMF slope (-29 mV decade" 0.1 M HEPES-NaOH buffer solution, pH 6.8) was much greater than in case of the electrode based on the macrocyclic pentaamine 1 (-15 mV decade" ) and corresponds to the slope as expected for a dianion according to the Nemstian equation. Extraction experiments confirmed that at this pH it is indeed the dianion that enters the organic phase. 

Electrodes based on neutral cytosine derivative 10a (1.3 wt%) and 9 (150 mol% relative to 10a) gave EMF slopes of-29 mV decade" for both 5 -GMP and5 -AMP (Figure 8b), suggesting that these electrodes respond in a Nemstian manner to nucleotides in their divalent form, just as in the case of the anion exchanger electrode. The potentiometric response was selective for 5 -GMP over 5 -AMP with... 

Potentiometric responses to cationic guests by neutral hosts can be interpreted on the basis of charge separation across the membrane interface with cationic host-guest complexes at the membrane side and hydrophilic counteranions at the aqueous side (Figure 3a). The amount of the cationic complexes at the membrane side of the interface is determined by the lipophilicity of the guest, the stability... 

Figure 14. Potentiometric responses to simple amine guests (33-37) by PVC matrix liquid membranes, (a) Membrane based on calix[6]arene hexaester (29 (R = H)/DOS/PVC = 5 68 27 wt%). (b) Membrane based on dibenzo-18-crown-6 (32/DOS/PVC = 2 66 32 wt%). Measured in 0.1 M Tris-HCI buffer (pH 7.0) at room temperature (ca. 20 °C) (reproduced with permission of American Chemical Society from Anal. Chem. 1993, 65,1079).

Figure 20. Potentiometric responses to anionic guest 78 by PVC matrix liquid membranes containing cyclophanes (74, 76,77) or acyclic reference compound (75). (a) Membrane potential as a function of pH in the presence ( ) or absence (o) of 1.00 X 10 M guest, (b) Membrane potential as a function of guest concentration at pH 5.0 (0.01 M AcONa-AcOH buffer) and room temperature (ca. 20 °C). Membrane composition DOS (95 mg) and PVC (44 mg) containing 6.2 x 10 mmol host.

Figure 24, Potentiometric responses toward RbCI of membrane 89 with various concentrations of ionophore 89 under UV and visible light irradiation. Membrane composition DBP/PVC = 2 1 wt/wt containing (a) 1 mM, (b) 0.1 mM, and (c) 0.01 mM of 89, respectively.

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