Neutral carrier sensors

More recendy, two different types of nonglass pH electrodes have been described which have shown excellent pH-response behavior. In the neutral-carrier, ion-selective electrode type of potentiometric sensor, synthetic organic ionophores, selective for hydrogen ions, are immobilized in polymeric membranes (see Membrane technology) (9). These membranes are then used in more-or-less classical glass pH electrode configurations. 

NADH, 121, 122, 180 Nafion coating, 118, 123, 124, 126 Nanometer electrodes, 116, 128 Nernst equation, 3, 15, 80 Nernstian behavior, 143 Nernst Planck equation, 5 Neuronal sensors, 188 Neurotransmitters, 40, 116, 124 Neutral carrier electrodes, 154 Nickel, 123... 

The highly dispersible calix[4]arene neutral carriers can also improve the durability for neutral-carrier-type ion sensors. Time-course changes in both sensitivity (slope for Na+ calibration graph) and selectivity (selectivity coefficient for Na" " with respect to K+) were followed in the Na -ISFETs based on ion-sensing membranes of silicone rubber-(l), plasticized PVC-(l), and plasticized PVC-(2) (Fig. 2). Deterioration proceeded quite quickly in the Na -ISFETs of plasticized PVC-(2) both the Na+ sensitivity and selectivity... 

As neutral carriers for the chemical modification, 16-crown-5 and calix[4]arene derivatives possessing a triethoxysilyl group (7) and (8) were designed for Na sensors. Triethoxysilylethyl-16-crown-5(7) was then mixed with a silicone-rubber precursor for the membrane fabrication accompanying covalent bonding of the neutral carrier. Comparison of IR spectra before and after extraction of the nonbonded neutral carrier... 

Appropriate fabrication of sol-gel-derived membranes encapsulating neutral carriers such as valinomycin can afford an excellent type of neutral-carrier-type ion-sensing membranes for ISFETs [27] as already mentioned. The simple encapsulation of neutral carriers in sol-gel-derived membranes, however, has a drawback the encapsulated neutral carriers are still apt to exude from the membranes into aqueous sample solutions, which thereby makes the resulting ion sensors less durable and more toxic. Incorporation of neutral carriers to sol-gel-derived membranes by covalent bonding is desirable. 

The design of bioeompatible (blood compatible) potentiometric ion sensors was described in this chapter. Sensing membranes fabricated by crosslinked poly(dimethylsiloxane) (silicone rubber) and sol gel-derived materials are excellent for potentiometric ion sensors. Their sensor membrane properties are comparable to conventional plasticized-PVC membranes, and their thrombogenic properties are superior to the PVC-based membranes. Specifically, membranes modified chemically by neutral carriers and anion excluders are very promising, because the toxicity is alleviated drastically. The sensor properties are still excellent in spite of the chemical bonding of neutral carriers on membranes. 

E. Bakker and E. Pretsch, Lipophilicity of tetraphenylborate derivatives as anionic sites in neutral carrier-based solvent polymeric membranes and lifetime of corresponding ion-selective electrochemical and optical sensors. Anal. Chim. Acta 309, 7-17 (1995). 

Fig. 20a. 10. Schematic of the sensing principle of a urea optical sensor based on an ammonium-sensitive membrane employing anionic dye and neutral carrier.

E. Wang, L. Ma, L. Zhu and C.M. Stivanello, Calcium optical sensors based on lipophilic dichlorofluorescein anionic dye and calcium organo-phosphate ionophore or neutral carriers, Anal. Lett., 30(1) (1997) 33-44. 

E. Wang, L. Zhu, L. Ma and H. Patel, Optical sensors for sodium, potassium and ammonium ions based on lipophilic fluorescein anionic dye and neutral carriers, Anal. Chim. Acta, 357 (1997) 85-90. 

Several classical ion-selective electrodes (some of which are commercially available) have been incorporated into continuous systems via suitable flow-cells. In fact, Lima et al. [112] used a tubular homogeneous crystal-membrane (AgjS or AgCl) sensor for the determination of sulphide and chloride in natural and waste waters. However, the search for new active materials providing higher selectivity and/or lower detection limits continues. Thus, Smyth et al [113] tested the suitability of a potentiometric sensor based on calix[4]arene compounds for use in flow injection systems. They found two neutral carriers, viz. methyl-j3-rerr-butylcalix[4]aryl acetate and... 

A plastic sodium membrane is now predominantly based on a neutral carrier (ETH 2120) that ensures sufficient sensitivity, selectivity and lifetime for the sensor. Some other compounds such as neutral carriers ETH 157, 227, 4120, calixarenes, crown ethers and hemisphe-rands have been proposed. Anionic influence observed during measurements in undiluted urine may be circumvented by dilution of the sample. 

The most widely used sensor for chloride ions in clinical analyzers is based on an ion-exchanger, a quaternary alkylammonium chloride, dispersed in a plastic membrane. It is not an ideal sensor due to the interference of lipophilic anions (e.g., salicylates, bromides) and lip-ophylic cations (e.g., bacteriostatic agents, anesthetics) and a relatively poor selectivity towards hydrogen carbonates (bicarbonates). However, compared to charged anion- and neutral carrier-based membranes that have been tested, it is still the best-suited for automated analyzers. 

The first representative of a potentiometric sensor was the pH-glass electrode invented in 1906 [35]. Decades of development resulted in the invention of many more ion-selective electrodes including more recently those based on neutral carrier membranes [36] and of the microelectronic fabricated ion selective field effect transistor (ISFET) [37]. 

Carrier based pH membranes (4-7) have traditionally required the addition of trapped, hydrophobic negative sites, typically tetraphenylborate (TPB) and p-chlorotetraphenyl borate (p-CITPB). In comparative studies we have frequently noticed the improved pH response of the membranes containing additional sites compared with those with only naturally occurring fixed sites, found in all the PVCs we have tested. Specifically, there is a distinctive deterioration in accuracy in the latter sensors at low pH. In addition, membranes prepared from aminated PVC with TPB have previously shown a good pH response (8). However, our preliminary impedance studies have shown that undoped aminated PVC membranes have a relatively low conductivity when compared with the neutral carrier designs above. 

Earlier sensors were made of plasticized PVC with neutral carriers TDDA (tridodecylamine) for H+ and valinomycin for K+. Accelerated deterioration tests of the proton sensor have been performed by heating TDDA in nitrogen, air and oxygen (9). Partially destroyed TDDA carriers have then been incorporated into the electrodes, and their responses were tested. Results demonstrated a deleterious effect of air oxidation on the response slopes. 

To confront these difficult tasks, we have incorporated old and new solutions. For example, strong yet flexible Kapton polyimide substrates are promising, and the neutral-carrier type ion-selective membranes offer the advantage of using similar fabrication methods for sensors of different ions of interest. We are also working on the biocompatibility problems, and all new designs are subject to in vivo tests. 

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... 

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