Electrodes proton-sensitive glass

Fig. 4. Schematic design of a sterilizable pH electrode made of glass. The pH-sensitive glass which develops a gel layer with highest mobility for protons is actually only the tip (calotte) of the electrode. Electrolytes can contain gelling substances. Double (or so-called bridged) electrolyte electrodes are less sensitive to poisoning of the reference electrode (e.g. formation of Ag2S precipitates)...

The dry glass core in a pH electrode exhibits solid electrolytic conductance, not semiconductive. The electrode is proton sensitive, but the small conductivity in the glass core stems from Li" ", Na" ", and K, which have mobilities 10 —10" larger than the protons. However, water is absorbed in the leached surface layers of the glass, and there the proton mobility is high and contributes to local DC conductance in an important way. 

To date, the majority of enzyme-based potentiometric sensors do not involve detection with an ionophore-doped selective membrane and fall outside of the scope of this chapter. The same is also true for most Severinghaus-type gas sensors, where a gas-permeable membrane covers an inner solution in which the gaseous analyte is determined with an ISE. Most Severinghaus-type electrodes use a pH-sensitive glass electrode to monitor the pH of this inner filling solution. However, ammonia has been detected indirectly with an ammonium-selective ionophore-based ISEs upon protonation in that inner solution, and the use of other ionophore-based ISEs for the more selective detection both in enzyme-based and Severinghaus-type ISEs is readily conceivable. 

The pH electrode is another suitable transducer for the construction of a urea biosensor. The classical glass electrode is sensitive to H ions and urease is attached in a gel of either polyacrylamide [107] or methacrylamide-aoylamide cqxdymer [108]. me metallic electrodes are also sensitive to H ions (for example, the antimony electrode) and can also be used in conjunction with a urease membrane [109]. The enzyme pH electrode detects a very weak variation in proton concentration arising from an enzymatic reaction, and the signal amplitude is determined by the buffering capacity of the solution. Both the nature of the buffer solution and the working pH value of the biosensor can reduce its practical use. A differential measurement at different pH values can be used to correct for any variation. 

The sensor is an ammonium ion-selective electrode surrounded by a gel impregnated with the enzyme mease (Figme 6-11) (22). The generated ammonium ions are detected after 30-60 s to reach a steady-state potential. Alternately, the changes in the proton concentration can be probed with glass pH or other pH-sensitive electrodes. As expected for potentiometric probes, the potential is a linear function of the logarithm of the urea concentration in the sample solution. 

The electrodes used in the above studies were double-barreled glass pH sensitive microelectrodes, and the spatial retinal pH profile was recorded by withdrawing the microelectrode tip at a rate of 1 //m/s or lOOpm/step across the retina in vivo or in vitro. In a typical retina pH profile (Fig. 10.9), measured in cat retina by the microelectrode, started from the choroids (Ph = 7.41, at distance Ojum). The pH steadily decreased to a minimum value (a maximum [H+] concentration) in the proximal portion of the outer nuclear layer (pH = 7.14 at —140 jum), then increased to —7.28 (at —310 pm) at the vitreous retinal border. The peak [H+] concentration in this layer indicated that a net production of proton occurred across the avascular outer retina [76],... 

Proton flux across lipid bilayers can be measured by a variety of techniques. For example, a buffered pH gradient can be established across liposome membranes, and the rate of decay of the gradient can be monitored by any of several methods. Early measurements were carried out by monitoring pH shifts in the external medium with a glass electrode (1) later measurements used pH-sensitive dyes such as pyranine, carboxyfluorescein, and 9-aminoacridine (4-6). Cafiso and Hubbell (7) used spin labels very effectively, and Perkins and Cafiso (8) conducted an extensive series of measurements with this system. 

Fundamentals. Based on the functional principles of the scanning electrochemical microscope, other scanning probe methods used to determine localized surface properties of the electrode under investigation or of the solution phase adjacent to this surface have been developed utilizing suitable microelectrodes. A pH-sensitive microelectrode based on a glass capillary filled with a pH-constant buffer solution and containing an internal reference electrode that has a tip filled with a proton-selective ionophor cocktail is scanned across the surface. The potential of the internal reference electrode with respect to an external reference electrode is directly correlated to the local pH value. A schematic cross section of this microelectrode is shown in Fig. 7.18. 

Glass electrodes sensitive to proton concentration were first introduced in 1909 and have long been the generally accepted way of determining pH. Similar electrodes which respond selectively to other ions are a much more recent development dating back only to the mid-1960s even so, ion-selective electrodes now have many applications in water and environmental analysis, for example the determination of pH, F, CN, NH3 and total hardness (Ca + Mg ). 

The pH dependence of the first flash proton uptake (Fig. 3B) coincides well with the data of McPherson et al. [8], also obtained with a glass electrode for Rb. sphaeroides R-26 RCs (see dashed line), but differs from the data of Maroti and Wraight [7], obtained using pH-indicators and by a conductivity method. The reasons for this discrepancy are unclear, but the binary oscillations are very sensitive to the experimental conditions, including quinone, exogenous donor, intensity of the flash, dark adaptation of the sample, and, as shown here, salt concentration. 

The catalytic hydrolysis of each molecule of these compounds releases two protons, the measurement and correlation of which to the OP concentration forms the basis of a potentiometric enzyme electrode. The basic element of this very simple enzyme potentiometric electrode is a pH electrode modified with an immobilized purified organophosphorus hydrolase (OPH) layer formed by cross-linking OPH with bovine serum albumin and glutaraldehyde. Thus, potentiometric OP biosensors were prepared by coupling OPH and a glass pH electrode. The sensors were constructed by immobilizing OPH on the surface of a pH-sensitive... 

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