Urea electrode

3 Urea Electrodes The physiologically important substrate urea can be sensed based on the following urease-catalyzed reaction  

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. 

Enzyme electrodes for other substrates of analytical significance have been developed. Representative examples are listed in Table 6-1. Further advances in enzyme technology, and particularly the isolation of new and more stable enzymes, should enhance the development of new biocatalytic sensors. New opportunities (particularly assays of new environments or monitoring of hydrophobic analytes) derive from the finding that enzymes can maintain then biocatalytic activity in organic solvents (31,32). 

The electrode is an ammonium ion-selective electrode surrounded by a gel impregnated with the enzyme urease [Fig. 6.13 (34)]. The generated ammo- 

Measured Species Enzyme Detected Species Type of Sensing Ref. 

Enzyme electrodes for other substrates of analytical significance have been developed. Representative examples are listed in Table 6.1. Further advances in enzyme technology, particularly the isolation of new and more stable 

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One example of an enzyme electrode is the urea electrode, which is based on the catalytic hydrolysis of urea by urease... 

In one version of the urea electrode, shown in Figure 11.16, an NH3 electrode is modified by adding a dialysis membrane that physically traps a pH 7.0 buffered solution of urease between the dialysis membrane and the gas-permeable... 

Another version of the urea electrode (Figure 11.17) immobilizes the enzyme in a polymer membrane formed directly on the tip of a glass pH electrode. In this case, the electrode s response is... 

Show that equation 11.12 for the urea electrode is correct. 

The pH-based urea electrode has the response shown in equation 11.13. Rewrite the equation to show the relationship between potential, E, and the concentration of urea. 

Such electrodes make use of an enzyme to convert the substance to be determined into an ionic product which can itself be detected by a known ion-selective electrode. A typical example is the urea electrode, in which the enzyme urease is employed to hydrolyse urea ... 

FIGURE 6-11 Urea electrode, based on the immobilization of urease onto an ammonium ion-selective electrode. 

Uranium, 85 Urea electrodes, 181 Urease, 181 Uric acid, 182... 

Among potentiometric enzyme sensors, the urea enzyme electrode is the oldest (and the most important). The original version consisted of an enzyme layer immobilized in a polyacrylamide hydrophilic gel and fixed in a nylon netting attached to a Beckman 39137 glass electrode, sensitive to the alkali metal and NHj ions [19, 2A Because of the poor selectivity of this glass electrode, later versions contained a nonactin electrode [20,22] (cf. p. 187) and especially an ammonia gas probe [25] (cf. p. 72). This type of urea electrode is suitable for the determination of urea in blood and serum, at concentrations from 5 to 0.05 mM. Figure 8.2 shows the dependence of the electrode response... 

Another urea electrode uses a carbon dioxide sensor covered with urease to measure the second product of the urea-urease reaction, HCOs. Na " and K" " had no influence on this electrode and the linear range was 0.1-10 mM (64). 

E524 Hirsch, C., Yu, G. and Dubinsky, E. (1989). Evaluation of the Nova-12 urea electrode. Clin. Chem. 35, 1100, Abstr. 163. 

George Guilbault introduced the potentiometric urea electrode [104]. 

Fig. 69. Schematic of the processes in an amperometric urea electrode based on the pH dependence of the anodic oxidation of hydrazine.

Fig. 70. Comparison of the signals of the amperometric urea electrode depicted in Fig. 69 (left axis) with those of a conventional pH glass electrode (right axis) on addition of sodium hydroxide to the measuring solution. (Redrawn from Kirstein et al., 1985b).

Hamann (1987) employed a potentiometric urea electrode in an enzyme difference analyzer for urea determination in serum. The difference between the potential changes of a urease-covered and a bare pH glass electrode is evaluated 30 s after sample injection. This fixed-time regime provides a measuring frequency of 20-25/h the linear range for 1 120 diluted samples is 1-20 mmol/l. These results are better than those of common potentiometric enzyme sensors. 

Guilbault G and Montalvo J 1970 Potentiometric urea electrode J. Am. Chem. Soc. 

Enzyme-Substrate Electrodes. Electrodes that can respond to a variety of organic and biological compounds are constructed by coating the surface of an appropriate ion-selective electrode with an enzyme immobilized in some matrix. Perhaps the most well-known of these is the urea electrode [10], which makes use of the enzyme urease to hydrolyze urea (the substrate ) ... 

In this case, the urease is physically entrapped in a polyacrylamide matrix polymerized on the surface of an ammonium-ion glass electrode. The enzyme-gel matrix is supported on the electrode by a sheer dacron or nylon gauze, about the thickness of a nylon stocking, or it is held by a thin semipermeable cellophane sheet. The urea diffuses to the urease-gel membrane, where it is hydroly d to produce ammonium ion. Some of the ammonium ion diffuses through the thin membrane to the electrode surface, where it is monitored by the ammonium-sensitive electrode. The urea electrode is fairly stable, sensitive, specific for urea, has a usable lifetime of 2-3 weeks before a new gel layer must be prepared, and has a fairly fast response time ( < 120 sec). The output of the electrode is linear from about 10 to 10" M urea. 

In the development of a urea electrode, the first approach was the use of a cation glass electrode, but the resulting probe could not be used for assaying urea in blood and urine because the sensor responded to sodium and potassium ions. 

The first urea electrode was prepared in 1969 by immobilizing urease in a polyacrylamide matrix on nylon or Dacron nets. These nets were then placed onto a glass cation-selective electrode. To improve the selectivity, an ammonium-ion selective electrode with... 

Another interesting immobilization technique based on the adsorption of suitability modified biomolecules on to fluorocarbon surfaces has been described by Kobos et al. [179]. This method minimizes nonspecific binding which affects the detection limit of biosensors unfavourably, because the transducer surface with immobilized enzyme can be treated with neutral fluorosurfactant. An enzyme can be immobilized directly onto the gas-permeable membrane of a gas-sensor. An urea electrode where perfluoroalkylated urease was immobilized onto an ammonia gas-sensing electrode may serve as an example [180],... 

Guilbault, G.G. and Nagy, G. (1973) Improved urea electrode. Anal Chem., 45, 417-419. 

Urea electrodes were among the Orst potentiometric enzyme sensors. They determine urea in blood and urine, and are based on the enzymadc decompK)sition of urea by urease ... 

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