Enzyme Electrode

A gas-sensing electrode can be modified to create a potentiometric electrode that respond to a biochemically important species, such as enzymes. One example of an enzyme electrode is the urea electrode, which is based on the catalytic hydrolysis of urea by urease

CO(NH)2(aq) + 2H2O(l) ↔ 2NH4+(aq) + CO32-(aq)

The schematic diagram here shows one version of the urea electrode, which modifies a gas-sensing NH3 electrode by adding a dialysis membrane that traps a pH 7.0 buffered solution of urease between the dialysis membrane and the gas permeable membrane.

Figure11.20

When immersed in the sample, urea diffuses through the dialysis membrane where it reacts with the enzyme urease to form the ammonium ion, NH4+, which, at a pH of 7, is largely present as NH3. The NH3, in turn, diffuses through the gas permeable membrane where a pH electrode measures the resulting change in pH. The response of the electrode to the concentration of urea is

Ecell = K – 0.05916log(aurea)

Another version of the urea electrode immobilizes the enzyme urease in a polymer membrane formed directly on the tip of a glass pH electrode, as shown here.

Figure11.21

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