Glucose detector

HIGHLY-SPECIFIC test strips for detecting glucose in urine or blood
are commonly used in doctors’ surgeries and in the home. All of these
tests utilise the enzymes glucose oxidase and peroxidase, immobilised
on a paper pad at the tip of the strip. The pad is covered with a thin
cellulose membrane which is permeable only to small molecules such
as glucose. In the presence of oxygen and glucose, the products of the
enzyme activity react with a chemical (also on the paper pad) to
produce a colour change. With some tests (such as Diastix or
Boehringer Diabur 5000), the intensity of the colour which develops
indicates the glucose concentration.
The principles behind these diagnostics are readily demonstrated,
using potassium iodide as the chromagen (colour-change reagent).
Different sugars can be used to indicate the specificity of the reaction
catalysed by glucose oxidase.
How the test strips work:
1) Glucose oxidase acts specifically on glucose to give
hydrogen peroxide and gluconic acid:
β-D-Glucose   +   Oxygen   +   Water
Hydrogen peroxide   +
(δ-Gluconolactone)                 Gluconic acid
2) Horseradish peroxidase then catalyses the reaction of
hydrogen peroxide with potassium iodide. The colourless
iodide is oxidised to brown iodine:
Hydrogen peroxide   +   Potassium iodide
Iodine   +   Water
Note: the reactions shown on the accompanying worksheet
have been simplified.
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