YSI 2700D, Руководство пользователя

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6-2
The face of the probe, covered by the membrane, is situated in a buffer-filled sample chamber
into which a sample is injected. Some of the substrate diffuses through the membrane. When it
contacts the immobilized oxidase enzyme, it is rapidly oxidized, producing hydrogen peroxide.
See Reaction 1, using glucose as an example.
The hydrogen peroxide (H
2
O
2
) is, in turn, oxidized at the platinum anode, producing electrons
(Reaction 2). A dynamic equilibrium is achieved when the rate of H
2
O
2
production and the rate
at which H
2
O
2
leaves the immobilized enzyme layer are constant and is indicated by a steady
state response (Figure 6.3). The electron flow is linearly proportional to the steady state H
2
O
2
concentration and, therefore, to the concentration of the substrate.
REACTION 1 (glucose):
β
-D-glucose + O
2
GOx
Glucono-
δ
-lactone + H
2
O
2
REACTION 2: H
2
O
2
Pt anode
2H
+
+ O
2
+ 2e
-
The platinum electrode is held at an anodic potential and is capable of oxidizing many
substances other than H
2
O
2
. To prevent these reducing agents from contributing to sensor
current, the membrane contains an inner layer consisting of a very thin film of cellulose acetate.
This film readily passes H
2
O
2
but excludes chemical compounds with molecular weights above
approximately 200.
The cellulose acetate film also protects the platinum surface from proteins, detergents, and
other substances that could foul it. However, the cellulose acetate film can be penetrated by
such compounds as hydrogen sulfide, low molecular weight mercaptans, hydroxylamines,
hydrazines, phenols and anilines.
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