Biosensors based on direct electron transfer of protein - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 565

the reduction of dissolved oxygen and glucose [252]. In the presence of oxygen, the
reduced enzyme is oxidized very quickly at the surface of the electrode:

GOD-FADH 2 O 2 → GOD-FAD H 2 O 2 (19)

In the O 2 -saturated buffer solution, an increase in reduction peak was observed, accom-
panied by a decrease in the oxidation peak current, which demonstrates that GOD
in the fi lm nicely catalyzes the oxygen reduction. When glucose is added to the
O 2 -saturated buffer solution, the reduction peak current decreases with the increase of
the glucose concentration. -(D)-glucose is the substrate of GOD and it will react with

the enzyme and decrease the concentration of the oxidized form of GOD on the elec-
trode surface:

β

Glucose GOD(FAD) → gluconolactone GOD(FADH 2 ) (20)

The decrease of the reduction current has been employed to determine the glucose
concentration and probe the enzyme activity of GOD. The peak current responds lin-
early to the glucose concentration ranging from 0.5 to 11.1 mM ( r

0.998), with a

A mM 1 and the estimated detection limit of 0.05 mM at a signal-
to-noise ratio of 3. The K m app is equal to 5.1 mM.
Liu [253] studied the direct electron transfer of glucose oxidase and glucose biosen-
sor based on carbon nanotubes-chitosan matrix. The characteristics of the biosensor
are investigated by chronoamperometric measurement under the optimum conditions.
From the current-time responses, we can know the linear range of the substrate. For
glucose, the calibration curve with linear range spans the concentration of glucose
from 0 to 7.8 mM (Fig. 17.5). And it deviates from linearity at higher concentration
representing a typical characteristic of Michaelis-Menten kinetics. K m app is evaluated
to be 8.2 mM.
GOD adsorbed on a colloidal gold-modifi ed carbon paste electrode can transfer
electrons directly between the protein and the electrode [254]. The immobilized GOD
could electrocatalyze the reduction of dissolved oxygen and resulted in a great increase
in the reduction peak current. Upon addition of

sensitivity of 7.0 µ

)-glucose to air-saturated PBS,
the reduction current response of GOD/Au/CPE decreased. The decrease increased
with the increase of

β -D(

)-glucose concentration and the decrease has a linear rela-
tionship with the concentration of

β -D(

)-glucose from 0.04 to 0.28 mM with a corre-
lation coeffi cient of 0.997. The detection limit is 0.01 mM at a signal-to-noise ratio of 3.
The sensitivity of GOD/Au/CPE to

β -D(

A mM 1 .
Wang [255] developed a novel biosensor which comprised an electrode of gold/
multi-walled carbon nanotubes-glucose oxidase (Au/MWNTs-GOD) to detect glu-
cose. The biosensor exhibits excellent bioelectrocatalytic oxidation activity for glucose
at a static applied potential (

β -D( )-glucose was found to be 8.4 µ

0.45 V vs Ag/AgCl). Addition of glucose to the buffer
solution results in a remarkable increase in the oxidation current, whereas the time
required to reach the 95% steady-state response is within 16 s. The current increased
linearly with the concentration of glucose in the range from 0.05 to 13 mM. The bio-
sensor also has a very low detection limit of less than 0.01 mM for glucose. Wu [256]
used colloid Au-dihexadecylphosphate composite to immobilize GOD and develop a