Biomedical Engineering Reference
In-Depth Information
is also the potential at which other substances that may be present (i.e. uric
and asorbic acids) are electroactive.
6
This can lead to electroactive interfer-
ence at the electrode, which can decrease the accuracy of measurement of
the analyte of interest. A selectively permeable coating can be used to
prevent the diffusion of other electroactive substances to the enzyme, and
these include electropolymerised fi lms (poly(phenylendiamine), polyphe-
nol, overoxidized polypyrole), cellulose acetate, negatively charged sulpho-
nated Nafi on, hydrophobic alkanethiol and lipid layers.
6
Third generation biosensors use direct electron transfer between enzyme
and electrode, in the absence of a mediator. The distance between the
immobilised enzyme and electrode should be as small as possible. The
promise of such biosensors is high selectivity. One potential method for
future amperometric electrochemical sensors is the use of conducting
organic salts, which form a crystal structure that can facilitate direct
electron transfer from glucose oxidase.
6
Oxidized boron-doped diamond
electrodes have also been investigated for their potential direct electron
transfer properties.
While amperometric biosensors are based on the measurement of the
current resultant from redox reactions, potentiometric biosensors use ion-
selective electrodes (ISE) combined with immobilised enzymes to measure
the concentration of specifi c ions. The potential produced at the electrode
surface is proportional to the logarithm of the analyte concentration.
4
Most common of this type is the pH electrode, which uses an H
+
selective
electrode. Urea sensors based on the detection of ammonium and bicar-
bonate ions have also been used.
7
The glass electrode is the classical
method for measuring pH. However it is seldom used in enzyme elec-
trodes, because it is brittle, has a high resistance and the sensitivity is
infl uenced by the buffer capacity of the internal measuring solution. Poten-
tiometric biosensors based on solid-state semiconductor fi eld effect tran-
sistors (FET) including ion-selective and enzyme-linked FETs have been
in development since the 1970s. The history of ISFET technology, includ-
ing theory, technology and applications, is reviewed by Bergveld,
8
who
introduced the ISFET concept in the 1970s. Biosensors based on these
technologies have signifi cant advantages, including small size, robustness,
high sensitivity, rapid response and small sample volume requirements.
9
These make them suitable for inclusion in point-of-care devices, lab-on-a-
chip systems and
in vivo
sensors. Typical biosensor applications of ISFET
technology include catheter-tipped pH sensors and fl ow-through sensors
for measurement of pH, Na
+
, K
+
, Ca
+
,
p
CO
2
and
p
O
2
.
8
The linking of an
enzyme to the ion-selective gate surface of the FET has also resulted in
biosensors for glucose, urea, creatinine and penicillin.
4,9
There is consider-
able interest in developing nanoscale ISFET devices, which may be useful
in microarray and
in vivo
devices
.
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