Biomedical Engineering Reference
In-Depth Information
H
2
O
2
P
E
O
2
S
1. Electrochemical a direct product
Mox
Ered
P
Mred
Eox
S
2. Mediated electron transfer
Ered
P
P:Product
S:Substrate
E:Enzyme
M:Mediator
Eox
S
3. Direct electron transfer
Fig. 4.2.
The development of amperometric enzyme sensors
Electrochemical methods were first used as transducers of biosensors. The
use of biosensors begin in 1962, when Clark and Lyons combined an oxygen
probe with glucose oxidase to determine glucose levels.
Electrochemical biosensors are formed by coupling a biochemical media-
tor, such as an enzyme, a microorganism, and an antibody, to a transducer
of the electrical signal, such as an ion-selective electrode, a pH electrode, a
gas-sensitive electrode, and so on. The general mechanism of electrochemical
biosensors is based on the interaction between the analyte present in the sam-
ple and the biochemical mediator, which is immobilized on the surface of the
electrode. The consequent formation of an electroactive species generates an
electrical signal or a variation of a preexisting electrical signal. This signal,
which is easily recorded by suitable electrochemical apparatus, is therefore
proportional to the activity of the chemical species to be determined.
Amperometric biosensors can be divided into three generations, which are
indicated in Fig. 4.2. First-generation enzyme electrodes are essentially mem-
brane electrodes, in which the enzyme is immobilized near the surface of an
electrode by the use of a semi-permeable membrane. Amperometric detection
of enzyme-catalyzed turnover of substrate to product is often limited to redox
enzymes, whereby the electrode responds to a change in the oxygen or hydro-
gen oxide concentration. In second-generation sensors, the electron transfer
activity of the redox enzymes is coupled to the electrode surface through
a low molecular weight electron transfer mediator. This arrangement lessens
interference from competing species in the analyte solution by requiring a less
