Biosensor technology in the treatment of cardiovascular disease - Biomaterials and Devices for the Circulatory System - page 282

Reaction
substrates,
including the
analyte of
interest

Reaction
products

Enzyme

e -

Electron
mediator

Polymer 'wire' connecting
enzyme to electrode

e -

Electrode

11.2 Methods of improving electron transfer between enzyme and
electrode in an electrochemical biosensor.

carefully designed as they infl uence the diffusion of the analyte to the
enzyme and the mass-transfer properties of the system.
First generation biosensors are dependent on the presence of oxygen for
the enzyme-catalysed reaction, and the production and measurement of
hydrogen peroxide. This dependence is a disadvantage for these biosensors,
as errors can result from limited physiological oxygen supply when com-
pared with glucose availability, which is known as the 'oxygen defi cit'. This
is a particular problem when making in vivo measurements. Second genera-
tion biosensors attempt to solve the problem of oxygen limitation by the
use of electron mediators, which reduce oxygen competition and transfer
electrons from the enzyme to the electrode surface. Electron mediators
used include ferro/ferricyanide, ferrocene derivatives, conducting organic
salts, hydroquinone and various redox dyes. 2,6 The main limitation of medi-
ator-based biosensors is that there may be interference from other redox
species. Figure 11.2 illustrates the use of an electron mediator, and a wired
enzyme in an electrochemical biosensor.
Wired enzyme electrodes are a non-diffusional method of improving
the contact between the enzyme and electrode. Glucose oxidase can be
'wired' to the electrode surface by a long fl exible hydrophilic polymer
backbone with a dense array of covalently linked osmium-complex elec-
tron relays. 6 Wired enzyme electrodes have a number of advantages; the
close coupling of the enzyme and electrode results in high sensitivity and
selectivity. There is no need for artifi cial mediators, which can be toxic
and are hence not suitable for in vivo applications in particular. Other
technologies have been used to obtain direct electrical connection
between the enzyme and electrode, including bonding of the mediator or
electron relays to the enzyme, and the use of gold nanoparticles and
carbon nanotubes.
Coatings can be used to improve the selectivity of enzyme electrodes.
The potential required for the anodic measurement of hydrogen peroxide

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