Biomedical Engineering Reference
In-Depth Information
been employed in various formats including the detection of ion concen-
trations (ISFET) 44,45 or immunochemical interactions (IMMUNOFET,
Figure 1.10b). 46 The imposition of such chemistry on the gate surface follows
a somewhat similar approach to methods used for conventional electrodes. In
addition, the strategies outlined above for biochemical immobilization are
generally employed in terms of attaching proteins and nucleic acids to the gate
region.
d n 4 t 3 n g | 1
1.4.1.2 Amperometry
Potentiometry does not involve transfer of charge at electrodes since there is a
mandated null-current. As mentioned above, the equilibrium potential of a
particular electrode is given by the famous Nernst equation (eqn (1.2)).
Alteration of an electrode potential with resulting disturbance of the
equilibrium condition will yield a transfer of charge and electron movement
across the liquid-electrode interface. In general terms, techniques that are
based on measurement of the resulting current versus applied potential are
called voltammetric methods. Anodic stripping and cyclic voltammetry are two
examples of such techniques. Amperometry has many definitions in the
literature but is more often than not simply seen as involving the measurement
of current associated with an oxidation-reduction reaction at an electrode.
As mentioned above, the beginning point of biosensor technology was the
oxygen-mediated electrode for the amperometric assay of glucose. Over the
years the electrochemical detection of this analyte, given its importance, has
been the subject of literally thousands of studies, the history of which has been
beautifully reviewed by Wang. 47,48 One of the most important advances in the
technology was the introduction of organometallic mediators such as ferrocene.
This iron p-arene complex composed of a cyclopentadiene sandwich of Fe acts
as a convenient electron acceptor for glucose oxidase, thus avoiding the
dependence on the role of O 2 (Figure 1.11). The mediator has a low redox
potential and can be conveniently immobilized on an electrode surface with the
enzyme. Years of further research have seen a number of alternatives employed
for same purpose, including derivatives of ferrocene, and efforts to improve the
proximity of the system to the enzyme and electrode. All this research has
contributed to the development of the point-of-care test strips for glucose assay
d n 3 .
Figure 1.11 Electron mediator for glucose oxidase.
 
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