Biomedical Engineering Reference
In-Depth Information
placed in close proximity to a platinum electrode polarized at a potential
of
þ
0.6 V. The substrate reacts with the enzyme according to:
GO
glucose
þ
O
2
!
gluconic acid
þ
H
2
O
2
d
n
4
t
3
n
g
|
1
The Pt electrode is designed to respond to the hydrogen peroxide produced by
the enzyme-glucose reaction. This progress led to a large number of O
2
mediated devices being developed which included a variety of enzymes and
substrates. A further early example connected to glucose detection was the
work of Updike and Dicks
8
who immobilized a gel impregnated with the
oxidase on a semi-permeable membrane. In this experiment, the amperometric
signal resulted from changes in the partial pressure of oxygen as it diffused
through the membrane to a sensor. Not surprisingly other components of the
basic oxidase reaction have been studied such as pH changes associated with
the production of gluconic acid; in this situation the electrochemistry involved
potentiometry.
Although a large number of enzyme-based systems have been developed over
many years, it is fair to say that by far the most research has centered on
glucose because of the obvious significance to the diabetic patient. Literally
hundreds if not thousands of scientific papers have been devoted to this
substrate. In modern times, O
2
as a redox mediator is avoided and other species
capable of electron transfer are employed to serve the same purpose.
9.10
Indeed
such a strategy is very much a component of the commercial test strips that are
so familiar to individuals suffering from diabetes.
We now turn to a concise look at contemporary biosensor technology using a
device outside-to-inside approach.
d
n
3
.
1.3 Probe Attachment to the Sensor Structure
In order for a biosensor to function it is mandatory to couple the probe or
receptor to the solid surface of the particular transducer being employed, and if
not bound at the device interface, it must generally be arranged be in close
proximity. Over the years a plethora of methods have been developed for this
purpose,
11,12
and although some of these are common to various probes, a large
number have been designed for a specific receptor type. The overall goal of
much of this chemistry is to achieve the maximum signal with respect to
operation of the device in use. In this respect there are a number of key factors
at play.
It is obviously critical to retain the binding activity of the probe for its
target. It is anticipated that certain proteins, for example, may be partially
denatured on attachment to the sensor surface. Others such as antibodies
may be more robust. Such is generally the case for DNA and oligo-
nucleotides whereas RNA moieties are considered to be more prone to
alteration in structure. When it comes to cells it is clearly important for
