Biosensors based on direct electron transfer of protein - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 549

coimmobilization of the corresponding oxidase on the electrode surface. A biosensor is a
detection system composed of a biological sensing element (e.g. enzyme) and a transducer
(e.g. electrode). The biosensor measures the signal caused by change in the concentra-
tion of a co-reactant which reacts with the analyte or a co-product which is produced with
the analyte of a biological reaction (e.g. enzyme reaction). When an electrode is used as a
transducer in a biosensor, the electrode converts the change in concentration of a product or
a reactant of a biological reaction into an electric signal.
The fi rst enzyme biosensor was a glucose sensor reported by Clark in 1962 [194].
This biosensor measured the product of glucose oxidation by GOD using an electrode
which was a remarkable achievement even though the enzyme was not immobilized
on the electrode. Updark and Hicks have developed an improved enzyme sensor using
enzyme immobilization [194]. The sensor combined the membrane-immobilized GOD
with an oxygen electrode, and oxygen measurements were carried out before and after
the enzyme reaction. Their report showed the importance of biomaterial immobiliza-
tion to enhance the stability of a biosensor.
The enzyme-based biosensor has come through three steps: (1) with oxygen for the
media; (2) with artifi cial intermediate for media; and (3) without media and based for
the direct electron transfer of redox proteins. The following is an example:

1. With oxygen for media:

In solution: GOD ox glucose → gluconolactone GOD re (4)

GOD red O 2 → GOD ox H 2 O 2 (5)

2H On electrode: H 2 O 2 → O 2 2e (6)

2. With artifi cial intermediate for media:

In solution: GOD ox glucose → gluconolactone GOD red (7)

GOD red M ox → GOD ox M red (8)

On electrode: M red → M ox ne (9)

3. Without media:

In solution: GOD ox glucose → gluconolactone GOD red (10)

On electrode: GOD red → GOD ox 2e (11)

In this chapter, we mainly describe the third-generation biosensor: mediator-free
protein-based biosensor. This biosensor has many advantages compared with other
two-generation biosensors. Many redox proteins and redox enzymes including cyto-
chrome c [195], myglobin [196], hemoglobin [197], horseradish peroxidase [198],
cytochrome P450 [199], catalase [200], glucose oxidase [201], and other active
enzymes have been used to make this type of biosensor in recent years.