Biomedical Engineering Reference
In-Depth Information
properties caused by biomolecular interactions or specifi c cell functions on the elec-
trodes. The electronic biochip, namely electronic microarray biosensors, is composed
of patterned multiple ultramicroelectrodes (UMEs). The electrode materials can be plat-
inum, gold, carbon, conducting polymers, metal oxides, carbides, nitrides, etc. In order
to increase the detection of electronic signals, the arrayed ultramicroelectrodes are often
porous for high specifi c surface area by specifi c fabrication processes. Electrochemical
processes occurring on arrayed UMEs are different from that on conventional elec-
trodes, due to the small size and porous properties. Theoretical considerations of the
electrochemical behaviors of the electronic microarrays can greatly provide data analy-
sis methods and optimized design.
The UMEs used in bioarrays can be divided into three types; disk, ring, and strip
electrodes. The theory of the disk, ring, and strip UMEs has been extensively studied
[97-100]. Due to the edge effect, the profi le of the mass diffusion to the ultramicro-
electrode surface is three dimensional, and can signifi cantly enhance the mass trans-
portation in comparison to the conventional large electrode with one-dimensional mass
transportation. The steady-state measurement at a planar UME can be expressed as
i
L
4
nFDC
0
r
(1)
where
n
is the number of electron transfers involved in the electrochemical reaction,
F
is
the Faraday constant,
D
is the diffusion coeffi cient of the reactant species in solution, and
C
0
and
r
are the bulk concentration of the reactant species and the radius of the UME,
respectively. For band UMEs, the steady-state currents are expressed as the following:
2
π
nFADC
w
64
Dt
w
o
(2)
i
ln
L
2
where
w
is the width of the band electrode and
A
is the surface area of the band UME,
which equals to
w
l
(
l
: the length of the band). The theory of transient measurements
on different UMEs is described in [97, 98].
The porous UME can be formed by coating a porous layer such as conductive poly-
mers, or by etching and growing porous materials such as nano carbontubes on a solid
UME surface. A porous UME can signifi cantly improve the sensitivity and detection
limit, due to its high specifi c surface area. Li and Cha reported that a porous UME
remarkably increased the amount of loading of enzyme and electron mediator of a
biosensor for great sensitivity improvement [101]. The theory and applications of the
porous UME have been investigated by Li and Cha [102-106]. Theoretical analysis
and experimental results of porous UME demonstrated that the porous UME could
be considered as a combination of two different types of electrochemical device. The
outer surface (end-surface) of the porous UME behaves as a planar microdisk electrode
of the same diameter, while the porous matrix and the electrolyte in the matrix pores
behave as a thin-layer device with a high reaction surface area. Thus, the steady-state
current of the thin-layer device is zero at any fi xed potential, and the steady-state rate
of diffusion at the powder microelectrode is also controlled by a mass transfer process
in the solution outside the end-surface of the porous UME; hence the same Eq. (1) is
applicable to both planar and porous UME. However, the transient response
I
of the
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