Biochips – fundamentals and applications - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 344

porous UME can be calculated as the sum of the responses I D , and I T , of the microdisk
electrode and the thin-layer device. Thus, the measured current using the porous UME
can be described by

K D v 1/2

I I D I T K T v ∆ E P(D) ∆ E P ∆ E P(T) (3)

where

1
2

⎛
⎝

⎞
⎠

⎡ ⎤

nF Dv
RT

π nF

RT

⎟

3
2

⎜

1
2

⎢⎢ ⎢ ⎥

I () Ac º ( EE ) Kv

D i D

⎣ ⎦

(4)

∆ E 59 /n mV

P(D)

22

′

nFvVc
RT

º exp[( nF RT / ( E E º ]

I Kv

T T

′

{ 1 exp[( nF / RT E ( E º )]}

2

(5)

∆ E 0

P(T)

where E 0 is the formal potential of the redox couple, E i is the initial scanning potential,
∆

E P(T) are the differences between the anodic and cathodic peak potentials
for the disk and the thin-layer device, respectively,

E P(D) and ∆

( E ) is the normalized dimension-
less current function for a sweep experiment with a reversible system, K D and K T are
constants independent of the scan rate, A is the surface area of the disk electrode, v is
the scan rate, and V is the volume of electrolyte within the porous matrix of the porous
UME. It is possible to achieve a signifi cantly enhanced electrode surface and homoge-
neous polarization simultaneously at a porous UME if the porous layer is under 50

χ

m.
Improved reversibility, a better developed limiting diffusion current and easier analysis
of experimental data should render the porous UME applicable in many areas of elec-
troanalytical chemistry and electrode kinetics studies in terms of Eqs (1)-(5).
The effects of the array pattern on UME current output of different packing densi-
ties have been studied by using steady-state and chronoamperometric responses [107].
When the interelectrode distance d

µ

2 r , the array was considered loosely packed

and Eqs (1)-(5) can be applied. When d 2 r , the array was considered densely packed

and more closely resembled the behavior from an UME of a similar surface area to a
large electrode. The implication is that only when d is large enough, the advantages of
using an array pattern can be achieved. The convection-dependent contribution to UME
array is also affected by the interelectrode distance. It was suggested that the theoretical
current response is in agreement with the experimental data when d 10 r [107, 108].

11.4.2 Fabrication technologies
11.4.2.1 Overview
Since the late 1970s, electroanalytical array sensors have been constructed by meth-
ods similar to the well-established semiconductor fabrication processes, which use