Biomedical Engineering Reference
In-Depth Information
4.3.3 Voltammetric ion-selective electrodes
Controlled potential methods have been successfully applied to ion-selective elec-
trodes. The term “voltammetric ion-selective electrode” (VISE) was suggested by
Cammann [60]. Senda and coworkers called electrodes placed under constant poten-
tial conditions “amperometric ion-selective electrodes” (AISE) [61, 62]. Similarly
to controlled current methods potentiostatic techniques help to overcome two major
drawbacks of classic potentiometry. First, ISEs have a logarithmic response function,
which makes them less sensitive to the small change in activity of the detected analyte.
Second, an increased charge of the detected ions leads to the reduction of the response
slope and, therefore, to the loss of sensitivity, especially in the case of large polyionic
molecules. Due to the underlying response mechanism voltammetric ISEs yield a lin-
ear response function that is not as sensitive to the charge of the ion.
This type of sensor often does not have a membrane; it instead utilizes the prop-
erties of a water-oil interface, a boundary between an aqueous and a non-aqueous
(organic) phase. Traditionally, sensors based on non-equilibrium ion-selective transport
phenomena were distinguished as a separate group and considered as the electrochem-
istry of the ion transfer between two immiscible electrolyte solutions (ITIES). Here,
we will not distinguish polymeric membrane electrodes and ITIES-based electrodes
due to the similarity in the theoretical consideration.
Theoretical insight into the interfacial charge transfer at ITIES and detection mecha-
nism of this type of sensor were considered [61-63]. In case of ionophore assisted trans-
port for a cation I the formation of ion-ionophore complexes in the organic (membrane)
phase is expected, which can be described with the appropriate complex formation
constant,
β
IL
n
I
.
Using the steady-state diffusion model described in section 4.3.2 one may defi ne
the parameter
q
as:
D
aq,I
δ
m
/
D
m,I
δ
aq
(14)
If
q
is negligibly small a stagnant diffusion layer in aqueous phase is a rate-limiting
step. This case is often the most useful from an analytical point of view. The well-
known equation for a reversible polarographic wave can be obtained as:
q
i
zF
RT
I,d
i
(15)
I
⎛
⎝
⎞
⎠
⎜
⎟
1
exp
(
EE
)
12
/
where the limiting (diffusion) current and half-wave potential are given by:
i
zFAa
(aq)
D
/
t
(16)
I,d
I
m,I
and
⎛
⎞
D
D
⎟
⎜
EE
RT
zF
aq,I
0
n
ln
[
ln
β
a
(m)
I
]
(17)
12
/
IL I
n
L
⎝
⎠
m,I
Search WWH ::
Custom Search