New trends in ion-selective electrodes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 101

(as in case of PVC for which the T g is 80ºC). The low T g provides the appropriate
mechanical properties, such as elasticity, and ensures suffi cient ionic mobilities in sensor
membranes. On the other hand, however, some polymers with low T g may be too soft
to be castable into membranes. The polymer itself is often considered an inert matrix;
however, it inevitably contains impurities that may infl uence, sometimes strongly, the
membrane properties. Membrane polarity depends on the polymer nature and content.
For a traditional PVC to plasticizer ratio of 1:2 (by mass) the relative permittivity of
DOS-plasticized membranes is 4.8, which is higher, and for NPOE-based ones the value
is 14, which is signifi cantly lower than the respective values for the pure plasticizers.
Non-traditional compounds such as perfl uorocarbons are now intensively tested
as new matrices for polymeric ISEs [93]. Properties of these species signifi cantly dif-
fer from those of other organics. Extremely low polarity and solubility in water, along
with very high chemical inertness and decreased affi nity to proteins and lipids make
them attractive materials for chemical sensors, especially for biomedical applica-
tions. However, due to compatibility reasons all sensor membrane components should
be fl uorinated, which narrows the range of available sensing materials. Moreover, the
knowledge of materials on ISEs accumulated over the years may be of limited appli-
cability for perfl uorinated membranes because of the widely different matrix proper-
ties. For example, ion-pair formation in fl uorous phases is many orders of magnitude
stronger than in conventional membranes and may compete with the ion-ionophore
complex formation, likely co-determining sensor selectivity. These novel membrane
materials are very promising but they require purposeful and comprehensive research.
4.4.2 Solid contact
An inner fi lling solution and internal reference electrode are used in macro ISEs due to
a very good stability of the potential at the inner membrane-solution interface in such
a setup (see Fig. 4.4). However, the presence of a solution inside a sensor could be a
serious limitation for development of microelectrodes and may be undesired for a vari-
ety of other reasons, including ionic fl uxes in the membrane and limited temperature
range of sensor operation. There are several requirements for such an inner contact.
First of all, a reversible change of electricity carriers ions-electrons must take place at
the membrane-substrate interface. The potential of the electrochemical reaction, ensur-
ing this transfer, has to be constant, stable, and must not depend on the sample compo-
sition. At last, the substrate must not infl uence the membrane analytical performance.
The fi rst and very simple solid contact polymeric sensors were proposed in the
early 1970s by Cattrall and Freiser and comprised of a metal wire coated with an ion-
selective polymeric membrane [94]. These coated wire electrodes (CWEs) had similar
sensitivity and selectivity and even somewhat better DLs than conventional ISEs, but
suffered from severe potential drifts, resulting in poor reproducibility. The origin of the
CWE potential instabilities is now believed to be the formation of a thin aqueous layer
between membrane and metal [95]. The dominating redox process in the layer is likely
the reduction of dissolved oxygen, and the potential drift is mainly caused by pH and
pO 2 changes in a sample. Additionally, the ionic composition of this layer may vary as
a function of the sample composition, leading to additional potential instabilities.