New trends in ion-selective electrodes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 102

Obviously, a well-defi ned redox process between metal substrate and membrane
is required for optimum sensor stability. In “pseudo” solid contact sensors a hydrogel
layer, entrapping a salt such as KCl, is applied between the membrane and the internal
reference electrode (Ag/AgCl). When the concentration of chloride ions in the hydro-
gel layer is kept constant the resulting reference potential remains stable. Importantly,
modern highly cross-linked hydrogels exhibit low swelling and prevents sensor mem-
brane mechanical damage and geometry change.
Various redox-active compounds such as salt-doped resins and lipophilic com-
plexes of silver were used to provide a reversible redox reaction at the inner interface.
Conducting polymers, due to their unique electrochemical properties are probably the
most important solid contact materials [12]. Conducting polymers, such as polypyr-
role, polythiophene, polyaniline, and their derivatives, not only provide defi ned redox
couples, but also form layers with mixed ionic/electronic conductivity that are often
compatible with the sensor membrane. On the back side of the polymer, however, an
aqueous layer still may appear, as in CWEs. The use of lipophilic monolayers assem-
bled on the metal surface could help avoid undesired formation of the aqueous phase.
While there is still work required to fully suppress the infl uences of sample pH, pO 2 ,
pCO 2 , and redox species on conducting polymer-based solid contact sensors, conduct-
ing polymers are very effective ion-to-electron transducers. Highly stable solid contact
ISEs for many ions have been reported, including those for trace level measurements
[96]. Furthermore, incorporation of the sensing materials directly into conducting pol-
ymer chains, either by covalent attachment or by doping with counter-ions containing
desired functionalities, is a very challenging approach to make a sensor and transducer
on the same and single molecule [97]. While the redox sensitivity, being an intrin-
sic property of conducting polymer, may interfere with the sensor response [98], this
approach opens new horizons in nanosensor technology.
4.4.3 Biocompatibility improvement
In addition to laboratory blood analyzers and portable point-of-care devices, which
require blood collection, continuous monitoring of ion activities in a blood stream via
implanted ion-selective electrodes is of great interest. The term “biocompatibility” refers
to the ability of a sensor not to cause toxic or injurious effects while being in contact with
living tissue. As dealing with any foreign object introduced into the human body, bio-
compatibility and hemocompatibility particularly are the most important requirements.
The fi rst aspect of biocompatibility is a natural immune response. When a foreign
object enters the blood stream, it can be attacked by the body's defense system. The
fi rst step is protein adsorption on an object surface. It is believed that the amount and
type of protein adsorption is one of the most important steps determining whether the
object is tolerated or rejected by the body. The next step is cell adhesion, which may
cause aggregation and activation of platelets and triggering of the blood coagulation
system with resulting thrombus formation. It may not only lead to sensor failure via
surface blocking but directly threatens the patient's health.
In order to improve the biocompatibility of ISEs and reduce adsorption of cells and
polypeptides several approaches have been used. Among them are immobilization of