New trends in ion-selective electrodes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 104

and well-developed microelectronics technologies may be more easily mass-produced
and assembled into sensor arrays for simultaneous multianalyte detection.
There are a variety of methods available for microfabrication of ion-selective elec-
trodes, from relatively simple dip-coating and casting to more sophisticated screen-
printing and lamination processes. The fi rst solvent polymeric membrane electrodes
proposed for intracellular measurements were made from micropipette tips fi lled with
a liquid membrane solution. This concept is still successfully used with capillary elec-
trode bodies and the reported electrode tip diameters are as small as 0.1 m. A recently

reported microelectrode array platform for intracellular calcium measurements [102]
is depicted in Fig. 4.14. During the past decade microelectrode development has been
shifting towards planar devices. Modern photolithography and etching techniques are
applicable for producing multilayer sensors using thin and thick fi lm technologies [13].
In-vivo measurements in the beating mammalian heart were performed with such sen-
sors (Fig. 4.15).
Most of the possible shortcomings of macroelectrodes are even more pronounced
in their micro counterparts. For instance, the challenges of a solid contact, discussed
above, are of great importance for microsensors. Layers of electrochemically depos-
ited Ag/AgCl or electropolymerized conducting polymers applied on an electron-
conducting substrate are used as solid contact layers, which are further covered with
an ion-sensitive fi lm. Unfortunately, the application of hydrogels in an inner reference
compartment, successfully employed in conventional ISEs, is technically more diffi -
cult with microsensors. The compatibility of materials may also be an issue, i.e. poor
adhesion of the sensing membrane to a transducer surface could lead to exfoliation and
limit the sensor lifetime. Various polymers with better adhesion properties than tradi-
tional PVC have been tested as matrices for microelectrodes [103].
Size-related problems may become important for all microsensors. Leakage of
sensing materials from a small membrane may lead to rapid deterioration of sensor
properties [104]. While the lipophilicity of membrane components cannot be increased
infi nitely, immobilization of ionophore and ion exchanger in the polymer by covalent
attachment or molecular imprinting along with utilization of plasticizer-free mem-
branes could help solve the leakage problem.

µ

4.5.2 Sensor arrays
Our multidimensional and fast-paced world does require robust instruments, capable
of tracing the constant changes. Discrete sensors, providing information on a single or
few parameters, cannot supply the growing needs. A general and complex approach
to gaining information on various systems and processes logically leads to develop-
ment of multisensor systems or sensor arrays. The signals from potentiometric sensor
arrays, comprising electrodes selective to different species in sample solutions, when
processed simultaneously using mathematical methods, may reduce the errors of deter-
mination of individual discrete sensors. Once the infl uences of interfering ions are well
understood, the mathematical data processing makes it possible to deconvolute the
contributions from primary and interfering ions and to introduce automatic correction