Biomedical Engineering Reference
In-Depth Information
of applications, including the biomedical, industrial, and environmental fi elds. The glass
pH electrode is the most widespread sensor, being present in an arsenal of virtually any
laboratory. Although the performance of the best glass and crystalline membrane sen-
sors remains unsurpassed, the chemical versatility of these materials is limited, which
imposes restrictions on the range of available analytes. During the last decades research
and development of potentiometric sensors has shifted primarily towards the more ver-
satile and tunable solvent polymeric membrane ISEs [2]. These sensors originated in the
early 1960s, completely replaced older analytical methods in various biomedical applica-
tions, and gained a foothold in clinical chemistry. Most of the advantages of solid state
ISEs, such as robustness and cost effectiveness, are still present in their polymeric coun-
terparts, but the unique versatile matrix allows a signifi cant increase in the number of
available analytes. Indeed, the list of detectible analytes is magnifi cent, today approach-
ing 100 species, a number hardly accessible with numerous other analytical techniques
[3, 4]. The extraordinary measuring range of ISEs (in some cases exceeding eight orders
of magnitude in concentration) is also an important intrinsic property of this analytical
method. Note that all these advantages are complemented by their simplicity, a distinct
characteristic of excellence for ISEs. The modern directions of solvent polymeric sensor
research are the focus of the present chapter.
The great success of ISEs achieved during the past decades has not reverted the
fi eld into a stagnating mature discipline. There are several dynamically develop-
ing research directions that continue to tempest the analytical chemistry community,
opening new horizons. Apart from the classical potentiometric response, new trans-
duction schemes for the solvent polymeric membranes have been proposed, making
the detection of new analytes accessible, allowing comprehensive instrumental con-
trol over sensing characteristics and introducing new detection principles [5, 6]. At the
same time, work on a detailed theoretical description of the sensors has kept pace with
the experimental progress, being a prerequisite for further development. The phase
boundary potential (PBP) model was shown to be a very powerful tool [7] that not
only allows one to quantify important sensor properties, but also inspires innovations
in the fi eld. The extremely low detection limits (DL), originally predicted by a detailed
theoretical analysis of the underlying membrane processes [8], were discovered for the
solvent polymeric ISEs [9] and revolutionized the fi eld. More than ten analytes (and
this number continues to grow) were reported so far to be detectable at the nanomolar
concentration level and some of them even down to picomolar activities [10], which
puts potentiometric sensors along with the most sensitive analytical methods.
Examination of the membranes with a variety of physicochemical techniques, from
related electrochemical approaches (as electrochemical impedance spectroscopy (EIS),
voltammetry and chronoamperometry) to more sophisticated characterization methods
(spectroscopy and microscopy), actually serves the same end as the theory and leads to
a deeper understanding of the chemistry behind the functioning of these sensors [5, 6].
Improvement and optimization of the characteristics of the existing sensors is an
important work that is constantly been addressed by many research groups, and is briefl y
reviewed here. The rational molecular design principles become important to the search
for new sensor materials to be more selective and sensitive, possess better detection limits,
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