New trends in ion-selective electrodes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 100

properties in terms of steric hindrance and lipophilicity. Control over the ion-pair for-
mation in the membrane, achieved through the QAS structure variation and selection
of an appropriate plasticizer, is an additional tool for anion selectivity enhancement, as
the latter is often highly desired [89].
The main classes of plasticizers for polymeric ISEs are defi ned by now and com-
prise lipophilic esters and ethers [90]. The regular plasticizer content in polymeric
membranes is up to 66% and its infl uence on the membrane properties cannot be
neglected. Compatibility with the membrane polymer is an obvious prerequisite, but
other plasticizer parameters must be taken into account, with polarity and lipophilicity
as the most important ones. The nature of the plasticizer infl uences sensor selectivity
and detection limits, but often the reasons are not straightforward. The specifi c solva-
tion of ions by the plasticizer may infl uence the apparent ion-ionophore complex for-
mation constants, as these may vary in different matrices. Ion-pair formation constants
also depend on the solvent polarity, but in polymeric membranes such correlations are
rather qualitative. Insuffi cient plasticizer lipophilicity may cause its leaching, which is
especially undesired for in-vivo measurements, for microelectrodes and sensors work-
ing under fl ow conditions. Extension of plasticizer alkyl chains in order to enhance
lipophilicity is only a partial problem solution, as it may lead to membrane component
incompatibility. The concept of plasticizer-free membranes with active compounds,
covalently attached to the polymer, has been intensively studied in recent years [91].
Dioctyl sebacate (DOS) with relative permittivity ε of 3.9 and 2-nitrophenyl octyl

ether (NPOE) with 23.9 are the traditionally used sensor membrane plasticiz-

ers. The choice of a plasticizer always depends on a sensor application. Thus, NPOE
appears to be more benefi cial for divalent ions due to its higher polarity, but for some
cases its lipophilicity is insuffi cient. Furthermore, measurements with NPOE-plasticized
sensors in undiluted blood are complicated by precipitation of charged species (mainly
proteins) on the sensor surface, which leads to signifi cant potential drifts. Although cal-
cium selectivity against sodium and potassium for NPOE-based membranes is better by
two orders of magnitude compared to DOS membranes, the latter are recommended for
blood measurements as their lower polarity prevents protein deposition [92].
Although widely employed and comprehensively studied, high molecular weight
poly(vinyl chloride) (PVC) as a polymeric matrix is not the only option. While PVC
is often preferred due to mechanistic aspects, a number of other polymers have been
tested, including functionalized PVC (with carboxyl, amino or hydroxyl groups), poly-
urethanes, silicon rubbers, polysiloxanes, polystyrenes, etc. with the main objective to
develop membranes with better biocompatibility and adhesion properties. While mem-
brane casting is widely used, development of other sensor manufacturing procedures is
of a great importance. Photocurable polymers and co-polymers based on various acr-
ylate derivatives not only may produce plasticizer-free membranes, but also allow one
to covalently attach other membrane components such as ionophore and ion exchanger
to the polymeric backbone. Furthermore, the simplifi ed manufacturing process is also
complemented with compatibility with microelectronics technology.
Of the formulated requirements, the glass transition temperature ( T g ) of a sensor
polymer must be below room temperature, otherwise the polymer should be plasticized

ε