New trends in ion-selective electrodes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 83

Aside from the factors accounted for in the simplifi ed Eq. (9), a deeper consideration
of the membrane selectivity reveals other infl uential parameters. The membrane polar-
ity, for example, which depends mainly on the nature of membrane plasticizer, may give
a selectivity modifying infl uence because of the improved solvation of high valence ions
by more polar media.
It is the selectivity that generally determines the lower detection limit (DL) of a sen-
sor, as background interferences may infl uence the electrode potential at low activities
of analyte. As the true selectivities of the ISEs were determined to be much better than
earlier believed, the detection limits could also be signifi cantly improved. For a long
time the sensitivity range of polymeric membrane ISEs was thought to be limited by sev-
eral orders of magnitude, from about one molar down to micromolar, which is already
uniquely wide for an analytical method. As mentioned above, the sensor detection limit
is determined by the activity at the cross-section of the two linear segments of the cali-
bration curve (Fig. 4.5). Note, this IUPAC defi nition for ISEs [28] differs from that of
other analytical methods (analyte concentration at which the signal is increased relative
to the background level by three times the standard deviation of the noise), which may
cause some confusion. Indeed, taking into account the low noise in potentiometric meas-
urements, the traditional analytical DLs of ISEs are orders of magnitude lower [10].
The main limitation of the lower DL was discovered to be caused by the diffusion of
the primary ion from the inner fi lling solution (where the concentration of the ions was
traditionally high) through the membrane, which leads to enrichment of the analyte ions
at the membrane-sample interface if the sample solution is more dilute. Under such con-
ditions further dilution of the sample does not change the phase boundary potential, deter-
mined mostly by the leached primary ions in the aqueous diffusion layer. Control over the
ion fl uxes in membrane and their minimization made it possible to signifi cantly reduce
the sensor DLs, and this led to a real breakthrough for the entire fi eld of ion-selective
electrodes. Many sensor parameters were varied in order to minimize the fl uxes: using
inner-fi lling solution with primary ions at low activities, buffered by chelating agents or
ion exchange resins; reducing ion diffusion coeffi cients in membrane by loading it with
high polymer content or by covalent attachment of the ionophore to the polymeric matrix;
using sample stirring or rotating electrodes, or wall-jet techniques to reduce the Nernstian
diffusion layer in the aqueous part of the outer interface and so on. The use of a monolithic
capillary fi lled with membrane cocktail as a matrix for ISE is another very recent approach
to suppress ionic fl uxes in the membrane and to achieve very low DLs (see Fig. 4.7).
The response of such sensors does not depend on the composition of the inner fi lling
solution [32]. Complete elimination of the inner fi lling solution and development of ISEs
with a solid inner contact is another successful approach to low DL sensors. Conducting
polymers are used as an intermediate layer between an electron-conducting substrate and
ion-conducting sensor membrane. Different polymers and sensor manufacture procedures
were described to improve the sensor characteristics and although not all experimental
challenges have been overcome so far, the concept offers new prospects to highly sensi-
tive electrodes, mainly due to its universality and applicability to different ion sensors.
So far ions as Na , K , NH 4 , Ca 2 , Ag , Cd 2 , Cu 2 , Pb 2 , Vitamin B1, ClO 4 and
I are detectable in the range of 10 8 -10 11 M. One of the challenges of the day is to