The possibility of shifting the detection limit of traditional, i.e. internal solution ion-selective electrodes (IS-ISE), down to picomolar levels has reached much attention in recent years[1]. The primary ions leakage from the plastic membrane phase into the sample solution is the origin of the detection limits observed for IS-ISE being in order of mM at the most. Thus, the control of ions transport through the membrane is the key to extend the linear responses range of ion-selective electrodes.
The ions leakage is also affecting the responses of other potentiometric sensors, in this number also those in which conducting polymer (CP) films are used as alternative membranes or all-solid-state ion-selective electrodes (ASS-ISE, where the internal solution was replaced by CP film). Due to the nature of the solid state electrodes the selection of remedies available to control (or reduce) the ion leakage is limited compared to their internal solution counterparts.
One of the possible approaches to prevent the leakage of ions from the ASS-ISE is to surface modify the CP transducer part of the sensor with complexing ligand[2].
The other attitude takes into consideration the nature of CP which affects the detection limits of both ASS-IS and CP membrane based sensors. This approach relates the local elevation of ions concentration at the CP surface to spontaneous processes of polymer charging/discharging. Taking into account the advantage of the electron and ion conductivity of CP, ion fluxes at the interface can be precisely adjusted by means of current control[3]. Thus, lower detection limits can be obtained both for ASS-IS and CP based sensors. The results obtained experimentally are in good accordance with those theoretically predicted.
References
1. T. Sokalski, A. Ceresa, T. Zwickl, E. Pretsch. J. Am. Chem. Soc. 119 (1997) 11347.
2. A. Michalska, A. Konopka, M. Maj-Zurawska, Anal. Chem. 75 (2003) 141.
3. A. Michalska, J. Dumanska, K. Maksymiuk, Anal. Chem. in press. |