Lipophilic Multi-walled Carbon Nanotube-based Solid Contact Potassium Ion-selective Electrodes with Reproducible Standard Potentials. A Comparative Study

The two most promising approaches for preparing solid contacts (SCs) for polymeric membrane based ion-selective electrodes (ISEs) are based on the use of large surface areas conducting materials with high capacitance (e. g., various carbon nanotubes) and redox active materials (e. g. conducting polymers). While many of the essential requirements for the potential stability of SCISEs were addressed, the E⁰ reproducibility and its predictability, that would enable single use of such electrodes without calibration is still a challenge, i. e., the fabrication of electrodes with sufficiently close E⁰ and slope values to enable the characterization of large fabrication batches through the calibration of only a small number of electrodes. The most generic solution seems to be the adjustment of the E⁰ potential by polarization prior to the application of the ion-selective membrane. This approach proved to be successful in case of conducting polymer-based solid contacts, but has to be still explored for capacitive solid contact based ISEs, which is the purpose of this paper. We have chosen a well-established highly lipophilic multi-walled carbon nanotube (MWCNT), i. e. octadecane modified MWCNT (ODMWCNT), that is investigated in the comparative context of a similarly lipophilic conducting polymer solid contact (a perfluorinated alkanoate side chain functionalized poly(3,4-ethylenedioxythiophene)). While, the OD-MWCNT based SCISEs had inherently small standard deviation of their E⁰ values (less than 5 mV) this could be further improved by external polarization and short circuiting the SCISEs.