ALL-SOLID-STATE POLY(VINYL CHLORIDE) MEMBRANE ION-SELECTIVE ELECTRODES WITH POLY(3-OCTYLTHIOPHENE) SOLID INTERNAL CONTACT

All-solid-state ion-selective electrodes were prepared by using poly(3-octylthiophene) (POT) as solid contact material. A film of POT (thickness approximately 0.25 mu m was deposited on a solid substrate (platinum, gold or glassy carbon) by electrochemical polymerization of 3-octylthiophene. The POT layer was subsequently coated with an ion-selective membrane (ISM) to produce a solid-contact ion-selective electrode (SCISE), SCISEs for several ions (Li+, Ca2+ and Cl-) were prepared and investigated. The compositions of the ion-selective membranes were the same as normally used for the same ions in poly(vinyl chloride) (PVC)-based ion-selective electrodes (ISEs) with internal filling solution. The potentiometric response of the SCISEs was studied and compared with that of coated-wire electrodes (CWEs) prepared by coating the bare substrate with the same ion-selective membrane. The potentiometric slopes, limits of detection and response times of the SCISEs were similar to those of the corresponding CWEs, but the long-term stability of the potential was different for the two type of electrodes. The SCISEs exhibited a more stable electrode potential than the corresponding CWEs. However, the stability of the SCISEs was found to be influenced by the substrate material and this was studied in detail for the Ca-SCISE and Ca-CWE. For comparison, a Ca-ISE with internal filling solution was also used. By using glassy carbon as the substrate it was possible to obtain a Ca-SCISE exhibiting a standard potential that was almost as stable (E(SCISE) = 259.3 +/- 1.3 mV, drift = 0.23 mV d(-1)) as for the conventional Ca-ISE (E(ISE) = 61.4 +/- 0.5 mV, drift = 0.16 mV d(-)) and significantly more stable than for the Ca-CWE, during a time period of 8 d. The most stable Ca-CWE, prepared by using glassy carbon as substrate, showed a potential drift of -3.8 mV d(-1) (E(CWE) = 269.6 +/- 10.2 mV) during testing for 8 d. Electrochemical impedance spectrometry was used to understand the charge-transfer mechanisms of the different types of ion-selective electrodes studied. The impedance response of the electrodes was modelled by equivalent electrical circuits.