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 (thick ness approximately 0.25 mu m was deposited on a solid substrate (plati num, gold or glassy carbon) by electrochemical polymerization of 3-oct ylthiophene. The POT layer was subsequently coated with an ion-selecti ve 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 wi th that of coated-wire electrodes (CWEs) prepared by coating the bare substrate with the same ion-selective membrane. The potentiometric slo pes, limits of detection and response times of the SCISEs were similar to those of the corresponding CWEs, but the long-term stability of th e potential was different for the two type of electrodes. The SCISEs e xhibited a more stable electrode potential than the corresponding CWEs . However, the stability of the SCISEs was found to be influenced by t he substrate material and this was studied in detail for the Ca-SCISE and Ca-CWE. For comparison, a Ca-ISE with internal filling solution wa s also used. By using glassy carbon as the substrate it was possible t o 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 th e 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 peri od 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 spectrom etry was used to understand the charge-transfer mechanisms of the diff erent types of ion-selective electrodes studied. The impedance respons e of the electrodes was modelled by equivalent electrical circuits.