Electrochemical study of electroactive reagent retention in overoxidised polypyrrole films

Abstract

The analytical performance of polypyrrole (PPy) films containing immobilised chelating reagents in stripping voltammetry-type applications is unsatisfactory in terms of the detection limits obtainable. This may be attributed to the large background currents due to the presence of a conducting PPy material on the working electrode surface. Overoxidation of PPy films is known to destroy their electronic conductivity. An electrochemical study of chelating reagent retention in overoxidised PPy (OPPy) has been undertaken in order to ascertain the retention, or otherwise, of such reagents in the OPPy film. Two electroactive reagents, 3-(2-pyridyl)-5,6-diphenyl-4,4′-disulfonate-1,2,4-triazine (PDTDS 2−) and anthraquinone-2,6-disulfonate (AQDS 2−), have been studied. These reagents were detected in the conducting (as grown) PPy film, exhibiting expected pH and sweep rate dependencies. The molar ratios of the pyrrole monomer-to-reagents in such films [11.8 (±1.9) : 1 for the PDTDS 2− system and 6.9 (±1.1) : 1 for the AQDS 2− system] were close to that expected for a PPy film with a 0.25 oxidation level per pyrrole moiety doped with di-anions. The reduction currents for the redox-active agents in the films after overoxidation were much decreased compared to prior to overoxidation. The molar ratios of pyrrole monomer-to-reagent in the OPPy films were not constant; rather, the amount of reagent detectable, via integration of the voltammetric peak, remained constant with increasing amounts of deposited pyrrole. This can be due to: (i) trapping of the anions in polymer regions inaccessible to the electrode surface so that they become immobile and cannot diffuse through the polymer for detection at the underlying electrode; (ii) the electroinactivity of OPPy, meaning that no redox species are available in the film to shuttle charge to/from the trapped agent at locations remote from the electrode surface; or (iii) expulsion of the anions from the film.