Electrochemical study of electroactive reagent retention in overoxidised polypyrrole films

The analytical performance of polypyrrole (PPy) films containing immobilise d chelating reagents in stripping voltammetry-type applications is unsatisf actory in terms of the detection limits obtainable. This may be attributed to the large background currents due to the presence of a conducting PPy ma terial on the working electrode surface. Overoxidation of PPy films is know n to destroy their electronic conductivity. An electrochemical study of che lating reagent retention in overoxidised PPB (OPPy) has been undertaken in order to ascertain the retention, or otherwise, of such reagents in the OPP y film. Two electroactive reagents, 3-(2-pyridyl)-5,6-diphenyl-4,4'-disulfo nate-1,2,4-triazine (PDTDS2-) and anthraquinone-2,6-disulfonate (AQDS(2-)), have been studied. These reagents were detected in the conducting (as grow n) 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 PDTDS2- system and 6.9 (+/- 1.1): 1 for the AQDS2- system] were clo se 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 ag ents in the films after overoxidation were much decreased compared to prior to overoxidation. The molar ratios of pyrrole monomer-to-reagent in the OP Py films were not constant; rather, the amount of reagent detectable, via i ntegration of the voltammetric peak, remained constant with increasing amou nts 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 underl ying electrode; (ii) the electroinactivity of OPPy, meaning that no redox s pecies are available in the film to shuttle charge to/from the trapped agen t at locations remote from the electrode surface; or (iii) expulsion of the anions from the film. (C) 1999 Elsevier Science B.V. All rights reserved.