Scavenging of DPPH free radicals by polypyrrole powders of varying levels of overoxidation and/or reduction

Polypyrrole (PPy) powders were synthesized chemically using the oxidant ammonium persulfate (APS). The free radical scavenging properties were evaluated using a form of the alpha,alpha-diphenyl-beta-picrylhydrazyl (DPPH) assay adapted for use with 1.0 mg amounts of conducting polymer powders. A lower scavenging capacity was obtained when a higher ratio of APS to pyrrole was used in the chemical synthesis which also led to a decrease in polymer conductivity and doping level as determined by the amount of (bi)sulfate present given by elemental analyses. The occurrence of overoxidation of polypyrrole at a higher oxidant level was confirmed in FTIR spectra by the presence of a carbonyl group. PPy prepared at a lower APS/pyrrole ratio contained more bipolaron charge carriers, which were converted to polarons upon initial reduction with hydrazine. More complete reduction with hydrazine led to a loss of(bi)sulfate dopant, but to improved DPPH radical scavenging, confirming that the conducting polymer is acting as a reducing agent in scavenging DPPH radicals. (C) 2008 Elsevier B.V. All rights reserved.