Electrochemical quartz crystal impedance study on the overoxidation of polypyrrole-carbon nanotubes composite film for amperometric detection of dopamine.

The electrochemical quartz crystal impedance (EQCI) method was used to study the overoxidation of polypyrrole (PPy)-multiwalled carbon nanotubes (MWCNT) nanocomposite film in neutral and alkaline solutions. The values of molar mass per electron transferred (M/n) obtained during the overoxidation of PPy in 0.10 mol L(-1) Na(2)SO(4) and 0.20 mol L(-1) NaOH aqueous solutions were estimated to be ca. 17 and 22 g mol(-1), respectively, suggesting the nucleophilic attack of solution OH(-) to the pyrrole units during the overoxidation, and the possible partial formation of carboxylic groups after the overoxidation in the NaOH solution. Also, the overoxidized PPy-MWCNT composite film prepared in the NaOH solution showed a notably larger affinity to dopamine (DA) dissolved in a neutral phosphate buffer than that prepared in the Na(2)SO(4) solution. The modification of the overoxidized nanocomposite film improved substantially the sensitivity for DA assay in a neutral phosphate buffer, as compared with the modification of overoxidized PPy or MWCNT alone. At a -6 kHz (201-nm thickness) nanocomposite film prepared in a polymerization bath containing 1.0 mg mL(-1) MWCNT and overoxidized in 0.20 mol L(-1) aqueous NaOH, the peak current response from differential pulse voltammetric assay of DA was linear with DA concentration from 4.0 x 10(-8) to 1.4 x 10(-6) mol L(-1), with a lower limit of detection of 1.7 nmol L(-1), good anti-interferent ability, as well as good stability and reproducibility.