Abstract

            In this study, the interaction between codeine and morphine with dsDNA was assessed at pH 7.0. Poly(diallyldimethylammonium chloride), PDDA, was used as a dispersant of MWCNTs. Using differential pulse voltammetry (DPV) at pencil graphite electrode (PGE) showed that both molecules were electrochemically oxidized due to the presence of phenolic and amino groups in their structures. When DNA was added to the solution, the electrochemical signal of codeine and morphine was decreased and shifted to more negative and positive potentials, respectively. The interaction modes were respectively electrostatic for codeine and intercalation for morphine with two anodic peaks of codeine being merged into them when DNA concentration was increased. At high DNA concentrations, a sharp anodic wave for codeine and a clear discrimination of codeine and morphine oxidation peaks were observed. Finally, a pencil graphite electrode was modified with carbon nanotubes and DNA was tested in order to determine codeine and morphine in solution. Electrochemical oxidation of codeine and morphine bonded on dsDNA/MWCNTs–PDDA/PGE was used to obtain an analytical signal. Allowing five min as an accumulation time, a linear dependence was observed between 0.05 and 40 µg mL^–1 for codeine and 0.05 and 42 µg mL^–1 for morphine. Detection limits of 0.041 and 0.043 µg mL⁻¹ were obtained for codeine and morphine, respectively. The biosensor was applied to validate its capability for the analysis of codeine and morphine in blood serum, urine samples and pharmaceutical formulations.