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A novel and sensitive biosensor employing immobilized DNA on a pencil graphite electrode modified with polypyrrole/functionalized multiwalled carbon nanotubes for the determination of 6-mercaptopurine (6-MP) is presented. In the first step, we modified the pencil graphite surface with polypyrrole and functionalized multiwalled carbon nanotubes (MWCNT/ COOH). The developed electrode was characterized by scanning electron microscopy, atomic force microscopy, reflection− absorption infrared spectroscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy. In the other step, we used decreases in the oxidation responses of guanine and adenine as a sign of the interaction of 6-MP with salmon sperm double-stranded DNA using differential pulse voltammetry. The signal of guanine oxidation was linear with respect to the 6-MP concentration in the range of 0.2−100 μmol L −1 with a detection limit of 0.08 μmol L −1 . The modified electrode was utilized for the determination of 6-MP in real samples.

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