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
