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An in situ evaluation of the dsDNA-methotrexate (MTX) interaction was performed by voltammetry using a DNA-electrochemical biosensor and characterized by atomic force microscopy (AFM) at a highly oriented pyrolytic graphite (HOPG) surface. Electrochemical experiments in incubated solutions showed that the interaction of MTX with dsDNA leads to modifications to the dsDNA structure in a time-dependent manner. The AFM images show reorganization of the DNA self-assembled network on the surface of the HOPG electrode upon binding methotrexate and the formation of a more densely packed and slightly thicker MTX-dsDNA lattice with a large number of aggregates embedded into the network film. The intercalation of MTX between complementary base pairs of dsDNA lead to the increase of purine oxidation peaks due to the unwinding of the dsDNA. The dsDNA-electrochemical biosensor and the purinic homo-polynucleotide single stranded sequences of guanosine and adenosine, poly[G] and poly[A]-electrochemical biosensors, were used to investigate and understand the interaction between MTX and dsDNA.

Methotrexate (MTX) is an antimetabolite of folic acid indicated in the treatment of a variety of cancers. The electrochemical behaviour of MTX on a glassy carbon electrode was investigated. The MTX oxidation is a complex, pH‐dependent, diffusion‐controlled irreversible process and proceeds with the transfer of two electrons and two protons and the formation of one electroactive product, 7‐hydroxymethotrexate that undergoes a reversible redox reaction. The MTX reduction is a pH‐dependent, quasi‐reversible process and involves the transfer of two electrons and two protons leading to the formation of an electroactive product.

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The electrochemical redox behavior of omeprazole (OMZ), a gastric acid pump inhibitor, was investigated at a glassy carbon electrode using cyclic, differential pulse and square-wave voltammetry over a wide pH range. The pHdependent oxidation occurs in two irreversible consecutive charge transfer reactions. Adsorption of the nonelectroactive product was also observed. The first oxidation involves removal of one electron, followed by deprotonation and leads to the formation of a hydroxylated species. The second oxidation process is related to the hydroxyl and amino groups in the benzimidazole moiety. The reduction is irreversible, also pH-dependent, and occurs in a single step at the sulfoxide group in a diffusion-controlled mechanism. The diffusion coefficient of omeprazole was calculated to be D OMZ ¼ 2.31 Â 10 À6 cm 2 s À1 .