Methylene Blue Interactions with Chromosomal and Plasmid DNA on Screen‐Printed Carbon Electrodes

Methylene blue (MB) is a typical photosensitizing agent and a DNA hybridization indicator, but its modes of interaction with the DNA molecules are not clearly described, particularly in relation to its electrochemical oxidation signals. To probe the DNA‐MB interactions we have used chromosomal salmon testes and supercoiled plasmid sc pUC19 DNA immobilized on home‐made screen‐printed electrodes (SPEs) and a wide range of MB concentrations, from nano‐ to micromolar. The applicability of the home‐made screen‐printed electrodes used for the DNA‐MB studies were tested using standard calf thymus DNA. Two MB oxidation peak signals: MB(I) at ca. ?0.18 V and MB(II) at 0 V vs. Ag/AgCl were detected within ±10–15% standard deviation, signals different from adsorbed MB signal (?0.25 V, pH 4.7). The MB(I) signal, seen when both DNAs were used, showed two plateaus, one at nano‐ and another at micromolar MB concentrations; these were accompanied by the changes in the oxidation signal at 0.98 V, usually attributed to guanine oxidation. In contrast, the MB(II) signal was only seen for salmon testes DNA, indicating various modes of MB interactions with chromosomal and plasmid DNA. In the presence of MB, the guanine related signal (G) at 0.98 V has been amplified significantly (10×), allowing for the identification of the DNAs at low DNA concentrations, the feature particularly useful in the plasmid sc pUC19 detection. The use of another DNA intercalator, riboflavin (RF), aided in the identification of the relation between MB(I), MB(II) and G oxidation signals.