Electrochemical properties of DNA-intercalating doxorubicin and methylene blue on n-hexadecyl mercaptan-doped 5'-thiol-labeled DNA-modified gold electrodes.

Interactions between DNA-intercalating molecules, methylene blue (MB) and doxorubicin (DOX), and gold surface modified by various DNA species and n-hexadecyl mercaptan (HDM) were investigated by cyclic voltammetry (CV). Hydrophilic DOX was completely blocked by the HDM film from contacting the gold electrode whereas hydrophobic MB could readily partition into the film. Unlabeled single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) underwent non-specific adsorption on gold surface but the adsorbed DNA can be partially displaced by HDM. Thiol-labeled ssDNA and dsDNA adsorbed on gold surface via both thiol-gold linkage and non-specific interactions between DNA strands and gold. The non-specific interactions could be interrupted by the addition of HDM, forming a mixed monolayer containing both HDM and DNA attached to the gold surface at 5'-thiol termini. The presence of ssDNA and dsDNA in the monolayer facilitated the redox reaction of MB and DOX on the modified electrode. Both MB and DOX diffuse along the ssDNA in the ssDNA-containing monolayers, and they additionally intercalate into the dsDNA in the dsDNA-containing monolayers. No sufficient evidence is shown to indicate that an organized monolayer is formed by the thiol-labeled dsDNA on gold surface, and that the redox reactions of MB and DOX were carried out by electron transfer through DNA helix.