A 20-mer oligonucleotide whose 5′-end phosphate was modified with ω-thiohexyl group was applied in order to modify a Au electrode with its duplex form. Cyclic voltammograms of ferrocyanide/ferricyanide redox couple with the DNA-immobilized electrode gave the peak currents due to the reversible electrode reaction, which were significantly enhanced on adding acridine orange (AO). The cathodic peak current showed almost a linear relationship with AO concentration in the range of 10-7 - 7×10-7 M, and then saturated beyond the concentration of 10-6 M. From the concentration-dependent profiles, an apparent binding constant of (9.1 ± 5.4) × 105 M-1 was obtained for the AO-DNA system. The obtained data agreed fairly well with the reported value (1.2 ± 0.3)×106 M-1.
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