DNA-Modified Electrodes: Molecular Recognition and Electrochemical Response

A biosensor for the detection of the compounds that bind to DNA is reported. A chemical derivatization of terminal phosphate ends of DNA double strands (ds) with 2-hydroxyethyl disulfide was made to immobilize DNA onto Au surfaces via chemisorption. By taking advantages of the redox couple-mediated artificial ion-channel principle, the DMA-modified electrode was successfully applied for a bioaffinity sensor. For example, cyclic voltammograms (CV) of ferrocyanide/ferricyanide couple with the DNA-modified electrode gave the redox wave due to the reversible electrode reaction. The CV peak currents were significantly enhanced on adding quinacrine. The peak currents showed almost a linear relationship with the concentration of quinacrine in the range of 10^-7 - 5 × 10^-7 M, and then saturated beyond the concentration of 8 × 10^-7 M. CV responses toward a variety of DNA-binding substrates including quinacrine, acridine orange, safranin, spermine and spermidine showed a reasonable selectivity order according to the DNA-binding affinity of these compounds, while the response was quite small for methyl viologen which binds to ds DNAs through a nonspecific electrostatic interaction. The sensitivity was diminished when the immobilized ds DNAs were heat-denatured. The voltammetric response is primarily due to a "titration" of the immobilized ds DNAs by DNA-binding molecules. Thus the response-concentration profile for quinacrine was analyzed by using the Langmuir's isotherm. The apparent binding constant thus obtained was 1.3 × 10⁶ M^-1 which agreed fairly well with that in literatures (1.5 × 10⁶ M^-1).