To examine DNA-modified surfaces, we have developed a simple, convenient, and reliable procedure based on the voltammetric response of multiply charged transition metal cations (such as [Ru(NH3)6]3+) bound electrostatically to the DNA probes. At micromolar concentrations of the redox molecules in the electrolyte, the reduction and oxidation waves resulting from the immobilized cations on DNA-modified electrodes are well defined, stable, and reproducible. The surface densities of both single- and double-stranded oligonucleotides were accurately determined by integration of the peak for reduction of [Ru(NH3)6]3+ to [Ru(NH3)6]2+. In addition, the binding constant and electron-transfer rate constant of [Ru(NH3)6]3+ on DNA-modified electrodes were evaluated with the help of classical models. The present research provides not only an applicable and simple protocol for the quantitation of DNA probes on chips but also a versatile and powerful tool for the investigation of the binding activity and electron-transfer kinetics of cationic analytes on DNA-modified surfaces.

The ion-exchange kinetics of metal cation binding to and dissociation from thiolate-DNA monolayers on gold can be monitored by a simple electrochemical protocol. The apparent first-order rate constants were obtained by analyzing the time-dependent voltammetric behavior of the redox cation [Ru(NH3)6]3+. It was found that the binding kinetics is dominated by the structural nature of the film; i.e., the apparent first-order rate constant (kapp) decreases significantly upon increasing the surface density of DNA strands. Dissociation rate constants were obtained by transferring the incubated electrode into redox-free buffer solution. The kinetic data augment our fundamental understanding of metal ion-DNA interactions and are critical to ensure the accuracy and reliability of experimental DNA detection protocols.

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