A novel microscale and surface-based method for the study of the interactions of DNA with other redox-active molecules using DNA-modified electrodes is described. The method is simple, convenient, reliable, reagent-saving, and applicable for DNA studies, especially those involving microsamples. Information such as binding site size (s, in base pairs), binding constant (K), ratio (K0x/KRed) of the binding constants for the oxidized and reduced forms of a bound species, binding free energy (delta Gb), and interaction mode, including changes in the mode of interaction, and "limiting" ratio K0x0/KRed0 at zero ionic strength can be obtained using only 3-15 micrograms of DNA samples. The method was developed using [Co(Phen)3]3+/2+ (Phen = 1,10-phenanthroline)/double-stranded DNA (dsDNA)-modified gold electrodes and [Co(bpy)3]3+/2+ (2,2'-bipyridyl)/dsDNA-modified gold electrodes as model systems. For the [Co(Phen)3]3+/2+/dsDNA-modified gold electrode system, a K2+ of (2.5 +/- 0.3) x 10(5) M-1 and an s of 5 bp were obtained in 5 mM pH 7.1 Tris-HCl buffer solution containing 50 mM NaCl. For [Co(bpy)3]3+/2+/dsDNA-modified gold electrodes, K3+ and s values of (1.3 +/- 0.3) x 10(5) M-1 and 3 bp, respectively, were obtained. While the s values are consistent with those reported in the literature obtained by solution methods, the K values are almost an order of magnitude larger. A transition in the nature of the interaction between dsDNA and [Co(Phen)3]3+/2+, from electrostatic to intercalative with increasing ionic strength, was found in our studies. Negative values of delta E0' for [Co(bpy)3]3+/2+ bound to dsDNA suggest that its interaction with dsDNA is predominantly electrostatic over the ionic strength range of 5-105 mM. The "limiting" ratio K3+0/K2+0 of 22 obtained for [Co(Phen)3]3+/2+ bound to dsDNA at zero ionic strength suggests that electrostatic interactions are predominant over intercalative ones under these limiting conditions. The ratio for [Co(bpy)3]3+/2+ of 16 also indicates that the 3+ form binds to dsDNA more strongly than the 2+ form at zero ionic strength. For [Co(Phen)3]3+/2+/single-stranded DNA (ssDNA)-modified gold electrodes, the nonuniformity of the surface structure of ssDNA-modified gold electrodes greatly complicates the analysis. A system consisting of a dsDNA-modified gold electrode and [Co(tppz)2]3+/2+ (tppz = tetra-2-pyridyl-1,4-pyrazine) was studied by this method, with a K2+ value of (5 +/- 1) x 10(5) M-1 and an 8 value of 7 bp being obtained.