Single-stranded oligonucleotides (ssDNA) containing guanine bases in their sequences were adsorbed onto gold nanoparticles (AuNPs) by electrostatic interaction. Cyclic voltammetry of Ru(bpy) 3 2+ in the presence of the ssDNA-AuNP complex resulted in an enhanced anodic current due to the oxidation of the guanine bases of DNA. The current obtained with ssDNA-AuNP appeared much smaller than the corresponding ssDNA alone. This current reduction was due to the decrease in solvent accessibility of the guanines in ssDNA immobilized to AuNPs. A progressive decrease in the current was observed in the titration of AuNPs to ssDNA, and a minimum current was eventually obtained, indicating complete binding of ssDNA. The size dependences of AuNPs on the interaction between ssDNA and AuNP were also studied, and the ssDNA adsorbed to 5 nm AuNPs was more solvent-accessible for the Ru mediator than 13 and 30 nm AuNPs.