The binding of the electroactive hexaammineruthenium (111) complex ions to anionic self-assembled monolayers (SAMs) has been investigated by means of chronocoulometry and ac voltammetry. From chronocoulometric data recorded in 10(-2) M LiClO4 containing different [Ru(NH3)(6)](3+) concentrations, we have established the adsorption isotherm of [Ru(NH3)(6)](3+) on a compact monolayer of 2-mercaptobenzimidazole-5-sulfonate (MBIS) self-assembled on Au(111). The data were satisfactorily fitted to the linearized Langmuir adsorption isotherm and a binding constant of 4.0 (+/- 0.4) x 10(6) M-1 has been determined. The electrostatic binding of [Ru(NH3)(6)](3+) to a dilute PNA-DNA monolayer formed after hybridization on a PNA-modified gold electrode by self-assembly from a mixed solution of mercaptobutan-1-ol and PNA oligonucleotides has been studied by ac voltammetry. The admittance of the PNA-modified electrode after hybridization with complementary DNA was measured in 0.01 M Tris-HCl buffer containing different [Ru(NH3)(6)](3+) concentrations. Based on these data, a binding constant of [Ru(NH3)(6)](3+) to the surface-confined PNA-DNA duplex was derived from the Langmuir isotherm and amounts to 2.9 (+/- 0.3) x 10(5) M-1. As the interactions between [Ru(NH3)(6)](3+) and the immobilized PNA-DNA hybrids on the gold surface are essentially electrostatic, the adsorption of the highly charged cationic redox complex at low concentrations to the negatively charged PNA-DNA modified surface is in large competition with other monovalent cations present in the electrolyte at higher concentrations. The influence of competing sodium cations was thus studied by adding different NaCl concentrations in the 0.01 M Tris-HCl electrolyte. (C) 2008 Elsevier Ltd. All rights reserved.