DNA-promoted electrochemical assembly of [Ru(bpy)2IP] at an ITO electrode

A controllable assembly technique of [Ru(bpy)₂IP]^3+/2+ (where bpy = 2,2′-bipyridine and IP = imidazo[4,5,f][1,10]phenanthroline) promoted by calf thymus DNA at an ITO electrode is proposed. The stable assembled layer containing [Ru(bpy)₂IP]^3+/2+ and double stranded DNA is obtained on the ITO electrode using repetitive voltammetric sweeping, confirmed by ex situ voltammetry, X-ray photoelectron spectroscopy (XPS) and the inverted fluorescence microscopy. There exist two pairs of diffusion-controlled waves and two pairs of prewaves for [Ru(bpy)₂IP]²⁺ in the voltammetric sweeping process. The half-wave potentials of the prewaves are far more negative than those of the diffusion-controlled waves. These experimental results suggest that double stranded DNA is enable to accelerate and increase the controllable assembly of Ru(bpy)₂IP]^3+/2+ by using the ITO surface. The fluorescence microscopy imaging reveals that [Ru(bpy)₂IP]^3+/2+ has the ability to bind with double strand DNA. The fluorescence intensity of [Ru(bpy)₂IP]^3+/2+ with DNA is stronger than that without DNA.     

DNA-promoted electrochemical assembly of [Ru(bpy)2dpp] on the ITO electrode by introducing copper(II) ion

The electrochemical assembly of [Ru(bpy)₂dpp]^3+/2+ (where bpy = 2,2′-bipyridine, dpp = 2,3-bis (2-pyridyl) pyrazine) promoted by calf thymus DNA on an ITO electrode based on the introduction of copper(II) ion has been investigated. There exists a diffusion-controlled wave and two prewaves for the complex in the differential pulse voltammetric sweeping process. The formal potential of the high prewave shift ca. 0.530 V negatively compared with that of the diffusion-controlled wave. Dpp ligand with two vacant chelating N sites in the complex can bite Cu²⁺ and the resultant heterometallic complex shows a weakened assembly in contrast to that of [Ru(bpy)₂dpp]^3+/2+ alone. Furthermore, double stranded DNA is able to accelerate the assembly of the ruthenium complex and heterometallic complex generated by chelating with Cu²⁺ by using the ITO surface, the prompted strength of the latter is far stronger than the former. Their assembled mechanism enhanced by DNA is proposed.     

Mediated oxidation of guanine by [Ru(bpy)2dpp] and their electrochemical assembly on the ITO electrode

Electrochemical oxidation of guanine mediated by [Ru(bpy)₂dpp]²⁺ (where bpy = 2,2′-bipyridine, dpp = 2,3-bis (2-pyridyl) pyrazine) and their electrochemical assembly at an ITO electrode prompted by guanine have been investigated with cyclic voltammetry and differential pulse voltammetry. It is found that [Ru(bpy)₂dpp]²⁺ can serve as an excellent mediator to induce the oxidation of guanine, and the mediated peak currents increase linearly with the rise of guanine concentration in the range from 0.01 to 0.20 mmol L⁻¹. Interestingly, with the increase of repetitive voltammetric sweeping numbers, [Ru(bpy)₂dpp]^3+/2+ can be assembled onto the ITO electrode and guanine has the ability to enhance the peak currents of prewaves. Also, with the rise of guanine concentration from 0.01 to 0.15 mmol L⁻¹, the peak currents of prewaves increase gradually. Meanwhile, the mediated mechanism of guanine oxidation by [Ru(bpy)₂dpp]²⁺ and the assembled process of [Ru(bpy)₂dpp]^3+/2+ on the ITO surface in the presence of guanine are discussed in detail.     

Electrochemical assembly of [Ru(bpy)<Subscript>2</Subscript>tatp]<Superscript>2+</Superscript> associated with surfactants on the MWNTs/GC electrode

The electrochemical assembly of [Ru(bpy)₂tatp]²⁺ (where bpy = 2,2′-bipyridine, tatp = 1,4,8,9-tetra-aza-triphenylene) on the multi-walled carbon nanotubes-modified glassy carbon electrode (MWNTs/GC) in the presence of anionic and cationic surfactants has been investigated. A diffusion-controlled wave and three prewaves are exhibited on the differential pulse voltammogram of [Ru(bpy)₂tatp]²⁺. The formal potential of the prewaves is found to be much negative than that of the diffusion-controlled wave. An appropriate amount of anionic surfactants including dihexadecyl phosphate (DHP) and deoxyribonucleic acid (DNA) can prompt the assembly of [Ru(bpy)₂tatp]²⁺ on the MWNTs/GC electrode by using the method of repetitive voltammetric sweeping. In contrast, cationic surfactant such as hexadecyl trismethyl ammonium chrolide (HTAC) dispersed on the MWNTs surface is found to inhibit the assembly of [Ru(bpy)₂tatp]²⁺. Meanwhile, the assembled principle of [Ru(bpy)₂tatp]²⁺ on the MWNTs/GC electrode with the participation of surfactants is discussed in detail.     

Electrochemical fabrication and potential-enhanced luminescence of [Ru(bpy)2tatp] incorporating DNA-stabilized single-wall carbon nanotubes on an indium tin oxide electrode

A simple method was developed for the preparation of [Ru(bpy)₂tatp]²⁺-based aggregates (where bpy = 2,2′-bipyridine, tatp = 1,4,8,9-tetra-aza-triphenylene) on an indium tin oxide (ITO) electrode in the presence of DNA-stabilized single-walled carbon nanotubes (DNA–SWCNTs). The presence of SWCNTs in the concentration range from 0.02 to 0.125 g L⁻¹ dispersed with 0.25 mmol L⁻¹ DNA was found to promote the immobilization of [Ru(bpy)₂tatp]²⁺ on the ITO electrode by the method of repetitive voltammetric sweeping. The photoluminescence of [Ru(bpy)₂tatp]²⁺ incorporating DNA–SWCNTs both in solution and on the ITO electrode was systematically investigated by emission spectra and fluorescence microscopic imaging. An excess amount of SWCNTs can quench the photoluminescence of [Ru(bpy)₂tatp]²⁺ enhanced by DNA. The anodic potentials combined with CW green laser via an optical microscope was found to significantly increase the emission intensity of [Ru(bpy)₂tatp]²⁺–DNA–SWCNTs aggregates on the ITO electrode. In addition, the electrochemical fabrication and photoluminescence principles of [Ru(bpy)₂tatp]²⁺–DNA–SWCNTs aggregates on the ITO electrode tuned by the external electric fields were discussed in detail.     

Characterization of electrochemiluminescence of tris(2,2′-bipyridine)ruthenium(II) with glyphosate as coreactant in aqueous solution

Glyphosate, a phosphorus-containing amino acid type herbicide was used as a coreactant for studying of electrochemiluminescence (ECL) reaction of tris(2,2′-bipyridyl)ruthenium(II) [Ru(bpy)₃²⁺] in an aqueous solution. In a phosphate buffer solution of pH 8, glyphosate itself was known to be electrochemically inactive at glassy carbon electrode, however, it participated in a homogeneous chemical reaction with the electrogenerated Ru(bpy)₃³⁺, and resulted in producing Ru(bpy)₃²⁺ species at the electrode surface. Kinetic and mechanistic information for the catalysis of glyphosate oxidation were evaluated by the steady-state voltammetric measurement with an ultramicroelectrode. The simulated cyclic voltammogram based on this mechanism was in good agreement with that obtained experimentally. ECL reaction of Ru(bpy)₃²⁺/glyphosate system was found to be strongly dependent on the media pH. In a pH region of 5-9, an ECL wave appeared at ca. +1.1 V vs. Ag/AgCl, which was caused by the generation of *Ru(bpy)₃²⁺ via a Ru(bpy)₃³⁺-mediated oxidation of glyphosate. When pH >10, a second ECL wave was observed at ca. +1.35 V vs. Ag/AgCl, which was believed to be associated with a reaction between Ru(bpy)₃³⁺ and the species from direct oxidation of GLYP at a GC electrode surface.     

Ru(bpy)3-doped silica nanoparticle DNA probe for the electrogenerated chemiluminescence detection of DNA hybridization

A sensitive electrogenerated chemiluminescence (ECL) detection of DNA hybridization, based on tris(2,2′-bipyridyl)ruthenium(II)-doped silica nanoparticles (Ru(bpy)₃²⁺-doped SNPs) as DNA tags, is described. In this protocol, Ru(bpy)₃²⁺-doped SNPs was used for DNA labeling with trimethoxysilylpropydiethylenetriamine(DETA) and glutaraldehyde as linking agents. The Ru(bpy)₃²⁺-doped SNPs labeled DNA probe was hybridized with target DNA immobilized on the surface of polypyrrole (PPy) modified Pt electrode. The hybridization events were evaluated by ECL measurements and only the complementary sequence could form a double-stranded DNA (dsDNA) with DNA probe and give strong ECL signals. A three-base mismatch sequence and a non-complementary sequence had almost negligible responses. Due to the large number of Ru(bpy)₃²⁺ molecules inside SNPs, the assay allows detection at levels as low as 1.0 × 10⁻¹³ mol l⁻¹ of the target DNA. The intensity of ECL was linearly related to the concentration of the complementary sequence in the range of 2.0 × 10⁻¹³ to 2.0 × 10⁻⁹ mol l⁻¹.     

A novel DNA light switch [Ru(bpy)2pzip]2+ activated by cobalt(II) ion

A novel ruthenium(II) complex, [Ru(bpy)₂pzip]²⁺ has been synthesized and characterized. The DNA-binding properties of this complex have been studied by spectroscopic and viscosity measurements. The results indicated that the complex [Ru(bpy)₂pzip]²⁺ bound to double-stranded DNA in an intercalation mode. In the presence of Co²⁺, the emission of DNA–[Ru(bpy)₂pzip]²⁺ can be quenched. And when EDTA was added, the emission was recovered. The experiment results show that [Ru(bpy)₂pzip]²⁺ exhibited the “on–off–on” properties of molecular “light switch”.     

Hydroxyl radical-related electrogenerated chemiluminescence reaction for a ruthenium tris(2,2′)bipyridyl/co-reactants system at boron-doped diamond electrodes

An electrogenerated chemiluminescence (ECL) reaction of the Ru(bpy)₃²⁺ (2,2′-bipyridyl, bpy)/co-reactant system in the extremely high-potential region (over 2.6 V versus Ag/AgCl) was probed using a boron-doped diamond (BDD) electrode. At the BDD electrode, three ECL waves (1.25, 2.30 and 3.72 V) were observed in cyclic voltammograms for 20 mM ascorbic acid (AA). For the ECL peaks observed at 1.25 V corresponding to the oxidation potential for Ru(bpy)₃²⁺ (1.15 V), the light intensities and current densities were found to depend on the square root of the AA concentration. This suggests that AA oxidation, followed by the formation of the reducing radical that is necessary for generating the excited state of Ru(bpy)₃^2+* occurred through homogeneous electron-transfer between Ru(bpy)₃³⁺ and the AA species. However, for the ECL peaks at 2.30 V, the current densities and light intensities linearly increased with increasing AA concentration, suggesting that the reducing radical was formed through the direct oxidation at the electrode surface. The ECL reaction at 3.72 V was observed only at the BDD electrode and not at other electrodes. The onset potentials for the light intensity were approximately 2.6 V, independently of the type of the co-reactants (e.g. 2-propanol and AA). The peak potentials exhibited linear relation with the co-reactant concentration. In the analysis of the ECL intensity for various co-reactants (alcohols) that show different reactivity for the hydrogen abstraction reaction, the order of the light intensities at the peaks for alcohols was found to be consistent with that for the rate constants of the hydrogen abstraction reaction. These results indicate that the co-reactant radical was formed through the hydrogen abstraction reaction with the hydroxyl radical (HO) generated during the oxygen evolution reaction.     

Preparation and characterization of thin films of amine functionalised mesoporous silica having cubic pore structures and their use for electrode surface modifications

Thin films of amine functionalised mesoporous silica were deposited on tin-doped indium oxide (ITO) slides by dip-coating mixtures of TEOS (tetraethyl orthosilicate), APTES ((3-aminopropyl)triethoxysilane), and the non-ionic surfactant Brij56. The pore structure of the films was found to depend on the temperature use during calcination to remove the templates. Films calcined at 350 °C had highly ordered mesostructures with face-centred cubic pore arrangements, while films calcined at 275 °C had primitive cubic pore structures. The as-prepared films, or films extracted with acidic ethanol solutions had very poorly ordered mesopores. Calcination at 275 °C was sufficient to remove the Brij56 templates, without damaging the organic amine chains. Calcination at 350 °C however, also resulted in the lost of the organic groups. The use of ITO substrates allowed the dip-coated films to be used directly as working electrodes. At pH 4.7, the voltammetric response of [Ru(bpy)₃]²⁺ was completely suppressed by the amine functionalised films. This was attributed to protonation of the amine groups that gave the films positive charges. At higher pH, the amine groups were no longer protonated and the voltammetric response of [Ru(bpy)₃]²⁺ was restored. Conversely, [Ru(bpy)₃]²⁺ was adsorbed by the unmodified films that carried negative charges at pH 4.7, resulting in a 4-fold increase in the peak currents compared to uncoated ITO. For [Fe(CN)₆]^3? at low pH the unmodified films blocked the response of the ions, while the functionalised films gave small but distinct voltammetric peaks.     

Detection of N-methyladenosine in urine samples by electrochemiluminescence using a heated ITO electrode

A sensitive and rapid electrochemiluminescence (ECL) method for the detection of N⁶-Methyladenosine (m6A) in urine samples on a heated indium-tin-oxide (ITO) electrode is presented. The ECL intensity of Tris(2,2′-bipyridyl) dichlororuthenium(II)hexahydrate (Ru(bpy)₃²⁺) can be enhanced by the presence of m6A. Experimental results showed that the change of ECL intensities (ΔI) of the Ru(bpy)₃²⁺ between before and after addition of m6A was affected by the working electrode surface temperature (T_e); the highest ΔI occurred at 31 °C. Under optimum conditions, the ΔI had a linear relationship with the m6A concentration in the range of 1.9 × 10⁻⁹-3.9 × 10⁻⁶ mol/L and a detection limit of 7.7 × 10⁻¹⁰ mol/L (S/N = 3) at T_e = 31 °C. The recovery of m6A standards added to urine samples verified the accuracy of the proposed method.     

Influencing factors on electrochromic hysteresis performance of ruthenium purple produced by a WO3/Tris(2,2′-bipyridine)ruthenium(II)/polymer hybrid film

A [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine)/WO₃ hybrid (denoted as Ru-WO₃) film was prepared as a base layer on an indium tin oxide electrode by electrodeposition from a colloidal solution containing peroxotungstic acid, [Ru(bpy)₃]²⁺ and poly(sodium 4-styrenesulfonate). A ruthenium purple (RP, Fe^III₄[Ru^II(CN)₆]₃, denoted as Fe^III-Ru^II) layer was electrodeposited on a neat WO₃ film or a Ru-WO₃ film from an aqueous RP colloid solution to yield a WO₃/RP bilayer film or a Ru-WO₃/RP bilayer film, respectively. The spectrocyclic voltammetry measurement reveals that Fe^II-Ru^II is oxidized to Fe^III-Ru^II by a geared reaction of [Ru(bpy)₃]^2+/3+ and Fe^III-Ru^II is reduced by a geared reaction of H_xWO₃/WO₃ in the Ru-WO₃/RP film. These geared reactions produced electrochromic hysteresis of the RP layer. However, the absorbance change in the hysteresis was smaller than that for the Ru-WO₃/Prussian blue bilayer film reported previously, resulting from the lower electroactivities of any redox component for the Ru-WO₃/RP film. The lower electroactivities could be explained by the specific interface between the Ru-WO₃ and RP layers. It might contribute to either an increase of the interfacial resistance between the Ru-WO₃ and RP layers, or formation of the physically precise interface between the layers to make it difficult for counter ions to be transported in the interfacial liquid phase involved in the redox reactions in the film. The specific interface at the Ru-WO₃ and RP layers could be formed possibly by the electrostatic interaction between [Ru(bpy)₃]²⁺ and terminal [Ru(CN)₆]⁴⁻ moieties of RP. It could be suggested by the decreased redox potential of [Ru(bpy)₃]²⁺ in the Ru-WO₃ layer from 1.03 to 0.61 V by formation of the RP layer.     

Effect of decreasing film thickness on the electrochemical responses of cations adsorbed in clay-modified electrodes

Clay-modified electrodes ranging in thickness from 3.4 μm to 8 nm, as estimated from the clay loadings, were prepared using three different smectites by spin-coating, solvent evaporation or electrophoretic deposition. For all three clays, the voltammetic waves obtained for [Ru(bpy)₃]²⁺ or [Os(bpy)₃]²⁺ adsorbed in these CMEs were independent of the film thickness for all films thicker than 100 nm. Only in very thin films, ≤40 nm were significant decreases in the peak currents observed. However, when the contributions to the peak currents from the electroactive concentrations, C* and effective diffusion coefficients, D_eff were separated, the values of C* were found to increase with decreasing film thickness, while D_eff decreased by several orders of magnitude. This was attributed to increase contributions to the electrochemical responses from less mobile electrostatically bound cations in the thinner films. Similar variations in C* and D_eff were obtained in films prepared by solvent evaporation. However, C* obtained in 20 nm thick electrodeposited films were significantly lower than in 40 nm spin-coated films. For [Ru(NH₃)₆]³⁺, the peak currents increased rapidly with the film thickness. However, no significant changes in the values of C* and D_eff with film thickness were found for this ion. This is consistent with the greater mobility of [Ru(NH₃)₆]³⁺ in clays films that allows a larger fraction of the adsorbed ions to remain electroactive even in thicker films. Results obtained for [Fe(bpy)₃]²⁺ were intermediate. While, the peak currents were independent of film thickness, the values of C* or D_eff obtained for this ion were also independent of the clay loadings.     

Electrochemiluminescent determination of nicotine based on tri(2,2′-bipyridyl) ruthenium (II) complex through flow injection analysis

This paper describes the electrogenerated chemiluminescence (ECL) processes of Ru(bpy)₃²⁺/nicotine system at ITO working electrode. An ECL-based method for rapid and sensitive detection of nicotine in phosphate buffer solution at pH 8.0 is established. Strong ECL emission was observed at a positive potential of 1.4 V vs. Ag/AgCl. A possible ECL mechanism is proposed for the Ru(bpy)₃²⁺/nicotine system, the oxidation product of nicotine at the electrode surface reacts with the 3+ state of ruthenium bipyridyl (2+) complex and form ruthenium complex exited state ions and thus releases photons. Effect of pH (medium/electrolyte), working potential, buffer composition, buffer concentration, reactant and co-reactant (nicotine) concentration, flow rate and loop size on the ECL spectrum of the Ru(bpy)₃²⁺/nicotine were studied. At the optimized experimental conditions, lower detection limit for nicotine was observed as 1.2 nmol L⁻¹ (S/N = 3). Linear relationship between ECL current and concentration of nicotine was observed (up to 100 μmol L⁻¹) with R-value of 0.997. The relative standard deviation with 5 μmol L⁻¹ concentration of nicotine for 20 analyses was only 1.4%. A 94% recovery rate was observed in a real sample analysis without any complications/disturbance in measurement. Interferences of humid acid, camphor and SDS were not observed in their presence in the sample solution. The established procedure for nicotine quantification manifests fascinating results and can be suggested for further applications.     

Electrocatalytic activity of [Ru(bpy)<Subscript>3</Subscript>]<Superscript>2+</Superscript> toward guanine oxidation upon incorporation of surfactants and SWCNTs

The homogeneous and mediated oxidation of guanine by [Ru(bpy)₃]²⁺ (2,2′-bipypyridine) in the presence of surfactants and single-walled carbon nanotubes (SWCNTs) has been investigated using cyclic voltammetry, repetitive differential pulse voltammetry and rotating electrode method. In acidic medium, the oxidation of guanine was controlled by mass transport process of [Ru(bpy)₃]²⁺ in solution, leading to a homogeneous electrocatalysis. In neutral medium, the result from emission spectroscopy suggested the formation of the aggregates containing [Ru(bpy)₃]²⁺, dihexadecyl phosphate (DHP) and guanine. The electrocatalysis of [Ru(bpy)₃]²⁺ toward guanine oxidation was promoted by anionic surfactant DHP and, however, hindered by an excess amount of hexadecyl trismethyl ammonium chloride (HTAC) or SWCNTs added to solutions. The electrocatalytic mechanism of [Ru(bpy)₃]²⁺ for guanine oxidation becomes evident, strongly depending on the presence of anionic or cationic surfactants and SWCNTs.     

Development and application of a poly(2,2′-dithiodianiline) (PDTDA)-coated screen-printed carbon electrode in inorganic mercury determination

In this work, monomer solutions of aniline (ANI) and 2,2′-dithiodianiline (DTDA), an aniline derivative containing -S-S- links, were prepared and used in the electrochemical copolymerisation of ANI and DTDA by cyclic voltammetry on a screen-printed electrode (SPE) in 1 M HCl. Electropolymerisation of aniline on the surface of the screen-printed working electrode was performed by sweeping the potential between −500 and + 1100 mV (vs. Ag/AgCl) at a sweep rate of 100 mV/s. Electrocopolymerisation was performed with a mixture of ANI and DTDA by sweeping the potential between −200 and + 1100 mV (vs. Ag/AgCl) at a sweep rate of 100 mV/s [J.L. Hobman, J.R. Wilson, N.L. Brown, in: D.R. Lovley (Ed.), Environmental Microbe Metal Interactions, ASM Press, Herndon, Va, 2000, p. 177]. The cyclic voltammogram (CV) for each of the electrochemically deposited polyaniline (PANI) and the mixture of ANI and DTDA for the co-polymer polymerisation on SPCE were recorded for electrochemical analysis of the peak potential data for the mono and copolymer. Anodic stripping voltammetry (ASV) was used to evaluate a solution composed of (1 × 10⁻⁶ M HgCl₂, 0.1 M H₂SO₄, 0.5 M HCl), in the presence of the co-polymer sensor electrode. The Hg²⁺ ions were determined as follows: (i) pre-concentration and reduction on the modified electrode surface and (ii) subsequent stripping from the electrode surface during the positive potential sweep. The experimental conditions optimised for Hg²⁺ determination included the supporting electrolyte concentration and the accumulation time. The results of the study have shown the use of a conducting polymer modified SPCE as an alternative transducer for the voltammetric stripping and analysis of inorganic Hg²⁺ ions.     

EQCM study of the ECL quenching of the tris(2,2′-bipyridyl)ruthenium(II)/tris-n-propylamine system at a Au electrode in the presence of chloride ions

Significant effect of chloride ions on the electrogenerated chemiluminescence (ECL) behavior of the ruthenium(II)tris(2,2′-bipyridine) (Ru(bpy)₃²⁺)/tri-n-propylamine (TPrA) system at a Au electrode was reported. At low concentrations (e.g., [Cl⁻] < 5 mM), the ECL was enhanced; at relatively high concentrations, however, the ECL intensity decreased with the increase of the [Cl⁻]. At [Cl⁻] = 90 mM, ∼50% and 100% ECL inhibition was observed for the first and the second ECL wave, respectively. The electrogenerated gold-chloride complexes (AuCl₂⁻ and AuCl₄⁻) which were verified using an electrochemical quartz-crystal microbalance (EQCM) method were found to be responsible for the ECL inhibition. This study suggests that care must be taken when a Au working electrode is used for ECL studies in chloride-containing buffer solutions (widely used in DNA probes) and/or with the commonly used chloride-containing reference electrodes since in these cases the ECL behavior may significantly disagree with that obtained using other electrodes and reaction media.     

Layer-by-layer assembly of functional silica and Au nanoparticles for fabricating electrogenerated chemiluminescence sensor

We described the use of silica nanoparticles as building blocks for the immobilization of electrogenerated chemiluminescence (ECL) reagent Ru(bpy)₃²⁺ and the fabrication of layer-by-layer assembly film by alternating the deposition of the Ru(bpy)₃²⁺-doped silica nanoparticles and Au nanoparticles. UV-vis absorption spectroscopy, scanning electron microscopy (SEM), cyclic voltammetry and ECL were used to characterize the uniform growth of the multilayer film. Since Ru(bpy)₃²⁺ could still maintain its ECL property when doped into the silica nanoparticles, the as-prepared multilayer film could be used as an effective ECL sensor, and the sensor showed high sensitivity and good stability.     

On the adsorption of hexaammineruthenium (III) at anionic self-assembled monolayers

The binding of the electroactive hexaammineruthenium (III) 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(1 1 1). The data were satisfactorily fitted to the linearized Langmuir adsorption isotherm and a binding constant of 4.0 (±0.4) × 10⁶ 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) × 10⁵ 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.